&EPA
United States
Environmental Protection
Agency
Office of Water
Office of Science and Technology
Washington, DC 20460
EPA-822-R-08-024
December 2008
www.epa.gov
METHODS FOR EVALUATING WETLAND CONDITION
#20 Wetland Hydrology
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vvEPA
United States Office of Water EPA-822-R-08-024
Environmental Protection Office of Science and Technolog December 2008
Agency Washington, DC 20460 www.epa.gov
METHODS FOR EVALUATING WETLAND CONDITION
#20 Wetland Hydrology
Principal Contributor
University of Georgia
Todd Rasmussen
Prepared jointly by:
The U.S. Environmental Protection Agency
Health and Ecological Criteria Division (Office of Science and Technology)
and
Wetlands Division (Office of Wetlands, Oceans, and Watersheds)
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NOTICE
The material in this document has been subjected to U.S. Environmental Protection
Agency (EPA) technical review and has been approved for publication as an EPA document.
The information contained herein is offered to the reader as a review of the "state of the
science" concerning wetland bioassessment and nutrient enrichment and is not intended to
be prescriptive guidance or firm advice. Mention of trade names, products or services does
not convey, and should not be interpreted as conveying official EPA approval, endorsement,
or recommendation.
APPROPRIATE CITATION
U.S. EPA. 2008. Methods for Evaluating Wetland Condition: Wetland Hydrology. Office
of Water, U.S. Environmental Protection Agency, Washington, DC. EPA-822-R-08-024.
ACKNOWLEDGEMENTS
EPA acknowledges the contribution of Todd Rasmussen of the University of Georgia
for writing of this module.
This entire document can be downloaded from the following U.S. EPA websites:
http://www.epa.gov/waterscience/criteria/wetlands/
http://www.epa.gov/owow/wetlands/bawwg/publicat.html
11
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CONTENTS
FOREWORD vi
LIST OF "METHODS FOR EVALUATING
WETLAND CONDITION" MODULES vn
WETLAND HYDROLOGY 1
INTRODUCTION 1
HYDROLOGIC MEASURES 2
WETLAND WATER BUDGETS 11
WATER BUDGET COMPONENTS 14
OPEN CHANNEL MEASUREMENTS 23
CONTROL STRUCTURES 24
EVOLUTION AND ALTERATION OF WETLAND HYDROLOGY 29
REFERENCES 35
LIST OF TABLES
TABLE 1 A: TIDAL WETLAND TYPES 3
TABLE 1 B: NON-TIDAL WETLAND TYPES 4
TABLE 2: ILLUSTRATIVE WATER BUDGET COMPONENTS
(MM/YEAR) FOR SELECTED WETLAND TYPES 16
TABLE 3: TYPES OF CHANNEL CONTROL STRUCTURES 25
TABLE 4: Two GENERAL TYPES OF WEIRS 26
111
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LIST OF FIGURES
FIGURE 1: MULTIPLE STAFF GAGES CAN BE USED TO DETERMINE WETLAND
WATER LEVELS FOR A WETLAND WITH HIGHLY VARIABLE STAGES.
THE HIGHER GAGE IS USED DURING WET PERIODS AND IS EASIER
TO READ THAN THE MORE DISTANT ONE. ONCE WATER LEVELS
FALL BELOW THE HIGHER GAGE, THEN THE LOWER GAGE IS USED. .
..5
FIGURE 2: PIEZOMETERS ARE USED TO MONITOR WATER LEVEL CHANGES
IN THE SUBSURFACE. MULTIPLE PIEZOMETERS (CALLED A NEST)
CAN BE INSTALLED NEXT TO EACH OTHER, ONE IN EACH SOIL
HORIZON, IF VERTICAL FLOW BETWEEN SOIL HORIZONS IS EXPECTED.
ADDITIONAL PIEZOMETERS CAN BE INSTALLED IN THE BEDROCK IF
SUCH MATERIALS AFFECT THE HYDROLOGY OF THE WETLAND 6
FIGURE 3: TIME DOMAIN REFLECTOMETER PROBES CAN BE USED TO
MONITOR SOIL MOISTURE SATURATION OVER TIME, PROVIDING
AN ESTIMATE OF THE WATER CHANGE IN THE SUBSURFACE 6
FIGURE 4: HYDROGRAPH FOR A SHORT PERIOD OF TIME SHOWING THE
WATER LEVEL VARIATION. NOTE THAT THE HYDROPERIOD IS
MARKED FOR A FEW STAGES 8
FIGURE 5: LEFT: WETLAND HYDROPERIOD PLOT SHOWING DURATION
OF FLOODING VERSUS STAGE. RlGHT! WETLAND STAGE-FREQUENCY
PLOT SHOWING NUMBER OF EXCEEDANCES PER YEAR. NOTE THAT
THE LONGEST DURATION OF FLOODING OCCURS AT THE LOWER
STAGES, AND VICE VERSA. LOWER STAGES HAVE A HIGHER
FREQUENCY OF BEING FLOODED THAN HIGHER STAGES 8
FIGURE 6: WETLAND STAGE-FREQUENCY-DURATION PLOT SHOWING DURATION
OF FLOODING VERSUS STAGE FOR A RANGE OF FREQUENCIES 9
FIGURE 7: STAGE-AREA (TOP) AND STAGE-VOLUME (BOTTOM) CURVES
SHOWING CHANGES IN WETLAND AREA AND VOLUME AS A
FUNCTION OF STAGE. FLOODED WETLAND AREA IS ZERO ONCE
WATER STAGE DROPS BELOW THE GROUND SURFACE IN THE
DEEPEST SECTION OF THE WETLAND. VOLUME OF WATER
STORAGE IS NOT ZERO BECAUSE THE BED SEDIMENTS MAY BE
ABLE TO STORE AND RELEASE WATER. WETLAND AREA ALSO
INCREASES RAPIDLY IF A LEVEE IS OVERTOPPED 13
IV
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LIST OF FIGURES (CONTINUED)
FIGURE 8: RELATIONSHIP BETWEEN HYDROLOGIC EXCHANGES
AND NONTIDAL WETLAND TYPES 15
FIGURE 9: EFFECT OF WETLANDS ON SURFICIAL AQUIFER MOVEMENT.
INFLOW TO WETLAND is ON SIDE WHERE WATER TABLE LEVELS
ARE HIGHER THAN WETLAND. OUTFLOW IS ON SIDE WHERE
WATER TABLES ARE LOWER 19
FIGURE 1O: NETWORK OF PIEZOMETERS REQUIRED TO MAP WATER LEVELS
IN THE VICINITY OF A WETLAND. NOTE THAT THE WATER LEVEL
CONTOURS CAN BE MADE BASED UPON INTERPOLATION BETWEEN
MEASUREMENTS WITHIN INDIVIDUAL PIEZOMETERS 21
FIGURE 11: WATER LEVEL BEHAVIOR SHOWING SLOW DECLINE IN BASEFLOW
ALONG WITH A DISCRETE STORM EVENT WITH ITS ASSOCIATED
STORMFLOW 22
FIGURE 12: RATING CURVE SHOWING RELATIONSHIP BETWEEN WATER LEVEL
STAGE AND STREAM DISCHARGE. NOTE CHANGE IN SLOPE I
N RELATIONSHIP AS DIFFERENT PARTS OF THE CHANNEL ARE
WETTED AS THE STAGE CHANGES 23
FIGURE 13: THREE TYPES OF WEIRS (RECTANGULAR, TRIANGULAR, TRAPEZOIDAL,
FLOODED ORIFICE) USED AS CONTROL STRUCTURES FOR MEASURING
STREAM DISCHARGE. WATER LEVEL (STAGE) IS MEASURED IN WEIR
BASIN UPSTREAM OF WEIR BLADE 26
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FOREWORD
In 1999, the U.S. Environmental Protection Agency (EPA) began work on this series of reports
entitled Methods for Evaluating Wetland Condition. The purpose of these reports is to help
States and Tribes develop methods to evaluate (1) the overall ecological condition of wetlands
using biological assessments and (2) nutrient enrichment of wetlands, which is one of the pri-
mary stressors damaging wetlands in many parts of the country. This information is intended
to serve as a starting point for States and Tribes to eventually establish biological and nutrient
water quality criteria specifically refined for wetland waterbodies.
This purpose was to be accomplished by providing a series of "state of the science" modules
concerning wetland bioassessment as well as the nutrient enrichment of wetlands. The individual
module format was used instead of one large publication to facilitate the addition of other
reports as wetland science progresses and wetlands are further incorporated into water quality
programs. Also, this modular approach allows EPA to revise reports without having to reprint
them all. A list of the inaugural set of 20 modules can be found at the end of this section.
This last set of reports is the product of a collaborative effort between EPAs Health and
Ecological Criteria Division of the Office of Science and Technology (OST) and the Wetlands
Division of the Office of Wetlands, Oceans and Watersheds (OWOW). The reports were
initiated with the support and oversight of Thomas J. Danielson then of OWOW, Amanda K.
Parker and Susan K. Jackson (OST), and seen to completion by Ifeyinwa F. Davis (OST). EPA
relied on the input and expertise of the contributing authors to publish the remaining modules.
More information about biological and nutrient criteria is available at the following
EPA website:
http://www. epa.gov/ost/standards
More information about wetland biological assessments is available at the following
EPA website:
http://www.epa.gov/owow/wetlands/bawwg
VI
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LIST OF "METHODS FOR EVALUATING WETLAND
CONDITION" MODULES
MODULE # MODULE TITLE
1 INTRODUCTION TO WETLAND BIOLOGICAL ASSESSMENT
2 INTRODUCTION TO WETLAND NUTRIENT ASSESSMENT
3 THE STATE OF WETLAND SCIENCE
4 STUDY DESIGN FOR MONITORING WETLANDS
5 ADMINISTRATIVE FRAMEWORK FOR THE IMPLEMENTATION OF
A WETLAND BIOASSESSMENT PROGRAM
6 DEVELOPING METRICS AND INDEXES OF BIOLOGICAL INTEGRITY
7 WETLANDS CLASSIFICATION
8 VOLUNTEERS AND WETLAND BIOMONITORING
9 DEVELOPING AN INVERTEBRATE INDEX OF BIOLOGICAL
INTEGRITY FOR WETLANDS
1O USING VEGETATION TO ASSESS ENVIRONMENTAL CONDITIONS
IN WETLANDS
11 USING ALGAE TO ASSESS ENVIRONMENTAL CONDITIONS IN WETLANDS
12 USING AMPHIBIANS IN BlOASSESSMENTS OF WETLANDS
13 BIOLOGICAL ASSESSMENT METHODS FOR BIRDS
14 WETLAND BIOASSESSMENT CASE STUDIES
15 BIOASSESSMENT METHODS FOR FISH
16 VEGETATION-BASED INDICATORS OF WETLAND NUTRIENT ENRICHMENT
17 LAND-USE CHARACTERIZATION FOR NUTRIENT AND SEDIMENT
RISK ASSESSMENT
18 BlOGEOCHEMICAL INDICATORS
19 NUTRIENT LOADING
2O WETLAND HYDROLOGY
Vll
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WETLAND HYDROLOGY
Jhe purpose of this module is to describe
and discuss the general hydrologic prop-
erties that make wetlands unique, and to pro-
vide an overview of the processes that control
wetland hydrologic behavior. The intent is to
provide a general discussion of wetland hy-
drologic processes and methods in the hope
of fostering an understanding of the impor-
tant attributes of wetland hydrology relevant
to the monitoring and assessment of these
systems. As such, it is not intended to address
the narrower definition of wetland hydrology
for jurisdictional or classification purposes.
Also, this module should not replace more
advanced wetland texts. If the need arises to
obtain more specific information, the reader
is advised to refer to wetland books or ar-
ticles, including those referenced within this
document.
INTRODUCTION
TJ/etlands are a unique hydrologic feature
f r of the landscape. One particularly im-
portant attribute is their position as the tran-
sition zone between aquatic and terrestrial
ecosystems. Wetlands share aspects of both
aquatic and terrestrial environments because
of this position. On one hand, most freshwa-
ter and marine aquatic environments, such
as lakes, rivers, estuaries, and oceans, are
characterized as having permanent water. On
the other hand, terrestrial environments are
generally characterized as having drier con-
ditions, with an unsaturated (vadose) zone
present for most of the annual cycle. Wetlands
thus occupy the transition zone between pre-
dominantly wet and dry environments.
A diagnostic feature of wetlands is the prox-
imity of the water surface (or water table be-
low the surface) relative to the ground surface.
In freshwater and marine aquatic habitats, the
water surface lies well above the land surface,
while in terrestrial environments it lies some
distance below the root zone as a water table
or zone of saturation. The shallow hydrologic
environment of wetlands creates unique bio-
geochemical conditions that distinguish it
from aquatic and terrestrial environments.
GEOMORPHIC POSITION
Wetlands are a fundamental hydrologic
landscape unit (Winter 2001) that generally
form on flat areas or shallow slopes, where
perennial water lies at or near the land surface,
either above or below. Wetlands tend to form
where surface and ground water accumulate
within topographic depressions, such as along
flood plains, within kettles, potholes, bogs,
fens, lime sinks, pocosins, Carolina Bays, ver-
nal pools, cienegas, pantanos, tenajas, and pla-
yas, and behind dunes, levees, and glacial mo-
raines. Seepage wetlands form where ground
water discharges on slopes, as well as near the
shores of streams, lakes, and oceans. Fringe
wetlands also form along shorelines, with pe-
riodic inundation not caused by ground wa-
ter discharges but, rather, by water exchanges
with adjacent waterbodies, such as by periodic
floods and tidal action. And, finally, perched
wetlands form above low-permeability sub-
strates where infiltration is restricted, such
as above permafrost, clay, or rock (Novitzki
1989).
Brinson (1993) provides a methodology for
using hydrogeomorphic indicators to classify
wetlands based on their unique hydrologic,
geomorphic, and hydrodynamic character! s-
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tics. In this way, the dominant landscape and
hydrologic factors can be synthesized to bet-
ter develop an understanding of wetland forms
and functions.
ENERGY AS THE DRIVING FORCE
The direction and rate of water movement
into and out of wetlands is controlled by the
spatial and temporal variability of energy. A
change in energy with distance generates a
force that causes water to move from zones of
high energy to zones of lower energy. Gravi-
tational forces account for most water move-
ment, in that water tends to flow from higher
to lower elevations. Resisting the gravitational
force are viscous (friction) forces that retard
the fluid velocity. Inertial (momentum) forces
resist a change in velocity, causing water to
move at a constant velocity and in a straight
line, unless additional energy is expended to
either accelerate, decelerate, or deflect the wa-
ter.
Water can also move due to a change in
pressure, from zones of high pressure to zones
of low pressure. This is common in ground
water systems, where confined aquifers flow
to the surface because of the greater pressure
at depth. Artesian flow from a confined aqui-
fer to the surface occurs when the recharge
area to the aquifer lies at a higher elevation
than the ground surface where the discharge
occurs. Classical artesian springs exist in low-
lying areas that are supplied with flows from
higher elevation areas.
Wetlands are normally found in low-energy
environments—that is, in areas where water
normally flows with a slow velocity. This re-
sults, in part, because the land surface is rela-
tively flat in these areas (Orme 1990). Because
wetlands lie in relatively flat landscapes, their
surface area expands and contracts as the wa-
ter stage changes, allowing for the storage of
large volumes of water. Wetlands therefore
serve as a moderator of hydrologic variabil-
ity—storing flood flows and reducing flow
velocities during wet weather in particular. In
addition, shallow depths and low slopes, con-
sistent with low energy environments, are im-
portant for trapping nutrients and sediments.
HYDROLOGIC MEASURES
rhree hydrologic variables can be denned
that are useful for characterizing wet-
land hydrologic behavior; the water level, hy-
dropattern, and residence time. Each of these
wetland descriptors are described in greater
detail in subsequent sections. What follows
here is a brief introduction of these concepts.
One hydrologic descriptor is the general
elevation of wetland water levels relative to
the soil surface. Open water usually occurs
in deeper areas with few, if any, emergent
macrophytes. Any vegetation present in these
areas is usually not attached to the wetland
bottom, but vegetation may be floating on the
water surface. An emergent zone may also
be present in areas shallower than the open
water zone, containing substantial quantities
of emergent macrophytic vegetation, either
living or dead. Yet, other wetlands may have
large areas of exposed, saturated soil that is
generally covered with macrophytic vegeta-
tion. The water level can, therefore, be used as
an indicator of the vegetation types likely to
occur in each of these zones.
2
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A second descriptor of wetland hydrology
is the temporal variability of water levels. The
timing, duration, and distribution of wetland
water levels are, together, commonly referred
to as the wetland hydropattern, which incor-
porates the duration and frequency of water
level perturbations. The hydropattern of some
systems, such as tidal marshes, fluctuate dra-
matically over short periods of time; other
systems, such as seasonally flooded bottom-
land hardwood communities, fluctuate more
slowly over time. Yet, other wetland systems
are more static and may not display substantial
short- or long-term variability. The wetland
hydropattern is a function of the net differ-
ence between inflows and outflows from the
atmosphere, ground water, and surface water.
A third descriptor of wetland hydrology is
the residence, or travel time, of water move-
ment through the wetland. Some wetland sys-
tems exchange water quickly, with water re-
maining within the wetland for only a short
duration of time, while water may travel very
slowly through other wetland systems. The
residence time is the ratio of the volume of
water within the wetland to the rate of flow
through the wetland. Short residence times oc-
cur when the flow through the wetland is large
compared to its volume—longer residence
times occur when the flow is small compared
to its volume. The residence time of a wetland
is often related to its hydropattern, in that wet-
lands with large water level fluctuations may
have shorter residence times, such as in tidal
marshes. On the other hand, some wetlands
may fluctuate rapidly due to large changes in
inflow, yet have very long residence times due
to slow loss rates.
WETLAND WATER LEVEL
An important feature of wetlands is the con-
dition of oxygen deficiency in wetland soils.
Anaerobic conditions develop more quickly in
saturated soils than in unsaturated soils due
to low oxygen solubility in water, slow rates
of water advection, and slow diffusion rates
of oxygen through water. Anaerobic condi-
tions in wetland soils affect vegetation by
creating adverse conditions for root survival
and growth. Thus, the presence of water sub-
stantially affects soil oxygen concentrations,
which affects plant growth and survival.
Yet, despite these low oxygen concentra-
tions, wetlands are among the most biologi-
cally productive ecosystems on the landscape.
They support a diverse assemblage of vegeta-
tive species having special physiological adap-
tations that enable them to survive and prosper
TABLE 1 A: TIDAL WETLAND TYPES
TIDALWETLANDS:
Subtidal - Tidal water permanently covers the land surface.
Irregularly Exposed - Tidal water usually covers the land surface, but is
Regularly Flooded - Tidal water alternately covers and daily exposes the
Irregularly Flooded - Tidal water covers the land surface less often than
not exposed daily.
land surface.
daily .
3
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TABLE IB: NON-TIDAL WETLAND TYPES
NON-TIDAL WETLANDS:
Permanently Flooded - Water covers the land surface throughout the year in all years.
Vegetation is composed of obligate hydrophytes.
Intermittently Exposed - Water covers the land surface throughout the year except in years of
extreme drought.
Semipermanently Flooded - Water covers the land surface throughout the growing season in
most years. The water table is at orverynp-:|rfn'21 Ql1 rf^r^ ™'hpn thp 1 anH en rfarp ic pvnncpH
' near the surface when the land surface is exposed.
Seasonally Flooded - Water covers the land surface for extended periods, especially early in
the growing season, but is absent by the end of the season in most years. The water table is at
or near the surface when the land surface is exposed. Saturated water never covers the land
surface, but the soil is saturated to the surface for extended periods during the growing season.
Temporarily Flooded - Water covers the land surface for brief periods during the growing
season, but the water table usually lies well be low the surface for mos t of the season. Plants
that grow both in uplands and wetlands are present.
Intermittently Flooded - Water covers the land surface for variable periods with no detectible
seasonal periodicity. Long periods of time separate periods of inundation. The dominant plant
communities under this regime may change as soil moisture conditions change. Some areas
may not exhibit hydric soils or support hydrophytes.
Artificially Flooded - The amount and duration of flooding is controlled by means of pumps
or siphons in combination with dikes or dams.
in these otherwise harsh growing conditions.
Many biogeochemical reactions occur within
these low oxygen zones, as noted elsewhere in
this document.
Water levels in wetlands serve as an indica-
tor of the dissolved oxygen state of the soil-
water system. Wetter systems generally have
higher water levels and lower soil dissolved
oxygen concentrations, while drier systems
have lower water levels and higher dissolved
oxygen concentration. A general relationship
between wetland water levels and hydric states
is shown in Table 1 (Cowardin, et al. 1979).
Note that a distinction is made between soil
saturation and soil surface inundation. Some
systems may be flooded for part of the year
and still have low pore water soil saturation,
and vice versa. Low soil dissolved oxygen
concentrations and reducing conditions may
result in both cases.
Wetland water levels (also called the stage)
may not be indicative of soil saturation. The
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zone of pore saturation may extend above
the water table due to capillary rise in fine-
grained materials (Black 1996). Capillary rise
results from the natural tendency of water to
adhere to soil surfaces and other water mol-
ecules. Because the capillary height of rise
can extend for several meters above the water
table in fine-grained materials, the soil may
be entirely saturated even when water levels
are below the ground surface.
HYDROGRAPHS
A hydrograph relates the stage, or water
level, as a function of time. Between storms,
water levels in wetlands normally decline
slowly over time, rising in response to precipi-
tation. The rising limb of the hydrograph cor-
responds to the period from when water levels
begin to rise following a precipitation event.
The peak stage corresponds to the time when
water levels reach their highest level. The fall-
ing limb of the hydrograph corresponds to the
period following the peak and lasts until the
next storm.
The time to peak is the length of time be-
tween the peak precipitation and peak stage.
Times to peak are short in urban areas with
large impervious surfaces and channels that
have been modified to increase stream veloci-
ties. Times to peak are longer in forested areas
with few impervious surfaces and channels
with many obstructions that slow the passage
of water. Another term, the time of concen-
tration, is the time required for flow to travel
from the most distant point on the watershed,
and is a function of the same factors that af-
fect the time to peak.
FIGURE 1: MULTIPLE STAFF GAGES
CAN BE USED TO DETERMINE
WETLAND WATER LEVELS FOR A
WETLAND WITH HIGHLY VARIABLE
STAGES. THE HIGHER GAGE IS
USED DURING WET PERIODS AND
IS EASIER TO READ THAN THE
MORE DISTANT ONE. ONCE WATER
LEVELS FALL BELOW THE HIGHER
GAGE, THEN THE LOWER GAGE
IS USED.
MONITORING WATER LEVELS
Water levels in wetlands can be determined
using a staff gage if the water level is above
the ground surface. A staff gage is a vertical
scale that serves to indicate the elevation of
water, or stage, with respect to a reference ele-
vation (Williams, et al.1996). The staff gage is
an inexpensive tool that should placed in the
wetland such that the base of the staff gage is
always submersed in water. For ease of mea-
surement, multiple staff gages can be placed
at different depths, such that the nearest one is
visible from the shoreline during wet weather.
The deeper staff gage is used once water lev-
els fall below the nearer gage (Figure 1). In
this way, one submerged staff gage is always
visible from the shoreline as water levels rise
and fall.
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In some wetlands, a water line can be ob-
served on periodically submerged vegetation.
The water line can indicate a high-water level
after a flood. In these cases, floating debris-
such as leaves, trash, or branches—is lodged in
the canopy of trees or bushes. In other cases,
the natural water level can be observed as a
horizontal line on the sides of trees. This line
typically represents the normal water level in
the wetland. This line would not be visible
during wet weather, but is more likely to be
observed during drier periods.
If water levels are below the ground surface,
then piezometers can be used to find the water
surface (Figure 2). A piezometer is a small-di-
ameter perforated tube that is installed within
the soil at a specified depth (Black 1996). The
perforated zone should be narrow to mini-
mize interference between layers, and placed
within a unique hydrogeologic unit such as a
soil horizon or geologic layer.
Nested
Piezomete
-HH
• •
• •
• •
r
s Soil
Horizon
A
B
ii C
^M^___ * *
Bedrock
FIGURE 2: PIEZOMETERS ARE
USED TO MONITOR WATER LEVEL
CHANGES IN THE SUBSURFACE.
MULTIPLE PIEZOMETERS (CALLED
A NEST) CAN BE INSTALLED
NEXT TO EACH OTHER, ONE
IN EACH SOIL HORIZON, IF
VERTICAL FLOW BETWEEN
SOIL HORIZONS IS EXPECTED.
ADDITIONAL PIEZOMETERS CAN
BE INSTALLED IN THE BEDROCK
IF SUCH MATERIALS AFFECT THE
HYDROLOGY OF THE WETLAND.
Water levels can be inexpensively deter-
mined by lowering a weighted, chalk-covered
steel measuring tape into the piezometer. The
tape is lowered until at least one part of the
tape is wet. The reading on the tape where the
chalk has been wetted is subtracted from the
reading taken on the tape at the top of the pie-
zometer. A slightly more expensive technique
is to use a depth-to-water detector, which pro-
vides an audio or visual signal when the water
level is encountered. Another option is to use
an automated water level recorder, such as a
float or pressure transducer. The advantage of
automated techniques is their suitability for
conditions when water levels are both above
and below the ground surface (Black 1996).
When water levels are below the ground
surface, the degree of soil saturation can be
FIGURE 3: TIME DOMAIN
REFLECTOMETER PROBES CAN
BE USED TO MONITOR SOIL
MOISTURE SATURATION OVER
TIME, PROVIDING AN ESTIMATE
OF THE WATER CHANGE IN THE
SUBSURFACE.
measured using Time-Domain Reflectom-
etry (TDR). TDR determines the water con-
tent using the electromagnetic properties of a
6
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wave pulse passing through a conducting set
of rods (such as 3-mm stainless-steel welding
rods) placed in the soil (Figure 3). TDRs pro-
vide the soil water content without the need
for calibration. Because the air porosity is not
measured using TDRs, additional measure-
ments of the total porosity are required, gen-
erally by collecting soil cores. The soil satu-
ration is the ratio of the water content to the
total porosity.
The principle of TDRs is that the velocity
of the electromagnetic wave along a conduc-
tor is a function of the dielectric coefficient of
the media around the conductor (Topp 1980).
Larger dielectric constants cause slower wave
velocities and, hence, longer travel times. Liq-
uid water has a dielectric constant of 80.2 at
20°C, while ice is 3.2, petroleum is 1.8 to 2.2,
quartz is 4.3, and air is only 1.00. It is clear,
therefore, that the wave velocity is substan-
tially retarded as the water content of a soil
increases.
HYDROPATTERN
The temporal variability of water levels in
wetlands results from dynamic changes in
hydrologic inputs and outputs, and temporal
changes associated with hydraulic controls
within the wetland. Temporal changes in wa-
ter level are important determinants for many
aquatic flora and fauna. The reproductive suc-
cess of these wetland species can be adversely
affected when fluctuations are not correctly
synchronized with their developmental stages.
The hydropattern is a distinctive feature of
the hydrologic variability that describes the
variation of water levels over time and space
(Acosta and Perry 2001; King, et al.2004).
Hydropattern is a recent term that is used to
expand the traditional concept of hydroperiod
(i.e., the frequency and duration of time that
the wetland is saturated) by incorporating ad-
ditional information about the aerial extent
and timing of inundation. The aerial extent is
important, especially for large, complex wet-
lands that contain a variety of wetland fea-
tures.
Several approaches can be used to charac-
terize temporal changes in wetland stage (i.e.,
water levels). The easiest approach is to plot
wetland stage as a function of time (called the
hydrograpK). The hydrograph shows the stage
for a period of time that captures the range
of possible hydrologic variability (Figure 4).
Inter-annual, seasonal, event, and daily water
level fluctuations may become apparent using
such an approach.
Using the observed water levels, a plot of
flooding duration versus wetland stage can be
constructed (Figure 5). This plot provides a
descriptive summary that indicates how long
a typical flood occurs for each stage. Lower
elevations have longer durations of flooding
than higher elevations. This approach is use-
ful for characterizing water level variability
by generating a stage-duration relationship
that quantifies the duration in time that a
specified water level is exceeded. In this case,
the period of time that water levels exceed
the specified stage (or range in stages) is de-
scribed. This approach should also consider
the seasonal nature of inundations by dividing
the data into specific time frames (Mitsch and
Gosselink 2000).
While the stage-duration approach success-
fully captures the duration of time that the
system is flooded, it fails to characterize the
7
-------
Hydroperiod
4 5
Days
7
9
FIGURE 4: HYDROGRAPH FOR A SHORT PERIOD OF
TIME SHOWING THE WATER LEVEL VARIATION. NOTE THAT
THE HYDROPERIOD IS MARKED FOR A FEW STAGES.
500x
Duration, days
Frequency, times per year
FIGURE 5: LEFT WETLAND HYDROPERIOD PLOT SHOWING DURATION OF
FLOODING VERSUS STAGE. RIGHT WETLAND STAGE-FREQUENCY PLOT
SHOWING NUMBER OF EXCEEDANCES PER YEAR. NOTE THAT THE LONGEST
DURATION OF FLOODING OCCURS AT THE LOWER STAGES, AND VICE VERSA.
LOWER STAGES HAVE A HIGHER FREQUENCY OF BEING FLOODED THAN
HIGHER STAGES.
8
-------
>l/100yrN >l/10yr ]
Duration
FIGURE 6: WETLAND STAGE-FREQUENCY-DURATION PLOT SHOWING
DURATION OF FLOODING VERSUS STAGE FOR A RANGE OF FREQUENCIES.
frequency with which this occurs. That is, the
number of times that a water level exceeds a
specified stage for a specific time period is
not quantified. An alternative approach is to
quantify the frequency in time that the wet-
land is observed to exceed a range of specified
stages. This approach yields a cumulative fre-
quency table or plot that can be used to calcu-
late exceedance probabilities (Figure 5, left).
The mean, median, and extreme stages (e.g.,
1, 10, 50, 90, and 99 percentile probabilities)
can be estimated using the exceedance prob-
ability plot (McCuen 1998).
A significant drawback using the exceedance
probability approach is that the correlation
structure between individual observations
may or may not be captured. That is, a system
which water levels vary slowly over time can
display the same frequency distribution of wa-
ter levels as does a system that varies quickly.
In effect, the amplitudes of the fluctuations
are described, but not the duration. An addi-
tional problem is that the frequency diagram,
in aggregate, may poorly convey daily and
seasonal behaviors. Partitioning or stratifying
data sets into seasonal or other periods may
improve the characterization of water level
conditions (Mitsch and Gosselink 2000).
To overcome these limitations, a stage-
duration-frequency (SDF) curve can be con-
structed (Figure 6). The SDF plot is analogous
to intensity-duration-frequency (IDF) curves
used in precipitation analysis (McCuen 1998).
9
-------
SDF curves indicate the frequency that a
depth-duration relationship is observed.
tion for a fully mixed system with constant
inputs over time (Law and Kelton 1991).
For large, complex wetlands the hydrody-
namic behavior may be very different from
one area to the next. Characterizing the hy-
dropattern for the entire wetland is a substan-
tially more difficult exercise than for a small,
uniform wetland.
HYDROLOGIC RESIDENCE TIME
The hydrologic residence time is used to
evaluate the time required for a hydrologic
input to pass through the wetland. The resi-
dence time, 'f for a system with constant vol-
ume and flow rate is simply the ratio of the
volume of water within the wetland, V, to the
flow rate, Q, or, 't' = V / Q.
The estimated residence time is only appro-
priate for conditions of: 1) piston-flow, such as
a First-In-First-Out (FIFO) queue; 2) steady
(i.e., constant) flow; 3) single locations of in-
flow and outflow; and, 4) no atmospheric or
ground water exchanges (Himmelblau and
Bischoff 1968). The estimated residence time
is not appropriate for conditions when water
within the wetland mixes, multiple inflows
and/or exchanges occur at different points
within the wetland, or flow into the wetland is
not constant over time.
If these conditions are not met, then the
above equation only provides an estimate of
the average residence time—actual residence
times now varying over time and space. Func-
tions for describing the distribution of resi-
dence times may be found for simple systems.
For example, the exponential function can be
used to determine the residence time distribu-
Different parts of a wetland may exhibit
different hydrologic residence times. Water
in active, flow-through sections of a wetland
may have shorter residence times than water
in inactive, isolated parts of a wetland. While
each section may have identical hydropatterns,
the flow is concentrated in one area, leav-
ing other areas with stagnant conditions. The
same equation can be applied regardless; in
this case, each section would be characterized
using the volume of water present in the sec-
tion, and the flow rate would be characterized
using the flow into the section of interest.
Residence times for dynamic systems are
more difficult to calculate than those with a
steady flow. In these cases, the residence time
changes—with increasing residence times
during periods when outflows exceed inflows,
and decreasing when inflows exceed outflows.
This is because adding so-called new water or
removing old water from the system decreases
the age.
While hydraulic residence times can be
calculated using the above equation, tracer
tests can also be conducted to confirm these
calculations. Conservative tracers (i.e., non-
reactive tracers that move passively with the
water velocity) can be added at the inflow
point of a wetland, and then tracer concentra-
tions can be monitored at the outflow location.
A breakthrough curve describes the resulting
tracer concentration over time. The time re-
quired for the median concentration—when
the outflow concentration equals half of the
input concentration—provides an estimate of
the average residence time of the system.
10
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WETLAND WATER BUDGETS
TJ/etland water levels, the hydropatterns,
f r and residence times are influenced and
controlled by hydrologic inputs and outputs.
In many cases, the wetland conditions ob-
served are influenced, in large part, by the
gains and losses of water. A water budget is
used to account for the inputs and outputs to
the wetland. The exchanges can be with the
atmosphere, with ground or surface water,
or by tidal action. The sum of all exchang-
es is what affects wetland water levels, i.e.,
if atmospheric exchanges cause an increase
in water storage but ground water exchanges
deplete these storages, then the total effect is
the balance of the two.
The water balance equation summarizes
this concept:
DQ=I-0 =
DV
Dt
(1)
where 'D'Q is the difference between inflows,
/, and outflows, O, and the 'D'V term repre-
sents the change in water storage over a pe-
riod of time, 'Z)'t. This equation means that
the volume in storage increases whenever the
inflows exceed the outflows, and vice versa.
Because water levels are directly related to the
storage volume, an increase in storage volume
always results in an increase in water levels:
Dh
DQ
A =
I-O
A
(2)
where 'D'V=^4D/z, D/z is the change in water
level, and A is the wetland area. This rela-
tionship holds because the change in storage
equals the product of the area of the water-
shed and the change in water level. This rela-
tionship becomes more complicated, as noted
below, whenever the water level falls below
the ground surface. In these cases, the min-
eral and organic soil materials release less
water because of their porosity and ability to
retain water.
BALANCING INFLOWS
WITH OUTFLOWS
Potential wetland inputs include precipita-
tion directly onto the wetland, direct overland
flow, surface water inputs from rivers, streams,
and marine sources, overbank flow, and
ground water sources including subsurface,
lateral unsaturated, and saturated flow from
uplands to toe-slope and flat landscapes.
Balancing the inputs are the possible outputs,
including evaporation, transpiration, ground
water recharge, and surface water outflows.
Water levels rise over time when hydrologic
inputs exceed outputs, and fall when outputs
exceed inputs. The change in water levels can
be described using the following equation:
(3)
where D/z is the water level change, DV=D<2
D^ is the net change in the input water volume
to the wetland, T>Q=I-O is the net change in
inputs less outputs, Dt is the time step, and A
is the wetland storage area, equal to the vol-
ume of water released per unit change in wa-
ter level.
Hydrologic inputs and outputs are expressed
in units of volume or depth per unit time. Wa-
ter levels, however, incorporate no explicit
11
-------
unit of time—only water level changes are
expressed in terms of units of depth per unit
time. Thus, observed water levels are the re-
sult of accumulated water level changes over
time and can be expressed as:
h(t)= lDh(t.-t) =
J=0
DQlyt.)
A
(4)
where Dt=t-t.. This integration of water level
changes means that wetlands reduce water lev-
el fluctuations by storing water during wet pe-
riods and releasing it during dry periods. The
fact that wetland water levels are accumula-
tors of hydrologic change over time and space
makes them sensitive to even small changes in
environmental conditions. That is, even small
alterations can manifest themselves as large
changes in wetland conditions when accumu-
lated over space and time.
In addition to a water balance equation, the
mass of dissolved and suspended matter car-
ried by the water can be balanced. The mass
balance equation is written as:
DM
Dt=DL=LrL2
where Mis the mass of dissolved or suspend-
ed matter carried by the water, 'D'M is the
change in mass between two points, 'D't is
the time interval, DL is the change in load, Ll
is the inflow load, and L2 is the outflow load.
Clearly, the rate of change in mass per unit
time is a function of the balance between in-
flows and outflows.
Most water quality measurements are not
based upon a load assessment. Instead, the
solute concentration is normally measured.
The relationship between the load, L, and the
concentration, C, is found by noting that:
L=C Q
(6)
where Q is the flow rate. This is because the
concentration is:
_L M.
^(TV (7)
or mass per unit flow rate, which is just the
mass, M, per unit volume, V.
STAGE-AREA-VOLUME
RELATIONSHIPS
Changes in water depth must normally be
converted to changes in water volume. This
conversion need arises because inflows and
outflows are measured in terms of water vol-
ume, while water levels within the wetland are
measured in terms of water depth. A conser-
vation of mass approach can be used to equate
the two quantities. The conversion from water
depth, h, to water volume, V, requires knowl-
edge of the effective storage area of the wet-
land, A:
A=
DV
Dh
(8)
where 'D'Fis the change volume of water and
'D7z is the change in water level.
For conditions when water levels are entire-
ly above the ground surface, the water volume
change per unit depth equals the wetland area.
The effective storage area may change as the
wetland grows in size during high stage, thus
-------
requiring the use of a table or plot of wetland
stage versus area.
SUBSURFACE WATER STORAGE
An additional complication arises when
wetland stages are below the ground surface.
In this case, the specific yield (i.e., the drain-
able porosity) of the organic and mineral sedi-
ments must be known. The specific yield is
the volume of water released per unit area
of wetland per unit decline in water level. In
many cases, organic and mineral sediments
may remain at or near saturation as water lev-
els fall. In these cases, only a small volume of
water is released from the sediments as they
drain.
Combining the storage above and below the
ground surface yields the following expres-
sion for the effective storage area:
A=A +S A
s y e
(9)
where As is the area of submerged wetland, Ae
is the area of exposed wetland, and Sy is the
specific yield of organic and mineral benthic
sediments, generally equal to the difference
between the saturated water content and field
capacity of the sediments.
Specific yields of sediments are strongly
influenced by their particle size distribution
and chemical composition. Sands have large
specific yields, while clays and mineral soils
have low specific yields (McCuen 1998). The
specific yield can be determined by extracting
core samples and determining their specific
yield, or reference texts can also be consulted
(Fetter 2001; Hillel 1971).
Ahove-Groiuid Storage
Below-Ground Storage
10
20 30
Wetland Area
•10
Above-Ground Storage
Below-Ground Storage
100 200 3UU
Wetland Volume
FIGURE 7: STAGE-AREA (TOP) AND
STAGE-VOLUME (BOTTOM) CURVES
SHOWING CHANGES IN WETLAND
AREA AND VOLUME AS A FUNCTION
OF STAGE. FLOODED WETLAND AREA
IS ZERO ONCE WATER STAGE DROPS
BELOW THE GROUND SURFACE IN THE
DEEPEST SECTION OF THE WETLAND.
VOLUME OF WATER STORAGE IS NOT
ZERO BECAUSE THE BED SEDIMENTS
MAY BE ABLE TO STORE AND RELEASE
WATER. WETLAND AREA ALSO
INCREASES RAPIDLY IF A LEVEE
IS OVERTOPPED.
The resulting areas are used to generate
stage-area and stage-volume relationships
from which changes in stage can be related to
net changes in volume. In general, the stage-
volume relationship shows a sharp change in
13
-------
slope once water levels fall below the ground
surface, as well as when water levels overtop
a natural bank or levee. Synthetic stage-area
and stage-volume curves are presented in
Figure 7.
DETERMINING AREAS AND
VOLUMES
If sufficient detail is present, then wetland
areas as a function of water elevation can be
determined using topographic maps. Other-
wise, surface mapping using a transit or level
can provide cross-sections from which the
volume and area can be determined. Alter-
natively, aerial photographs taken at different
water stages can be used to estimate the stage-
area relationship.
One method for determining the volume of
wetland is to add a known mass of tracer, al-
low the tracer to thoroughly mix in the wet-
land, and then measure the tracer concentra-
tion. Because the tracer concentration, C, is
equal to the mass of tracer per unit volume of
water, C = M / V, the volume of water within
the wetland, V, is equal to the mass of tracer,
M, divided by the tracer concentration, V =
M/C.
If evapotranspiration is the only outflow
and there are no inflows, then the tracer con-
centration increases over time as the volume
of water within the wetland decreases. Like-
wise, if precipitation or surface and ground
water inflows are present with no correspond-
ing outflows, then the tracer concentration de-
creases over time, allowing the calculation of
the wetland volume.
Water levels can be used in conjunction
with the tracer data to obtain an estimate of
the water balance. In effect, there are three
mass balance relationships that can be used to
estimate water budget components:
Water Balance Equation:
Q\-Q2=DV/Dt
where Ql is the total inflows, Q2 is the total
outflows, DVis the change in storage volume,
and D^ is the change in time.
Mass Balance Equation:
where Ll is the total mass input, L2 is the
total mass output, and 'D'M is the change in
mass.
Concentration Equation:
C=M/V=L/Q
where C is the concentration, Mis the mass,
V is the volume, L is the load, and Q is the
discharge.
WATER BUDGET
COMPONENTS
TT/ater budgets are an important tool for
r f characterizing the behavior of wetland
systems. There are four general types of wa-
ter sources and sinks in wetlands. The first
includes atmospheric inputs and outputs,
including rainfall, snow, evaporation, and
transpiration. The second type of water ex-
change includes subsurface inflows and out-
flows. Another exchange mechanism results
from interaction with surface water, includ-
ing overland flow, as well as from rivers and
streams. A final type of exchange occurs in
marine systems that respond to tidal varia-
tions.
14
-------
Subsurface Exchanges
Atmospheric Exchanges
Surface Water Exchanges
FIGURE 8: RELATIONSHIP BETWEEN
HYDROLOGIC EXCHANGES AND
NONTIDAL WETLAND TYPES.
In other cases, exchanges may be between
different types of waterbodies and have a
hybrid character. That is, inflows may be of
one type (e.g., subsurface inputs) and outflows
may be of another (e.g., evapotranspiration).
Regardless, it is clear that identifying the key
wetland hydrologic inflow and outflow com-
ponents is a useful tool for understanding and
managing wetlands. Table 2 provides water
budget components for illustrative purposes
only; the values should not be used for spe-
cific wetlands. Instead, an estimate of actual
values for the wetland of interest would be
required in order to understand the types of
exchanges present.
Figure 8 presents a general characterization
of wetland hydrologic exchanges for three of
the four types of hydrologic exchanges (tidal
wetlands are not included). The figure was
adapted from Brinson (1993), who originally
characterized wetlands based on the type of
hydrologic inflow. The figure has been modi-
fied to show that wetlands are affected by ex-
changes of water—both inflows and outflows.
For example, atmospheric exchanges include
evapotranspiration components as well as
precipitation. Surface water exchanges incor-
porate releases from wetlands.
As noted by Brinson (1993), wetlands have
different types of inflow and outflow patterns.
That is, some wetlands have simple exchanges
with adjacent waterbodies, such as with a river
during a flood such that the wetland receives
water from the river during the rising stage of
the flood and returns water to the river during
the falling stage. Another example is the tidal
exchanges along the coast, where the water
moves in and out of wetlands to its original
source.
ATMOSPHERIC EXCHANGES
Atmospheric exchanges, i.e., precipitation,
evaporation, and transpiration, also need to
be estimated for the water budget. Rain, snow,
hail, sleet, freezing rain, fog drip, dew, and
frost are various forms of precipitation result-
ing from the condensation of tropospheric
water vapor. Water levels in wetlands that are
dependent on atmospheric exchanges tend
to be more affected by climatic signals than
wetlands dependent on ground water sources
(Orme 1980). Lakes, like wetlands, tend to in-
tegrate climatic signals over time because of
the longer residence time in these systems.
Precipitation generally occurs as a discrete
event, characterized by using intensity, dura-
tion, frequency, and areal extent. In aggre-
gate, precipitation events can be described
using monthly and seasonal averages along
with longer-term variability associated with
climatic fluctuations. While regional precipi-
tation networks can be used to estimate site
conditions, the large spatial heterogeneity of
precipitation patterns generally means that
-------
TABLE 2: ILLUSTRATIVE WATER BUDGET COMPONENTS
(MM/YEAR) FOR SELECTED WETLAND TYPES.
Southeastern Swamp
Northern Bog
Floodplain
Prairie Pothole
Atmospheric
Precip. ET
1100
900
1000
600
1600
700
1400
400
Surface Water
Inflows Outflows
10
0
3000
0
30
200
2900
0
Ground Water
Inflows Outflows
100
0
300
100
500
0
0
300
onsite precipitation measurements (either con-
ventional or recording raingages) are needed
when trying to obtain information for water
balance analysis.
The loss of water from a wetland by evapo-
ration from the water surface, as well as by
transpiration from plant leaf and stem sur-
faces, can have large effects on water levels.
The combined processes are called evapo-
transpiration. Evaporation dominates when
open water is present and vegetation is not.
Saturated soils may lose nearly as much as
open water, but not if a litter or mulch layer
is present. Transpiration dominates in systems
with little open water and large coverage of
living vegetation. Evapotranspiration rates
are affected by leaf and stem area, air, water,
and plant temperatures, atmospheric humid-
ity, wind speed, and the water potential of ex-
posed soils.
Evapotranspiration losses from wetlands in
close proximity are generally similar (Kadlec,
et al.1988). This is because the source of en-
ergy for vaporization (i.e., the sun) is region-
ally uniform, and the availability of water for
vaporization is similar owing to the lack of
water limitations in wetlands. Forested wet-
lands may have greater evapotranspiration
rates, however, due to higher leaf areas. Also,
wetlands covered with dead vegetation may
have lower evapotranspiration rates due to a
lack of transpiration, a reduction in evapora-
tion from shading, and poorer wind exchange.
If increasing eutrophication leads to increased
plant leaf area, then increased evapotranspi-
ration water losses to the atmosphere could
result.
EVAPORATION THEORY
Whether water evaporates to—or condenses
from—the atmosphere is a function of the en-
ergy state of water in the liquid and gaseous
16
-------
forms. The vapor pressure of water (a mea-
sure of the water content of the atmosphere)
is the primary measure of the energy state.
Dalton's law relates the rate of evaporation to
the difference in vapor pressure between the
air-water interface and the vapor pressure in
the atmosphere at some distance from the in-
terface:
= c[RH1es(T1)-RHaes(Ta)]
(10)
where E is the evaporation rate, c is an evapo-
ration rate coefficient, e{ is the vapor pressure
at the air-water interface, ea is the vapor pres-
sure in the atmosphere at some distance away
from the interface, and e (T ) is the saturation
' sv a'
vapor pressure, which is a function of the air
temperature, Ta.
The evaporation rate coefficient, c, is a func-
tion of wind speed and the type of evaporation
surface, either soil or water. The vapor pres-
sure in the air, e=RH^ es(T^ equals the prod-
uct of the relative humidity of the air, RHa,
and the saturation vapor pressure, es, based on
the air temperature, Ta.
The relationship between the saturated va-
por pressure and temperature for the range of
liquid water at standard atmospheric pressure
is:
e =6.11 exp
17.3T
+ 237.3
(11)
where es is the saturated vapor pressure in hPa
(1 hPa = 1 millibar) and T is the temperature
in°C.
The energy state of the liquid at the air-water
interface is a function of the fluid potential, or
pressure, at the interface. If a free surface is
present, then the fluid pressure at the air-water
interface is zero, and the water potential in the
atmosphere just above the surface equals the
saturated partial pressure of water within the
atmosphere (termed the saturated vapor pres-
sure). In this case, the relative humidity of the
air just above the interface is equal to 100%
(i.e., saturated with water vapor). If the water
potential at the water surface is negative, due
to osmotic potentials or negative pressures
within soil pores or within plant stomata, then
the relative humidity above the surface is no
longer saturated. The equilibrium relative hu-
midity, RH, as a function of fluid potential,
'y', and temperature, T, is:
RH=exp
RTJ
(12)
where R is the water vapor gas constant.
MONITORING ATMOSPHERIC
EXCHANGES
Precipitation and evaporation can be read-
ily measured using raingages and evaporation
pans, respectively. These are relatively inex-
pensive and provide reliable estimates of dai-
ly atmospheric exchanges. A single raingage
is usually sufficient for small wetlands (e.g.,
smaller than 100 ha), but multiple raingages
may be required for larger wetlands, especial-
ly if significant spatial variation in rainfall is
present.
Measured evaporation rates can be used to
estimate evapotranspiration rates. A single
evaporation pan is probably sufficient for
17
-------
all but the largest wetlands. While potential
evapotranspiration derived from pan estimates
(either manual or recording) can be used to
estimate site conditions, the local effects of
shading and wind shelter can adversely affect
the accuracy of the measurements. Pan coef-
ficients (the ratio of actual evapotranspiration
to pan measurements) are reported to range
from 0.54 to 5.3 (Carter, et al. 1979).
Automated raingages are available but more
expensive than manual raingages. Automated
evaporation pans are less reliable, and addi-
tional research is needed to improve their ac-
curacy. If pan measurements are not available,
then evaporation can be calculated using auto-
mated measurements of solar radiation, tem-
perature, relative humidity, and wind speed.
Daily precipitation data should be plotted,
along with daily evaporation. The difference
between precipitation and evaporation can be
compared to observed wetland water levels. In
systems where atmospheric exchanges domi-
nate the wetland hydrology, water levels rise
during precipitation events and fall at a rate
controlled by the evaporation rate.
SUBSURFACE EXCHANGES
Subsurface inflows to wetlands (also called
ground water discharge to wetlands) may re-
sult from shallow, topographically induced
drainage from nearby uplands or from dis-
charges of regional, confined aquifers (Ma-
ley and Peters 1999; Stuurman and de Louw
1999). Subsurface outflows from wetlands
(also called ground water recharge) may re-
sult from downward and lateral flow from the
wetland to underlying surficial aquifers, and
to deeper, confined aquifers where the con-
fining layer has been locally breached due to
collapse or subsidence.
Shallow inflows may result from perched, or
interflow, drainage on top of lower-permeabil-
ity units within the unsaturated zone, such as
clay beds, soil horizons, or even permafrost.
Shallow subsurface inflows may also arise
when the water surface within the wetland
lies below the water table in the underlying
surficial (unconfined) aquifer. In this case, the
direction and magnitude of the hydraulic gra-
dient can be estimated using aquifer water lev-
els obtained in piezometers positioned in the
vicinity of the wetland. Besides the hydraulic
gradient, the water flow rate is also dependent
on the permeability of the aquifer and any
organic and mineral benthic sediments. The
inflows may be concentrated at one or several
points within the wetland. Inflows may also
result from diffuse upward leakage, in which
case the leakage is more uniformly distrib-
uted across the benthic materials.
Subsurface inflows from deeper sources may
arise when confined aquifers discharge into
the wetland. These discharges occur when the
confining layer is breached due to subsidence
or collapse, such as in karst areas. In this case,
wetland water levels are controlled by the pi-
ezometric surface in the confined aquifer.
In addition, confined aquifer discharges can
occur when diffuse upward leakage moves
through the confining layer into the overlying
unconfined aquifer, and from there to the wet-
land. Discharges from deeper sources are less
likely to respond rapidly to individual storm
events, tending to be more responsive to sea-
sonal and longer-term changes.
Shallow inflows may respond more rapidly
to individual storm events, as well as to sea-
18
-------
sonal and climatic changes. This is because
interflow and water levels in shallow aquifers
tend to be more sensitive to net changes in at-
mospheric flux (precipitation less evapotrans-
piration in nearby upland areas).
Shallow subsurface outflows may occur if
the wetland is underlain by a layer of low per-
meability that allows the water to perch. In
these cases, a low point on the perimeter of
the wetland allows water to exit the wetland
as either overland flow, channel flow, or in-
terflow. These systems have water levels that
are perched above the regional water table,
and may also have an unsaturated (vadose)
zone present between the water table and the
perched wetland. Recharge to the underlying,
surficial aquifer also occurs through the low
permeability layer.
Downward movement of water is prevented
in permanently frozen soils, i.e., permafrost,
because any liquid water is converted to sol-
id form by heat exchange in the underlying
frozen unit. Yet, the ability of the underlying
frozen unit to freeze water can be overloaded
over time, resulting in a loss of the confining
ability of the unit. The entire loss of the per-
mafrost layer is possible if too much heat is
added to the unit.
In some cases, wetland water levels are con-
tiguous with those within the surficial aquifer
(Figure 9). In these cases, flow through the
surficial aquifer may be affected by the wet-
land. Normally, surficial aquifer water levels
dip in the direction of water flow, while the
water levels in the wetland are more horizon-
tal. Thus, wetland water levels lie below the
water table in the upgradient direction, while
they lie above the water table in the downgra-
dient direction. As a result, aquifer discharge
Unsaturated
Zone
V
Inflow
Saturated Zone
FIGURE 9: EFFECT OF WETLANDS
ON SURFICIAL AQUIFER MOVEMENT.
INFLOW TO WETLAND IS ON SIDE
WHERE WATER TABLE LEVELS ARE
HIGHER THAN WETLAND. OUTFLOW
IS ON SIDE WHERE WATER TABLES
ARE LOWER.
conditions are present at the upstream end of
the wetland, and aquifer recharge conditions
are present at the downstream end of the wet-
land. This type of flow-through wetland may
account for most of the flux of water through
a wetland that has no readily apparent inflows
or outflows.
Finally, recharge to deeper, confined aqui-
fers may occur when subsidence or collapse
has breached the confining layer that isolates
the aquifer from the surface. In this case, wa-
ter level increases in the wetland during wet-
weather periods may cause direct recharge
to the deeper aquifer (Mitsch and Gosselink
2000).
GROUND WATER GRADIENTS
The gradient of ground water potentials
governs the flow of water in the subsurface
because the potential is a measure of the en-
ergy status offerees that cause water to move,
and the direction of the forces controls this
movement. The gradient is calculated using:
19
-------
r r r r [^ ^ ^1
G=[Gx'Gy'Gz]=LDx' Dy' DzJ (13)
where G is the hydraulic gradient, composed
of components in three directions, Gx, Gy,
and Gz, which in turn are determined using
the change in head, 'D'h, in each of the three
directions, Gx, G , and Gz. Due to the layered
nature of many geologic deposits, the hydrau-
lic gradient can be simplified into horizontal
and vertical components, with each layer hav-
ing a unique horizontal flow pattern.
GTT= r G , G n =
L * yJ
TT
H
Dh Dh"!
Dx' DyJ
(14)
For flow through and across layered media,
the horizontal component of flow, qrr, can be
determined using the horizontal hydraulic
conductivity, KTT, while the vertical compo-
nent, q uses the vertical conductivity, Ky\
G
H
(17)
and
These equations are the horizontal and ver-
tical forms of Darcy's law. The negative sign
indicates that flow is from regions of higher
hydraulic head to regions where the head is
lower. The estimated quantities are for flow at
a point. The flows must be multiplied by the
cross sectional area of the unit in question to
estimate flow across the area.
r r
G=G =
V=[z]
(15)
The total flow, Q, is calculated using:
where 'G'H is the horizontal component and
'G 'v is the vertical component of the hydraulic
gradient. This approach is appropriate when-
ever horizontal layering is present. The mag-
nitude of the horizontal component within
each layer is found by determining the change
in water levels with distance, while the verti-
cal gradient between layers is found using the
change in water level with depth between two
adjacent layers.
The hydraulic gradient must be combined
with the hydraulic conductivity to determine
the ground water flux, or rate of volume flow.
Like the hydraulic gradient, ground water flux
can be separated into three components, two
horizontal and one vertical:
qH=[Vqy] qv=[qz] (16)
where ATT is the profile, or cross-sectional,
area of flow, Ay, is the map-view area of flow,
and QTT and Qy are the horizontal and verti-
cal components of total flow, respectively.
MONITORING
SUBSURFACE FLOWS
Ground water gradients—and flows—nor-
mally vary over both space and time. Thus, a
high resolution of temporal and spatial sam-
pling is required to determine the flow field
with any accuracy. This means taking mul-
tiple vertical and horizontal measurements at
sufficiently frequent time intervals in order to
capture any variability present in the system.
20
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Monitoring ground water flows can be ac-
complished by placing an array of piezom-
eters within and around the wetland. Multiple
piezometers can be placed at different depths
at each location to evaluate the magnitude
of vertical flow. A nest of piezometers, com-
posed of multiple piezometers placed at dif-
ferent depths, is needed whenever complex
hydrostratigraphic conditions (e.g., layering)
are present.
Piezometers are placed at multiple distances
away from the wetland to determine the wa-
ter table configuration in the neighborhood
of the wetland. This network of piezometers,
deployed at various locations and depths, is
required to determine the three-dimensional
characteristics of water potentials. Ideally, the
locations of these water levels should form an
equilateral triangle—not acute or obtuse tri-
200 Water level contours
o
180
Piezometer
FIGURE 10: NETWORK OF
PIEZOMETERS REQUIRED TO MAP
WATER LEVELS IN THE VICINITY
OF A WETLAND. NOTE THAT THE
WATER LEVEL CONTOURS CAN BE
MADE BASED UPON INTERPOLATION
BETWEEN MEASUREMENTS WITHIN
INDIVIDUAL PIEZOMETERS.
angles (Figure 10). Otherwise, the colinearity
of the wells interferes with the estimation of
the gradient.
Estimating the total flow of water into or
out of the wetland requires an independent es-
timate of the hydrologic conductivity. Either
aquifer tests can be conducted or standard
tables of values can be used (Fetter 2001).
Undisturbed core samples or field testing can
also be used to estimate hydraulic conductivi-
ties. Care must be taken when using core data
to estimate hydraulic conductivities in that
the spatial variability of most geologic media
is very high. In general, core samples tend to
underestimate field hydraulic conductivities
due to the difficulty in obtaining core samples
for the very highest flow paths within the sys-
tem.
Verry and Boulter (1979) report that the
hydraulic conductivity of peat can be readily
determined from its bulk density or unrubbed
fiber content. They report that fibric peats have
three-orders of magnitude higher horizontal
hydraulic conductivity than sapric peats due
to their larger pore sizes. Daniel (1981) notes
a similar effect of decomposition on hydraulic
conductivity.
Once the directional hydraulic conductivi-
ties and gradients have been found, then the
total flow within the system can be calculated.
There are often many uncertainties with this
method, however, in that the spatial variabil-
ity of both gradients and conductivities can be
high.
Water quality sampling can be used as an
independent method to determine the source
of water within wetlands. For example, if shal-
21
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low ground water is moving laterally into the
wetland, and ground water is also moving up-
ward into the wetland from a deeper aquifer,
then the geochemical signature of each source
can be used to evaluate the relative magnitude
of each inflow relative to the total.
flowing before the rainfall (a typical situation),
stormflow is the flow that occurs in addition
to the baseflow that would have occurred if it
had not rained.
FLOOD ATTENUATION
SURFACE WATER EXCHANGES
Surface water exchanges with wetlands re-
sult from a large number of mechanisms, in-
cluding overland—or sheet—flow, direct ex-
change when the channel of a river or stream
flows through the wetland, overbank flooding
during wet weather when the channel is sepa-
rated from the wetland by a levee or flood-
plain, and along the edges of lakes, estuaries,
and the ocean. These surface water exchanges
result in either constant or episodic hydrolog-
ic communication between the surface water
and the wetland.
Streamflow can be divided into two types,
baseflow and stormflow (Figure 11). Baseflow
is that component of flow found during low
flow periods, while stormflow refers to the re-
sponse to precipitation events. If a stream was
<2,
per unit change in stream cross sectional area,
'DM:
Dv
(19)
where Q=A v is the stream discharge, A is the
stream cross-sectional area, and v is the mean
stream velocity. This equation indicates that
the flood wave velocity equals the water ve-
locity plus a second term that is positive if the
water velocity increases as the cross-sectional
area (or, equivalently, stage) increases, and is
negative if the water velocity slows as the area
or stage increases. In other words, the wave
velocity is faster than the water velocity if the
water velocity increases with stage, and vice
versa.
This concept can also be demonstrated us-
ing the ratio of the wave velocity to the water
velocity, termed the kinematic ratio, k:
£ A Dv Dlnv
v=1+v DA=1+DlnA
(20)
22
-------
•s
w
p
Streambank
overtopped
Stream fills
channel
Streambed
exposed
Stage
FIGURE 12: RATING CURVE SHOWING
RELATIONSHIP BETWEEN WATER
LEVEL STAGE AND STREAM
DISCHARGE. NOTE CHANGE IN SLOPE
IN RELATIONSHIP AS DIFFERENT
PARTS OF THE CHANNEL ARE WETTED
AS THE STAGE CHANGES.
This relationship again illustrates the con-
cept that the flood wave velocity is faster
than the water velocity, k>l, when the second
term is positive, and vice versa. The second
term in the equation is the critical parameter
needed to control damaging flood waves. In
effect, wetlands serve to sequester flood wa-
ters, thus slowing the average and incremental
water velocities and flood wave travel times
and reducing peak discharges. The flood wave
velocity could actually be less than the water
velocity if the velocity decreases with increas-
ing depth.
MEASURING SURFACE FLOWS
Surface water flows can be estimated using
flow measurement control devices, such as
weirs (which require a pool upstream and are
not satisfactory when elevated sediment con-
centrations are present), flumes (which tend to
flush sediments more effectively than weirs),
and culverts (which are less accurate). The re-
lationship between stage and streamflow dis-
charge is called the rating curve (Figure 12).
Once a rating curve has been developed, the
stream discharge is readily found by observ-
ing the stage and then consulting the rating
curve.
OPEN CHANNEL
MEASUREMENTS
Otream discharge, Q, is obtained by estab-
kJ lishing a stream cross section, and then
measuring the stream velocity, v, across the
section. Because the stream velocity and depth
vary across the section, the total discharge is
approximated by the sum of the discharge of
subsections within the total section:
Q=
..
(21)
where A is the cross-sectional area of the sec-
tion, n is the total number of subsections, v. is
the average velocity in each subsection, and
A. is the area of each subsection, equal to the
product of the width and average depth of the
subsection.
For shallow streams, the average velocity is
found at approximately 60% of the depth of
the stream, measured from the surface down.
For deeper streams, the average velocity is
found by averaging two velocity measure-
ments, at 20% and 80% of the depth.
Upstream and downstream variations in
channel shape, as well as obstructions, may
cause rapid changes in velocities within the
cross-section. Thus, it is important to select
23
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a location where the channel appears to be of
uniform width and depth and free of obstruc-
tions.
The site should also be selected so that
backwater effects from downstream inflows
are avoided. Another source of error occurs
when the channel shape changes over time,
so a solid bottom is preferred over a mobile
bottom. Finally, a site located upstream of a
knickpoint (a narrowing or shallowing of the
river) is preferred over a site located down-
stream of a knickpoint. The knickpoint may
cause subcritical (slow velocity) conditions
upstream and supercritical (high-velocity)
conditions (and even a possible hydraulic jump)
downstream.
To construct the rating curve, the observed
stream discharge is related to the river stage,
measured using a staff gage. The staff gage is
a vertical rule placed in a protected location.
Repeated measurements of discharge over a
range of stages is required.
CONTROL STRUCTURES
Control structures avoid the vagaries of
channel geometry by creating a uniform
section. Table 3 provides a summary of the
various types of control structures along a
brief summary of their attributes. A flume
can be readily constructed with a uniform
cross sectional area so that Q=W h v where
W is the width of the flume, h is the depth of
water in the flume, and v is the water velocity
through the flume. In most cases, no unique
relationship between depth and velocity can
be established, being a function of the slope
of the flume and the upstream and down-
stream conditions.
An improvement on the standard flume is
to place a constriction in the flume (called the
throat) that forces the flow to be subcritical
(low velocity) upstream of the constriction,
and supercritical (high velocity) downstream.
Examples of this type include the Parshall
Flume and the H-Type flume. The H-type
flume was developed by the U.S. Department
of Agriculture for measuring discharge in
sediment-laden streams. Flumes require no
upstream stilling basin and allow sediment
to pass unimpaired through the structure. Ice,
leaves, and other debris can still affect the
reading, however.
Yet another control structure is the weir
(Figure 13). A weir has a stilling basin up-
stream of a constriction (normally called the
weir blade), and a free-fall below the constric-
tion. The stilling basin is used to eliminate the
velocity head, yielding H=z in the stilling ba-
sin, where z is the measured water surface el-
evation above the lowermost point on the weir
blade.
Water flows out of the weir over the weir
blade, which can take a variety of shapes, in-
cluding triangular, rectangular, and trapezoi-
dal. A submerged, circular orifice can also
be used. The discharge is calculated using
Q=vA=v (WH\ where A=WH is the cross-
sectional area perpendicular to flow above the
weir blade, Wis the width of the weir, and H
is the depth of flow above the weir blade. So
the general weir formula is:
Q=aH
(22)
where a accounts for the cross-sectional area
as well as contraction and energy losses, H is
the water surface elevation in the stilling basin
-------
TABLE 3: TYPES OF CHANNEL CONTROL STRUCTURES
WEIRS:
stilling basin is located upstream of weir
water level recorder is used to measure stage in stilling basin
outlet structures include rectangular, triangular (v-notch), and Cipolletti (trapezoidal) shapes
weir crests can be broad (flat lip) and sharp (knife-blade) crested
flow is subcritical upstream of crest, supercritical downstream
weirs collect sediments in the stilling basin, debris on weir blade
FLUMES:
no stilling basin, only a narrow throat
• regular approach section
passes sediment easily
woody debris can be a problem
CULVERTS:
Four combinations of flow equations, flooded vs. open upstream, flooded vs. open
downstream
• Culvert should be a regular shape, round or rectangular, with no debris
above the weir blade, and b=0.5 for a flooded
orifice, b=1.5 for a rectangular weir, and b=2.5
for a triangular weir. This equation holds for the
broad-crested weir as well, with b=1.5.
total head under subcritical conditions. Weirs
tend to be more accurate than flumes but suf-
fer from sediment accumulation in the stilling
basin and debris obstructing the weir crest.
Weirs may not provide accurate estimates in
several situations. One source of error occurs
when the weir blade becomes blocked by ice
or floating debris, such as leaves and branches.
Another source of error arises when the weir
basin fills with sediments, resulting in an inac-
curate estimate of the total head. For all weirs,
a staff gage or a water-level recorder is placed
upstream of the constriction to measure the
Culverts under roads can also be used as a
control structure. Four types of flow condi-
tions can be found for most culverts; upstream
intake either submerged or open, and down-
stream discharge conditions either submerged
or open. When the upstream end is flooded,
and the downstream end is open, then the
orifice solution for the weir equation can be
used.
25
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Rectangular Triangular
Trapezoidal
Orifice
\
FIGURE 13: THREE TYPES OF WEIRS (RECTANGULAR, TRIANGULAR,
TRAPEZOIDAL, FLOODED ORIFICE) USED AS CONTROL STRUCTURES FOR
MEASURING STREAM DISCHARGE. WATER LEVEL (STAGE) IS MEASURED IN
WEIR BASIN UPSTREAM OF WEIR BLADE.
When both upstream and downstream open-
ings are submerged, then pipe flow conditions
are present and discharge can be found using
the difference in head between the two ends,
the culvert length and diameter, and the type
of culvert (smooth, corrugated, etc.). When
the upstream end is open and the downstream
end is either open or flooded, then the flow
can be found using indirect techniques such
as the Manning's equation, noted below.
Regardless of flow conditions, it is better if
the culvert has a uniform shape throughout
its length and is not obstructed with debris.
Elevation measurements can be made
Type
Flooded Orifice
Rectangular§
Triangular
Trapezoidal
Weir Equation:):
Q=CAH°-5
Q=Ca WH1-5
Q=CbtanaH2-5
Q=Ca WHl-5+Cb tana//2-5
T Neglects contraction effects along weir blade edges
* H is elevation of water surface in stilling basin
§ Applies to broad-crested weirs as well
TABLE 4: TWO GENERAL TYPES OF WEIRS INCLUDE BROAD-CRESTED AND
SHARP-CRESTED. AS THE NAME IMPLIES, THE BROAD-CRESTED WEIR HAS
A BROAD CONSTRICTION IN THE DIRECTION OF FLOW, WHILE THE SHARP-
CRESTED WEIR HAS A KNIFE-EDGE BLADE THAT FORMS THE CONSTRICTION.
A BROAD-CRESTED WEIR CONSISTS OF AN OUTFLOW STRUCTURE OVER
WHICH WATER FLOWS FOR SOME DISTANCE BEFORE FALLING OVER THE
DOWNSTREAM EDGE. A SHARP-CRESTED WEIR IS CONSTRUCTED SO THAT
THE FLOWING WATER PASSES OVER A VERTICAL, KNIFE-EDGE,
THUS MINIMIZING RESISTANCE WITH THE WEIR BLADE.
26
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mechanically using a water level recorder or
visually using a staff gage.
INDIRECT MEASUREMENTS
For situations in which control structures
are not present and instream measurements
not possible, Manning's equation is com-
monly employed to indirectly measure water
velocity:
- ID2/3cl/2
v= R S
n
(23)
where n is the Manning's roughness coeffi-
cient, R is the hydraulic radius, and ^is the gra-
dient in total head. The roughness coefficient
is normally found in tables and is based on
stream channel characteristics such as stream
bed materials, amount of vegetation within
the channel, variation in channel shape, and
sinuosity. The hydraulic radius is defined as
the ratio of the stream cross-sectional area to
the wetted perimeter, R=A/P, which is approx-
imately equal to the water depth in a shallow,
wide channel. The total head gradient is the
drop in total head per unit distance of stream
channel.
Once the average water velocity, v, has been
determined, the stream discharge, Q, can be
estimated using:
Q=vA
(24)
where A is the cross-sectional area of the
channel.
ESTIMATING OVERBANK FLOWS
Wetlands adjacent to riverine systems are
often affected by overbank flows during
stormflow periods. In these cases, the river
spills out of its normal channel and overflows
onto adjacent floodplains. The period of time
that wetlands on the floodplains are affected
by the duration of stormflows, and their am-
plitude. Instrumentation to monitor water lev-
els in wetlands adjacent to riverine systems
can be installed using techniques mentioned
previously. Additionally, the U.S. Geological
Survey provides estimates of overbank flood-
ing frequencies for ungaged sites (Jennings, et
al. 1994).
TIDAL EXCHANGES
Coastal wetlands are similar in many ways
to freshwater wetlands, except that they are
transitional between marine and terrestrial
environments. Coastal wetlands have unique
attributes that distinguish them from both ter-
restrial and marine systems. It is, in fact, the
combination of flooding and soils near the
water surface that promotes ecologic diversity
and productivity in coastal wetlands.
Coastal wetlands occupy similar landscape
positions as lacustrine wetlands. Great vari-
ability exists, however, among coastal wet-
lands. Some coastal wetlands are dominated
by the ebb and flow of the water levels of the
ocean due to tides, termed tidal wetlands.
The large magnitude of the daily tides, along
with their regularity, result in unique wetland
conditions. Other marine wetlands are more
sheltered from tidal effects. Still others may
be affected by water quality changes resulting
from freshwater tributaries.
27
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TIDAL EFFECTS
Many coastal wetlands function to dampen
tidal and wave energies. Because viscous and
turbulent drag through coastal wetlands dis-
sipates energy, the magnitude of fluctuations
generally diminishes with distance from the
coast. The equation for a harmonic wave
height with constant energy dissipation as a
function of distance is (Carslaw and Jaeger
1959, Eq. 2.6.8):
h=hQ e Xcos(wt-kx)
with k=
,^
where h is the wave or tidal height, h is the
maximum magnitude of the fluctuation at the
shoreline, '&' is a damping coefficient that ac-
counts for the dissipation of energy with dis-
tance, x is distance from the shoreline, w is the
frequency of the fluctuation, t is time, and D
is the hydraulic diffusion coefficient. The am-
plitude of the oscillation, \h\=ho e~ *, decreases
with increasing frequency and distance from
the shoreline, as does the lag, 'k'x. This means
that low-frequency waves, such as daily and
twice-daily tidal fluctuations, propagate deep-
er into coastal regions and are attenuated less
and lagged more than high-frequency waves.
While the harmonic wave height equation
may not be suitable for many coastal areas,
it does combine most of the important fac-
tors (e.g., time, distance, wave frequency,
and hydraulic resistance) that affect water
and energy movement. An understanding of
the local coastal morphology and vegetation
is important, as are the energy inputs from
marine sources and terrestrial surface water
and ground water inflows (Dronkers 1986).
Together, these external factors influence how
coastal wetlands are affected by hydrologic
tidal exchanges.
Tides can affect wetlands many miles from
any saline water sources. The hydrodynamic
conditions imposed by a changing sea level
cause rivers to flow more slowly during high
tides, and vice versa. Thus, water levels in riv-
ers upstream of the coast rise and fall in tan-
dem with the tides. The magnitude of this ef-
fect is diminished with distance upstream and
is a function of local channel features.
OTHER INPUTS
Many coastal wetlands are affected by
nearby freshwater inputs, especially in es-
tuarine environments. In these cases, occa-
sional, large stormwater inflows can cause
rapid changes in the salinity, temperature,
dissolved oxygen, and sediment concentration
within the wetland. In some cases, these in-
puts can be beneficial, such as historical sedi-
ment deposition in the Mississippi River delta
region of southern Louisiana—in contrast to
the current practice of diverting stormflows
away from coastal wetlands, which has led to
regional subsidence and salt water intrusion.
In other cases, these inputs can be detrimen-
tal, as when increased urban wastewater and
stormwater inputs to coastal estuaries alter
the natural conditions.
Ground water inputs to coastal wetlands may
also be significant, and can take two forms,
point and diffuse. Point discharges—such as
springs—form when an underlying confin-
ing layer for an artesian aquifer is breached,
allowing the upward flow of water. Diffuse
upward leakage occurs when the confined ar-
tesian aquifer discharges over a large region.
28
-------
In these cases, the leakage is more spatially
uniform, with greater amounts of leakage oc-
curring in low-elevation areas. Ground water
exchanges may be more difficult to character-
ize than other sources, however. One prom-
ising method is the use of radioisotopes to
differentiate between groundwater and other
inputs (Charette, et al.2003).
MONITORING COASTAL
WETLANDS
In some cases, distinguishing tidal from
freshwater and ground water inputs can be
achieved using geochemical information. The
tidal water quality is clearly distinguished by
its high concentrations of sodium-chloride
type water, while freshwater inputs normally
have markedly lower specific conductivities
and total dissolved solids. Depending upon
location, ground water inputs are intermedi-
ate, with possibly distinct geochemical signa-
tures. For example, if carbonate aquifers are
present, then a calcium signal may be present
in these waters.
The degree of tidal flushing can thus be
monitored using water quality data to charac-
terize the residence times of water within the
system. Estimates of fluxes can then be esti-
mated based on the water balance equation.
EVOLUTION AND ALTERATION
OF WETLAND HYDROLOGY
TT/etlands change over time. Natural pro-
r r cesses such as sediments that fill wet-
lands and beaver activity, as well as acceler-
ated processes such as upstream development
and direct alteration of the wetland, all cause
changes that affect wetland hydrologic be-
havior. This section discusses some of these
effects, focusing on a few of the many factors
that cause wetlands to change over time.
NATURAL FORCES OF CHANGE
Wetlands are formed as the result of many
geologic forces. Rivers form flood plains that
provide a landscape position that enhances
wetland development. Glaciers scour the
landscape, leaving behind features that pro-
mote wetlands. Tectonic uplift and subsidence
create depressional features that are favorable
to wetland formation. Carbonate aquifers dis-
solve over time, leaving behind depressions
where wetlands can form. Also, accelerated
erosion transports sediment out of natural
channels, leading to down-cutting and deep-
ening of channels, which leads to a lowering
of riparian water tables and the reduction of
overland flows, both of which alter wetland
saturation.
Wetlands can modify their environment as
they mature. Peats may substantially modify
the original landscape by filling in the depres-
sion they originally formed in (Daniel 1981).
Other biological forces also promote wetland
formation. Beaver create impoundments,
which form natural wetlands in habitats fa-
vorable to their needs. Large, woody debris
also forms natural dams that impound shal-
low wetlands.
Once formed, wetlands can also age over
time, slowly filling in with external sources of
materials such as sediments from upland ero-
sion, as well as with detrital materials from
wetland vegetation. Rates of deposition of
these materials can be slow, such as in olig-
29
-------
otrophic systems with small upstream catch-
ment areas. Or they can be rapid, such as in
nutrient-rich systems with large upstream ar-
eas marked by extensive erosion.
dikes, revetments, and jetties obstruct or al-
ter natural hydrologic patterns. Many of these
alterations resulted from efforts to drain wet-
lands.
Wetlands in a natural setting, therefore, are
constantly being formed and lost—depending
on the balance of forces. So, too, wetland hy-
drology changes over time. Reducing the vol-
ume of storage within a wetland decreases the
residence time, and can also reduce the depth
and hydropattern by removing storage volume
within the deep-water areas that would nor-
mally remain wet under drought conditions.
The dynamic nature of hydrology—espe-
cially when applied to wetlands—means that
wetlands can not be investigated apart from
their regional environment. Hydrologic altera-
tion upstream of the wetland affects wetland
evolution.
HUMAN ALTERATION OF
WETLAND HYDROLOGY
Humans have substantially increased hy-
drologic disturbances within watersheds
(Azous and Horner 2001; Fisk 1989). These
changes generally cause increased sediment
production and transport, as well as increases
in nutrient concentrations and loads. Such in-
creases naturally cause reductions in wetland
storage volumes due to sediment trapping and
nutrient uptake with subsequent deposition of
organic sediments.
Surface water inflows to wetlands can be
increased by routing stormwater runoff into
them from urban, industrial, and agricultural
areas. Inflows are also altered when hydraulic
structures such as reservoirs, canals, levees,
Outflows from wetlands were increased by
the construction of drainage ditches, chan-
nels, and canals, or the removal of natural
barriers such as vegetation and by straighten-
ing streams. Other efforts to drain wetlands
used ground water extraction techniques such
as underground tile drains and pumping wells
that led to lowered ground water levels. Low-
ering of water tables can affect wetlands by
increasing subsurface drainage from the wet-
land to the point of ground water extraction.
Ditches and tile drains increase the discharge
of shallow ground water, thus lowering water
tables in the vicinity of the drain. Water levels
increase with distance away from the drains,
reaching a maximum midway between the
drains. Tile drainage systems increase the
rate of shallow ground water flow, thus fa-
voring drier conditions within the wetland.
Tile drainage systems are more effective for
removing water resulting from low-intensity,
long-duration storms, and are less effective for
draining water resulting from short-duration,
high-intensity storms (Galatowitsch and van
der Valk 1994).
Drainage of fields for agriculture may re-
duce surface water inflows, lower water ta-
bles, and reduce the seasonal period of soil
saturation. Ground water pumping in the vi-
cinity of the wetland can lead to a reduction in
shallow aquifer water levels while irrigation
may increase water levels, resulting in either
decreases or increases in wetland water levels,
respectively. The effects of regional ground
water pumping tend to manifest themselves as
slow (and in some cases, rapid) declines in re-
-------
gional, confined aquifer levels. These declines
are then transmitted by reductions in diffuse
upward leakage or direct connections to wet-
lands, resulting in the lowering of water levels
in wetlands.
The effect of the alteration of channels and
canals can be two-fold—not only do the new
excavations convey more water out of the wet-
land, the spoils (i.e., the materials removed
from the excavated areas) are commonly piled
near the excavations and may concentrate or
otherwise alter the natural drainage through
the wetland (Chabreck 1988).
As mentioned above, beaver ponds form
natural wetlands that once dotted the land-
scape. Beaver trapping and eradication efforts
may therefore have reduced the formation of
new wetlands. Also, reducing the availability
of large, woody debris may reduce wetland
formation. Harvesting of riparian vegetation—
particularly the larger diameter trees—could
result in poorer recruitment of large, woody
debris. While wetlands clearly affect vegeta-
tion, fish, and wildlife, it is also true that these
biological factors affect wetlands.
Obstructions (such as beaver dams, roads,
channels, dams) to surface water exchanges
alter the hydrology by requiring a higher stage
to pass the same flow. Obstructions to inflows
may deprive the wetland of natural flows. In
some cases, obstructions may not substantially
alter total wetland outflows; they may just al-
ter the stage-discharge relationship, requiring
a higher water level in order to pass an equiv-
alent discharge. This may have both positive
and negative effects on wetlands. Increased
water levels can alter the natural storage abil-
ity (a negative effect), but may increase the
residence time (a positive effect).
Some wetlands can have a large hydraulic
effect by mitigating flood flows. Wetlands can
retard floods by slowing the average water ve-
locity, as well as the flood peak velocity. Rapid
flood waves cause greater damage downstream
because they have less time to dissipate their
peaks. Slower floods have smaller peaks and
lower velocities, resulting in less downstream
damage. This is accomplished by the addi-
tional friction, or resistance to flow, that wet-
lands provide, along with the increased water
storage capacity associated with their area.
Efforts to mitigate stormwater runoff have
resulted in endeavors to design and construct
artificial wetlands to mimic the beneficial hy-
drologic effects of natural wetlands (Kadlec,
et al.1993; Walker, 1995; Walker and Kadlec
2002; Moustafa and Kadlec 2002). These en-
deavors focus on using hydrodynamic models
to achieve specific management goals that re-
quire reductions in nutrients, sediments, and
peak flows.
As noted earlier, Manning's equation sug-
gests that several factors affect the water ve-
locity, including the flow roughness of flooded
ground, the hydraulic radius (i.e., the effective
water depth), and the water energy slope. Wet-
lands with substantial macrophytic vegetation
can increase the hydraulic roughness, thus
decreasing flow velocities. Also, shallow wa-
ter bodies reduce the hydraulic radius, again
decreasing flow velocities. Removing wetland
vegetation thus decreases the hydraulic radius,
causing increased water velocities.
It is also apparent that increased wetland
loading rates, along with decreased retention
times, substantially decreases the effective-
ness of wetlands in storing water during flood
periods and subsequently releasing it during
31
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dry periods. The resulting hydrologic perfor-
mance of affected wetlands is fundamentally
compromised.
Irrigation can increase water tables if the
amount of irrigation exceeds plant needs. In
many arid areas, surplus irrigation is required
to remove dissolved salts from the root zone.
Under these conditions, recharge from irriga-
tion, precipitation, and/or surface flows may
increase water levels to the point where sur-
face inundation results, forming saline wet-
lands (Mitsch and Gosselink 2000).
As noted earlier, some coastal wetlands are
dominated by nearby freshwater inputs, es-
pecially in estuarine environments. In these
cases, occasional, large stormwater-driven in-
flows can cause rapid changes in the salinity,
temperature, dissolved oxygen, and sediment
concentration within the wetland. Alteration
of the coastal morphology by dredging can
adversely affect natural wetlands by increas-
ing saltwater intrusion rates (Chabreck 1988).
Density-dependent stratification of estua-
rine waters may prevent salt-water presence
in coastal areas. Construction of deep-water
navigation channels may allow for salt-water
to gain inland access, which can result in in-
creased salinities.
Coastal ground water pumping affects coast-
al wetlands by reducing artesian pressures in
underlying confined aquifers, which may then
cause a reduction in point and diffuse upward
leakage. Also, ground water pumping may
cause coastal subsidence, resulting in the ef-
fective lowering of the ground surface relative
to the sea level, causing the intrusion of saline
water into coastal wetlands.
Pumping from shallow aquifers can also
lower coastal zone water levels, causing local
dewatering of coastal wetlands. Shallow dis-
posal of septic wastes can alter local ground
water quality by increasing organic and nutri-
ent loading and decreasing dissolved oxygen
concentrations. These changes can affect lo-
cal wetlands if and when this ground water
discharges into them.
WETLAND RESTORATION
Efforts toward restoration of the hydro-
logic function of compromised wetlands are
currently expanding (Hey and Philippi 1999;
Means and Hinchee 1999). Additional ef-
forts are being undertaken to create artificial
wetlands that take advantage of the natural
functions that wetlands provide (Kadlec and
Knight 1995; Hammer 1996). Regardless of
whether impaired wetlands are being restored
or new wetlands are being created, the intent
is to recreate the hydrologic behavior that we
find so important (Greeson, et al.1979). The
emphasis in these cases is the design and
evaluation of alternative strategies for wetland
restoration (Marble 1992).
As shown earlier, water levels in wetlands
can be controlled by manipulating the stage-
discharge relationship. Changing the elevation
of an outflow structure, e.g., by raising the
base of an outlet weir elevation, changes the
wetland water levels and flooded areas. The
base elevation, along with the rate of change
in discharge with elevation, can be adjusted
using outflow structures of different sizes and
shapes, depending upon the desired outflow
characteristics.
32
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Other hydrologic alteration possibilities in-
clude closing of ditches and drains, thus re-
ducing wetland outflows. Removing artificial
obstructions such as roads and berms can also
improve flow through the wetland by recre-
ating natural hydrologic communication with
neighboring waterbodies.
These principles apply not only to fresh-
water wetlands but also to the restoration of
tidal wetlands, which requires the recreation
or simulation of natural hydraulic conditions
(Zedler 2001). Weirs and plugs are devices
used in tidal marshlands to maintain mini-
mum water levels (Chabreck 1988). Weirs are
useful because the bottom elevation of the
weir controls the minimum elevation on the
upstream side but allows higher flows to pass
unaffected. Ditch plugs provide the same con-
trol, but are more susceptible to destruction
during high flows.
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34
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