Impact Assessment Study of Climate Change on
Evapotranspiration and Irrigated Agriculture
in the San Luis Valley, Colorado
Finnerty, B., and J. A. Ramirez, 1995: ‘Impact Assessment Study of Climate Change on Evapotranspiration and Irrigated Agriculture in the San Luis Valley, Colorado’, AWRA 31st Annual Conference and Symposia, Houston, TX Nov.
Office of Hydrology
NOAA/National Weather Service
1325 East-West Highway
Silver Spring, Maryland 20910
Jorge A. Ramirez, Ph.D.
Hydrologic Science and Engineering Program
Deptartment of Civil Engineering
Colorado State University
Fort Collins, Colorado
ABSTRACT: The impacts of CO2, temperature, precipitation, and
water
table variations on evapotranspiration (ET) and irrigated
agriculture
were assessed. The sensitivity of ET to both CO2 and air
temperature
changes was evaluated using the modified Penman-Monteith
equation.
This equation accounts for the effects of atmospheric CO2 on
plants'
stomatal resistance, as well as air temperature changes on
land-surface-atmosphere water vapor exchanges. A root zone soil
water
balance was performed using a real-time, physically-based
soil-crop-climate model to analyze the sensitivity of soil
moisture to these
climate-induced changes. The economic sensitivity of potato
production to potential changes in available irrigation water and
agro-economic parameters was analyzed and compared with the
potential
climatic impacts on agriculture.
INTRODUCTION
The San Luis Valley is of great importance to Colorado's agricultural economy
and contains vast water resources which are of interest to agricultural, urban
and down stream water users. Anthropogenic impacts on land-surface features, emissions
into the atmosphere of greenhouse gases such as carbon dioxide [CO2], and natural
climate variability have a significant effect on water mass and energy budgets,
thus affecting hydrologic system response, weather, and climate (Cotton and Pielke,
1992).
Atmospheric CO2 levels are expected to double pre-industrial
revolution levels by some point in the next century, and are
expected
to have many potential impacts on climate, vegetation, and
hydrology.
General Circulation Model (GCM) simulations under 2xCO2 scenarios
simulate a global climate with temperature increases from 2 C to
5
C, with regional temperature changes from -3 C to +10 C.
Precipitation is expected to vary in the range of +20% to -20%
from
current regional averages (Peterson and Keller, 1990).
Variations in
depth to the water table may result from climate induced changes
in
groundwater recharge or from human consumption. Increased
atmospheric
CO2 is also known to effect many plant species' stomata, which
control
transpiration, as well as increase plant biomass production by
enhancing photosynthesis (Morison, 1987; Rosenberg, 1981, 1988).
To more effectively manage the water resource of the San Luis
Valley in a changing climate this work assessed the possible
impacts
of many different climate scenarios. Given the uncertainty in
predicting climate changes and climatic variability, a wide range
of
potentially plausible climate change scenarios were analyzed:
both a
3 C increase and decrease in air temperature; 50% and 100%
increase
in atmospheric CO2 concentrations; both a 25% increase and
decrease in
precipitation volumes; and water table depths ranging from 1 to 3
meters below the soil surface. The fertilization effects of CO2
on
crop production was also analyzed by assuming a 50% increase in
biomass production for the 2xCO2 scenario.
EVAPOTRANSPIRATION
Evapotranspiration (ET) is driven and controlled by the
physical
climatic conditions that exist at the land surface-atmosphere
boundary, and the physiological characteristics of the
vegetation.
Potential evapotranspiration (PET) is the upper bound on actual
evapotranspiration (AET), and represents the vertical flux rate
that
exists if water supply in the soil-plant system is not
constrained.
PET is controlled by atmospheric demands while AET is controlled
by
available soil moisture. Aside from small experiments,
evapotranspiration can not be measured at large scales. However,
there are many ET estimation techniques available at various
temporal
and spatial scales.
The development of a physically and physiologically based
soil-plant-climate evapotranspiration model requires all energy,
mass
transfer, stomatal resistance, and crop aerial resistance terms.
The
modified Penman-Monteith equation is such a model, capable of
assessing the impacts of CO2 and temperature climate change
scenarios
on evapotranspiration (Monteith, 1965). Many GCMs use this
equation
to assess energy and vapor exchanges at the Earth's surface,
making it
well suited for analyzing the sensitivity of PET to climate
change
scenarios. The Penman-Monteith equation is considered to be
universally applicable because it is derived from the energy
conservation equations (Fennessey and Kirshen, 1994). Details
concerning the data requirements and application of the PET model
over
the course of the growing season were presented by Finnerty
(1994).
Morison (1987) compiled the results of the effects of 2xCO2
experiments performed on 16 C3 species, and found stomatal
resistance
rs increased 67% over present values. The data indicated a
general
linear relationship between atmospheric CO2 concentrations (Ca)
and rs,
and was assumed to apply to potatoes. This relationship was used
to
assess the impacts of increased CO2 on PET rates of potatoes
which have
C3 type photosynthetic pathways. The results of this research
may be
applicable toward other C3 species grown in the San Luis Valley.
Accounting for seasonal changes in crop roughness height,
stomatal resistance, and climate parameters, mean daily PET rates
for
each growth period (GP) of the potato crop were evaluated.
Climate
change scenarios for temperature, CO2, and combinations of the
two were
analyzed. The modified Penman-Monteith equation estimates daily
PET
rates using daily or monthly data with 5-15% accuracy of measured
field data (Van Bavel, 1966; Szeicz et al., 1967; Jensen et al.,
1971). Measured PET rates for potatoes in the San Luis Valley
range
from 4.5 to 7.7 mm/day, which is very consistent with the
historical
PET estimates presented in Table 1 (Troolen, 1988). The results
presented in Table 1 were obtained by changing only those
parameters
of the Penman-Monteith equation which are a function of
temperature or
a function of CO2. The combined climate scenarios were assumed
to have
an independent and additive effect on PET, and neglect any
feedback
processes between temperature, stomatal resistance, and PET.
This
analysis neglected climate change scenarios related to changes in
solar radiation, relative humidity, and wind speed. These issues
were
not addressed because of the uncertainty associated with
CO2-induced
changes to these variables in mountainous regions.
The results of the analysis presented in Table 1 indicated
that
the PET rates of a potato crop are very sensitive to changes in
climatic temperature, atmospheric CO2 concentrations, and
combinations
of the two. All singular climate change scenarios analyzed
reduced
PET, with the exception of a temperature increase. The most
interesting results were those related to the combined changes of
temperature and CO2. These illustrated that the CO2 climate
changes
had a greater effect on PET than temperature changes, for the
cases
considered, and neglecting possible feedback processes. An
increase
in temperature does increase PET; however the combination of a 3
C
temperature increase with a doubling of CO2 resulted in a 18.5%
decrease in PET. In addition, a 3 C temperature decrease
combined
with a doubling of CO2 showed a 39% decrease in PET (Ramirez and
Finnerty, 1995a).
In conclusion, the analysis demonstrated PET rates for
potatoes
and other C3 vegetation species growing in the San Luis Valley
were
reduced in a CO2 rich atmosphere, regardless of potential
temperature
changes. The effects of increased CO2 on PET rates dominated
those of
increased temperature for the climate change scenarios analyzed.
Table 1: Potential Evapotranspiration Rates, mm/day
CLIMATE SCENARIO |
GP1 |
GP2 |
GP3 |
GP4 |
% CHANGE |
HISTORICAL |
4.47 |
6.70 |
7.24 |
5.84 |
0.0 |
Temp +3.5 C |
5.10 |
7.52 |
8.08 |
6.51 |
+13.8% |
Temp -3.5 C |
3.89 |
5.91 |
6.43 |
5.19 |
-13.8% |
1.5xCO2 |
3.70 |
5.67 |
6.18 |
4.97 |
-15.2% |
2xCO2 |
3.16 |
4.92 |
5.40 |
4.33 |
-29.4% |
Temp +3 C, 2xCO2 |
3.65 |
5.61 |
6.12 |
4.91 |
-18.5% |
Temp -3 C, 2xCO2 |
2.71 |
4.28 |
4.72 |
3.79 |
-39.3% |
SOIL MOISTURE DEPLETION
A water balance was performed to investigate the impacts of
climate driven PET and precipitation changes, water table
fluctuations, and CO2 fertilization effects on soil moisture
depletion
processes and agricultural benefits. These changes modified the
temporal evolution of soil moisture content throughout the
growing
season and consequently had a large impact on optimal irrigation
decisions and agricultural benefits. Ramirez and Bras (1982,
1985)
presented the details of the physically-based, theoretical
soil-crop-climate model capable of incorporating changes in
precipitation, PET
rates, and depth to the water table into optimal irrigation
decisions.
Derivation of the capillary rise model as a function of soil
moisture
content and depth to the water table was presented by Finnerty
(1994).
Impacts of Temperature-CO2 Changes on Agriculture
The results displayed in Table 1 for the cases of combined
temperature-CO2 scenarios on PET were used to derive AET curves
as a
function of time. Figure 1 shows how decreasing PET acted to
increase
the ratio of AET/PET in time. The crops were evapotranspiring at
lower rates, but that rate was closer to their potential rate.
This
resulted in a slowing of the soil moisture depletion rate as
shown in
Figure 2. The ratio of AET/PET was a surrogate measure of crop
moisture stress in the crop model, and was used to evaluate
optimal
irrigation schedules required to obtain maximum expected
agricultural
benefits. Figure 3 shows the increase in the AET/PET ratio
resulted
in lower crop stress, water conservation, and higher crop yields,
which translated into higher expected agricultural benefits.
Figure 3
also shows that increasing available irrigation water increased
soil
moisture through irrigation applications, which resulted in
reduced
crop moisture stress and increased agricultural benefits.
Increasing
available irrigation water reduced the relative impact of
climatic PET
changes on agricultural production, while decreasing irrigation
water
increased the impact of climate-induced PET changes (Ramirez and
Finnerty, 1995a).
Figure 1. Combined Effects of Temperature and CO2 on %AET/PET
Ratios.
Wmax=0.88 mm/day.
Figure 2. Combined Effects of Temperature and CO2 on Soil
Moisture
Depletion Curves. Wmax=0.88 mm/day.
Figure 3. Combined Effects of Temperature, CO2, and Available
Water on
Agricultural Benefits. Wmax=0.88 mm/day.
The combined impacts of temperature and CO2 are very
important
when considering that temperature changes are uncertain as to
their
sign and magnitude, while CO2 is expected to double in the next
century. This analysis indicated that increasing atmospheric CO2
concentrations had a positive effect on irrigated agriculture,
regardless of potential temperature changes, for the climate
change
scenarios analyzed. Analysis of the single climate change
scenarios
of temperature and CO2 can be found in Ramirez and Finnerty
(1995a).
Water Table Fluctuations
A shallow water table exists at the study site, ranging from
1 to
4 meters below the soil surface (Yenter et al., 1980). Water
table
variations may be caused by natural dynamics in groundwater
recharge,
or from irrigation pumping schedules and other consumptive uses.
The
contribution to root zone soil moisture from a shallow water
table
increases as the depth to the water table decreases and is a
major
component in the soil water balance. Table 2 illustrates the
effect
of water table fluctuations on the maximum rate of capillary
rise,
Wmax. The effect of capillary rise on soil moisture depletion
curves
is illustrated in Figure 4. As the soil moisture content
decreased
the capillary potential in the soil increased, and consequently
so did
capillary rise. The depletion curves exhibited asymptotic
behavior,
converging in time to a soil moisture concentration where
capillary
rise was balanced by actual evapotranspiration. The same
behavior was
found for AET functions, converging on the point where AET of
water
out of the soil was constant and equal to the contribution of
capillary rise into the soil (Ramirez and Finnerty, 1995b).
Increases in AET attributed to decreasing the depth to the
water
table reduced crop water stress and increased crop yields.
Capillary
rise was most important to agriculture when there is limited
irrigation water because capillary rise substitutes for
irrigation
water requirements and is provided at no cost to the farmers. As
available irrigation water increased, capillary rise became less
significant because irrigation water satisfied the plant water
use
requirements.
Table 2: Capillary Rise with Depth to Water Table
DEPTH, cm |
100 |
150 |
200 |
250 |
300 |
Wmax, mm/d |
4.44 |
1.29 |
0.54 |
0.27 |
0.16 |
Figure 4. Effects of Depth to the Water Table Z on Capillary Rise
and Soil Moisture Depletion.
Precipitation Changes
The high variability of the spatial distribution of global
precipitation causes a large degree of uncertainty concerning
regional
and local precipitation changes resulting from various
temperature and
CO2-induced climate change scenarios. There is also a lack of
understanding of potential changes in rainfall characteristics
such as
storm intensity, duration, and arrival rates. The impact
assessment
of precipitation changes on agriculture analyzed the following
scenarios: both a 25% increase and decrease in storm intensity,
storm
duration, and storm arrival rates. For the analysis
precipitation
characteristic changes were made independently, while holding all
other processes constant. The precipitation changes were applied
to
the entire growing season for all homogeneous precipitation
periods,
and were shown to impact soil moisture dynamics and agricultural
benefits.
The results obtained from the analysis showed agriculture to
be
relatively insensitive to climatic precipitation changes for the
cases
analyzed (Ramirez and Finnerty, 1995b). Precipitation provides a
minor portion of crop water use requirements in the San Luis
Valley's
irrigated agricultural production. The valley's low
precipitation
quantity of 76 mm/acre/season is small as compared to the 381
mm/acre/season of irrigation water used. Varying precipitation
by 25%
(19 mm/acre/season) had little or no effect on irrigation water
use
requirements and agricultural benefits.
CO2 FERTILIZATION IMPACTS
Doubling of atmospheric CO2 has been experimentally proven to
increase crop yield and biomass production by 43% to 75% in
potato
crops (Collins, 1976). Root and tuber crops were found to
increase
marketable yield by an average of 52%, as observed in 17
experiments
of doubled atmospheric CO2 concentrations (Kimball and Idso,
1983).
This result shows that increasing CO2 will significantly increase
maximum crop yields and expected agricultural benefits given the
same
production and irrigation costs that currently exist.
Actual evapotranspiration was reduced even when biomass and
plant
leaf area were increased due to increased atmospheric CO2
concentrations (Morison and Gifford, 1984; Idso et al., 1986).
However, there is uncertainty concerning the effect of increased
biomass on PET rates for CO2 fertilized plants. Because of this
uncertainty three cases were analyzed to investigate the issues
of CO2
fertilization and crop water use efficiency. Case 1 is a 50%
increase
in crop yield, combined with the historical PET rate. Case 2 is
a 50%
increase in yield, coupled with the reduced 1.5xCO2 PET rate.
The
third case is a 50% increase in yield, coupled with the reduced
2xCO2
PET rate. The 1992 maximum potato yield was equal to 375
100lbs/acre,
the 50% increase in maximum yield was 562.5 100lbs/acre (Colorado
Agricultural Statistics, 1992).
Table 3 shows CO2 fertilization had a very large impact on
irrigated agricultural benefits. The crop yield increase of 50%
combined with a 15% to 29% reduction in PET (see Table 1),
resulted in
significant increases in agricultural benefits for all cases of
available irrigation water. The results illustrate how
agricultural
benefits almost doubled under doubled CO2 concentrations, due to
a
large increase in crop yield coupled with increased crop water
use
efficiency. CO2 fertilization could make agriculture
economically
feasible in regions with high production and irrigation costs,
and
where available irrigation water is constrained. In addition,
CO2
fertilization had a much greater positive effect on agricultural
benefits than the effects of CO2 on stomatal resistance and PET
(Finnerty, 1994).
Table 3: CO2 Fertilization Effects on Agricultural Benefits
($/ac),
Wmax=0.88 mm/day
AVAILABLE IRR. WATER |
HISTORICAL YIELD |
1.5xYIELD |
HIST. ET |
1.5xCO2 ET |
2xCO2 ET |
00 mm |
370.1 |
555.2 |
640.8 |
723.9 |
300 mm |
1365.3 |
2081.9 |
2109.5 |
2206.9 |
ECONOMIC SENSITIVITY OF AGRICULTURE
The laws of supply and demand establish crop market values,
and
influence production decisions concerning acres to be planted,
irrigation water requirements, and production costs. These
decisions
are made prior to the start of the growing season when a large
degree
of uncertainty exists. Farmers are at financial risk due to the
uncertainty associated with future crop market prices, natural
weather
variability, unforeseen production costs, and natural disasters.
The
field of agro-economics is very complex and often site specific.
Therefore, this analysis only addresses the main economic issues
related to potato production in Conejos County, in the San Luis
Valley, Colorado.
Production Costs
Production costs are difficult to predict at the beginning of
the
season because of unforeseen production problems. The primary
objective of agriculture is to minimize production costs while
maximizing crop yields, so as to maximize financial benefits.
These
two objectives are in direct opposition because increasing
spending on
production generally increases crop yield. This makes it
difficult to
evaluate the marginal value of money invested in crop production.
Given no information concerning future production costs, only
short-term climate variability in temperature and precipitation
were
analyzed. These climate changes were made while assuming current
atmospheric CO2 concentrations. The results of the analysis
showed the
maximum expected benefits for all climate change scenarios to be
greater than the production costs, given sufficient available
water
(400 mm/acre/season) and a significant contribution of capillary
rise
(0.88 mm/day). However, a 10% increase in production costs
removed
all profits for all temperature and precipitation change
scenarios
analyzed. Conversely, a decrease in production costs, or an
increase
in production efficiency would increase profit margins (Ramierz
and
Finnerty, 1995b). A 10% increase in production costs had a
larger
impact on agriculture than +/-14% changes in PET or +/-25% change in
precipitation.
Market Value of Crops
Farmers generally plan for an average crop market value at
harvest time. However, the price of Colorado potatoes has
fluctuated
around the ten year mean of $4.65/100lbs, from a fifteen year low
of
$2.10/100lbs in 1987, to a high of $8.10/100lbs in 1989 (Colorado
Agricultural Statistics, 1992). This extreme annual variation in
market value makes it difficult for farmers to plan a production
strategy, especially when high production years may result in low
crop
market values because of the excess supply at harvest time.
The results of the analysis on the impacts of crop market
price
variations on agricultural benefits are displayed in Table 4.
The
table shows a 50% increase in crop market price caused a very
significant increase in agricultural benefits, while a 50%
decrease in
market value resulted in a devastating reduction of financial
benefits, causing the industry to loose money regardless of ample
water supply and high productivity (Ramirez and Finnerty, 1995b).
Table 4: Impact of Crop Market Value on Benefits, $/acre
IRR. WATER |
HISTORICAL |
+50% MARKET |
-50% MARKET |
PRODUCTION |
0 mm |
370.13 |
555.59 |
185.46 |
1315.00 |
400 mm |
1427.81 |
2182.86 |
677.07 |
1315.00 |
SUMMARY AND CONCLUSIONS
-
1. All temperature and CO2 climate change scenarios had a
favorable
impact on evapotranspiration, soil moisture depletion, and
irrigated
agriculture, with the exception of a temperature increase alone.
2. Long-term expected changes in CO2 had a larger impact than
temperature changes, for the scenarios analyzed.
3. CO2 fertilization effects had a significantly larger positive
impact on agricultural production than any of the other climate
induced changes to agricultural benefits.
4. Small variations in the depth to the water table
significantly
impacted the contribution of capillary rise to root zone soil
moisture
and agricultural benefits.
5. Agricultural sensitivity to agro-economic parameters had a
larger
impact on agricultural benefits than any of the climate change
scenarios analyzed.
6. Irrigated agriculture in the San Luis Valley was essentially
insensitive to plausible precipitation changes.
7. Available irrigation water was crucial to the irrigated
agricultural economy of the San Luis Valley, Colorado. The crop
water
use requirements had to be met either from precipitation,
capillary
rise, or irrigation. Irrigation was capable of supplementing any
reduction of soil moisture caused by increased PET, lowering of
the
water table, or decreased precipitation. However, those
irrigation
water resources needed to be available to reduce agricultural
risks
attributed to damaging climatic or economic conditions if
agricultural
production was to remain profitable in the region.
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