Abstract—Efficient irrigation systems and water
management practices can help maintain farm profitability in an
era of increasingly limited and more costly water supplies. Improved
water management practices may also reduce the impact of irrigated
production on offsite water quantity and quality, and conserve water
for growing nonagricultural demands. The effectiveness of public
water conservation programs depends on how such programs account
for diverse farm types and the farm-size characteristics of irrigated
agriculture.
Why Manage Irrigation Water?
Agriculture, which accounts for about 90 percent of freshwater consumption
in the Western States and over 80 percent nationwide, is increasingly
being asked to use less water in order to meet societal demands
for other uses (see AREI
Chapter 2.1). Water demands are increasing for municipal and
industrial uses, recreation, fish and wildlife habitat, and Native
American trust responsibilities. For example, conservation of farm
irrigation water was a key component of recent water transfer agreements
between the Imperial Irrigation District and the San Diego County
Water Authority, expected to account for 200,000 acre-feet of annual
water transfers during 2021-2047 (Schaible, 2004a).
Farm-level irrigation water management (IWM) involves the managed
allocation of water and related inputs in irrigated crop production
to enhance economic returns and minimize environmental impacts.
USDA identifies improvements in IWM as essential to meeting its
national priorities for reducing agriculturally induced nonpoint-source
pollution, including surface- and groundwater contamination, reductions
in soil erosion and sedimentation, and conservation of ground and
surface water (USDA, 2004b). The National Research Council in A
New Era for Irrigation (NRC, 1996) highlights the importance
of IWM "to allocate limited water resources equitably."
Improved IWM can help reduce loadings of nutrients, pesticides,
and trace elements in irrigation runoff to surface waters, and leaching
of agrichemicals into groundwater supplies (Schaible and Aillery,
2003). Strategies to improve the Nation's water quality (see
AREI Chapter 2.2) must
address the effect of irrigation on surface- and groundwater resources
(NRC, 1996).
Improvements in IWM can also help maintain the long-term viability
of the irrigated agricultural sector. Irrigated cropland is an important
and growing component of the U.S. farm economy, accounting for almost
half of total crop sales from just 16 percent of the Nation's
harvested cropland in 1997 (USDA, 2001). Water savings at the farm
level can help offset the effect of rising water costs and limited
water supplies on producer income. Improved water management may
also reduce expenditures for energy, chemicals, and labor, while
enhancing revenues through higher crop yields and improved crop
quality. Strategic IWM may also enable producers to better withstand
the downside risks of drought.
Use of Improved Irrigation Technology and Management
Producers may respond to limited water supplies through various
means, with differing implications for crop production, farm returns,
resource use, and environmental quality. Water use per acre may
be reduced by applying less than a crop's full consumptive
requirement, by shifting to alternative crops or varieties that
use less water, or by adopting more efficient irrigation technologies
and management practices. Producers may even convert from irrigated
to dryland farming or retire land from production.
With water increasingly scarce, irrigators will likely continue
to rely on improved technologies and water management practices
to conserve water. Irrigation efficiency, broadly defined at the
field level, is the ratio of irrigation water beneficially used
(crop consumptive use plus an allowance for leaching of salts) to
that applied, expressed as a percentage (USDA, 1997). (See irrigation
efficiency definitions.)
Irrigation application systems may be grouped under two broad
types: gravity flow and pressurized. Gravity-flow systems
distribute water across the field via land treatments—such
as soil borders and furrows—that control lateral water movement
and channel it in the field. Water is conveyed to the field by means
of open ditches, above-ground pipe (including gated pipe and flexible
tubing), or underground pipe, and released along the upper end of
the field through siphon tubes, ditch gates, pipe valves, or pipe
orifices. Pressurized systems include
a variety of sprinkler and low-flow irrigation techniques to distribute
water across a field.
With rare exceptions, the pressure to distribute water involves
pumping, which requires energy. Sprinkler systems—in which
water is sprayed over the field surface, usually from above-ground
piping—may be operated on sloping or rolling terrain unsuited
to gravity systems. (See irrigation
system definitions.)
Gravity-Flow Irrigation
Although total acreage in gravity systems has declined by 26 percent
since 1979, gravity-flow systems still account for 44 percent of
irrigated acreage nationwide, down from 62 percent in 1979 (fig.
4.6.1). Gravity-flow systems, used in all irrigated areas, are particularly
dominant in the Southwest, Central Rockies, Southern Plains, and
Delta regions (USDA, 2004a). Furrow application comprises about
half of the acreage in gravity-flow systems; border/basin and uncontrolled-flood
application account for the remaining acreage (table 4.6.2). Much
of the uncontrolled flooding is used for hay and pasture production
in the Northern and Central Rockies.
Table
4.6.1—Changes in irrigation system acreage, 1979-2003
System
1979
1998
2003
Change
1979-98
Change
1998-2003
Million
acres
Percenta
All
systems
50.2
54.2b
52.6
8
(3)
Gravity-flow
systems
31.2
26.8
23.1
(14)
(14)
Sprinkler
systems
18.4
24.6
26.9
34
9
Center
pivot
8.6
18.5
21.3
115
15
Mechanical
move
5.1
3.0
2.7
(41)
(10)
Hand
move
3.7
1.9
1.7
(49)
(11)
Solid/permanent
set
1.0
1.2
1.2
20
0
Low-flow
irrigation
(drip/trickle and micro-spray)
0.3
2.2
3.0
633
36
Subirrigation
0.2
0.6
0.3
200
(50)
aNumbers
in ( ) indicate a decrease.
bBased
on USDA-NASS 2004 revised estimate for 1998 due to re-weighting
for undercoverage. (The sum of subcategories will differ slightly
from aggregates because of rounding error.)
Source:
USDA-ERS, based on Farm and Ranch Irrigation Surveys for 1979,
1998, and 2003 (USDC, 1982; USDA, 1999, and USDA, 2004a).
Table
4.6.2—Irrigation application systems, by type, 1998 and
2003
1998
2003
Area
Share
of all
systems1
Area
Share
of all
systems1
Million
acres
Percent
Million
acres
Percent
All
systems
54.2
100
52.6
100
Gravity-flow
systems2
26.8
50
23.1
44
Row/furrow
application
13.8
25
11.7
22
Open-ditch
delivery systems
4.6
9
4.4
9
Pipe/poly-tubing
delivery systems
9.2
17
7.4
14
Border/basin
application
8.3
15
8.8
17
Open-ditch
delivery systems
4.8
9
5.5
10
Pipe/poly-tubing
delivery systems
3.5
7
3.3
5
Uncontrolled
flooding application
3.2
6
2.3
4
Open-ditch
delivery systems
2.8
5
2.1
4
Pipe/poly-tubing
delivery systems
0.4
1
0.1
*
Other
gravity
(mostly with unlined ditches)
1.5
3
0.3
*
Sprinkler
systems2
24.6
45
26.9
51
Center-pivot
18.5
34
21.3
41
High-pressure
(60 psi or more)
1.9
4
1.9
4
Medium-pressure
(30 to 59 psi)
7.4
14
9.7
18
Low-pressure
(under 30 psi)
9.2
17
9.7
18
Other
sprinkler systems
6.1
12
5.6
9
Low-flow
irrigation (drip or trickle)
2.2
4
3.0
6
Subirrigation
0.6
1
0.3
*
1Numbers
may not add due to multiple systems on some irrigated acres
and incomplete survey responses.
2For
a more detailed breakout of irrigation systems, see the ERS
Briefing room on "Irrigation
Water Management".
*
= less than 1 percent.
Source:
USDA-ERS, based on Farm and Ranch Irrigation Surveys for 1998
and 2003 (USDA, 1999 and 2004a).
Water losses are comparatively high under traditional gravity-flow
systems due to percolation losses below the crop-root zone and to
surface-water runoff. Field application efficiencies typically range
from 40 to 65 percent, although improved gravity systems with proper
water management may achieve efficiencies of up to 80-90 percent
(USDA, 1997).
Various land treatments, system improvements, and water management
measures have been developed to reduce water losses under gravity-flow
systems. For example, precision laser-leveled irrigation is practiced
on 3.7 million acres (16 percent of gravity acres), mostly in the
Southwest, Delta, and Northern Rockies (Montana, Idaho, and Wyoming)
regions. Improved gravity systems generally involve onfarm water
conveyance upgrades that increase uniformity of applied water and
reduce percolation losses and field runoff. However, open-ditch
systems still account for 53 percent of gravity acreage served (table
4.6.2; USDA, 2004a). Improved ditch systems, lined with concrete
or another impervious substance, account for only 20 percent of
gravity acres served by open ditches. Above-ground, pipeline delivery
systems—including gated pipe and flexible (poly or lay-flat)
tubing—account for 34 percent of all gravity acreage served,
with underground pipe delivery systems serving the remaining 13
percent. Surge-flow and cablegation systems—designed to control
water deliveries from gated pipe—were used on 0.4 million
acres, representing 2 percent of gravity-flow acres in 2003 (fig.
4.6.2).
Use of improved water management practices for gravity irrigation,
while increasing, remains an area of significant growth potential.
Alternate-row irrigation is practiced on only 11 percent of gravity-flow
acres; special furrowing practices (wide-spaced, compacted, or diked)
on 6 percent; and shortened-furrow water runs on 2 percent. Tailwater-reuse
pits, designed to recirculate field drainage flows, are used on
about 7 percent of gravity acres, while reduced irrigation set-times
are observed on 12 percent. Polyacrylamide—a water-soluble
soil amendment designed to reduce soil erosion, enhance water infiltration,
and improve nutrient uptake—is used on 2 percent of gravity-flow
acres.
Pressurized Irrigation
Sprinkler irrigation has been adopted in many areas as a labor-
and water-conserving alternative to gravity-flow systems. Field
application efficiencies for properly designed and operated sprinkler
systems range from 50 to 95 percent, with most systems achieving
75 to 85 percent (USDA, 1997). Acreage for all pressurized systems
expanded from 19 million acres (37 percent of total irrigated acreage)
in 1979 to 30 million acres (57 percent) in 2003 (table 4.6.1).
Sprinkler systems alone accounted for 27 million acres, or 51 percent
of all irrigated acreage in 2003 (table 4.6.2). Acreage in sprinkler
systems has continued to expand in recent years, with an increase
of nearly 9 million acres (46 percent) since 1988 (USDC, 1990; USDA,
2004a).
Center-pivot sprinkler systems accounted for roughly 79 percent
of sprinkler acreage in 2003, or 41 percent of total irrigated acreage
(table 4.6.2), increasing by nearly 13 million acres from 1979.
Nearly two-thirds of the increase is attributable to net increases
in irrigated area under sprinkler, while about a third reflects
the net replacement of other sprinkler types with center-pivot systems
(table 4.6.1). The more advanced low-pressure center-pivot and linear-move
systems, including low-energy precision application (LEPA) systems
(below 30 pounds per square inch), combine high application efficiencies
with reduced energy and labor requirements. These systems account
for 46 percent of center-pivot acreage, and are especially popular
in the Southern Plains where irrigation relies heavily on higher-cost
groundwater pumping. Current advances in sprinkler technology focus
on the variable application of spray heads, as well as remote control
of individual sprinklers and nozzles for precision agriculture.
Low-flow systems—including drip, trickle, and micro-sprinklers
(with application efficiencies of 95 percent or greater)—were
used on 3 million acres in 2003, or just 6 percent of irrigated
cropland acreage (table 4.6.2), up from 300,000 acres in 1979 (table
4.6.1). The annual rate of growth (7 percent) was slower during
1998-2003 than the explosive 74-percent rate during 1979-88 (table
4.6.1). Low-flow systems are most commonly used for vegetables and
perennial crops such as orchards and vineyards (primarily in California
and Florida), although experimentation and limited commercial applications
are occurring with some row crops (e.g., cotton).
Irrigation Scheduling and Water-Flow Measurement
Proper irrigation scheduling and precise measurement of water flow
help producers match water applied to crop needs. Most irrigated
farms continue to use a combination of less sophisticated methods
to schedule irrigations (USDA, 2004a). Nearly 80 percent of irrigated
farms use mere visual observation to evaluate the "condition
of the crop," while some farms (ranging from 6 to 35 percent)
simply "feel-the-soil," irrigate "when their neighbor
irrigates," use a "personal calendar schedule,"
use "media daily weather/crop evapotranspiration (ET) reports,"
or irrigate consistent with "scheduled water deliveries."
Most irrigated farms do not use the more advanced, information-intensive
methods to schedule irrigation; less than 8 percent of irrigated
farms use soil and/or plant moisture sensing devices, commercial
or government-sponsored irrigation scheduling services, or computer
simulation models. These current statistics suggest a significant
potential for greater agricultural water conservation through public
policy that promotes broader understanding and more extensive application
of such scheduling techniques.
Water-flow measurement devices, for both on- and off-farm conveyance,
include weirs, flumes, and in-canal flow meters for open ditches,
internal/external meters for pipe delivery systems, and flow meters
in wells to monitor groundwater pumping. Of the 380,000 wells used
in 2003 to pump ground water for agriculture, only 61,000 (16 percent)
used flow meters. While this is a 32-percent increase since 1994,
flow meters on wells account for just 1 in 5 acres irrigated with
ground water.
Potential for Improvement in Irrigation Conservation
Significant potential still exists for expanding agricultural water
conservation. How much can be achieved depends on the combined use
of conserving water-management practices and irrigation systems
(Schaible, 2004b; USDA, 2004a). Of the 23.1 million gravity-irrigated
acres in 2003, only 56 percent benefited from the use of one or
more water management practicesaccounting for just 53 percent
of gravity-irrigated farms (USDA, 2004a). While not all water management
practices can (or should) be applied to all gravity-irrigated acres
simultaneously, at least 40-60 percent of gravity irrigation could
benefit from improved water management (fig. 4.6.2). In addition,
while use of low-pressure sprinkler systems increased to 38 percent
of total irrigated acres in 2003, at least 39 percent of irrigated
acreage likely remains available for improved conservation (fig.
4.6.3). The combined effect of improved systems and water management
practices, along with more extensive use of advanced irrigation
scheduling and water-flow measurement practices across all irrigation,
would likely translate to even greater agricultural water conservation
potential.
Farm Size and Water Conservation
An ERS analysis of structural characteristics of Western irrigated
farms found that size matters in how well water conservation programs
serve both USDA conservation and small-farm policy goals (Schaible,
2004b). In the 17 Western States, which account for 77 percent of
U.S. irrigated acres, nearly 81 percent of irrigated farms are small, with less than $250,000 in annual farm sales (FS) (fig. 4.6.4).
However, large irrigated farms (FS > $250,000) account for
61 percent of irrigated crop acres, nearly 85 percent of irrigated
farm sales, and 66 percent of the total farm water applied. The
largest 9.5 percent of irrigated farms (FS > $500,000) account
for 48 percent of total farm water applied. Average annual water
applied ranges from less than 150 acre-feet for the smallest irrigated
farms (FS < $100,000) to more than 2,500 acre-feet for the largest
farms.
In aggregate, "water-conserving/higher-efficiency"
irrigation in the West ranges from 46-78 percent of acreage for
pressurized (sprinkler) irrigation to 40-57 percent of acreage for
gravity irrigation (Schaible, 2004b). For both categories, relative
conservation improvement potential is generally greater for smaller
irrigated farms. However, larger farms irrigate many more acres,
so aggregate water savings due to a conservation program could be
much greater for these farms. While "perceived economic benefits"
and "lack of financing ability" are two commonly reported
barriers to irrigation system improvements across all irrigated
farms, "not investigating the merits of system improvements"
is an additional critical barrier to system improvements for smaller
irrigated farms.
Producers' Incentives
While survey results demonstrate that irrigators do implement irrigation
system improvements to meet environmental goals, improved farm returns
is likely the dominant motivating factor (table 4.6.3). From a private
economic perspective, producers generally invest in improved irrigation
technologies when perceived benefits are greater than additional
(net) producer costs. However, Kim et al. (2000) demonstrate that
from a public perspective where water quality benefits accrue largely
off-farm, public cost-share funding of a more conserving technology
may be warranted. For example, in Merrick County, NE, adoption of
tailwater recovery or surge-flow gravity systems may be more profitable
to the producer, although, even with these systems, groundwater quality
would continue to deteriorate. A center-pivot sprinkler system would
significantly reduce the accumulation of nitrates in ground water
after 15 years. However, adoption of center-pivot systems would
reduce producer profits by about $9 per acre (in 1990 dollars),
so cost-sharing or other incentives might be necessary to encourage
adoption of systems that contribute more to improving water quality.
Table
4.6.3—Producer reasons for irrigation conservation improvements,
1999-2003
Farms
Irrigated
farms implementing irrigation improvements during 1999-2003:
70,336
Percent
Reason
for/effect of improvements:
Improved
crop yield or quality
57.6
Reduced
energy cost
39.0
Reduced
water applied
58.5
Reduced
labor cost
39.2
Reduced
fertilizer/pesticide losses
14.2
Reduced
soil erosion
30.8
Reduced
tailwater
22.9
Other
8.4
Source:
USDA-ERS, based on the Farm and Ranch Irrigation Survey (2003),
Vol. 3, Special Studies, Part I, table 39 (USDA, 2004b).
Federal, State, and local cost-share programs that address farm
water delivery, field-level irrigation systems, and farm water management
practices are key to improving irrigation efficiency. Only about
13 percent of irrigated farms in the West participated in public
cost-share programs for water conservation between 1994 and 1998.
Smaller irrigated farms make up 77 percent of participants in USDA
cost-share programs designed to encourage irrigation or drainage
improvements. Given that such farms account for only 34 percent
of farm water applied in the West, these results indicate that farm
size matters in the effectiveness of current agricultural water-conservation
programs. Cost-share programs that target larger farms would likely
conserve more water, making more water available to meet environmental
and other objectives, especially when integrated with State-sponsored
water markets, water banks, and conserved-water-rights programs (Schaible,
2004b). Integrated Federal/State conservation policy would likely
increase opportunities to better balance alternative farm policy
objectives—i.e., resource efficiency and potential gains in
water saved, with distributional considerations involving cost-share
funding allocations.
References
Kim, C.S., G.D. Schaible, and S.G. Daberkow (2000). "An Efficient
Cost-Sharing Program to Reduce Nonpoint - Source Contamination:
Theory and an Application to Groundwater Contamination,"
Environmental Geology, Vol. 39, No. 6 (April): pp. 649-659.
National Research Council (NRC) (1996). A New Era For Irrigation,
Water Science and Technology Board—Committee on the Future
of Irrigation in the Face of Competing Demands, National Academy
Press, Washington, DC.
Schaible, Glenn D., and Marcel P. Aillery (2003). "Irrigation
Technology Transitions in the Mid-Plains States: Implications for
Water Conservation/Water Quality Goals and Institutional Changes,"
International Journal of Water Resources Development, Vol.
19, No. 1, pp. 67-88.
U.S. Department of Agriculture (1997). National
Engineering Handbook, Part 652, "Irrigation Guide", Natural
Resources Conservation Service.
U.S. Department of Agriculture (1999). Farm and Ranch Irrigation
Survey (1998), Vol.
3, Special Studies-Part 1 of the 1997 Census of Agriculture,
AC97-SP-1, Natural Agricultural Statistics Service.
Schaible, Glenn D. (2004a). Agricultural
Risks in a Water-Short World: Producer Adaptation and Policy Directions—A
Workshop Summary, Glenn D. Schaible (ed.), Econ. Res. Serv.
(Nov.).
U.S. Department of Agriculture (2004a). Farm
and Ranch Irrigation Survey (2003),
Vol. 3, Special Studies-Part 1 of the 2002 Census of Agriculture,
AC-02-SS-1, National Agricultural Statistics Service, (Nov.).
U.S. Department of Agriculture (2004b). Farm Bill 2002: Environmental
Quality Incentives Program, Fact Sheet, Natural Resources Conservation
Service (Oct.). (See also Public Law 107–171–May 13,
2002, Farm Security and Rural Investment Act of 2002, Subtitle
D– Environmental Quality Incentives, Sections 2301 and 1240I.)
U.S. Department of Commerce (1982). 1979 Farm and Ranch Irrigation
Survey, Vol. 5, Special Reports—Part 8, AC78-SR-8, Bureau
of the Census.
U.S. Department of Commerce (1990). Farm and Ranch Irrigation
Survey (1988), Vol. 3, Part 1—Related Surveys of the
1987 Census of Agriculture, AC87-RS-1, Bureau of the Census.