Impact of Agricultural Water Conservation on Water Availability Bert Clemmens USDA-ARS Maricopa, AZ
Outline for Talk • Context of Irrigated Agriculture • Water Balance • Irrigation Uniformity • Case Studies • water capture and reuse • efficiency improvement incidental groundwater recharge
Context of U.S. Irrigated Agriculture 2002 Census of Agriculture
Context of U.S. Irrigated Agriculture • Water withdrawals for irrigation (2000) • 153 million ac-ft (AZ 6 million ac-ft) • Value of production from irrigated land (2002) • $55 billion (AZ $1.6 billion) • Market value per unit of water withdrawn • $360/ac-ft (AZ $270/ac-ft)
Irrigation Hydrology – Water Balance Evaporation + Transpiration Irrigation + Precipitation Pumpage Surface Runoff Root zone Soil Water Storage Ground Water River GW Discharge to River
Destinations of Applied Water • Water consumed - beneficial • Water consumed - not beneficial • Water flows out - recovered and reused. • Water flows out - not recoverable or not reusable. • Water in storage
Opportunities for conservation • Reduce consumption of water outside the cropped area (e.g., phreatophytes) • Reduce non-productive consumption within the cropped area (e.g., weed ET) • Reduce outflow of water that cannot be recovered (with reasonable quality) • Reduce degradation in quality of water that leaves the watershed (more water is required when salinity is higher)
What is Crop ET? • Basal versus Total ET • How much E is beneficial? Unavoidable?
Irrigation and rainfall events increase total ET Irrigating more frequently consumes more water
All irrigation systems have some non-uniformity,which influences yield and irrigation efficiency
Some nonuniformity is self created. • Most non-uniformity tends to be systematic!
Irrigators apply extra water to avoid deficits caused by non-uniformity, but there is diminishing return.Most non-uniformity tends to be systematic.
Improving uniformity reduces water applied and reduces the deficit, and may also increase consumption
Improving Irrigation Efficiency • If you increase irrigation efficiency by improving irrigation uniformity, the following will almost always happen • Decrease • water applied • Increase: • Crop growth and yield (larger fraction of area has high growth) • water consumption (less area under deficit)
Irrigation Efficiency • Burt et al. 1997 redefined IE • requires water balance with well defined boundaries • water is only considered beneficial or not when it leaves the boundaries • IE is defined in terms of a time interval
Application Efficiency • Burt et al. (1997) made minor adjustments to definition • Event based • Primarily to add water to soil storage • A target application amount is implied
IE versus AE • Numerator • AE includes all soil E • IE includes only beneficial soil E • e.g. seed-bed prep, germination, crop quality • can’t include all soil E plus these other uses • IE includes beneficial leaching • Denominator • AE includes all water applied • IE only considers water that is used (leaves boundaries) • Event versus Time period
Field Example • Water applied to commercial field
Spatial organization of irrigation scheduling on the farm Physical constraints and labor schedules preclude theoretical irrigation scheduling
Chaos dominates such large-scale irrigation water distribution systems. • These systems are naturally dispersive.
Chaos rules! • What appear to be minor problems at the top of the distribution system, can end up as extreme differences in delivery – or chaos -- at the bottom. • Rarely is the poor distribution the result of poor performance by operators.
These results are what one would expect for a large-scale open-channel water distribution system, even one with reasonable infrastructure and reasonable management.
The answer is to change the management philosophy. • The only way to overcome this scenario is to reestablish physical control of the water at intermediate points within the system.
Improved physical and administrative controls • Establish agreed upon water delivery criteria (rate, volume, flexibility) • Measure and monitor deliveries • Water supply side held accountable for delivery service and quality • Water user side responsible for payment
Improved physical and administrative controls • Corrective actions taken on supply side to meet delivery criteria – e.g. appropriate volumes to all offtakes -- no excuses • Approach is to not only stop the chaos but also to reverse it effects! • Approach provides feedback from bottom to top of system
Improved Administrative Control • To the extent possible, chaos is not passed on to the farm level. • These administrative controls force the needed physical and operational controls to be implemented.
Example from Humid Region Grand Prairie Project in Eastern Arkansas • Farmers currently pump from deep groundwater – Sparta Aquifer being overdrafted • Rice is main crop, with soy bean rotation • Deep percolation is minimal and little reaches aquifer being pumped • Historically, runoff from fields was not collected • White River diversion, proposed to help stop groundwater overdraft, met environmental opposition
Rainfall adds tremendous uncertainty to irrigation practices and ability to quantify performance Grand-Prairie Project in Eastern Arkansas • Watershed water balance: paper & pencil study • Groudwater overdraft is prompting river diversions • On-farm reservoirs and tailwater pits being constructed to store rainfall and irrigation runoff
Reservoirs can capture ½ of off-season rainfall runoff – 30% of irrigation water needIn-season captured rainfall runoff can satisfy 30% of irrigation needDiversion needs to satisfy 40% of irrigation water needIE above 70% only saves in power cost of recycling water
Example from Arid Region Wellton-Mohawk Project in Southwest Arizona • Project has no surface water outflow • Deep percolation goes to saline aquifer • Saline drainage water pumped from groundwater into Gila River • Rainfall is insignificant (5% of reference ET) • USBR took some land out of production • SCS improved farm irrigation efficiency • Laser land grading • Flow measurement • Level-basin irrigation • USBR constructed drainage canal to bypass Gila/Colorado Rivers
On-Farm ImpactWellton-Mohawk On-Farm Improvement Program • Reduced water applied by 20% • Reduced labor by $18/ha • Increased yields by 12 to 20%
Project-Level ImpactWellton-Mohawk On-Farm Improvement Program • Water diversions from Colorado River reduced. • Drainage water pumping reduce by roughly half. • Water consumption reduced?? • All water saved by improving efficiency (reducing drainage) is considered a benefit! • Drainage canal flows to Sea of Cortez, where it has an environmental benefit (but it’s Arizona’s water?!) • (Recent cropping changes have change this impact).
Example from Semi-Arid Mountain ValleySnake River PlainBenefits of incidental recharge
Other (0.5 million ML) River Losses (0.9 million ML) Precipitation (0.9 million ML) Incidental Recharge from Tributary Underflow irrigation with river (1.7 million ML) water (5.9 million ML) Breakdown of annual recharge to the East Snake River Plain Aquifer
Result of “Losses”: - Increased Aquifer Recharge - Increased Stream Discharge Start of “inefficient” Irrigation Development Conversion of some to Sprinkler + GW Pumping
Water Balance Example • Imperial Irrigation District • 200,000 ha irrigation project • Southern California on Mexican Border • surface irrigation • extensive subsurface drainage system • closed basin • all drainage goes to Salton Sea
IID Water Use Assessment • Use water balance to determine “total consumption of water” as remainder. • Use Weather-based Kc-ETo approach to partition consumption into component parts. • Develop sub-system water balances to create better understanding of water uses. • Use accuracy of each measured or estimated volume to understand accuracy of water balance and performance parameters