Purpose of HUA
The Idaho Snake-Payette Rivers Hydrologic Unit Area (HUA) is one of 74 projects funded nationally by USDA. These 5-year projects have the purpose of accelerating the transfer of best management practice (BMP) technology necessary to protect both ground and surface waters while still maintaining farm profitability. Program efforts focus on irrigation, nutrient, and pesticide management for groundwater protection. The HUA projects offer agriculture the opportunity to demonstrate that education coupled with a voluntary BMP implementation program can protect and even enhance existing water quality.
The Snake-Payette Rivers HUA comprises over 840,000 acres in Canyon, Gem, Payette, and Washington counties in southwestern Idaho. Virtually all of the productive farmland on the 3,400 farms in the project are irrigated. Agriculture within the HUA is very diverse as over 50 different high-value crops are grown.
Idaho is the nation's second largest water user. Agriculture accounts for 97 percent of the 23 billion gallons of water used in the state every day.
Most of the intensively irrigated crop production areas in the U.S. have some degree of groundwater nitrate-N (NO3-N) problem. The Environmental Protection Agency (EPA) drinking water standard for NO3-N of 10 parts per million (ppm) is exceeded by between 5 to 8 percent of the wells in the HUA. This compares unfavorably to the U.S. average where the nitrate drinking water standard is exceeded about 2.4 percent of the time. Some pesticides have also been detected in wells within the HUA.
The public is demanding that agriculture minimize potential environmental impacts associated with irrigation -- such as leaching losses of NO3-N into groundwater and sediment losses to rivers. Sound irrigation management is essential to address water conservation, erosion control, and water quality issues and maintain farm profitability.
This survey of grower irrigation management practices was a necessary first step for the development of both education and implementation plans related to irrigation in the HUA. Specific survey objectives included:
|Crop||Acres in HUA||Fields surveyed|
|-- Irrigation method --|
Number of Water Applications
Crops in the HUA are irrigated anywhere from 3 to 14 times a season. In general, shallow rooted crops like onions and potatoes are irrigated more frequently than deeper rooted crops. The number of irrigations is also positively related to the length of growing season.
Amounts of Water Applied
The project area has a semi-arid continental climate with less than 4 inches of precipitation during a typical growing season. The warm, dry summers necessitate frequent irrigation to fulfill seasonal consumptive use of the crops grown in the HUA. The average amount of water applied to each crop was based on average reported irrigation plus rainfall during the 1991 growing season.
|------------- (acre inches) -------------|
Crop consumptive use values are based on evapotranspiration values from the USDA-SCS Idaho Irrigation Guide. Water applications range from 19 to 56 acre inches for HUA crops over the growing season. Actual crop consumptive use values are crop dependent and range from 16.0 acre inches of water for small grains to 33.3 acre inches of water for orchards.
Based on consumptive use, all crops received more water by irrigation than actually required by evapotranspiration. This excess ranged from 3 inches for alfalfa seed to over 30 inches for sweet corn production. Based on evapotranspiration values, sweet corn uses only 36.9 percent of the water applied in an average year (48.4 - 17.9 = 30.5). Crops using less than half the water applied include: potatoes (49.8%), hops (48%), dry beans (46.9%), small grains (41%), and onions (39.2%).
On average, 28 percent less water was applied with sprinklers than with furrow irrigation. The runoff from surface irrigated fields is generally reapplied to fields downstream. Irrigation set times averaged 19.5 and 13.5 hours for furrow and sprinkler methods, respectively.
Total Water Use
Over 1,500,000 acre feet of water are annually applied in the HUA. Almost half of this water is applied to alfalfa and small grain crops. Sugar-beets and field corn also consume significant amounts of water.
The primary method for scheduling irrigations in the HUA was reported to be experience. Irrigation scheduling based on evapotranspiration and soil moisture monitoring provides an avenue to improve irrigation efficiency in the HUA. Fourteen percent of fields surveyed can only be irrigated when water is available to the grower.
Upstream storage reservoirs supply 85 to 90 percent of the water demands of irrigated agriculture in southwestern Idaho. Future demands on the water in these resevoirs may increase the cost and reduce the quantity of water available for irrigation. This will necessitate improvement in irrigation efficiency for agriculture to remain profitable.
Based on information gained from this survey, cost assistance programs and informational efforts are being targeted toward practices to improve the water use efficiency in the HUA. Practices being promoted to decrease deep percolation and runoff from surface systems include: surge irrigation, land-leveling, straw mulching, basin irrigation, improved scheduling methods, and tailwater re-use systems. Conversion of furrow systems to sprinkler and drip systems will be promoted where technically and economically feasible or when requested by producers to meet water conservation or crop production objectives.
This brochure, WQ-26, was prepared by T. D. Steiber and R. L.
Mahler. Mahler is the University of Idaho extension water quality coordinator,
located in the Soil Science Division, University of Idaho, Moscow, Idaho
83844-2339. Stieber is an Extension agent with the Idaho Snake-Payette Rivers
USDA Water Quality Project. The project office is located at 1630 Third Ave. S.
#3, Payette, ID 83661. Telephone: (208) 642-6128.
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All contents copyright © 1997-2003. College of Agricultural and Life Sciences, University of Idaho. All rights reserved. Revised: January 3, 2003