Upper Snake Rock Watershed, Idaho

Characteristics

The Upper Snake-Rock (USR) watershed was one of eight special emphasis CEAP watersheds selected in 2004. This 6300 km² watershed is located in south-central Idaho along the Snake River. Land use within the USR is 37% irrigated agriculture, < 1% dryland agriculture, and 60% rangeland and forest land. Since the USR watershed was chosen to specifically assess the effects of conservation practices in irrigated agriculture, watershed monitoring focuses on the Twin Falls irrigation tract, an 82,000 ha agricultural area in which the irrigation water is managed and delivered by the Twin Falls Canal Company (TFCC).

The Twin Falls irrigation tract is bounded on the north and west by deeply incised canyons (100 to 150 m deep) of the Snake River and Salmon Falls Creek. The Twin Falls Main Canal forms the eastern boundary and the High Line Canal forms the southern boundary. Rock Creek is the only stream contributing significant flow to the Twin Falls tract, and this stream often does not flow from June through September. Other streams flowing within the Twin Falls tract to the Snake River originate from furrow irrigation runoff, unused irrigation water, and subsurface drainage within the tract.

Irrigation diversions into the Twin Falls irrigation tract are about 5 times greater than the 250 mm average annual precipitation. The TFCC has supplied irrigation water to this area since 1905. Water is diverted from the Snake River at Milner Dam and routed through 180 km of main canals and over 1600 km of smaller channels and laterals. The TFCC has over 3000 service gates (headgates) for delivering water to fields. Irrigation water flows by gravity from the Snake River throughout the 82,000 ha watershed. Natural channels or coulees often convey water to laterals, and collect runoff and unused irrigation water from fields. Water only flows in these coulees during the irrigation season (April through October) unless subsurface drains contribute flow during the non-irrigation season.

All crop production within the Twin Falls irrigation tract is irrigated. Major crops grown include alfalfa, corn, small grains, sweet corn, dry beans, sugar beets, and potatoes. Approximately 30% of the crop land is sprinkler irrigated with the remaining 70% irrigated by furrow irrigation. Many fields have been converted to sprinkler irrigation since the mid-1990’s with financial assistance from the Environmental Quality Incentive Program (EQIP) and other special projects. EQIP has provided over $4 million for conservation practices in Twin Falls County from 2002 to 2006, mostly for converting to sprinkler irrigation.

Environmental Impacts

Irrigation-induced soil erosion remains the predominant natural resource concern in this watershed. Water flowing in irrigation furrows detaches and transports soil. Eroded sediment and associated nutrients are then transported to the Snake River with irrigation return flow. Impacts of irrigation diversions on water temperature are also a concern for meeting beneficial uses for cold water biota and salmonid spawning.

Management Practices

Irrigation System, Sprinkler (442)

Irrigation Water Management (449)

Anionic Polyacrylamide (PAM) Erosion Control (450)

Nutrient Management (590)

Sediment Basin (350)

Waste Utilization (633)

Research Objectives

The two main objectives of this study are to 1) determine the effects of conversion from furrow irrigation to sprinkler irrigation on water quality and quantity and 2) compare current water and salt balances for the Twin Falls irrigation tract with data collected from 1968 to 1970, prior to the conversion to sprinkler irrigation. An additional objective is to modify/adapt an existing watershed water quality model for use in an irrigated watershed.

Approaches

Watershed monitoring began in 2005. Twenty-six water quality monitoring sites have been established for determining water and salt balances. These sites are categorized as primary, secondary, or tertiary sites. Data loggers record flow stage at weirs or control sections for primary and secondary sites. Flow rates at tertiary sites are calculated from a weekly staff gage measurement on a weir or from a weir stick measurement on a concrete TFCC structure. Primary sites have automatic water samplers collecting time-composite water samples while weekly grab samples are collected at secondary and tertiary sites.

A multiple sub-watershed approach is being used to assess the water quantity and quality effects of converting from furrow to sprinkler irrigation. Five small watersheds within the Twin Falls irrigation tract, each having a well defined inflow boundary and a single outlet, are being monitored. It is common within the Twin Falls irrigation tract for unused irrigation water and field runoff to be diverted from drainage channels to other fields, making the surface water hydrology very complex. Water is not re-diverted within these five watersheds, which vary from 150 to 600 ha and had 5 to 50% of the cropland sprinkler irrigated in 2005.

Inflow to each sub-watershed is calculated from TFCC daily records for all headgates in each watershed. Outflow from each watershed is measured with a flume. A data logger records flow stage and an automatic sampler, controlled by the data logger, collects flow proportional water samples. Crop production and irrigation practices on the five sub-watersheds are recorded through monthly field surveys during the irrigation season. Two or three additional surveys are conducted during the winter and early spring to record tillage practices and manure application.

Collaborators and Cooperating Agencies and Groups

NRCS, Twin Falls Canal Company, Idaho Department of Environmental Quality, Idaho Department of Water Resources, University of Idaho.