Abstract
Abstract. The 82,000 ha Upper Snake Rock watershed has been irrigated with water from the Snake River since 1905. Furrow irrigation was used on 95% of the cropland until the 1990‘s when conversion to sprinkler irrigation increased due in part to cost-share programs. Water quantity and quality monitoring from 2006-2008 showed that irrigation supplies four to seven times more water to the watershed than precipitation and 37% of the water returns to the Snake River as irrigation return flow. Irrigation water delivered 550 kg ha-1 of sediment, 0.3 kg ha-1 of nitrate-N (NO3-N), 0.5 kg ha-1 of dissolved phosphorus (P), and 3500 kg ha-1 of dissolved salts into the watershed. Irrigation return flow transported 420 kg ha-1 of sediment, 13.9 kg ha-1 of NO3-N, 0.3 kg ha-1 of dissolved P, and 2650 kg ha-1 of dissolved salts back to the Snake River. There was net deposition of 130 kg ha-1 of sediment, 0.2 kg ha-1 of dissolved P, and 850 kg ha-1 of dissolved salts, and a net loss of 13.6 kg ha-1 of NO3-N based on differences between inflow and outflow. Converting to sprinkler irrigation and installing sedimentation ponds greatly reduced the sediment loss compared to 1971 when 1380 kg ha-1 of sediment returned to the Snake River and there was a net loss of 460 kg ha-1 of sediment from the watershed. Data indicated that leaching has decreased since 1970, but NO3-N concentrations in irrigation return flow have approximately doubled since 1968-1970. Conservation practices have greatly reduced sediment losses but additional practices are needed reduce NO3-N leaching and improve water use.
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