Sediment and nutrient losses from the Upper Snake Rock watershed in southern Idaho

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.