Phosphorus Losses From An Irrigated Watershed in the Northwestern U.S.: Case Study of the Upper Snake-River Watershed.

Approximately 14% of US cropland is irrigated with irrigated farms accounting for 55% of the total value of crop sales. Idaho, Washington and Oregon have 13% of total US irrigated area and 40% is surface irrigated. Unlike rainfall, irrigation is a scheduled activity with controlled application rates and durations and runoff is often planned to improve infiltration uniformity. Twenty to 50% of applied water may run off a field and be reused within an irrigation district. Runoff that is not captured carries sediment and nutrients to receiving waters. We will review 38 years of research in the Upper Snake Rock watershed focusing on nutrient movement within surface irrigated fields and nutrient loads returning to the Snake River from the 820 km² Twin Falls irrigation tract.  Irrigation water diverted from the Snake River supplies 80% of the input water to the watershed with approximately 27 % returning to the Snake River.  From 1968 to 2006 there have been substantial decreases in sediment and total P loads due to irrigation and field management practices.  The irrigation tract is a sink for total and soluble P during the irrigation season and a source of total and soluble P (via subsurface drainage water) during the non-irrigation season.  However, soluble P concentrations in the return flow are frequently over the TMDL threshold of 0.075 mg L^-1. Understanding processes and management practices at the field and system level have helped to identify management practices to reduce these loads such as sedimentation ponds, the use of polyacrylamide, alum, and buffer strips. New work in the area of irrigation erosion modeling will also enable water users and managers a way to better predict sediment and nutrient loading from these artificially drained systems.