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Phosphorus losses from an irrigated watershed in the Northwestern U.S.: Case study of the Upper Snake Rock Watershed
- Bjorneberg, David L., Leytem, April B., Ippolito, James A., Koehn, Anita C.
- Journal of environmental quality 2015 v.44 no.2 pp. 552-559
- case studies, conservation practices, drainage water, irrigation, irrigation water, monitoring, nutrients, phosphorus, ponds, sediments, surface water, suspended sediment, total maximum daily load, water quality, watersheds, Northwestern United States, Snake River
- Watersheds utilizing surface water for irrigation often return a portion of the water to a water body. This irrigation return flow often includes sediment and nutrients that reduce the quality of the receiving water body. Research in the 82,000 ha Upper Snake Rock (USR) watershed from 2005 to 2008 showed that, on average, water diverted from the Snake River annually supplied 547 kg/ha of total suspended sediment (TSS), 1.1 kg/ha of total phosphorus (TP) and 0.50 kg/ha of dissolved phosphorus (DP) to the irrigation tract. Irrigation return flow from the USR watershed contributed 414 kg/ha of TSS, 0.71 kg/ha of TP and 0.32 kg/ha of DP back to the Snake River. Significantly more TP flowed into the watershed than returned to the Snake River while there was no significant difference between inflow and return flow loads for TSS and DP. Average TSS and TP concentrations in return flow were 71 and 0.12 mg/L, respectively, which exceeded the TMDL limits of 52 mg/L TSS and 0.075 mg/L TP set for this section of the Snake River. Monitoring inflow and outflow for five water quality ponds constructed to reduce sediment and phosphorus losses from the watershed showed that TSS concentrations were reduced 36 to 75%, but DP concentrations were reduced only 7 to 16%. This research showed that continued implementation of conservation practices should result in irrigation return flow from the USR watershed meeting the TMDL limits for the Snake River.