Water quality trends in irrigation return flow from a southern Idaho watershed.
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The quality of irrigation return flow in an 82,000 ha watershed in southern Idaho has changed since 1970 when return flow water quality was first measured. Converting from furrow irrigation to sprinkler irrigation and installation of sedimentation ponds have greatly reduced sediment losses. There is now more sediment entering the watershed with irrigation water from the Snake River than returns to the river, compared to a net loss of 460 kg/ha sediment in 1971. The irrigation water has <0.5 mg/L of nitrate-N. Runoff from furrow irrigated fields does not increase nitrate-N concentrations. Subsurface drainage, however, does contribute nitrate-N from shallow groundwater to irrigation return flow, resulting in 10 to 15 kg/ha of nitrate-N flowing to the Snake River each year. Nitrate-N concentrations in four main return flow streams approximately doubled from 1970 to 2005 to 2-2.5 mg/L. Recent data indicate about a 10% decrease in nitrate-N concentrations from 2005 to 2015. There are more soluble salt flowing into the watershed with irrigation water than returning to the river. While continually adding salts to the soil is not desirable, the overall average sodium adsorption ratio (SAR) in the irrigation water from 2005-2008 was only 0.73 and the average soluble salt concentration was 280 mg/L or 435 µS/cm, which are low enough to not impact crop production. Nitrate-N has become a higher natural resource concern in this watershed as sediment losses have been addressed. Continued efforts to improve irrigation and nitrogen management will be needed to reduce nitrate-N concentrations in shallow groundwater which will reduce nitrate-N in irrigation return flow.