Changes in Groundwater Quality and Agriculture in Forty Years on the Twin Falls Irrigation Tract in Southern Idaho.

Understanding long-term impacts of agricultural landscapes on shallow groundwater quality is needed to improve soil and water management in the southern-Idaho, Twin Falls Irrigation Tract. In 1999 and 2002-07 we resampled 10 of the 15 tunnel drains monitored in a late-1960s study to determine how nutrient concentrations and outflow rates have changed over time in response to changes in land management or climate. Since the late-1960s, an 8-fold increase in the dairy herd has driven shifts toward increased feed cropping, which, along with improved hybrids and production, increased inorganic and manure fertilizer use. The late-1960s to early-2000s period saw a consistent 1.4-fold increase in mean tunnel-drain outflow nitrate nitrogen (NO₃-N) concentrations, a 10% decrease in mean Cl, and an overall 14% decrease in dissolved reactive P (DRP). However, 3 of the 10 tunnels exhibited increased DRP concentrations during the period, and the rate of DRP increase was attributed to a change in land use near the tunnel. Decreases in tunnel flow between sampling periods were linearly related to corresponding increases in the fraction of sprinkler irrigation employed on lands drained by the tunnels. However, further conversion from furrow to sprinkler irrigation is unlikely to reduce tunnel drain NO₃-N concentrations since the latter were unrelated to changes in sprinkler coverage. Implementation of management practices that more precisely control the amount and timing of applied N are needed to match N availability to crop uptake and preclude nitrate leaching. This requires that nitrogen made available from previous manure applications and preceding alfalfa crops be more efficiently utilized.