Changes in groundwater quality and agriculture in forty years on the Twin Falls irrigation tract in southern Idaho

Abstract

Understanding long-term impacts of agricultural landscapes on shallow groundwater quality is needed to improve soil and water management in the Twin Falls irrigation tract in southern Idaho. In 1999 and 2002 through 2007, we resampled 10 of the 15 tunnel drains monitored in a late-1960s study to determine how nutrient concentrations and flow rates of outflows 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 (from 3.06 to 5.06 mg L⁻¹ [ppm]), a 10% decrease in mean chloride (Cl) (from 49.2 to 44.2 mg L⁻¹ [ppm]), and an overall 14% decrease in dissolved reactive phosphorus (DRP) (14 to 12 μg L⁻¹ [ppb]). However, 3 of the 10 tunnels exhibited increased DRP concentrations during the period, and the rate of DRP increase was positively related to increasing encroachment of confined animal feeding operations or residential development. 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 (p = 0.01). 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. The amount and timing of applied N and availability for crop uptake or leaching should be more carefully managed in these soils to prevent continued increases in groundwater NO₃-N concentrations. It is recommended that alfalfa (Medicago sativa L.) rotations be followed by deep-rooted crops with large N requirements to better utilize N mineralized from killed alfalfa roots.