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

Authors: Lentz, Rodrick; Carter, David; HAYE, STANLEY - Natural Resources Conservation Service (NRCS, USDA)

Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2017
Publication Date: 3/15/2018
Publication URL: http://handle.nal.usda.gov/10113/5934968
Citation: Lentz, R.D., Carter, D.L., Haye, S.V. 2018. Changes in groundwater quality and agriculture in forty years on the Twin Falls irrigation tract in southern Idaho. Journal of Soil and Water Conservation Society. 73(2):107-119.

Interpretive Summary: Optimal light and temperature conditions in the western U.S. produce near maximum yields for crops grown under irrigation. Heavy fertilizer applications are needed to attain these high yields, which may cause soil nutrients to leach into ground water.Increased use of organic fertilizers (dairy manure) may also increase nutrient leaching risks. When nutrient rich groundwater seeps into surface waters, it promotes algal growth in downstream waters and depletes them of life-giving oxygen, resulting in dead zones, which have dramatically reduced aquatic production. Long-term impacts of agriculture on groundwater quality are difficult to ascertain, but this knowledge is crucial for protecting and preserving Earth’s water supply. Historic data on shallow groundwater quality in Idaho’s Twin Falls irrigation tract provided an opportunity to study such effects across a 40-year span. The study determined how conversion from furrow to sprinkler irrigation, changes in crop types, and increases in dairy herds and inorganic/organic fertilizer applications have altered the spatial and seasonal nitrate-nitrogen and dissolved phosphorus concentrations in shallow groundwater. These data shed light on complex agriculture-groundwater relationships and document a problem of increasing nitrate concentration in groundwater. It also warns land managers of a future long-term issue of increased dissolved phosphorus leaching in some areas, likely due to phosphorus saturation of key soil water pathways. The study points resource managers toward a solution, clearly indicating that agricultural fertilizers of all kinds need to be more efficiently utilized in the Twin Falls irrigation tract to prevent further degradation of water supplies.

Technical Abstract: Better understanding agriculture’s effect on shallow groundwater quality is needed on the southern Idaho, Twin Falls irrigation tract. In 1999 and 2002-2007 we resampled 10 of the 15 tunnel drains monitored in a late-1960s study to determine the influence of time on NO3-N, dissolved reactive P (DRP), and Cl concentrations, and flow rates of shallow groundwater outflows.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 NO3-N concentrations (from 3.06 to 5.06 mg/L), a 10% decrease in mean Cl (from 49.2 to 44.2 mg/L), and an overall 14% decrease in DRP (14 to 12 µg/L). 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). Further conversion 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.