EVALUATING SUBSURFACE DRAINAGE HYDROCLIMATOLOGY AND IMPACTS ON STREAMFLOW ACROSS THE CORN BELT

Abstract: Agricultural subsurface drainage is installed across vast extents of the United States’ Corn Belt. While extremely productive, these drained agricultural lands may influence flooding and are the primary contributor to downstream nutrient loading in the Great Lakes and Gulf of Mexico. To better understand and quantify the seasonality of drainflow across the region and impacts to streamflow, a macroscale hydrologic modeling approach was used. A framework was developed to simulate historical subsurface drainage across the Corn Belt. Estimates of regional drain system design (depth and spacing) were made from extension recommendations, guidelines, and physical soil properties; across the region the mean estimated drain depth and spacing based on saturated hydraulic conductivity and depth to any restrictive layer were 1.17 and 24.63 meters, respectively. Soils with greater conductivity had deeper drain depths and wider drain spacings compared to soils with more restriction to water movement. The estimated drain system designs were used in simulated regional hydroclimatology of subsurface drainage at 1/16-degree resolution from 1986‑2015 using the field site and regionally-evaluated Variable Infiltration Capacity (VIC) model. The seasonality of drainage volume and timing was found to vary with both climate and soil, with less interaction between climate and soils in the northern parts of the region where cold season processes have greater influence on subsurface drainage regime. Precipitation minus evapotranspiration was highly correlated with drainage volume and timing metrics, and frost depth was an influential indicator of cold season impacts. Drainage hydroclimate zones were informed by these findings to highlight seasonal differences in subsurface drainage regime based on climate and soils. A combination of observations and simulations were used to isolate influences of subsurface drainage on streamflow and investigate potential mechanisms. Annual streamflow metrics were assessed for 21 large watersheds with subsurface drained area ranging from 1 – 65% of the basin. Evidence for subsurface drainage reducing minimum and low flows and increasing high, but not extreme, flows was found. The relationship for more extreme flows was complex and requires further investigation on the influence of antecedent conditions. Overall, this body of research harmonized findings of field scale studies and provides insight on regional patterns of drainflow volume and timing. Future work investigating the ability of drainage conservation practices to mitigate flooding and nutrient loading attributed to conventional drainage, historically and under future climate scenarios, is recommended to work towards potential management and policy recommendations.