The widespread adoption of subsurface drainage in the Midwest United States coupled with fertile land and abundant rainfall has made this region the largest producer of corn and soybean in the nation. Although subsurface drainage helps reduce the waterlogging stress on crops by removing nutrient-enriched water from the field, it could contribute to harmful algal blooms in freshwater ecosystems. Controlled drainage (CD) practices can reduce the drainage volume leaving the field and have a positive effect on nutrient load reduction. Although the efficiency of CD under the present climate has been widely studied, it is essential to evaluate its performance in the future to build resilient agricultural systems. In this study, the efficiency of two CD practices was evaluated in reducing drainage discharge for the future based on the height and timing of the weir management. We used the Root Zone Water Quality Model (RZWQM2) to predict the CD management effects on drainage discharge. To obtain reliable simulation results from the RZWQM2 model, it is important to use measured soil-water characteristic parameters and flow data. We used HYPROP to measure the parameters of the soil water characteristic curve which served as input to the model. Additionally, we developed a stage-discharge equation for an AgriDrain metal-edge sharp-crest 45℗ʻ V-notch weir to accurately estimate the drainage discharge from the field. A reliable estimate of the drainage discharge was necessary to accurately estimate the nutrient loss.The recently developed P module of the RZWQM2, known as RZWQM2-P was used in this study to predict drainage discharge and P loss from a subsurface-drained field with clay loam soil. We used the Nash-Sutcliffe model Efficiency (NSE) and percentage bias (PBIAS) statistics to evaluate model performance. While the model showed "good" and "satisfactory" performance in predicting drainage discharge and total phosphorus (TP) load, respectively, it performed unsatisfactorily in predicting the dissolved reactive phosphorus (DRP) load for both calibration and validation periods. The underperformance of the model in simulating DRP load may be due to the inability of the model to partition fertilizer P into different P pools.We predicted the efficiency of two CD management scenarios (i.e., common and aggressive management) in reducing drainage discharge for future climate using the calibrated RZWQM2 model. The CD management scenarios were performed by maintaining the weir height at a higher (i.e., 15 cm for non-growing season and 40 cm for growing season) or lower (i.e., 30 cm for non-growing season or 50 cm for growing season) level inside a control structure and by altering the timing of the CD management based on the planting and harvesting dates. While both common and aggressive management was efficient in reducing drainage discharge for both historic and future periods, the percent reduction of drainage discharge with aggressive management was about 11% higher than the common management. The projected increase in precipitation and temperature in the future would cause increased drainage discharge during fall and winter. The aggressive management will be able to completely restrict the flow during the non-growing season which would reduce the nutrient loss of the surface water bodies. In the future, farmers should plant early to benefit from the projected increase in spring rainfall and avoid dry summers. In conclusion, it is evident from the results of this study that both common and aggressive CD management will continue to be effective in reducing drainage discharge in a changing climate.
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