Predicting Field Water Balance, Crop Yield, and the Economics of Drainage Under Various Cropping Systems Using Drainmod

Subsurface drainage received considerable attention during the recent few years in South Dakota. While subsurface drainage is a widely accepted water management practice for increasing crop yield, research implicated tile drainage in surface and groundwater quality problems. Conservation practices such as crop rotation and controlled drainage may decrease tile flows and improve water quality. A two-year (2014-2015) subsurface drainage study was conducted at South Dakota State University Southeast Research Farm (SERF) near Beresford, South Dakota to evaluate the effectiveness of selected conservation practices in reducing drainage volume and nitrate losses. Six experimental plots, under corn-soybean rotation, divided into drained and undrained plots, were monitored for baseline data (i.e. drainage discharge, water table depth, infiltration, bulk density, and rainfall) collection. DRAINMOD was used with the baseline data to quantify the long-term hydrologic impacts of subsurface tile drainage on field water balance for different drainage conditions (conventional drainage, controlled drainage, and undrained condition) and cropping practices. Long-term simulations for 12-year period (2004-2015) were conducted to predict annual and monthly water balance, crop yield response under conventional drainage, controlled drainage, and undrained conditions for continuous corn, corn-soybean, soybean-corn, corn-wheat, wheat-corn, soybean-wheat, and wheat-soybean cropping practices. Average annual subsurface drainage results for continuous corn, corn-soybean, soybean-corn, corn-wheat, soybean-wheat, wheat-corn, and wheat-soybean cropping practices under controlled drainage showed drainage volume reduction of 28%, 24%, 24%, 52%, 37%, 54%, and 40%, respectively, compared to conventional drainage. Similarly, average annual surface runoff results for continuous corn, corn-soybean, soybean-corn, and wheat-soybean rotation under conventional drainage indicated runoff volume reduction of 72%, 75%, 71%, and 76%, respectively, compared to undrained conditions, and under controlled drainage runoff volume reductions for same cropping practices were 65%, 68%, 65%, and 66%, respectively, compared to undrained conditions. Average monthly water balance showed high ET water loss during the month of May to August and high drainage water loss during month of May and June. Drainage volume reduction of 57.0% was observed in June for wheat-corn rotation under controlled drainage compared to conventional drainage. Likewise, surface runoff volume reduction of 86.7%, and 70.0% in conventional drainage and 86.6% and 63.3% in controlled drainage for May and June was observed in soybean-corn rotation compared to undrained conditions. Predicted relative crop yield percentage showed high yield in soybean-corn, and corn-soybean rotation under conventional drainage and controlled drainage compared to all other cropping practices. Relative crop yield for soybean-corn and corn-soybean under conventional drainage was 81.6% and 80.9%, respectively, and under controlled drainage, relative yield was 81.8% and 81.7%, respectively. Crop relative yield results indicated better yield for soybean-corn followed by corn-soybean production under both conventional and controlled drainage compared to undrained conditions but economic analysis results showed better net annual return form soybean-corn rotation under controlled drainage compared to all other cropping practices in controlled drainage, conventional drainage, and undrained conditions