Modeling effects of drainage water management in fields with rolling topography

Abstract. A gently sloping agricultural field in the Upper Big Walnut Creek (UBWC) Watershed in central Ohio had two adjacent systematically drained zones with separate outlets, both of which were monitored for drainage discharge and nitrate concentrations during 2005 through 2012. Both outlets were subject to conventional drainage during the first four years (i.e. 2005-2008). From 2009 through 2012, one of the outlets was retrofitted with a water table control structure, and subjected to Drainage water management (DWM) during November thru March each year. The observed flow volume and nitrate concentration data were used to calibrate and validate DRAINMOD-NII under conventional and controlled drainage scenarios, respectively. The model simulations indicated average reduction of 11.9% (2.9 cm) in drainage outflow and 4.5% (1.7 kg/ha) in NO₃-N loads under DWM compared to those under conventional drainage mode. A DEM-based delineation suggested that only about 15% of the total area of the field accounted for the zone of influence of the DWM. Furthermore, the zone with DWM treatment was at a higher elevation, and hydraulically connected with the conventional drainage treatment zone. The hydraulic interdependence of the two zones may have affected the water and nutrient balance, and hence the estimates of reductions. The reductions in discharge and NO₃-N loads at the entire field level were then considered by combining flow volumes and NO₃-N loads data from both zones. This approach suggested that, implementing DWM on only 15% of area reduced the annual drainage discharge by about 26% and NO₃-N loads by about 9.4%. The study concluded that for assessing the impact of DWM on drainage outflows from sloping fields that are adjacent and/or hydraulically inter-dependent, an approach based on combined discharge and loads from both zones may be a better indicator of effectiveness of DWM.