Apex Model to Assess Conservation Efforts of Mrbi Farm Sites in Missouri.

Mississippi River Basin Healthy Watershed Initiative (MRBI) provides incentives for voluntary implementation of conservation practices at farm level to reduce pollutant loadings and conducts water quality monitoring for impact assessment. Such assessment is not only important for return of expenditure but also for policy and decision making. A calibrated and validated farm-scale hydrological model, the Agriculture Policy Environmental eXtender model (APEX) was used with the objective of quantifying the pre- and post benefits of conservation practices of terraces, winter cover crops (WCC), and grass waterways (GWW) on flow, sediment, and total phosphorus (TP), and total nitrogen (TN) losses at four farm locations (1.8 to 31.7 ha) in Knox, Audrain and Chariton Counties of Missouri (NRCS-MLRA, 109 and 113) with row-crops. Multi-variable model calibration was conducted with event-data (2012-2013) from a watershed (2.69 ha) in Chariton County, without any conservation practice, with acceptable model performance indicators for all outputs: coefficient of determination (r²): 0.6-0.9, Nash-Sutcliff efficiency (NSE): 0.5 to 0.8, and percent bias (Pbais): -26%, -11%, 34%, and -65%, for runoff, sediment, TP and TN, respectively. The model was validated with acceptable model performances (r²>0.6, NSE>0.5 and Pbias <25% for runoff and <65% for sediment, TN and TP), except for NSE of TN, using event-data (2011-2013) from a nearby watershed with terraces, WCC, and a GWW (31.7 ha). The calibrated and validated model simulated pre- and post management/s scenarios for long-term water quality benefits of exiting practices at other locations. Most dominant effects were seen on sediment and nutrient loadings. Hundred percent terraced watersheds (Chariton and Knox) showed the largest reductions in sediment (~72%), TP (48 to54%), and TN (67 to69%) loads compared to ~50% terraced watersheds in Audrain County (37 to 46% reductions in sediment, TP and TN). However this loss was compensated by having winter wheat CC in addition to terraces, to increase the reductions by ~20% in sediment and nutrient loads in 50% terraced watersheds. The combination of terraces, WCC and GWW reduced 89%, 63%, and 78%, sediment, TP, and TN loads, respectively. The conservation practices implemented with MRBI incentives showed beneficial returns in water quality at farm-levels in Missouri. Multi-variable calibrated and validated APEX model was a useful tool for water quality assessment of edge of the field conservation practices.