Apex Model Simulation of Runoff and Non-Point Source Pollutants From Watersheds Managed With No-Till Management.

The study was conducted to measure the level of confidence in the ability of Agriculture Policy eXtender (APEX) model to simulate small scale changes at the field scale (0.79 ha). The small watershed, located in Coshocton, Ohio has two periods of land management, a no-till (NT) corn (Zea mays L.)-soybean (Glycine max L.) practice with a rye (Secale cereal L.) cover crop (2000-2004), followed by a period of continuous no-till corn (2005-2010). Specific objectives of this study are to: (i) simulate runoff and water quality data measured from small watershed managed with NT management, and (ii) identify long-term scenarios for best management practices (BMPs) benefits for improving water quality.  This research reports model runoff synthesis comparisons.  The soils of the watershed are classified as Coshocton silt loam. The annual average runoff measured for the 2000-2010 yr was 17 cm yr^-1. Data extraction of all the watershed scale data, digital elevation model (DEM), land use information, and soil information for the whole watershed was completed and map layers of these databases were created using ArcGIS software. Sensitive analysis showed that Hargreaves potential evapotranspiration (PET) equation exponent, SCS curve number index coefficient , soil evaporation-plant cover factor, and CN retention were among the few parameters that were sensitive to the APEX model output. The model calibration and validation periods of the runoff and water quality were reasonable. Long-term scenario analysis results showed that NT management when used for longer duration reduces runoff and nutrient losses compared with that of traditional tillage systems.