Simulating Runoff from Small Grazed Pasture Watersheds Located at North Appalachian Experimental Watershed.

Simulation models are one of the decision making tools for the land manager and the conservationist. The Agricultural Policy Environmental Extender (APEX), a process based hydrologic model, is used to evaluate the grazing impacts on runoff. The sensitivity analysis and calibration of model was done using the inverse modeling approach of Parameter Estimation (PEST) model. The specific objectives of the study were to: (i) calibrate and validate runoff from three small grazed pasture watersheds using the inverse modeling approach and, (ii) simulate the long-term impact of different management scenarios on runoff response. The study was conducted using data from three small grazed pasture watersheds located on the North Appalachian Experimental Watersheds (NAEW) near Coshocton, Ohio. The WS103 and WS110 were modeled for 2003-2011 and WS129 for 1976-1984 period with same management system. The management on all three watersheds during simulation period includes rotational grazing. The inverse modeling approach increased the efficiency of sensitivity analysis and provided better calibration and validation of the APEX model for most of the measured runoff values. For daily runoff calibration and validation during the growing season, R² and Nash-Sutcliffe (NSE) values ranged from 0.49 to 0.72 and from 0.25 to 0.60, respectively. After calibrating and validating, model was run for 37 years (1975 to 2011) for long-term scenario analyses. Long term scenario indicates that meadow management significantly reduced runoff by 60%, 67% and 58% for watersheds 103, 110 and 129, respectively, compared to grazed pasture management. Similarly, replacing soil properties of the upslope with those of downslope significantly reduced runoff by 0.3, 2.4 and 5% at alpha level of 0.05 compared to the antecedent condition for WS103, WS110 and WS129, respectively. The results from this study demonstrate the benefits of using the inverse modeling approach, the long-term benefits of meadow management system on runoff compared to those of grazed system, and the sensitivity of soil properties change along different landscape positions (upslope and downslope) on water runoff.