Simulating hydrologic response of a pasture hillslope in North Alabama using the Hortonian Infiltration and Runoff/On model

Abstract

A physically based, fully distributed Hortonian Infiltration and Runoff/On hydrologic model was used to model infiltration excess as the dominant runoff generation mechanism on a pasture hillslope. The model was evaluated for its applicability to simulate spatial and temporal variability of runoff generation areas observed on a pasture hillslope in the Sand Mountain region of North Alabama, United States. Three rainfall events of varying intensity and duration were simulated for a highly instrumented pasture hillslope to study the dynamics of runoff generation and runon areas. Calibration and cross validation were performed on all three rainfall events. Performance of the Hortonian Infiltration and Runoff/On–simulated hydrographs was evaluated using root mean squared error, coefficient of determination and Nash-Sutcliffe coefficient of efficiency. The calibrated model for the first event resulted in a root mean squared error of 1.18 m³ (41.7 ft³) for runoff volume; the next two events resulted in root mean squared errors of less than 1 m³ (35.3 ft³). Similarly, the coefficient of determination and Nash-Sutcliffe coefficient of efficiency values for all three events were greater than 0.70 for the calibrated model. Results from cross validation showed that the Hortonian Infiltration and Runoff/On model-simulated runoffs were in agreement with the observed data. In addition, the model simulated spatial and temporal variations in runoff generation, and runon areas were in agreement with observed variations. Model results helped explain the interactions among hydrologic and climatic characteristics, such as topography, soil parameters, and rainfall variations, and their connections to surface runoff–generation processes. Although the model does not simulate subsurface lateral flow, it shows promise for identifying runoff generation and runon areas for controlling nonpoint source pollution from pasture hillslopes in this and similar regions.