| A process based, small rural watershed surface hydrology model, with emphasis on the winter hydrology of the Pacific Northwest (PNW), was developed. The model divides the watershed into homogeneous cells, hydrologically interconnected, and runs a full cropping systems submodel in each of them. The model simulates the runoff generated under different management scenarios and environmental conditions. Several hydrological components were developed using simple and suitable algorithms that adequately mimic the process and the outputs of a more complex model. Soil surface temperature, frost depth, infiltration, and runoff were among the most important. The components were integrated into Cropsyst (Stockle and Nelson, 1994) and linked with a GIS system to account for spatial variability. Soil surface temperature was simulated for conditions of residue, crop canopies and snow cover using a modification of Parton's equations. The soil temperature profile was estimated using a simplified numerical solution of the heat equation (SNS). The modeling of soil frost depth was based on one of the solutions of the Stefan's equation, (Modified Ruckili's solution) and compared with estimations given by the SHAW model. Infiltration and runoff were treated using a finite difference scheme, where surface storage was determined using the concept of random roughness. This methodology was compared, for several management scenarios, with the original SCS curve number method, and with a modification that included a function to account for the soil frost in the curve number value. A 10.4 ha watershed, located near Pullman, with 10 x 10 m cells, was used to evaluate the hypothetical watershed's hydrological response to management scenarios, using the developed watershed model. The run-time fluctuated between 240 and 400 minutes to run 365 days of simulation using a Pentium™ II PC running at 450 MHz. The maximum runoff was estimated when the soil was frozen during most of the winter and no residues were present. When residue was present, total runoff decreased to one-third. Future development of the model will include a subsurface hydrology simulator, processes of transport of sediment leading to soil erosion estimation and transport of chemicals on surface and in subsurface flow. |
