Conceptualization and simulation of runoff generation from rainfall for three basins in Thurston County, Washington

A method to conceptualize and simulate the generation of runoff from rainfall was applied to three small drainage basins in north-central Thurston County, Washington. Because the study basins face continued urban development, this method was developed to provide a technique for assessing the effects of alternative urban-development plans on runoff characteristics. A conceptual model of runoff generation and three numerical simulation models, called basin models, were constructed for the study basins. Hydrologic Simulation Program- FORTRAN was the program used to construct the basin models. The basin models were constructed using parameter values based on qualitative rainfall- runoff relations defined in the conceptual model. The features of the conceptual model, incorporated in the basin models, were assessed by calibration and evaluation of the basin models with observed streamflow data collected from March 1988 through March 1990. The simulation results from the basin models generally confirmed the study's conceptual model of runoff generation. Absolute differences between simulated and observed streamflows were less than 6 percent for total runoff volumes, equal to or less than 32 percent for daily mean discharges, less than 32 percent for storm runoff volumes, and less than 33 percent for peak discharges. Although the conceptual model adequately described runoff generation from rainfall, it did not adequately describe the actual flow paths from land segments to streams.