MODELING HYDROLOGICAL PROCESSESS IN THREE MOUNTAINOUS WATERSHEDS IN THE U.S. PACIFIC NORTHWEST
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Assessment of water yield from mountainous watersheds is critical for managing aquatic life while meeting downstream water supply demands. Baseflow plays a key role in regulating the streamflow during low-flow periods. Quantification of baseflow from lands with different watershed characteristics is beneficial in the management of water resources.For this doctoral research, I developed an algorithm for estimating groundwater baseflow following the linear-reservoir approach, and incorporated this algorithm into a commonly used watershed hydrologic and erosion model, WEPP (Water Erosion Prediction Project, v2012.6). I applied the modified WEPP model to simulate the hydrological processes in three mountainous forest watersheds in the US Pacific Northwest. The first study demonstrated the relevance of a linear-reservoir approach in simulating baseflow from the Priest River watershed, ID. The WEPP-simulated streamflow was in good agreement with the observed streamflow when baseflow was considered; without including the baseflow, WEPP under-predicted streamflow. The second study aimed at developing a stand-alone algorithm to estimate groundwater baseflow following the linear-reservoir approach, and incorporating it into WEPP, a commonly used watershed model. The Levenberg-Marquardt algorithm was implemented to estimate the groundwater storage and baseflow coefficients by fitting simulated streamflow to observed streamflow data. WEPP was applied to the Upper Cedar River watershed, WA, and its performance evaluated using observed SWE and streamflow data. WEPP reproduced the majority of observed streamflow peaks and the general trend of the hydrograph, demonstrating its applicability to large watersheds where baseflow contributions to streams are significant.The third study evaluated the effect of wildfire prevention treatments on water balance in the East Deer Creek watershed, WA. FlamMap and Online GIS BAER WEPP were used to identify critical hillslopes for thinning and prescribed-burn treatments. Changes in water yield and balance from pre- treatment conditions were obtained. Simulated water balance components at the treated hillslopes showed substantial changes, suggesting alterations in hydrological processes at these hillslopes. Despite variations at the hillslope scale, thinning and prescribed-burn treatments resulted in only a slight increase in water yield at the watershed scale.