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Title: Modeling streamflow in a snow-dominated forest watershed using the Water Erosion Prediction Project (WEPP) model

Author
item SRIVASTAVA, A - Purdue University
item WU, J - Washington State University
item ELLIOT, W - Forest Service (FS)
item BROOKS, E - University Of Idaho
item Flanagan, Dennis

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2017
Publication Date: 8/29/2017
Citation: Srivastava, A., Wu, J.Q., Elliot, W.J., Brooks, E.S., Flanagan, D.C. 2017. Modeling streamflow in a snow-dominated forest watershed using the Water Erosion Prediction Project (WEPP) model. Transactions of the ASABE. 60(4):1171-1187.

Interpretive Summary: Soil erosion by water is a serious problem around the world, affecting cropland, rangelands, forests, and other types of managed lands. Computer simulation models are often used to estimate the amount of soil erosion that will occur from areas of land, typically field-sized or smaller (scale of around 1-100 acres). This size of simulation area can usually be characterized by a single climate (rainfall, temperatures, etc.) input file, along with soil(s), topography, and types of plant/crops and tillage or other disturbance, and surface runoff is driven by the amount and rates of rainfall, and resulting overland flow. Erosion prediction models, like the USDA Water Erosion Prediction Project (WEPP) were originally designed to function at this type of small field scale. However, as the size of a simulated area becomes larger, and especially when land management is composed of large amounts of vegetative cover (such as in forests), some (or most/all) of the rainfall may infiltrate into the soils, and then move laterally underground towards streams and channels, emerging as baseflow. New uses for WEPP include application to larger watersheds, particularly in forested areas to address problems such as remediation after burning by wildfires. In order to better account for baseflow, runoff, and channel erosion in larger areas, this research modified the WEPP model and added a special baseflow component. Testing results using observed data from a larger watershed (about 26,000 acres) in Washington State showed that adding a baseflow component improved the ability of WEPP to simulate runoff and baseflow. These results impacts scientists, action agency personnel, and others involved in simulation of hydrologic and erosion processes in larger watersheds, where baseflow is a significant component of the water balance and flow in streams and channels.

Technical Abstract: The Water Erosion Prediction Project (WEPP) model was originally developed for hillslope and small watershed applications. The model simulates complex interactive processes influencing erosion, such as surface runoff, soil-water changes, vegetation growth and senescence, and snow accumulation and melt. Recent improvements to the model led to enhanced computations for deep percolation, subsurface lateral flow, and frozen soil. In addition, the incorporation of channel routing has made the WEPP model well suited for large watersheds with perennial flows. However, the model is still limited in modeling forested watersheds where groundwater baseflow is substantial, and where snow accumulation and melt dominate winter hydrology. The objectives of this study were to (i) incorporate nonlinear groundwater baseflow algorithms in WEPP (v2012.8) for estimating groundwater baseflow; (ii) auto-calibrate the modified WEPP using a model-independent parameter estimation tool, and (iii) evaluate and compare the performance of the modified WEPP model with baseflow (WEPP-Mod), and the current version of WEPP without baseflow (WEPP-Cur) by application to a snow-dominated watershed in the US Pacific Northwest. A subwatershed of the Upper Cedar River watershed was chosen for WEPP application and assessment. The Upper Cedar River watershed (105 km2) is a gauged, snow-dominated mountainous watershed that provides drinking water to the greater Seattle area. Simulations were conducted for two periods, 1997–2003, to calibrate the model, and 2004–2011, to validate the model. The WEPP-Mod model reproduced hydrograph recessions during the low-flow periods and the general trend of the hydrographs, demonstrating its applicability to a watershed where groundwater baseflow was significant.