Incorporating Forest Regeneration Routines in the Water Erosion Prediction Project (WEPP) Model

The Water Erosion Prediction Project (WEPP) is a process-based continuous simulation water erosion prediction model. The US Forest Service uses WEPP for a variety of applications in the forest landscape and has developed a suite of interfaces (Forest Service WEPP or FS WEPP) for various applications including undisturbed forests, timber harvest, post-wildfire, and road analysis scenarios. In the current version of WEPP (v2012.8), the model application is restricted to the first-year post-disturbance conditions due to the lack of adequate forest regeneration routines involving live biomass, rooting depth, canopy cover, canopy height, Leaf Area Index (LAI), which are sensitive to hydrologic and erosion calculations. Canopy cover is an essential variable in WEPP to simulate raindrop impact and snow water equivalent on the ground. LAI, rooting depth, and canopy height are other sensitive variables in the model for evapotranspiration computations that are responsible for changes in soil-water content affecting runoff generation. The growth and senescence factors of live biomass determine the accumulation of ground cover affecting the hydraulic and erodibility parameters critical for soil erosion. We incorporated improved forest regeneration routines in WEPP to simulate successional forest vegetation dynamics. This included developing relationships of canopy cover, LAI, rooting depth, canopy height with live biomass for different forest-stand ages and species, and modify hydraulic and erodibility parametric equations to account for temporal variability of vegetation growth and groundcover. We derived forest vegetative characteristic versus age relationships of tree species of the US Pacific Northwest (PNW) using the 1) freely-available remote sensing satellite imagery data, and 2) derived data of the Environmental Monitoring, Analysis and Process Recognition Lab, Oregon State University. Forest Inventory and Analysis (FIA) datasets were used for validation. The derived relationships of forest stands were incorporated and adapted into WEPP. We assessed the modified WEPP by applying it to several watersheds in the US Pacific Northwest with available observed streamflow and sediment delivery data. The associated vegetation growth parameters will be incorporated into WEPPCloud interface, a forest management decision-support tool, which will allow users to study and evaluate the cumulative effects of forest management on water and sediment yield for post-timber harvest and post-wildfire activities at the watershed scale.