Modeling forestry management effects on water and sediment yield from nested, paired watersheds in the interior Pacific Northwest, USA using WEPP

Authors: SRIVASTAVA, ANURAG - Purdue University; BROOKS, ERIN - University Of Idaho; DOBRE, MARIANA - University Of Idaho; ELLIOT, WILLIAM - Forest Service (FS); WU, JOAN - Washington State University; Flanagan, Dennis; GRAVELLE, JOHN - Pine Orchard, Inc; LINK, TIMOTHY - University Of Idaho

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/2019
Publication Date: 11/1/2019
Citation: Srivastava, A., Brooks, E.S., Dobre, M., Elliot, W.J., Wu, J.Q., Flanagan, D.C., Gravelle, J.A., Link, T.E. 2019. Modeling forestry management effects on water and sediment yield from nested, paired watersheds in the interior Pacific Northwest, USA using WEPP. Science of the Total Environment. 701:134877. https://doi.org/10.1016/j.scitotenv.2019.134877.
DOI: https://doi.org/10.1016/j.scitotenv.2019.134877

Interpretive Summary: Runoff, soil erosion, and sediment losses are important processes in forested watersheds in mountainous states of the U.S. and other areas in the world. Human activities such as timber harvesting can remove protective vegetation cover, and leave the soil exposed and disturbed, and at much greater risk of erosion and sediment loss. Computer simulation models are often used to represent erosion processes, and the effects of land management practices. In this study we applied the Water Erosion Prediction Project (WEPP) model to seven experimental watersheds in northern Idaho, where there were observed data available on streamflow and sediment losses, as well as detailed climatological data and land management records of timber harvesting operations. WEPP was able to predict the streamflow and sediment losses very well after calibration, though the results showed areas of the model that could be improved. This research impacts scientists, forest managers, conservation agency personnel and others involved in assessment of land management in forested regions. Modeling can provide another tool for managers to evaluate the impacts of planned timber harvesting activities on on-site erosion and off-site stream water quality.

Technical Abstract: The Water Erosion Prediction Project (WEPP) model was applied to seven paired, nested watersheds within the Mica Creek Experimental Watershed located in northern Idaho, USA. The goal was to evaluate the ability of WEPP to simulate the direct and cumulative effects of timber harvest activities on water and sediment yield. WEPP was modified to represent Leaf Area Index (LAI) for ET changes to reflect vegetative recovery during post-harvest conditions. Daily streamflow simulations were conducted without parameter calibration for 1992–2007 for watersheds with and without treatments. For sediment yield simulations, pre-treatment years were used for calibration and post-treatment years for model performance assessment. The channel critical shear stress was the only model parameter used to calibrate sediment transport in the channels. Change in simulated water and sediment yield was assessed against observed data. Overall, for the seven watersheds, the Nash-Sutcliffe Efficiency (NSE) and deviation of runoff volume (DV) between observed and simulated daily streamflow ranged from 0.58 to 0.71 and from -4 to 9%, respectively. For sediment yield, NSE and DV for the calibration period ranged from 0.62 to 0.97 and from -2 to 2%, respectively. For the model performance period, NSE and DV varied from 0.61 to 0.93 and from -24 to 13%, respectively. Statistical analysis of observed and WEPP-simulated increase in water and sediment yield following harvest treatment was similar, however, the WEPP-simulated increase was lower compared to observations particularly from the partial-cut watershed. Overall, the simulated results demonstrate the potential effectiveness of WEPP as a forestland watershed management modeling tool. Future efforts should be devoted to improving modeling inputs to better represent physical processes such as forestland vegetation ET complexities by deriving an adequate relationship between aboveground live biomass and LAI of various tree species for use in WEPP or evaluating an evapotranspiration simulation approach that explicitly accounts for stomatal resistance dynamics.