Hydrologic impacts due to land cover change in the Yellowwood Lake watershed
The Yellowwood Lake watershed in Southern Indiana has experienced land cover change due to forest harvest throughout the last century. A group of local stakeholders have identified sedimentation into the lake and surface erosion as major concerns for the watershed. The main objective of this study is to better understand how forest harvest methods applied within the watershed effect hydrologic and soil erosion processes. Such knowledge is required to develop a more comprehensive plan to protect the watershed. The Distributed Hydrology-Soil-Vegetation Model (DHSVM) was used for this analysis. This is a physically based, distributed hydrology model that simulates the water and energy balance at the scale of the digital elevation model (DEM). The DHSVM sediment model also simulates hillslope erosion by overland flow and raindrop impact. A sensitivity study was conducted on the model to better understand the effect of forest thinning on the hydrology of the watershed, which was simulated by adjusting the user input fractional coverage parameter of the forest vegetation. Updates were made to the calculation of aerodynamic roughness to produce a more continuous change in displacement height with thinning forest density. Current harvest management, as prescribed by the Indiana Department of Natural Resources, was input to the model using a mixture of fractional coverage values to represent the change in canopy density due to harvest prescriptions throughout the watershed. The simulated output from the forest harvest scenario was compared to output produced using a non-harvested scenario for water years 1961-2013. The results indicate that harvest resulted in statistically significant increases to streamflow metrics related to high and low flow frequency. Flow magnitudes for 1.1 year return period flows also increased by as much as 12%. Results from the DHSVM sediment model showed that the annual sediment load into the lake increased after forest harvest. The watershed also experienced greater loss of soil in areas with steep slopes and under the clear-cut harvest prescription. It is recommended that the forest managers avoid a clear-cut prescription and harvesting on slopes steeper than 7.5% in order to reduce some of these effects.
Cherkauer, Purdue University.
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