A coupled hydrologic-geomorphic model for evaluating effects of vegetation change on watersheds

This work describes the modeling framework and initial results for CLAWS (CoupledLandscape And Water System), a physically-based, spatially-distributed hydrologic-geomorphic model that has been coupled with a vegetation dynamics simulation. Spatialvariability of topography, soil, vegetation, and climate drive a hydrology module thatcalculates the water budget at given time steps and with a spatial resolution defined bydigital elevation data. Important hydrologic processes, including snow accumulation andmelt, interception, evapotranspiration, infiltration, subsurface flow, overland flow andchannel flow routing are incorporated in CLAWS. The hydrology drives hillslope andchannel geomorphic processes, including probability of mass movements (landslides,debris flows). Both the hydrologic and geomorphic modules are linked to forest growthsimulation so that effects of changing forest stand structure in response to cutting or otherdisturbances through time can be examined. Dynamic integration of CLAWS withgeographic information systems enhances model parameterization and result analyses.Calibration and testing of CLAWS from three small watersheds at HJ AndrewsExperimental Forest in western Oregon shows that watershed hydrologic behavior andhillslope stability can be simulated accurately. The module for snow accumulation andmelt has been tested using four years of point measurements of snow water equivalentand showed that this simple energy and mass balance model accurately simulated snowaccumulation and melt processes. Application of CLAWS to land use managementpermits long-term simulations of effects of alternative forestry practices on hydrologicand geomorphic responses of watersheds. Results from Monte Carlo simulations in smallwatersheds in Western Oregon, show long-term effects of forest harvesting over 100years, which are in agreement with long-term observations. Simulations show that 40years is needed for watershed recovery to pre-clearcutting status with respect to oldgrowth through an exponential trend and changes in watershed responses show strongseasonal and storm variations. Models such as CLAWS, that successfully incorporatehydrologic, geomorphic and forest dynamic processes into a single model, are an
effective complement to traditional paired-watershed studies of hydrologic andgeomorphic responses.