Erosion modelling for land management in the Tahoe basin, USA: scaling from plots to forest catchments

Abstract

Land management and its effects on water quality are a concern whereregulatory agencies work to establish sediment and/or nutrient loadings.Runoff and erosion measurement in the field and modelling at thecatchment scale are often the only means of generating realistic dataand results for subsequent analyses. As such, it is critical to linklocal-scale field measurements associated with the range of land uses orsoil restoration efforts with the catchment-scale sediment loading. Adistributed hydrological model with locally-derived, slope-dependentsediment yield (erodibility) equations developed from rainfallsimulation (RS) studies at the 1-m(2) scale across the Tahoe basin, USA,is employed to determine the runoff-dependent scaling factors (SFs)necessary to predict daily stream sediment loading from the foresteduplands. Data from three ``paired{''}, adjacent, west-shore Lake Tahoetributary catchments are considered for the period 1994-2004 at timescales ranging from daily to annual. At all time scales, the SF wasdependent on runoff (R), particularly at smaller values, but was readilysimplified as an approximately inverse square-root function. OptimizedSF-runoff regressions for each watershed were equivalent when modifiedby ratios of watershed area. As a result, a single daily SF-runoffequation was determined ( through minimization of sediment loadprediction errors) that could be successfully applied to all threewatersheds with an accuracy consistent with the predictive errorassociated with any one of the watersheds alone. Sensitivity analysesindicated that sediment loading predictions were more sensitive to theSF-runoff equation coefficient rather than the exponent. Annual sedimentload prediction errors of similar to 30% might be expected for low orhigh runoff years.