Effects of Watershed Land Use Data on HSPF Water Quality in the Upper Opequon Watershed in northern Virginia, USA

Abstract

Land use data source can contribute to errors in watershed modeling. This paper evaluated the effects of using site-specific versus county-level aggregated land use data on Hydrologic Simulation Program-Fortran (HSPF) simulated contaminant losses. Site-specific land use was derived from the local watershed land use inventory while aggregated land use was derived from county-level data (percentage of county land in various land use categories and sub-categories). County level data are useful when modeling large watersheds such as the Chesapeake Bay Watershed when collection and use of site-specific data may be cost prohibitive. The study site was the 14,941 ha predominately rural Upper Opequon Watershed in northern Virginia, USA. Percentage relative errors in model output were calculated and compared using the two land use data sources. Results showed that use of aggregated land use data resulted in 13, 3 and 4 percent higher simulated sediment, and total nitrogen and phosphorus losses, respectively due to overestimated cropland area. The higher contaminant losses would suggest the need for more management measures to meet water quality goals. This study suggests that while the use of county-level aggregated land use data may be appropriate for developing basin scale pollutant reduction goals such as those in total maximum daily load (TMDL) plans, it should be used with extreme caution for watershed planning and implementation activities on smaller watersheds that may mandate site-specific changes in land management and costs for landowners. For smaller watersheds, TMDLs and their watershed implementation plans should utilize local site-specific spatial data that accurately reflects watershed conditions. This will help target resources where they are most needed and maintain credibility with local stakeholders while improving the accuracy of the developed pollution reduction plans.