Evaluation of Nitrogen Nonpoint-source Loadings using High Resolution Land Use Data in a GIS: A Multiple Watershed Study for the State of Maryland.

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2008-08-14

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The performance of three monitoring perspectives, for the detection of watershed compliance with water quality standards, was evaluated. In order to compare performances 35 watershed nonpoint-source nitrogen loading cases were calculated within a GIS. Calculations showed that the probability of loads exceeding a criterion at the watershed outlet is more representative of upstream conditions than a nominal mean load comparison at the watershed outlet. Combined outlet compliance interpretations were found to isolate loading conditions that on average did not exceed the criterion; however, variations within loading distributions were large such that compliant conditions were threatened. The whole watershed perspective mapped the relationship between stream network structure, land cover/land use, and loadings. Comparisons between the perspectives suggested that both outlet perspectives usually are consistent with whole watershed conditions. Semivariograms were demonstrated to characterize spatial variability in loadings and predict the accuracy with which monitoring sites represent loads at upstream locations.

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