REGIONAL INTERPRETATION OF WATER-QUALITY MONITORING DATA

We describe a method for using spatially referenced regressions of con taminant transport on watershed attributes (SPARROW) in regional water -quality assessment. The method is designed to reduce the problems of data interpretation caused by sparse sampling, network bias, and basin heterogeneity. The regression equation relates measured transport rat es in streams to spatially referenced descriptors of pollution sources and land-surface and stream-channel characteristics. Regression model s of total phosphorus (TP) and total nitrogen (TN) transport are const ructed for a region defined as the nontidal conterminous United States . Observed TN and TP transport rates are derived from water-quality re cords for 414 stations in the National Stream Quality Accounting Netwo rk. Nutrient sources identified in the equations include point sources , applied fertilizer, livestock waste, nonagricultural land, and atmos pheric deposition (TN only). Surface characteristics found to be signi ficant Predictors of land-water delivery include soil permeability, st ream density, and temperature (TN only). Estimated instream decay coef ficients for the two contaminants decrease monotonically with increasi ng stream size. TP transport is found to be significantly reduced by r eservoir retention. Spatial referencing bf basin attributes in relatio n to the stream channel network greatly increases their statistical si gnificance and model accuracy. The method is used to estimate the prop ortion of watersheds in the conterminous United States (i.e., hydrolog ic cataloging units) with outflow TP concentrations less than the crit erion of 0.1 mg/L, and to classify cataloging units according to local TN yield (kg/km(2)/yr).