Use of SPARROW Ratios with Limited Tributary Monitoring to Estimate Loading from the Entire Great Lakes Basin

Abstract:

Because of degradation in water quality of the Great Lakes, efforts are being made throughout their watersheds to reduce nutrient loading. To evaluate the success of these efforts and whether water quality improvements can be expected, it is important to continually estimate total nutrient loading to each of the Great Lakes. However, tributary monitoring to the Great Lakes, like most aquatic systems, consists of monitoring at a limited number of sites. In this study, we developed a technique to extrapolate the monitored loads at selected tributaries to the total loading using SPARROW ratios (ratios of nonpoint loads between monitoring tributaries and nearby unmonitored areas using published SPARROW models), annual point source inputs throughout the basin, and annual (or shorter term) monitored loads from tributaries in consistent ongoing monitoring programs (such as the Great Lakes Restoration Initiative Tributary Monitoring Network). The SPARROW-ratio approach incorporates differences in point and non-point nutrient sources, watershed characteristics (such as soil types, slope, runoff, etc.) and spatial differences in runoff. Nutrient loading from the monitored tributaries are computed using surrogate regression techniques to better describe short-term loads and improve their accuracy. To demonstrate how loading estimates from this SPARROW-ratio approach compare with estimates of the previous approach used to estimate total watershed loading (extrapolation based on unit-area yields from nearby monitored tributaries), total loading from the entire U.S. part of the Lake Erie Basin are compared for water years 2012 and 2013 (last years with phosphorus loads estimated for the entire Lake Erie Basin).