The SPARROW16 model is NAWQA’s most popular and visible regression model.17 The SPARROW model is a watershed based model designed to predict patterns in water quality, concentration, and amount of constituents, across spatial extents ranging from entire regions of the United States to smaller watersheds. The model is perhaps best known for contributing to understanding of key parameters that affect hypoxia in the Gulf of Mexico by determining nutrient load to the Gulf and pinpointing which watersheds or which of the 31 state drainage basins are the greatest contributors. Specifically, the SPARROW effort highlighted that nine states18 making up one-third of the Mississippi River drainage area contribute 75 percent of the nitrogen and phosphorus to the Gulf (Alexander et al., 2008). This study also filled gaps in the understanding on the sources of phosphorus in the Gulf; phosphorus associated with animal manure contributes almost as much phosphorus as cultivated crops (37 versus 43 percent) (Alexander et al., 2008).
Currently NAWQA is in the process of developing fine-scale, regional water-quality models in each MRB. Nutrients are the focus of these modeling efforts, except in the arid southwest, where dissolved solids are of greater importance. To do this, NAWQA is using local ancillary data and refining the SPARROW model to reflect the unique environmental conditions and smaller scale of each MRB. At this time, models have been developed for six of the eight MRBs. Regional models for the remaining basins, California and the Southwest, are planned for the future. The preliminary findings from this effort show the promise of future regional SPARROW modeling of water-quality conditions in the United States. The October 2011 issue of the Journal of American Water Resources Association provides a featured collection of articles on the regional SPARROW effort.19
NAWQA is exploring uncertainty in all the modeling efforts, i.e., associating uncertainty with all the estimates the program produces. For example, Robertson et al. (2009) examined approximately 800 watersheds in the Mississippi River basin and assigned a ranking that indicated whether nutrient yields from the basin were among the highest delivering of nutrients contributing to hypoxia in the northern Gulf of Mexico (Figure 2-11, top). This involved a robust statistical procedure applied to the results from a previous application of SPARROW to identify the top 150 watersheds. Once identified, scientists incorporated information on confidence intervals
17 Development of SPARROW was initiated by the Branch Systems Analysis working on new and emerging technical issues and techniques used within the former Water Resources Division. The branch was dissolved in the late 1990s because of funding shortfalls, and the individuals developing SPARROW joined NAWQA and continued their work.
18 Illinois, Iowa, Indiana, Missouri, Arkansas, Kentucky, Tennessee, Ohio, and Mississippi.