Regional Transport Of Point And Nonpoint Source Nitrogen To The Gulf Of Mexico

The quantification of the regional transport of nutrients to the Gulf of Mexico is important to developing management strategies for reducing the hypoxic zone observed in recent summers on the Louisiana coastal shelf. Although existing research clearly identifies the Mississippi and Atchafalaya Rivers as the primary conduits for nutrients, the origin (type and location) of the sources of nutrients in these rivers is less certain. Better estimates of the quantities of point- and nonpoint-source nutrients delivered to the Gulf of Mexico from interior watersheds could improve the efficiency of management strategies. To assist in identifying the origin of stream nutrients nationally, we developed a water-quality model of nutrient flux in rivers of the United States. This model allows us to estimate the origin of point and nonpoint source nutrient flux at numerous locations on the coastal margin including the outlets of the Mississippi and Atchafalaya Rivers. The regression-based water-quality model relates monitored nutrient flux from 430 watersheds to various measures of upstream pollutant loadings, such as industrial and municipal discharges, fertilizer application, animal manure, and atmospheric deposition. The monitored watersheds range in size from several hundred to several tens of thousands of square miles. Flux estimates are developed from regularly-collected season nutrient measurements and daily estimates of streamflow using log-regression rating curve techniques. The estimated loadings of nonpoint-source nutrients to streams include the effects of watershed physical characteristics including precipitation, soil permeability, and topography. The model also estimates the first-order decay of nutrients during the transport of point and nonpoint sources through a digital stream network of nearly one million kilometers and 60,000 reaches. These decay rates reflect time-of-travel estimates from field studies and the residence time of water in major reservoirs. Through application of the model to unmonitored reaches, we estimate the quantities of point and nonpoint source nutrients delivered to the Gulf from several interior watersheds of the Mississippi and Atchafalaya Basins, including the Missouri, Arkansas, Upper Mississippi, and Ohio River Basins. All model predictions are accompanied by estimates of statistical error. This document is in PDF format.