Hypoxia in the Northern Gulf of Mexico

"Chapter 3 Nutrient Fate, Transport, and Sources (p. 51-52)

The Study Group was asked to review the available literature and information, especially that developed since 2000, that would allow them to assess any changes and improvements in the understanding of nutrient sources and flux estimates within the Mississippi and Atchafalaya River basins (MARB) (see Fig. 1.2) and the current ability to use watershed models to route and predict nutrient delivery to the Gulf of Mexico. The following sections discuss the current levels of understanding and provide brief summaries of the Study Group’s key findings and recommendations.

3.1 Temporal Characteristics of Streamflow and Nutrient Flux

The research needs identified in the Integrated Assessment to understand and document the temporal characteristics of MARB riverine nutrient loads included (1) studies on small watersheds to better document nutrient export on the short timescales needed; (2) detailed information on tile drainage intensity; (3) increased monitoring of stream sites; and (4) measurements of point source discharges rather than estimates from permits. Only a limited number of these needs have been met. However, more recent estimates of agricultural drainage appear to be more representative than those used in the original assessment (e.g., see Sands et al., 2008), and new procedures for load calculations have resulted in changes in estimates of nutrient fluxes. A brief discussion of each of the improvements follows. Current extent and patterns of agricultural drainage.

The Integrated Assessment relied largely on the 1987 USDA-ERS report (Pavelis, 1987), which based estimates of agricultural drainage on land capability class and crop information from the 1982 Natural Resources Inventory (NRI). NRI estimates were dropped after 1992, and NRI is statistically valid only at a watershed or county level. Based on the USDA surveys, some degree of subsurface drainage is present on 13 million hectares (over 32million acres) in the Midwest states. However, there is considerable uncertainty with respect to the actual extent and distribution of drainage of cultivated cropland.

In the absence of additional survey data, more recent estimates of the extent of drained agricultural land have been developed based on land use and soil class/characteristics (Jaynes and James, 2007; Sugg, 2007). This general approach needs further development and validation but seems to provide the best current estimate of the extent of agricultural drainage. The approach takes advantage of the now extensive and detailed GIS coverages and provides a considerably finer level of spatial resolution than previously available.

In the following example, USDA STATSGO soil data were used to estimate the extent of agricultural drainage based on the distribution of row crops (primarily corn and soybean) on soils with a drainage class of poorly drained soils and slopes 2% or less (Fig. 3.1, per D. Jaynes, National Soil Tilth Lab, Ames, IA). These patterns of agricultural drainage predicted using this approach are generally similar to patterns in land use (Fig. 3.2) and in-stream nitrate concentration estimated from STORET data selected to exclude point source influences (Fig. 3.3). Drainage estimates could be further refined by using improved land-use data and by using SSURGO rather than STATSGO data."