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A regression-based approach to understand baseline total nitrogen loading for TMDL planning

Conference Proceedings
By Gerard McMahon and Chris Roessler

Abstract

Requirements under Section 303(d) of the Clean Water Act have led North Carolina environmental officials to prepare a total nitrogen (TN) total maximum daily load (TMDL) plan for the entire Neuse River Basin. The SPARROW (SPAtially Referenced Regression On Watershed attributes) watershed model was used to develop baseline estimates of TN inputs and delivery and a TN budget for the Neuse River Basin and two adjoining basins, the Cape Fear and Tar-Pamlico. The model explained 94% of the variability in log-transformed stream TN flux. Estimates of stream yield typically were within 25% of the observed values at the 44 monitoring stations used to calibrate the model. The model indicates that landscape factors, such as soil drainage characteristics, and channel transport factors, such as aquatic processes in streams and reservoirs, both exert a large influence on the transport of TN at both the reach and whole-basin scale. TN losses associated with in-stream processes occurred at a rate of about 5% per kilometer in streams with a mean annual discharge less than 1.04 cubic meters per second; losses in larger streams occurred at a rate of 0.2% per kilometer.

McMahon, Gerard, and Roessler, Chris, 2002, A regression-based approach to understand baseline total nitrogen loading for TMDL planning: National TMDL Science and Policy 2002 Speciality Conf., Water Environment Federation, 27 p.