Assessment of Total Maximum Daily Load Implementation Strategies for Nitrate Impairment of the Raccoon River, Iowa

摘要

The state of Iowa requires developing total maximum daily loads(TMDLs) for over 400 water bodies that are listed on the 303(d) list of the impaired waters 1 he Raccoon River watershed, which covers approximately 9400 km(2) of prime; agriculture land and represents a typical Midwestern corn-belt region in west-central Iowa, was found to have three stream segments impaired by nitrate-N The Soil and Water Assessment Tool (SWAT) was applied to this watershed to facilitate the development of a TMDL The modeling framework integrates SWAT with supporting software and databases on topography, land use and management, soil, and weather in Annual and monthly simulated and measured steamflow and nitrate loads were strongly correlated The watershed response was evaluated for a suite of watershed management scenarios where and use and management changes were made uniformly across die watershed A scenario of changing the entire land to row crop resulted in an increased mu ate load of about 12% over the baseline condition at the watershed outlet Results from the I 5 nitrate load reduction strategies were found to reduce nit rate from <1% to about 85%, with the greatest potential reduction associated with changing the row crops to grassland This research demonstrated the use of a modeling system to facilitate the analyses of TMDL implementation strategies, including the ability to target the most efficient allocation of alternative practices on a subwatershed basis