Modeling Sediment and Phosphorus Yields Using the HSPF Model in the Deep Hollow Watershed, Mississippi

Author(s): Diaz-Ramirez, J.; Martin, J.; McAnally, W.; Rebich, R.

The impact of excess nutrient loads on eutrophication of waterbodies, including the increasingly frequent occurrences of harmful algal blooms and hypoxia, is well known and well documented. The Mississippi River/Gulf of Mexico Hypoxia is also a major environmental issue, and a key component of the Gulf Hypoxia Action Plan is the development and implementation of state nutrient reduction strategies. Effective implementation of nutrient load reductions requires that analytical tools be available to accurately estimate loads from watersheds and waterbodies as a function of hydrologic conditions. Hydrologic models have widely been used to accurately estimate outflows from watersheds, and to a lesser degree sediment and nutrient loads. Factors impacting runoff of nutrients are not well understood and as a consequence predictions of nutrient loads are highly uncertain. This research evaluated the ability of the Hydrological Simulation Program—FORTRAN (HSPF) to simulate storm, seasonal, and long-term runoff, sediment, and phosphorus transport at the farm scale in the Deep Hollow drainage area, Mississippi. The main goal was to demonstrate the usefulness of HSPF as a computer tool for future environmental management and planning in the Mississippi Delta region. When analyzing the datasets developed by U.S. Geological Survey, 69 events were selected to setup and evaluate the HSPF model. Model evaluation consisted in splitting the available data in two different time periods, calibration from 1997 to 1998 and validation in 1999. Runoff processes were evaluated using 45 events for calibration and 24 events for validation. In evaluating sediment export processes, 29 and 11 storm events were utilized in calibration and validation periods, respectively. Phosphorus simulations (dissolved and total) were evaluated using 19 and four storm events in calibration and validation periods, respectively. The HSPF model was setup to evaluate runoff, soil erosion, dissolved & total phosphorus to storm, monthly, and annual time scales.

This study concluded that the HSPF runoff model's simulations of storm-by-storm, long term monthly, and annual intervals were very good. In simulating suspended sediment loads, HSPF performance was poor for storm-by-storm analysis. However, long term monthly, and annual suspended sediment load simulations were tracked fair and good, respectively. Simulations of dissolved phosphorus of storm-by-storm and long term monthly intervals were good. Simulated annual dissolved phosphorus loads correlated very good with observed data. Similarly, HSPF performed good in simulating long term monthly total phosphorus loads and showed very good results in modeling annual total phosphorus loads exported from Deep Hollow drainage area.
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