Watershed Assessment using an Integrated Modeling Approach
The emergence of information technologies and watershed management approaches enable decision-makers to establish predictive relationships be-tween different land-use activities and the corresponding sediment and nutrient loads discharged from the watershed. Furthermore, these approaches are extended to assess the impacts of watershed waste loads on the receiving aquatic systems. For example, modeling of water quality provides the means to assess the effectiveness of nutrient reduction programs on the eutrophication process in the receiving water body. This is a complex process to manage because it requires tying the environmental outcomes of a management policy to land-use and socioeconomic implications.
This chapter presents a research study of the impacts of land-use activity on sediment and nutrient loads. A case study on the simulation of Tangipahoa Watershed and Lake Pontchartrain Ecosystems is emphasized. Surface runoff from the watersheds represents a major component in the water balance of Lake Pontchartrain. Estimates of waste loads entering the lake have to be made using a water quality model accounting for land-use and agricultural processes. This requires the use of a continuous time model to simulate watershed hydrology, surface and subsurface water quality, and impacts of crop management. The Soil and Water Assessment Tool, SWAT, and the Geographical Resources Analysis Support System, GRASS, were used in the case study. The United States Geological Survey (USGS) stream flow records and the Louisiana Department for Environmental Quality (LADEQ) water quality data set were used for the calibration and verification of the model.
The baseflow component of the surface stream was quantified using the modified recursive digital filter technique. The curve-number and the available moisture capacity proved to be the most important calibration parameters in the hydrologic component calibration, while the soil erosion factor and the rain water quality were important in the sediment and water quality components. The accounting of spatial variability of watershed parameters is one of the major strengths of the SWAT-GIS model. Using the integrated approach involving SWAT, GRASS, and a relational database containing temporal weather information, the hydrology, soil, and crop management conditions renders study of large watershed surface and sub-surface processes possible.
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