The utility of modeling water quality: finding the best practices for the Squam Watershed, New Hampshire

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Hanson, Rebecca K.
Watershed planning involves identifying pollutants and developing management strategies to reduce impacts of pollutant loading. Current federal and state guidelines for watershed planning focus on using simple models to determine pollutant loads and estimate the benefits from implementing management strategies. These models often use simple assumptions and readily available information to make predictions about phosphorus loading to receiving water at the watershed level. This study examines two water quality models applied to the Squam Watershed in New Hampshire-one simple, one complex-and investigates the usefulness of these models for watershed planning and management. Both models provide reasonable estimates of annual phosphorus yields in Squam Watershed subbasins. However, results indicate that the simple model-STEPL (Spreadsheet Tool for Estimating Pollutant Loads)-lacks the complexity to be useful in predicting watershed dynamics. The more complex, and data intensive, model-SWAT (Soil and Water Assessment Tool)-also provides reasonable estimates of annual yield. However, without sufficient data for calibration and validation, SW AT does not provide a full understanding of the dynamics of phosphorus loading. These models are widely used and acceptable for meeting state and federal requirements for watershed planning for determining annual pollutant loads, but do little to target how, where, and when a pollutant such as phosphorus moves through the watershed. It is recommended further study of tributary phosphorus concentrations and flow accompany modeling for watershed planning.