Development and enhancement of web-based tools to develop Total Maximum Daily Load

Flow and load duration curves (FDCs and LDCs) are commonly used to develop total maximum daily loads (TMDLs). A web-based tool was previously developed to facilitate development of FDC and LDC, allowing use of USGS streamflow data via web access. In the research reported here, the tool has been upgraded to retrieve water quality data from STORET/WQX and USGS, because significant effort is often required to obtain water quality data, and additional tools were developed to assist in decision making for best management practices (BMPs) selection. The Web-based LDC Tool employs LOADEST and LOADIN to estimate daily pollutant loads using intermittent water quality data; therefore, LOADEST and LOADIN were evaluated for annual pollutant load estimations. Daily nitrogen, phosphorus, and sediment concentration data were collected and subsampled using six sampling strategies. Since the water quality parameters showed different relationships with streamflow for each sampling strategy, it was concluded that pollutant regression models need to be selected based on water quality parameters. In addition, water quality data used to estimate annual pollutant loads need to include an appropriate proportion of water quality data from storm events, with 20-30% of water quality data from high-flow (i.e. the upper 10 percent of flows for a given analysis period) providing the closest estimated sediment and phosphorus loads to measured loads. After the Web-based LDC Tool identifies pollutant loads exceeding standards and computes the required pollutant reduction to meet standard loads, a model capable of simulating BMPs was required. The Spreadsheet Tool for the Estimation of Pollutant Load (STEPL), a spreadsheet model to estimate annual pollutant loads, was evaluated as the basis for the BMP model. STEPL computes annual direct runoff using the Soil Conservation Service Curve Number (SCS-CN) method with average rainfall per event. Annual direct runoff using the EPA STEPL approach showed large differences compared to the annual direct runoff computed by general use of SCS-CN method. However, annual direct runoff computed from daily precipitation data generated from CLIGEN showed smaller differences than values computed from EPA STEPL approaches. Therefore, a web-based model to simulate BMPs, STEPL WEB, was developed to compute annual direct runoff obtained using daily precipitation data generated by CLIGEN. STEPL WEB establishes a priority list of BMPs based on implementation cost per mass of pollutant reduction, and then the model performs iterative simulations to identify the most cost-effective BMP implementation plans