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About STEPL

Objective

STEPL (Spreadsheet Tool for Estimating Pollutant Load is a customizable spreadsheet-based model for use in Excel. Using simple algorithms, it calculates nutrient and sediment loads from different land uses and the load reductions from the implementation of best management practices (BMPs). Annual nutrient loading (nitrogen, phosphorus and 5-day biological oxygen demand) is calculated based on the runoff volume and pollutant concentrations. The annual sediment load from sheet and rill erosion is calculated based on the Universal Soil Loss Equation (USLE) and the sediment delivery ratio. Loading reductions resulting from the implementation of BMPs are computed from the known BMP efficiencies.

Developer

Tetra Tech, for U.S. EPA http://it.tetratech-ffx.com/steplweb/default.htm

Contact information

Aileen Molloy, aileen.molloy@tetratech.com, 703-385-6000, 10306 Eaton Pl, Suite 340, Fairfax, VA 22030

Inputs

Inputs can be derived from local data provided by the user or from the Online Data Server. Required inputs are county level land use acres, agricultural animal numbers, number of septic system, hydrologic soil group. Optional inputs include irrigation acres, manure application, impaired stream banks, gullies and BMPs. BMPs include low impact development options.

STEPL provides default values for USLE parameters, soil nutrient concentrations, runoff curve numbers, nutrient concentration in runoff, nutrient concentration in shallow groundwater, and urban land use distribution.

Outputs

STEPL provides the nitrogen, phosphorus, BOD and sediment loads with and without BMPs applied, on a subwatershed basis. Loads are calculated on an annual basis. STEPL calculates the load reductions by subwatershed and by land use category. Tables and graphs are provided to illustrate the results.

User

STEPL is easy to use and is intended for the non-modeler, such as planners or watershed groups. The User Guide should provide sufficient training.

Scales of Relevance

STEPL is designed for use at the HUC 12 level. Default data in the model is provided at the county level. Accuracy is primarily limited by the wide variability in event mean concentrations (EMCs) across watersheds. EMCs drive the water quality calculations.

Other

STEPL has a coarse resolution and is intended as a planning tool. STEL can help evaluate different BMP implementation options. The more local data that is added to STEPL by the user, the more representative the results will be.

Sponsors/funders

U.S EPA Office of Water, Grants Reporting and Tracking System