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Models/Load Reductions

There are a wide variety of scientific and technical models to support decisions and policies. Models increase the level of understanding about natural systems and the way in which they react to varying conditions. While many models exist, the ones listed below are readily available to perform the necessary calculations for estimated current loads when hard data in unavailable; estimated load reductions based upon natural and man-made influences such as soil types and BMP installations; load duration and flow duration curves and other simple equations for estimating other potential factors in a watershed.

L-THIA Model

The Long-Term Hydrologic Impact Assessment (L-THIA), model uses land use and soil characteristics from the user along with thirty years of precipitation data to determine the average impact that a particular land use change or set of changes will have on both the annual run-off and the average amount of several nonpoint source pollutants. L-THIA is a web-based application that is available through the Purdue University Engineering Department. Detailed information and a tour are available for using this model on the Purdue website.

This tool is designed to help you quantify the impact of land use change on the quantity and quality of your surface water by estimating long-term average annual run-off for land use and soil combinations, based on actual long-term climate data for your area. The area is visualized using the Online Watershed Delineator Tool that delineates the area that flows to the waterway “pour” point and allows you to send that watershed outline, and the soil and land-use data within the outline, to other online models such as L-THIA.


The Spreadsheet Tool for Estimating Pollutant Load (STEPL) can be downloaded from the U.S. EPA website. STEPL employs simple algorithms to calculate nutrient and sediment loads from different land uses and the load reductions that would result from the implementation of various best management practices (BMPs). STEPL provides a user-friendly (Visual Basic) interface to create a customized spreadsheet-based model in Microsoft Excel. It computes watershed surface run-off; nutrient loads, including nitrogen, phosphorus, and 5-day biological oxygen demand (BOD5); and sediment delivery based on various land uses and management practices. For each watershed, the annual nutrient loading is calculated based on the run-off volume and the pollutant concentrations in the run-off water as influenced by factors such as the land use distribution and management practices. The annual sediment load (sheet and rill erosion only) is calculated based on the Universal Soil Loss Equation (USLE) and the sediment delivery ratio. The sediment and pollutant load reductions that result from the implementation of BMPs are computed using the known BMP efficiencies. The STEPL 4.0 Users Manual and Guides include illustrations of the application and results worksheets to help you start using the this model.

Region 5 Load Estimation Spreadsheet Model

The Region 5 Model is an Excel workbook that provides a gross estimate of sediment and nutrient load reductions from the implementation of agricultural and urban BMPs. The algorithms for non-urban BMPs are based on the "Pollutants controlled: Calculation and documentation for Section 319 watersheds training manual" (Michigan Department of Environmental Quality (DEQ), June 1999). The algorithms for urban BMPs are based on the data and calculations developed by Illinois EPA.

It is recognized that this system has limitations, but it does provide a uniform system of estimating relative pollutant loads. The methods are simple in concept and workable within a field office. The training manual [PDF] includes instructions and examples regarding the calculation and documentation of pollutant reductions for:

  1. sediment;
  2. sediment-borne phosphorus and nitrogen;
  3. feedlot run-off; and,
  4. commercial fertilizer, pesticides, and manure utilization.

The Region 5 Model does not estimate pollutant load reductions for dissolved constituents. Also, water quality impacts from wind erosion will not be estimated. The dynamics of wind erosion and resulting atmospheric deposition do not perform like water erosion and quantifying these relationships for water quality is currently not possible. Likewise, the impacts of BMPs on ground water quality are not well enough understood to make pollutant reduction estimates feasible.

Load Duration Curve Modeling

The use of flow duration curves provides for enhanced targeting, both in TMDL development and in water quality restoration efforts. Duration curves can add value to the planning process by identifying targeted areas, targeted programs, targeted activities, and targeted participants. Flow duration curve analysis identifies intervals, which can be used as a general indicator of hydrologic condition (i.e., wet versus dry and to what degree). This indicator can help point problem solution discussions towards relevant watershed processes, important contributing areas, and key delivery mechanisms. These are all important considerations when identifying those controls that might be most appropriate and under what conditions. In addition, duration curves also provide a context for evaluating both monitoring data and modeling information. This offers another way to look at identifying data needs where adaptive management is being considered or utilized. Below are links to more information and the tools for your consideration.

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