AQUA TERRA Consultants
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Development of TMDL USLE, a Visual Basic Spreadsheet Model for Estimating Watershed Sediment Loadings

(EPA Contract No. 68-C-98-010, Work Assignments No. B-10, 1-10, 2-10)


Of the thousands of Total Maximum Daily Loads (TMDLs) that will be required by the Clean Water Act, the predominant pollutant of concern is sediment. Consequently, development of sediment TMDLs is currently a priority for EPA. One promising area of EPA assistance to the States is refining the most promising of the simplified approaches for sediment TMDL development that are currently recommended. In many cases simplified methods are appropriate tools for developing TMDLs for watersheds that are not classified as heavily impacted. The purpose of this work assignment was to improve and expand guidance materials and tools related to simplified techniques that are useful for developing sediment TMDLs.

To support this objective, AQUA TERRA Consultants developed a Visual Basic spreadsheet tool named TMDL USLE. This spreadsheet is useful for estimating the expected relative magnitude of land surface sediment loadings (tons per year) from different land use types within a watershed. Applications of the spreadsheet vary from a very simple computation of the annual edge-of-field sediment loading from a single parcel of land to more complex applications designed to characterize grouped sediment sources throughout a watershed. Estimates of sediment loadings for grouped sources are particularly useful for TMDL analysis, since similar control measures can be considered for similar sources throughout the watershed. The spreadsheet methodology can also be used to estimate the relative magnitude of loadings from grouped sources to the edge-of-stream by utilizing a delivery ratio approach. The spreadsheet methodology is a useful tool for determining preliminary, or target, estimates of land surface erosion that can be used as a starting point for more detailed watershed sediment analysis performed via modeling.

The strength of the methodology is in estimating sediment loadings generated by erosion on agricultural lands. However, the USLE has also been applied to cropland, rangeland, forest lands, landfills, construction sites, mining sites, reclaimed lands, military training lands, parks, and other land uses where mineral soil material is exposed to the erosive forces of raindrop impact and overland flow. The geographic scope of the spreadsheet methodology is the continental United States. Maps, recommended value tables for USLE factors, and other information useful in deriving appropriate values for the USLE and delivery ratios are provided, to the extent that it is practical, throughout the U.S. The tool includes an on-line tutorial and active links to Internet web sites containing supplemental information that can assist users in evaluating USLE factors. AQUA TERRA performed all aspects of design and implementation of TMDL USLE.

  Download TMDL USLE (16.7 MB, Zip File).

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