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)
Clients:
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