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Polluted Runoff: Nonpoint Source (NPS) Pollution

Spreadsheet Tool for Estimating Pollutant Loads (STEPL)

The Spreadsheet Tool for Estimating Pollutant Load (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).
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STEPL provides a user-friendly Visual Basic (VB) interface to create a customized spreadsheet-based model in Microsoft (MS) Excel. It computes:

STEPL can be used to evaluate loading and load reductions at a variety of scales. Although the terminology used throughout STEPL to define land use areas is “watershed,” these same input fields can also be treated as smaller areas, such as a project area or individual agricultural fields. For instance, if a user needs to estimate the load reductions from BMPs on individual farm fields, the “watersheds” can be further defined as “Field 1”, “Field 2”, “Field 3”, etc. There is no functional difference in the way STEPL calculates loads when watersheds or fields are used. The user defines the size and characteristics of each area being evaluated, based on the total acreage of each landuse entered into STEPL. This flexibility is not limited to agricultural fields, the “watersheds” could similarly be defined by the user as city blocks in an urbanized area.

For each watershed, the annual nutrient loading is calculated based on the runoff volume and the pollutant concentrations in the runoff 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)HelpUniversal Soil Loss EquationAn equation used to predict the average rate of erosion of an area on the basis of the rainfall, soil type, topography, and management measures of the area. 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.

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Model Download and Documentation

STEPL 4.4b

(Last updated: 10/27/2020)

  • STEPL 4.4b Spreadsheet Model for 10 Watersheds (Microsoft Excel 2016-ZIP)(4 MB, 10-27-2020)   - This download includes STEPL version 4.4b which is fully functional for modeling up to 10 watersheds at a time. It does not require users to have administrator rights for installing STEPL on their computers.  Changes from version 4.4 were made to address Excel functionality issues and inconsistencies between the full installation version and the 10 watersheds version. Note: This  update replaces a version posted 10/05/2020. The only substantive change fixed a cell reference error in the ag irrigation calculations. 


(Last updated: 03/15/2018)

Note: This update includes new BMP types (cropland and pastureland) and this version of STEPL has been upgraded to be compatible with Windows 10 and Excel 2016. It is also compatible with Windows 7 and Excel 2013.

STEPL 4.3b

(Last updated: 01/16/2017)

  • STEPL 4.3b Installation Package(9 MB, Jan 2017) Note: This .zip file download contains a 'setup.exe' file that installs STEPL version 4.3 for full functionality on a customizable number of watersheds; however, it requires Administrator Rights for model installation.

Note: This version of STEPL has been upgraded to be compatible with Windows 10 and Excel 2013.

Input Data Server for STEPL

The Input Data Server for STEPL uses a waterbody selection interface based on the 12-digit Hydrologic Unit Code (HUC 12) of the waterbody in question and allows the user to download data for one or multiple HUCs.

Key features of this data server include:
  • 2011 Landuse area distribution at the Subwatershed (HUC12) level.
  • 2012 Agricultural animal count at the Subwatershed (HUC12) level.

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Training Materials

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