Pollutant Load Estimation Tool (PLET)

Questions or Comments?

Contact email-based Help Desk for PLET Model support.

The Pollutant Load Estimation Tool (PLET) is replacing the Spreadsheet Tool for Estimating Pollutant Loads (STEPL). PLET uses the same underlying formulas as STEPL, but in a more user-friendly web interface. Both tools employ 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).

The archived page for STEPL can be found here.

Overview

PLET provides a user-friendly web interface to create a customized watershed model. It computes:

watershed surface runoff;
nutrient loads, including nitrogen, phosphorus, and 5-day biological oxygen demand (BOD5); and
sediment delivery based on various land uses and management practices.

PLET can be used to evaluate loading and load reductions at a variety of scales. Although the terminology used throughout PLET 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 PLET 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 land-use entered into PLET. 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) 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.

PLET includes an integrated combined BMP calculator, which calculates the aggregate BMP efficiency of combinations of multiple BMPs that can then be applied in the model. The calculator can be used to represent BMPs both in series and parallel. In addition, the calculator can also save BMP configurations created by the user.

Model Documentation

PLET Version 1.0

Link to PLET
User's Guide: Pollutant Load Estimation Tool (PLET) version 1.0 (pdf) (6.34 MB, April 2022)

Input Data Server for PLET

The Input Data Server for PLET 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 Land-use area distribution at the Subwatershed (HUC12) level, and
2012 Agricultural animal count at the Subwatershed (HUC12) level.

Training Materials

PLET Training Video: For ease of navigation through this training video, you can use the bookmarked chapters below:

Questions and Answers

Question 1: What are the differences between PLET, STEPL, and Region 5 Model?

Question 2: The septic system failure rate seems low, are there other alternative data?

Question 3: How can I add my own BMPs to PLET’s BMP list?

Question 4: Can you help me use the BMP Calculator?

Question 5: What are other models that can be used for load reduction calculations?

Question 6: If I don't know exactly where all my BMPs for a particular sub-watershed are located, am I better off calculating them in serial or in parallel configuration for that sub-watershed?

Question 7: Why is the PLET-calculated sediment loading rate (t/ac/yr) from my watershed much lower than the erosion rate (> 1 t/ac/yr) reported for my study area? Can you explain?

Question 8: Some of the data I'm gathering to input into the PLET model is not consistent. For filter strips, I have some data coming to me as actual acres of filter strip, and some coming to me as acres affected by the filter strip. What is the best way to deal with these discrepancies in the model?

Question 9: My general question is whether it is appropriate to use PLET to compare the loading effects of changing a particular site from one land use to another, e.g., agricultural to commercial. I also wonder how it applies the USLE to "soil loss" from a parking lot.

Question 10: Can PLET be used to model bacteria?

Answers:

Question 1: What are the differences between PLET, STEPL, and the Region 5 Model?

Below are the major differences among the three models:

The Region 5 Model

Calculates load at the source level
Sources are independent (no relationship between worksheets)
Users can not specify and update BMPs used in the model

STEPL and PLET

Calculates load for different sources at source and watershed level
Sources are related in watershed
Users can specify and update BMP list
Users can use BMP calculator to estimate combined BMP efficiencies for complex BMP arrangements
On-line input data server for initial model setup (Caution: Do not substitute the on-line data for real local data!)

PLET

Web-based modeling tool
Scenarios are saved online
Excel versioning and compatibility issues are no longer a problem
Updated BMP Calculator tool, with improved user interface and ability to directly import efficiencies into a selected scenario
Scenarios can be shared with other PLET users

Question 2: The septic system failure rate seems low, are there other alternative data?

Yes. The failure rate data have very high uncertainty. Users are encouraged to obtain the septic failure rate from their local health departments.

Besides the data from the ‘Input Data Server’ (National Environmental Service Center formerly National Small Flow Clearinghouse, 1992 and 1998), Electric Power Research Institute conducted a state-by-state survey on septic failure rate. Below is the summary:

State	Estimated system failure rate (in %)	Failure definition
Alabama	20	Not given
Arizona	0.5	Surfacing, backup, surface or ground water contamination
California	1-4	Surfacing, backup, surface or ground water contamination
Florida	1-2	Surfacing, backup, surface or ground water contamination
Georgia	1.7	Public hazard
Hawaii	15-35	Improper construction, overflow
Idaho	20	Backup, surface or ground water contamination
Kansas	10-15	Surfacing, nuisance conditions (for installations after 1980)
Louisiana	50	Not given
Maryland	1	Surfacing, backup, surface or ground water contamination
Massachusetts	25	Public health
Minnesota	50-70	Cesspool, surfacing, inadequate soil layer, leaking
Missouri	30-50	Backup, surface or ground water contamination
Nebraska	40	Nonconforming system, water quality
New Hampshire	<5	Surfacing, backup
New Mexico	20	Surfacing
New York	4	Backup, surface or ground water contamination
North Carolina	15-20	Not given
North Dakota	28	Backup, surfacing
Ohio	25-30	Backup, surfacing
Oklahoma	5-10	Backup, surfacing, discharge off property
Rhode Island	25	Not given
South Carolina	6-7	Not given
Texas	10-15	Surfacing, surface or ground water contamination
Utah	0.5	Surfacing, backup, exceeding discharge standards
Washington	33	Public health hazard
West Virginia	60	Backup, surface or ground water contamination
Wyoming	0.4	Backup, surfacing, ground water contamination

Question 3: How can I add my own BMPs to PLET?

To create a User defined BMP, from the BMPs Module, click on Create a User Defined BMP. This opens a dialog box where the BMP name and pollutant reduction efficiency values can be entered. After clicking Save and Close, the BMP will be saved. To add the new BMP to the scenario, click Add BMP and select the watershed and land use where the practice will be applied. Then select the practice from the drop-down list in the BMPs column. Custom BMPs are distinguished from default BMPs with the naming convention “User Defined/Custom BMP (Custom BMP Name)” where the Custom BMP Name is the name entered in the Custom BMP creation dialog box. See more information in the PLET User’s Guide.

Question 4: Can you help me use the BMP Calculator?

Yes. The PLET User’s Guide has two detailed sections (BMP Calculator and Appendix A) that describe the procedures and examples of using the BMP Calculator.

Question 5: What are other models that can be used for load reduction calculations?

PLET, STEPL, and the Region 5 Model are some of the simplest models for calculating pollutant load reductions. There are many other models that can be used to calculate the load reductions, and these models are described in a document in the PLET website. You can find out more about these models in the document "Nutrient and Sediment Estimation Tools for Watershed Protection (PDF)".

Question 6: If I don't know exactly where all my BMPs for a particular sub-watershed are located, am I better off calculating them in serial or in parallel configuration for that sub-watershed?

Most non-structural BMPs should be used in parallel configuration (with the BMP Calculator) if you do not know their locations and spatial relationship. Some downstream structural BMPs or on stream BMPs may be in serial connection with the upstream BMPs; for example, a sediment settling pond may be in serial to upstream BMPs such as no-till practices. If you do not know the BMP locations in a watershed, assuming parallel BMP configurations will be better than assuming serial configurations if most of the BMPs are non-structural (except filter strips) or not in-stream.

Question 7: Why is the PLET-calculated sediment loading rate (t/ac/yr) from my watershed much lower than the erosion rate (> 1 t/ac/yr) reported for my study area? Can you explain?

The sediment output from a watershed is different from on-site erosion. For cropland in your calculation, erosion rate may be larger than 1 t/ac/yr; however, the sediment output from your watershed is much lower. The PLET model calculates sediment output using "erosion x sediment delivery ratio." If the watershed area is large, it results in a low sediment delivery ratio.

Question 8: Some of the data I'm gathering to input into the PLET model is not consistent. For filter strips, I have some data coming to me as actual acres of filter strip and some coming to me as acres affected by the filter strip. What is the best way to deal with these discrepancies in the model?

The important data that PLET needs is the acreage affected by a filter strip. For example, if you have 20 acres of cropland affected by a filter strip, you can choose "Cropland-filter strip" as the BMP in PLET and apply it to the affected area for load reduction calculation. The actual area of a filter strip is less important than the area it controls because:

the area of the filter strip is usually significant smaller than the affected area, and
the filter strip is not a source of nonpoint source pollution (in PLET, you may exclude it from the load calculation or treat it as a type of forest).

Question 9: My general question is whether it is appropriate to use PLET to compare the loading effects of changing a particular site from one land use to another, e.g., agricultural to commercial. I also wonder how it applies the USLE to "soil loss" from a parking lot.

For assessment and planning purposes, you can use PLET to compare effects of land use changes on pollutant loads (N, P, and sediment). However, PLET was created to only give rough estimates of loads and load reductions, and it is not intended for making very precise predictions. For urban land uses, PLET does not use USLE. PLET uses "annual runoff volume x pollutant concentration" to estimate pollutant loads, including sediment load.

Question 10: Can PLET be used to model bacteria?

PLET should not be used to model bacteria. There are other models available to model bacteria. These include models like HSPF, SWAT, and PLOAD, which is now part of BASINS.