Pollutant Load Estimation Tool (PLET)
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:
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nutrient and sediment loads from different land uses, and
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the load reductions that would result from the implementation of various best management practices (BMPs).
On this page:
Overview
PLET provides a user-friendly web interface to create a customized watershed model. It computes:
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watershed surface runoff;
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nutrient loads, including nitrogen, phosphorus, and 5-day biological oxygen demand (BOD5); and
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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
Access PLET!
Date | Document | Notes |
---|---|---|
October 2024 | PLET User's Guide Version 2.0 (pdf) | Current |
April 2023 | BMP Descriptions (pdf) | Current |
April 2022 | PLET User's Guide Version 1.0 (pdf) | Historical |
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 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 (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.