This document summarizes watershed and receiving water models. It describes watershed models that estimate pollutant loads from land areas, such as SWAT and GWLF. It also describes receiving water models that estimate environmental impacts on waterbodies, such as LAKE2K and QUAL2K. The document provides examples of using models to estimate pollutant concentrations in streams before and after calibration. It also describes how watershed models vary in complexity from pollution indices to simulation models like SWAT. Key pollutants and transport pathways are outlined. The modeling process is summarized as pre-processing GIS data, running the core model GWLF-E, and using optional tools to visualize and evaluate outputs and BMP scenarios.

1. Barry M. Evans, Ph.D.
Penn State Institutes of Energy and the Environment
Pennsylvania State University
Stroud Water Research Center (Adjunct)

2. Watershed (Landscape) Models
Generally used to estimate flows and pollutant loads
produced by a given geographic area
SWAT, GWLF, AGNPS, HSPF, PLOAD, WAM, SWMM
Receiving Water (Waterbody) Models
Used to estimate environmental impact of flows and loads
to a given waterbody (lake, river, estuary, etc.)
LAKE2K, QUAL2K, Bathtub, SWAT, HSPF

3. Receiving Water Models: In-Stream
0.1 mg/l
1.5 mg/l
0.6 mg/l

4. Receiving Water Models: In-Stream
0.1 mg/l
1.5 mg/l
(0.8 mg/l)
0.6 mg/l
(0.3 mg/l)

5. Receiving Water Models: In-Stream
0.1 mg/l
1.5 mg/l
0.6 mg/l

6. Watershed Models
Vary in Complexity
 Pollution potential indices (GIS-based)
 Unit area loading models (STEPL)
 Statistical models (SPARROW)
 Simulation models (SWAT, GWLF)

8. Watershed Models: Pollutants and Pathways
Primary Pollutants





Nutrients (nitrogen and phosphorus)
Organic matter (e.g., BOD)
Sediment/TSS
Pathogens
Exotics (pesticides) and heavy metals
Primary transport pathways





Dissolved or washed off solids in surface runoff
Attached to eroded soil in surface runoff
Leached and transported via subsurface flow
Streambank erosion
Direct disposal in surface water (e.g., WWTPs, cows)

9. Water Quality Models: Calibration
Pre-calibration (N)
Post-calibration (N)

10. • GIS “pre-processor” used to overlay and manipulate
GIS layers, statewide weather files, and other data for
purpose of creating input file for the core watershed
simulation model (GWLF-E)
•GWLF-E model is then used to run input files to
estimate nutrient (N and P), sediment, and pathogen
loads for a given watershed
•Various ancillary tools can be used to visualize and
analyze model output, and to evaluate BMP effects

11. Pre-Processor

12. Pre-Processor
….then use MapShed to derive input data for the GWLF-E model

13. Run Model
When input data have been prepared, open up the GWLF-E model

14. Run Model
Review and edit input data

15. Run Model
Run the GWLF-E model

16. Run Model
Various types of monthly
and daily output created

17. Run Model

18. Run Model
All output written to Excel
files to provide additional
control in mangement and
display of data.

19. Optional Tools
Various optional tools can be used view and evaluate output.

20. Optional Tools

21. Optional Tools
Input can be edited to reflect BMP implementation levels.

22. Optional Tools
Specify various BMP
settings for a given
scenario, and then re-run
GWLF-E to estimate
potential load reductions….

23. Optional Tools
... and compare the results

24. Some Key Questions
Planning or regulatory?
Need to compare results against observed data?
Focus on urban, agriculture or mixed land use?
How large is the area/watershed?
What pollutants/processes are to be modeled?
Level of detail for model processes?
How readily available are data?
Can “what-if” scenarios be analyzed?
How important is acceptance by state agency?
How user-friendly is it?