Water Quality Modeling: A Quick Synopsis of Concepts and Considerations

Barry M. Evans, Ph.D.Penn State Institutes of Energy and the Environment

Pennsylvania State UniversityStroud Water Research Center (Adjunct)

Water Quality Models

Watershed (Landscape) ModelsGenerally used to estimate flows and pollutant loads produced by a given geographic areaSWAT, GWLF, AGNPS, HSPF, PLOAD, WAM, SWMM

Receiving Water (Waterbody) ModelsUsed to estimate environmental impact of flows and loads to a given waterbody (lake, river, estuary, etc.)LAKE2K, QUAL2K, Bathtub, SWAT, HSPF

Receiving Water Models: In-Stream

0.1 mg/l

1.5 mg/l

0.6 mg/l

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)

Receiving Water Models: In-Stream

0.1 mg/l

1.5 mg/l

0.6 mg/l

Watershed Models Vary in Complexity

Pollution potential indices (GIS-based)

Unit area loading models (STEPL) Statistical models (SPARROW) Simulation models (SWAT, GWLF)

Watershed Models: Pollutants and PathwaysPrimary 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)

Water Quality Models: Calibration

Pre-calibration (N)

Post-calibration (N)

• 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

MapShed

Pre-Processor

Pre-Processor

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

Run Model

When input data have been prepared, open up the GWLF-E model

Review and edit input data

Run Model

Run the GWLF-E model

Run Model

Various types of monthly and daily output created

Run Model

Run Model

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

Run Model

Various optional tools can be used view and evaluate output.

Optional Tools

Optional Tools

Optional Tools

Input can be edited to reflect BMP implementation levels.

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

Optional Tools

Optional Tools

... and compare the results

Some Key QuestionsPlanning 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?