General Overview of Watershed Modeling

1. Eric Berntsen, PH, CPESC, CPSWQ State Water Resources Control Board Watershed Modeling with HEC-HMS May 28, 2008 General Overview of Watershed Modeling

2. State Board Modeling Needs and Interests • •Quick Overview of Event-Based and Continuous Simulation Modeling • Program Modeling Needs/Interests • Water Rights • TMDL • Storm Water

3. Hydrologic Cycle From Lake (2004)

4. Native Soil

5. Disturbed Soil

6. Event-Based vs. Continuous Simulation Modeling • •Event-based models • e.g., NRCS Runoff Curve Number Method • Rational Method (Q=CIA) • Input rainfall, desired simulation period, and watershed characteristics

7. Event-Based Models • Rainfall • Usually based on statistical analysis • Sometimes, historical storm information used • Example: 10-year, 24-hour storm

11. Event-Based Models • Simulation period • Period typically ranges from 5 minutes to 24 hours • Shorter durations for peak flow calculations (e.g., 10-year, 30-minute storm)

12. Event-Based Models • Watershed Characteristics • Relationship between rainfall and runoff identified (e.g. Rational Method “C” factor, Runoff Curve Number). • These coefficients/factors depend on soil infiltration rate, vegetation, land use, imperviousness, etc.

13. From V.T. Chow (1964)

16. Continuous Simulation Models • Use long term rainfall record (20-30 years) and can simulate flows for entire period of record • Incorporate evapotranspiration and infiltration estimates – simulate the water balance • Major ones being used these days include HEC-HMS, HSPF, SWMM

17. Continuous Simulation Models • Continuous simulation models are better at predicting variability in flow and pollutant loads because they are based on long term observed hydrologic data • Output from these models can help assess expected variability and inform water resource decisions

18. Program Needs – Water Rights • Two fundamental questions for each application to appropriate water: • Is there enough water available to supply applicant with the requested water amount? • Will diverting the requested amount affect existing users and beneficial uses? • Currently, no approved procedure to estimate flows

19. Program Needs – Water Rights • Flow estimation procedures currently used: • Extrapolate gage data to gaged and ungaged watersheds • Modified Rational Method used in North Coast Streams • Desire to develop a suite of methods to estimate flow on daily time step (i.e., continuous simulation methods)

20. Program Needs – Water Rights • Continuous simulation procedures would allow applicants and water rights staff to: • Estimate unimpaired runoff • Simulate various diversion scenarios • Estimate the range of geomorphically-significant flows

21. Program Needs – TMDL • Accurate estimates of flow and pollutant loads are necessary to determine assimilative capacity of waterbodies and load allocations for land uses in watershed • Calculating flow and load on a daily timestep desireable

22. Program Needs – Storm Water • Compliance with post-construction treatment requirements usually verified through modeling • Example-requirement to capture/treat 80% annual runoff volume • Projects may have specific load reduction targets • Event-based and continuous simulation models can be used

23. Program Needs – Storm Water Sources of Impairment (USEPA 2006)

24. Program Needs – Storm Water Hydromodification-historic approach (still done in many places)

25. Program Needs – Storm Water • Need modeling tools that will allow us to assess and mitigate impacts from hydromodification • Event-based models are good at predicting large, infrequent flood flows, may not be as good at simulating the range of geomorphically-significant flows • Continuous simulation modeling is a promising approach