State Board Modeling Needs and Interests

Eric Berntsen, PH, CPESC, CPSWQState Water Resources Control Board

CWEMF Hydrology and Watershed Modeling Workshop

June 22, 2007

•Quick Overview of Event-Based and Continuous Simulation Modeling

•Program Modeling Needs/Interests

Water Rights

TMDL

Storm Water

State Board Modeling Needs and Interests

Hydrologic Cycle

From Lake (2004)

From King County Department of Natural Resources

From King County Department of Natural Resources

•Event-based models

e.g., NRCS Runoff Curve Number Method

Rational Method (Q=CIA)

•Input rainfall, desired simulation period, and watershed characteristics

Event-Based vs. Continuous Simulation Modeling

Rainfall

•Usually based on statistical analysis

•Sometimes, historical storm information used

Example: 10-year, 24-hour storm

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)

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.

Event-Based Models

Type of Drainage Area Runoff Coefficient (C)

Lawns:

Sandy soils, flat, 2% 0.05 – 0.10

Sandy soils, average, 2 -7 % 0.10 – 0.15

Sandy soils, steep, 7% 0.15 – 0.20

Heavy soil, flat, 2% 0.13 – 0.17

Heavy soil, average, 2 -7% 0.18 – 0.22

Heavy soil, steep, 7% 0.25 – 0.35

Business:

Downtown areas 0.70 – 0.95

Neighborhood areas 0.50 – 0.70

Residential:

Single family 0.30 – 0.50

Multi units, detached 0.40 – 0.60

Multi units, attached 0.60 – 0.75

Suburban 0.25 – 0.40

Apartment dwelling areas 0.50 – 0.70

From V.T. Chow (1964)

•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

•We’ll talk about the major ones being used these days (HSPF, SWMM, etc.)

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

Continuous Simulation Models

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

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)

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

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

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

Program Needs – Storm Water

Sources of Impairment (USEPA 2006)

Program Needs – Storm Water

Hydromodification-historic approach (still done in many places)

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