Chesapeake Bay Modeling Perspectives for the Regulated Community

Clifton Bell, P.E., P.G.

Chesapeake Bay Modeling Perspectives for the Regulated Community

Themes

Chesapeake Bay modeling framework is an remarkable set of tools.• Impressive capabilities• Important limitations

TMDLs lead to an overreliance on models. Be prepared to advocate local achievements in

model world.

Primary purposes of the Bay modeling framework:

Identify the:

1. Nutrient and sediment loads that will meet water quality standards in tidal waters.

2. Management actions that will achieve these loads.

“The model” is actually many linked models and data processing tools

Models developed, refined over 25+ years

Models developed, refined over 25+ years

Originally used to predict “hypoxic volumes” in Bay

Estimate watershed-scale reductions (e.g., 40% reduction by 2000)

Track progress over large areas

Use of the model has also evolved

Now trying to predict water quality at very specific locations and depths

Predict ≤1% changes in attainment. Estimate local loads

Use of the model has also evolved

Some Important Strengths

Watershed model relatively well calibrated at Baywide and major tributary basin level

Water quality model relatively well calibrated for dissolved oxygen in critical deep water segments

Calibration ValidationPredictive

Management Scenarios

Modeling Process

Uncertainty

Modeling Framework is Conservative with a Implicit Margin of Safety Attainment controlled

by small area, timing. All WWTPs

discharging at full permitted load

Conservative assumptions

Conservative BMP Efficiencies

BMP Text from BMP ReportsRiparian buffers “…a 20% reduction in the effectiveness values is applied to

efficiencies from literature sources…”Urban wet ponds and wetlands

“…recommendation to use a more conservative percent removal estimate.”

Bioretention “The 10% TN concentration reduction [is] a conservative judgment…”

Vegetated open channel

“A more conservative value …was selected…”

Permeable pavement

“…a conservative approach is taken to estimating…performance.”

Infiltration basins and trenches

“…a 15% reduction in TN is used here …to be…conservative.”

Categories of Model Limitations

Limitations of the basic algorithms Calibration errors Overparameterization Scale limitations Input errors Poor model behavior Imprecision of management predictions

Limitations of Basic Algorithms

Examples from watershed model:• Groundwater component crude• No explicit simulation of stream bank erosion• No mass balance of fertilizer

Calibration issues

No calibration is perfect. Quality of Bay model calibration varies greatly

by parameter and location. Watershed model partially calibrated to another

model.

Overparameterization

Complex nutrient cycling algorithms

Overparameterization

x + y =100

Highly Empirical Regional Transport Factors

RegionalDeliveryFactors

Edge of Stream

In Stream Concentrations

21

ungaged basins

TN calibrated Factors0.25 - 0.50.5 - 0.6670.667 - 0.8330.833 - 1.21.2 - 1.51.5 - 2 2 - 4

Phase 5.0 TP Calibrated Regional Factors

Scale Issues

Watershed model lack resolution for accuracy at the local scale• Segmentation• Input data• Calibration

Hoffman County

DianeRiverBasin

STAC Peer Review: 2008

23

“[The] current [watershed model]… is not appropriate for development and implementation of TMDLs at the local watershed scale. A major barrier appears to be the scale of information built into the [model]…”

Input Errors

No benefit of agricultural nutrient management Urban land use

Poor model behavior

Many segments where the model doesn’t “behave”.

e.g., poor calibration e.g., non-intuitive

trends Often the cause and

its extent is undiagnosed.

Summary so far

The model is• Complex• Conservative• Imprecise

So how precise are model predictions of future attainment, anyway? Impossible to accurately quantify. Bay program instituted the “1% rule”. Field measurements are not this precise. Laboratory measurement are not this precise. Model is nowhere near this precise. Lowest realistic estimates:

• 5% for DO attainment.• 15% for chlorophyll-a attainment.

USEPA’s Justification for “1 % Rule”

How Will the Model be Used Post-2010?

Phase II WIPs• Quantify local loads?

Model “locked down” until 2017 Tracking progress

• Baywide• Major state tributary basin• Local level?

Community Model Scenario Builder

Phase 5.3 watershed model publically available.

Scenario Builder• Tool for creating input

to watershed model• Web version planned.• Can’t refine model

scale.

How Should Stakeholders Use the Model and Scenario Builder? Don’t

• Use current watershed model for local TMDLs.• Let current watershed model output drive Bay TMDL

implementation at local level.• Let MS4 permits base compliance on current

watershed model predictions.

Do• Track BMPs for input to watershed model.• Use current watershed to track progress at major

tributary, state, and Baywide scale.• Base MS4 permit requirements on MEP.• Use refined models for local TMDL planning.

How Should Stakeholders Use the Model and Scenario Builder

Do• Use watershed model to identify offsets and trades• Advocate new BMPs for inclusion in the Baywide

model New structural BMPs Non-structural BMPs

– Ordinances– Public education and outreach– Improved BMP maintenance

How Should Stakeholders Use the Model and Scenario Builder

Questions?