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

 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

CalibrationValidation Predictive 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 BMPText from BMP Reports Riparian 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 Regional Delivery Factors Edge of Stream In Stream Concentrations

21 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 Diane River Basin

STAC Peer Review: “[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?