Clifton Bell, P.E., P.G. Chesapeake Bay Modeling Perspectives
for the Regulated Community Slide 2 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. Slide 3
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. Slide 4 The model is actually many linked models and
data processing tools Slide 5 Models developed, refined over 25+
years Slide 6 Slide 7 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 Slide 8 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 Slide 9 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 Slide 10 CalibrationValidation Predictive Management
Scenarios Modeling Process Uncertainty Slide 11 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 Slide 12 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
estimatingperformance. Infiltration basins and trenches a 15%
reduction in TN is used here to beconservative. Slide 13 Categories
of Model Limitations Limitations of the basic algorithms
Calibration errors Overparameterization Scale limitations Input
errors Poor model behavior Imprecision of management predictions
Slide 14 Limitations of Basic Algorithms Examples from watershed
model: Groundwater component crude No explicit simulation of stream
bank erosion No mass balance of fertilizer Slide 15 Calibration
issues No calibration is perfect. Quality of Bay model calibration
varies greatly by parameter and location. Watershed model partially
calibrated to another model. Slide 16 Slide 17 Overparameterization
Slide 18 Complex nutrient cycling algorithms Slide 19
Overparameterization x + y =100 Slide 20 Highly Empirical Regional
Transport Factors Regional Delivery Factors Edge of Stream In
Stream Concentrations Slide 21 21 Phase 5.0 TP Calibrated Regional
Factors Slide 22 Scale Issues Watershed model lack resolution for
accuracy at the local scale Segmentation Input data Calibration
Hoffman County Diane River Basin Slide 23 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] Slide 24 Input Errors No benefit of agricultural nutrient
management Urban land use Slide 25 Poor model behavior Many
segments where the model doesnt behave. e.g., poor calibration
e.g., non-intuitive trends Often the cause and its extent is
undiagnosed. Slide 26 Summary so far The model is Complex
Conservative Imprecise Slide 27 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. Slide 28
USEPAs Justification for 1 % Rule Slide 29 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? Slide 30 Community Model Scenario Builder Phase
5.3 watershed model publically available. Scenario Builder Tool for
creating input to watershed model Web version planned. Cant refine
model scale. Slide 31 How Should Stakeholders Use the Model and
Scenario Builder? Dont 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. Slide 32 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 Slide 33 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 Slide 34
Questions?
