Review of the Edwards Aquifer Habitat Conservation Plan Report 2 (2017) / Chapter Skim
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2 Hydrologic Modeling
2 Hydrologic Modeling
Pages 25-48
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From page 25... ...
In addition, the EAA created a Five-Year plan for hydrologic modeling. The objective of the Five-Year plan is the continued updating of the hydrologic model, including such steps as conducting uncertainty analysis with the ensemble method, documenting the model, and obtaining a peer review.
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From page 26... ...
In addition, the lessons learned from incorporating the contributing zone in FEFLOW will be useful for recharge estimation in the MODFLOW model and should be articulated now. Conduit and barrier features in the MODFLOW model were adjusted based on FEFLOW modeling, but additional evaluation of these features could be considered (see below)
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From page 27... ...
While there are additional costs associated with running these smaller models, they are small compared to the cost of the FEFLOW effort, which involved developing a second fully calibrated model. Modeling smaller areas can address some of the RRWG's concerns about cost and feasibility in testing conceptual models because there is no need to reconceptualize the entire HCP model.
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From page 28... ...
. Recharge estimates are a good place to start an ensemble uncertainty analysis, because it is clear that recharge originating in the contributing zone and the interformational inflows are some of the most uncertain inputs and have a large influence on water levels and spring flows.
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From page 29... ...
Therefore, peaks in the recharge estimates will be arbitrarily reduced in the ensemble of recharge estimates. Beyond Puente, there appear to be at least two additional recharge estimation methods that have been applied to the groundwater modeling
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From page 30... ...
A map of prob ability of a travel time less than 50 days (a defined compliance level) showed that the 10% probability of exceedance extended beyond the existing protection zone in the west but there was more coverage than needed in the south.
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From page 31... ...
The Committee recommends using as many different recharge estimation methods as feasible, and varying uncertain recharge parameters within these methods, to create the ensemble. The ensemble will provide a range of possible outcomes for spring flows; this range can be examined for calibration periods and validation periods, and, most importantly, for future scenarios predicted by the model.
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From page 32... ...
SCENARIOS FOR HYDROLOGIC MODELING The MODFLOW model is expected to continue to be the primary groundwater modeling tool for the HCP. It is essential that the EAA strives to improve the predictive skills of the model for the anticipated refinements to the flow protection measures that may be necessary in Phase 2.
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From page 33... ...
Several modeling scenarios that can be run are then described, including lesser and more severe droughts as compared to the drought of record, optimization of the EAA's so-called "bottom-up package" of the four spring flow protection measures, understanding the influence of spatial patterns of pumping, and potential implications of land-use changes in the contributing zone on the water budget in the region. These scenarios are designed to ensure that the EAA meets the requirements of the adaptive management phase of the HCP by having a model that has been tested under a variety of conditions and by optimizing the flow protection measures in order to ensure that they meet specified goals.
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From page 34... ...
This type of approach for using the model for conditions that have not been observed is not atypical and probably the only way to develop water resources management plans. To improve the predictive skills of the model further, the MODFLOW model should be tested for periods outside the drought of record but under less extreme conditions and where more accurate data are available.
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From page 35... ...
In addition, scenario testing should provide information on relative effects of withdrawals and effectiveness of management measures that were implemented during this period. Performance of the System under a Variety of Drought Conditions To date, the only scenario that the EAA has sought to run in the hydrologic model is the management program that includes the four spring flow protection measures (aka the bottom-up package)
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From page 36... ...
The climate scenarios should be designed considering the results of climate-model predictions available from regional climate models that are nested within general circulation models. Spatial variability in rainfall within the Edwards Aquifer region, and the variations in pumping patterns, both of which will impact spring flows, should also be explored in scenario investigations.
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From page 37... ...
NCDC = National Climate Data Center. Optimize the Bottom-Up Package Another useful scenario for the hydrologic modelers to run can answer the question "Can implementation of the four spring flow protection measures of the HCP be optimized?
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From page 38... ...
38 FIGURE 2-3 Observed flows at Comal Spring. SOURCE: www.edwardsaquifer.org/dataflow/api/chart.
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From page 39... ...
For instance, in certain situations, using only the ASR option, which appears to have the greatest "lift" in terms of improving spring flows, may be adequate. The optimization exercises described above will provide the necessary information for decision making either in an adaptive management setting or for revisions of flow protection measures that may be necessary in Phase 2 of the HCP.
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From page 40... ...
By applying GWM to the EAA's MODFLOW model, minimum spring flow thresholds and groundwater levels could be better managed, as influenced by the rate and timing of groundwater withdrawal, ASR, and anthropogenic recharge. For example, a groundwater management problem could consist of decisions about pumping rates and pumping periods for multiple wells to help maintain minimum flow rates from Comal and San Marcos Springs.
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From page 41... ...
A scenario with projected land-use changes and likely change in climate (but no change in water withdrawals by well pumping) over the next two to three decades should be simulated to answer the question "How would a changes in recharge amount due to changing land use impact spring flows?
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From page 42... ...
The model archive should be made available to other government agencies and interested parties for the purpose of simulating specific scenarios of interest or for confirming the EAA's published simulation results. Decision Support System To ensure minimum continuous spring flows, the HCP specifies flow protection measures, some of which are triggered at specific ground water elevations at selected index wells.
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From page 43... ...
The EAA is encouraged to incorporate additional recommendations from the first Committee report, such as more extensive uncertainty analysis, testing conceptual models on subgrids, and better documentation of model updates (including incorporation of new field data)
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From page 44... ...
ing, and predicting how significant growth and land-use change in the recharge area might affect spring flows. Testing a variety of scenarios will not only improve the confidence in the model itself but will also help develop strategic decisions associated with adaptive management and revisions to minimization and mitigation measures.
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From page 45... ...
2015. Applied Groundwater Modeling: Simulation of Flow and Advective Transport.
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From page 46... ...
2016. Evaluation of the Predictive Ecological Model for the Edwards Aquifer Habitat Conservation Plan: An Interim Report as Part of Phase 2.
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From page 47... ...
Journal of Water Resources Planning and Management ASCE 123(6)
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