Summary
The Edwards Aquifer in south-central Texas is the primary source of drinking water for over 2.3 million people in San Antonio and its surrounding communities, and it supplies irrigation water to thousands of farmers and livestock operators in the region. A karst aquifer with extremely high-yield wells and springs and rapid groundwater transport, the Edwards responds quickly to both rainfall events (known as recharge) and withdrawals, such as pumping for irrigation and water supply. The two largest springs emanating from the Edwards Aquifer are home to a number of endemic fish, amphibians, insects, and plants found nowhere else in the world. Because of the potential for reduced spring flow during drought, which the region has suffered from periodically, eight of these species are listed as threatened or endangered under the federal Endangered Species Act: the fountain darter, the San Marcos gambusia (presumed extinct), the Texas blind salamander, the San Marcos salamander, the Comal Springs dryopid beetle, the Comal Springs riffle beetle, the Peck’s Cave amphipod, and Texas wild rice.
To protect the listed species, the Edwards Aquifer Authority (EAA) and four other entities created a 15-year Habitat Conservation Plan (HCP), which outlines a broad array of programs that when implemented will help to maintain the endangered species while managing withdrawals from the aquifer. The programs that make up the HCP range from long-term biological monitoring of the springs to restoration of native aquatic vegetation to the building of mechanistic models of the aquifer region. Given the diversity and complexity of the HCP, in 2013 the EAA requested the input of the National Research Council (NRC) during plan implementation. This
report is the second of a three-phase study to provide advice to the EAA on various scientific aspects of the HCP. The Committee convened to conduct the study addressed the following tasks (with the chapters containing the material indicated in parentheses):
- Evaluate progress and modifications implemented as a result of the Committee’s first report. (Chapter 2 Hydrologic Modeling, Chapter 3 Ecological Modeling, and Chapter 5 Applied Research Program)
- Continue to assess the methods of and data collected through the water quality monitoring and biomonitoring programs. (Chapter 4)
- Identify those biological and hydrologic questions related to achieving compliance with the HCP’s biological goals and objectives that the ecological and hydrologic models should be used to answer, specifically including which scenarios to run in the models. These questions shall help generate information needed to make the HCP Phase 2 strategic decisions about the effectiveness of minimization and mitigation measures. (Chapter 2 Hydrologic Modeling, Chapter 3 Ecological Modeling, and Appendix A Ecological Model)
- Provide an evaluation of how the Phase 1 minimization and mitigation measures in the HCP (including flow protection measures and habitat restoration measures) are being implemented and monitored. Specifically, the Committee will discuss if the proper method of implementation is being utilized to achieve the maximum benefit to the covered species. (Chapter 6)
The reader is referred to (1) Chapter 1 for a description of the hydrology and ecology of the Edwards Aquifer and its springs, events that led to the creation of the HCP, and the plan’s many elements; and (2) subsequent chapters for conclusions and recommendations not found in this summary.
HYDROLOGIC MODELING
The HCP calls for improvements to existing groundwater models of the Edwards Aquifer so that they can predict the effects of future hydrologic conditions (such as climate change and droughts) on spring flow and predict how management actions (like conservation measures) will affect water levels and spring flows. The Committee’s first report recommended devoting future resources to a single model that incorporates the best concepts from existing models, rather than developing two “competing” models. It suggested that whatever model is selected should have features that advance the conceptual model of the system, such as telescoping meshes to accommodate shorter time scales and features for representing conduits and
barriers. The Committee also stressed the need to quantitatively assess and present model uncertainty in formal EAA documents, using one or more of the following techniques: conducting more explicit sensitivity analysis; validating the groundwater model by testing its predictive abilities using data from a time period not included in the model calibration; using additional calibration and validation metrics; using PEST predictive uncertainty analysis; using the ensemble method; and having confidence intervals presented with all modeling results.
Subsequent to the first Committee report, the EAA created a Five-Year plan for hydrologic modeling, the objective of which is the continued updating of the hydrologic model, including conducting uncertainty analysis with the ensemble method. The Five-Year plan involves further development of the MODFLOW model, but not the second model of the groundwater system based on FEFLOW. A goal for the EAA will be to incorporate the learnings from the FEFLOW effort while maintaining a focus on the MODFLOW model. In addition, the Committee hopes that more of the improvements recommended in its first report are incorporated into the modeling effort, including more emphasis on conceptual model improvements, more careful evaluation of recharge estimation, further extension of uncertainty analysis, and improved descriptions of the modeling plans. Finally, several scenarios are suggested for the hydrologic model to improve its reliability and predictive capability.
The groundwater model should be tested against the 2011 to 2015 period, which was not used in model calibration. This period, which includes both very dry and very wet years, offers a remarkable opportunity to validate the model and enhance confidence in the model for future applications. Testing the model using the 2011-2015 period is likely to reveal the limitations of the current model. In addition, it should provide information on relative effects of withdrawals and effectiveness of management measures that were implemented during this period. The hydrologic, climatic, and well withdrawal data and the information on management actions for 2011-2015 should be more accurate than those from prior years, allowing for a more reliable assessment of the model.
Several scenarios are suggested for the hydrologic model, including optimizing the bottom-up package of the four spring flow protection measures,1 evaluating spatial variations in pumping, 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
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1 These measures are the Voluntary Irrigation Suspension Program Option, the Regional Water Conservation Program, Stage V Critical Management Period, and Aquifer Storage and Recovery.
associated with adaptive management and revisions to minimization and mitigation measures.
The Five-Year plan for the hydrologic model should include formal versioning and a decision support system that will be useful in future phases of HCP. The model should be updated every five years, with each new version including a peer-reviewed report and permanent archive of the numerical model that is available to the public. A decision support system will help minimize the subjectivity of management decisions that require a rapid response and should be included in Phase 2 of the HCP.
ECOLOGICAL MODELING
One of the major efforts set forth by the HCP is the creation of predictive ecological models for the Comal and San Marcos Spring systems. The models are expected to be able to account for impacts to the ecosystems from both management measures and natural variations, including such things as groundwater withdrawal, recreation activities, and restoration actions. The initial efforts of the ecological modeling team have focused on modeling the population dynamics of the fountain darter and key submersed aquatic vegetation (SAV) species.
In its first report, the Committee discussed the basic design of the fountain darter model, including the decision to develop an individual-based model, and it opined on several precursors to the model, such as the habitat suitability analyses done for fountain darter, Texas wild rice, and the Comal Springs riffle beetle. A subsequent interim report of the Committee, published earlier this year (see Appendix A), reviewed the first complete report from the ecological modeling team on what is now expected to be the sole product—models that predict the abundance of SAV and fountain darter, each run separately and also run in a coupled mode.
Chapter 3 addresses the EAA’s response to its first report, and it suggests scenarios for the fountain darter model to run, now that a calibrated version is available. The comments and suggestions for scenarios presume that the recommendations in the interim report have been sufficiently addressed. In general, the Committee feels that the ecological modeling efforts have made good progress and that scientifically sound frameworks and approaches for the SAV and fountain darter models are in place. Model development is an iterative process. It is hoped that the models will continue to reflect new knowledge and understanding (beyond the originally anticipated time frame) in order to fully reap their benefits.
The EAA has now provided a scientifically sound basis for the development of a generalized ecosystem-based conceptual model. The conceptual diagrams produced to date for the fountain darter and SAV ecological models will help to guide further development of whole-system conceptual
models. This collection of conceptual models will provide a communication tool for the HCP, will aid in coordination of the diverse expertise found across EAA’s multiple advisory committees and contractors, and will serve an important function, along with the predictive ecological models, to evaluate the appropriateness and efficacy of the minimization and mitigation measures.
Armed with a fully capable fountain darter model, the scenarios analyzed should be designed and documented consistent with several concepts. These include careful designing of the scenarios and use of terminology to ensure transparency, confirming scenarios are within the domain of applicability, associating uncertainty with model predictions, and properly interpreting predictions and providing model-based mechanistic explanations for model responses.
Seven scenarios are described for the fountain darter model, which can be either diagnostic based (e.g., varying process rates) or evaluative (e.g., running the bottom-up package). The scenarios offered demonstrate how the model can be used to examine how extreme flows, process rates, environmental factors, SAV habitat, and episodic population reductions affect fountain darter population dynamics. These results can then be merged with the expected effects of minimization and mitigation measures to identify the robustness and redundancies of the entire suite of actions.
Only general guidance is given on possible scenarios for the SAV model, as it is not appropriate to provide detailed advice at this stage of development. Nonetheless, given the recently proposed adaptive management actions related to changing SAV species coverage goals in the HCP, it would be timely to evaluate the longer-term impact of these decisions on the stability of the SAV populations. The prospect of having such a valuable quantitative tool to better understand the effects of minimization and mitigation measures and predict future states will hopefully motivate those involved to continue developing the SAV model.
BIOLOGICAL AND WATER QUALITY MONITORING
The biological and water quality monitoring programs are intended to provide the observational data needed to assess whether the HCP is meeting its goals of protecting the covered species. Monitoring in the Edwards Aquifer spring systems has been ongoing since 2000 and is now even more comprehensive as a result of the HCP.
In its first report, the Committee commented on the design, purpose, integration, and adequacy of the two monitoring programs. In particular it raised concerns about the lack of integration between the water quality and biological monitoring programs, the difficulty of making system-wide estimates of target species population densities and trends given the reli-
ance on non-randomized sampling of selected index reaches, the inability to assess whether changes in nutrient status are leading to changes in the frequency and magnitude of algal blooms because of insufficient detection limits of phosphorous and nitrogen, and the inability to determine population densities of invertebrates such as the Comal Springs riffle beetle.
In response, the EAA established two working groups to assess the water quality and biological monitoring programs, respectively, and make necessary modifications. Many of the Committee’s recommendations were addressed, and the Comal Springs riffle beetle was made the subject of all Applied Research for 2016. Although the Committee feels that the monitoring programs remain strong, it identified areas for continued improvement. The following recommendations should be considered under the overarching goal of integrating the water quality and biological monitoring programs into a single program that provides the basic information needed to assess compliance with the HCP.
The monitoring program should include the measurements needed to monitor the performance of the broad suite of minimization and mitigation measures. Relying on the individual Applied Research projects or minimization and mitigation activities to provide these data is unrealistic, as these projects and measures are not designed nor funded over the long term, even though it may well take multiple years for the effects of these projects to be realized.
The monitoring program should include the long-term data required to test and inform continuous refinements of the ecological model. The ecological model will need to be continuously assessed and refined, and long-term data collected by the monitoring program will be critical to this effort. It is important that the modeling team be involved in the design of the monitoring program to ensure that the variables being measured are the ones that are most important for model assessment.
The EAA is making progress on addressing the sampling deficiencies that may limit the ability to estimate the distribution and abundance of the Comal Springs riffle beetle populations. The focus on the beetle in the Applied Research Program is a substantial effort for gaining knowledge about the distribution and life history features that will be important for understanding how the beetle responds to environmental variation, including changes in flow and responses during drought conditions. If the Comal Springs riffle beetle is to remain an indicator taxon for other listed invertebrate and vertebrate species, these gaps in life history and distribution will need to be addressed. Alternatively, the EAA should begin to develop monitoring plans for the other listed species.
APPLIED RESEARCH PROGRAM
The Applied Research Program created by the HCP has several goals, including filling gaps in knowledge about particular listed species, increasing understanding of key processes that affect their population dynamics, and providing data and information that can be used to parameterize and validate the ecological models. The overall goal of the program is to generate useful information during Phase 1 of the HCP to be able to make well-informed decisions about the overall direction of the HCP during Phase 2. Projects to date have been evenly split between the fountain darter, Texas wild rice, other SAV species, and the Comal Springs riffle beetle.
In its first report, the Committee provided a number of broad recommendations and conclusions about the Applied Research Program covering three general areas: improving the process used to solicit, review, and manage the Applied Research Program; adopting and implementing a data management system; and increasing understanding of the Comal Springs riffle beetle. The Committee was pleased that the EAA responded positively to its recommendations in these areas and has continued to devote resources to this program. Starting in 2018, the Applied Research Program will be used as a mechanism to assess the effectiveness of minimization and mitigation measures such as removal of exotic species, SAV restoration, and sediment control. The following additional conclusions and recommendations are made for the Applied Research Program.
The Committee applauds the changes made by the EAA regarding the procedures to identify, solicit, and review the projects in the Applied Research Program. The program as modified should be continued and could be expanded to facilitate additional multi-year studies in the future. To encourage more involvement of outside experts, the EAA should look for ways to ease barriers to participation in the Applied Research Program.
The Committee is supportive of EAA’s attempts to develop an effective database management system that will provide data storage, curation, and access into the future. Resources for ongoing data management activities will need to be allocated throughout the lifetime of the HCP.
Monitoring the effectiveness of minimization and mitigation measures such as removal of exotic species, sediment control, and riparian conservation should be done through integration into the existing biological and water quality monitoring programs, rather than through one-off studies conducted through the Applied Research Program.
Modeling efforts should become more integral to consideration of future Applied Research projects. Projects in the Applied Research program can provide data and information to help design model scenarios, to improve parameter estimation and model formulation, and to enable model calibration and validation. For example, the Committee’s previous
recommendations that nutrients be considered in the ecological submodel of SAV would be easier to implement with nutrient data collection and more explicit consideration of nutrients in Applied Research projects on SAV.
MINIMIZATION AND MITIGATION MEASURES
The HCP lists 38 minimization and mitigation measures that when implemented are meant to protect the listed species from the impacts of both anthropogenic and natural disturbances to the Edwards Aquifer spring systems. Chapter 6 reviews the following minimization and mitigation measures and their implementation to date:
- SAV restoration/invasive plant removal in both the Comal and San Marcos systems
- Sediment removal at specific locations
- Dissolved oxygen management in Landa Lake
- Voluntary Irrigation Suspension Program Option (VISPO)
- Regional Water Conservation Program (RWCP)
- Stage V Critical Management Period
- Aquifer Storage and Recovery (ASR)
The first three measures were specifically identified for review by the EAA as a result of uncertainties about their implementation. The latter four, spring flow protection measures, were selected because of their importance to reaching the biological goals and objectives of the HCP.
In general, the Committee feels that implementation of key minimization and mitigation measures is moving in the right direction, with the various programs being characterized by competent project teams, sustained effort, and adequate initial performance monitoring. For every minimization and mitigation measure implemented, performance monitoring should be done not only for the first year, but regularly during implementation, with a comprehensive synthesis of the monitoring data about every five years that goes beyond the simple trends analyses found in the HCP annual reports. The following recommendations pertain to individual minimization and mitigation measures. Details can be found in Chapter 6.
SAV Removal and Restoration. Substantial progress has been made removing non-native vegetation from both the Comal and San Marcos systems and replacing it with native SAV species. Nonetheless, despite this sustained effort, there is not enough new habitat from native plantings to maintain populations of fountain darter to balance non-native SAV removal. This should be verified by considering the carrying capacity of the various SAV species (both native and non-native) for fountain darter.
Sediment Management. In general, sediment removal activities should be limited to areas where ongoing upland sources or natural stream dynamics will NOT lead to deposition of new sediment within a matter of years.
Dissolved Oxygen Management in Landa Lake. The Committee recommends that aeration not be used routinely as a mitigation measure. If floating mats cover more than 25 percent of the surface of Landa Lake and dissolved oxygen concentrations decrease, then manual breaking up and removal of the floating mats should be considered as a mitigation measure. Monitoring of dissolved oxygen concentrations using the miniDOT oxygen sensors in selected areas of Landa Lake and Upper Spring Run should be incorporated into an integrated water quality and biological monitoring program.
Voluntary Irrigation Suspension Program Option. When the HCP is reviewed for renewal, it may be appropriate to re-evaluate the time period that the VISPO trigger is based on using a decision support system. Consideration should be given to redefining the trigger to use additional information, such as groundwater elevation from a longer time frame, precipitation and recharge data, and groundwater model projections of future conditions.
Aquifer Storage and Recovery. The Committee recommends that the following activities related to aquifer storage and recovery be initiated: (1) at a minimum of annually, determine specific injection at each ASR well to assess if there are any long-term changes in ASR well performance, (2) design and implement water quality monitoring for arsenic and related constituents in monitoring wells during recharge and storage events, and (3) design and implement water quality monitoring in ASR wells during recovery events.
All Spring Flow Protection Measures. The total expense to implement the HCP in 2015 was $16,397,097, with the spring flow protection measures accounting for 67 percent of the total. Due to the high expense of the spring flow protection measures and their importance to the HCP’s success, the Committee recommends that compliance of the parties participating in the spring flow protection measures be audited so that there is assurance that parties are complying with the terms of the program and the program will operate as designed.
