The Edwards Aquifer in south-central Texas is one of the most productive karst aquifers in the United States. The aquifer, which contains over 25,000,000 acre-feet of useable water, is the primary source of drinking water for over 2.3 million people in San Antonio and surrounding communities. It also supplies irrigation water to thousands of farmers and livestock operators in the region. Given its karst hydrogeology, the Edwards Aquifer is extremely responsive to both rainfall and to withdrawals (e.g., pumping for irrigation and water supply), such that large volumes of groundwater are rapidly transported through the system.
The two largest freshwater springs in Texas—Comal Springs and San Marcos Springs—emanate from the Edwards Aquifer and currently account for about 45 percent of its annual discharge (withdrawals like pumping account for the other half). These springs and their associated river systems are heavily used for recreation and are also home to a number of endemic species found nowhere else in the world. Because of the potential for reduced flows to the springs during times of drought, eight of these species are listed under the federal Endangered Species Act (ESA): the fountain darter, the San Marcos gambusia (presumed extinct), the Texas blind salamander, the Comal Springs dryopid beetle, the Comal Springs riffle beetle, Peck’s cave amphipod, Texas wild rice, and the San Marcos salamander. To protect the listed species, the Edwards Aquifer Authority (EAA) and four other local entities applied for an Incidental Take Permit under the Endangered Species Act, creating a 15-year comprehensive Habitat Conservation Plan (HCP) as part of the process.
Central Texas is currently experiencing drought conditions, although
not as severe as the drought of record in the 1950s, during which flows at Comal Springs ceased for 4 months and flows at San Marcos Springs were severely reduced. At current pumping levels, a similar drought today would be catastrophic to the ESA-listed species of the Edwards Aquifer and its springs. Given the uniqueness of these ecosystems, the many diverse projects that make up the HCP, and the persistence of drought conditions across the region, the EAA has requested the input of the National Research Council as the HCP is implemented. This report is the first product of a three-phase study to provide advice to the EAA on various scientific aspects of the HCP.
The National Research Council was charged with constituting a committee of experts that would review and provide advice on four scientific initiatives within the HCP: (1) ecological modeling, (2) hydrologic modeling, (3) biological and water quality monitoring, and (4) applied research. In particular, the Committee’s first report addresses:
- hydrological and ecological modeling approaches;
- accuracy and reliability of the assumptions used to support development of both conceptual and quantitative models;
- adequacy of data for model calibration and verification;
- identification and description of uncertainties;
- additional monitoring data needs;
- additional research needs; and
- other issues deemed relevant by the Committee.
Later reports will review the performance of minimization and mitigation measures found in the HCP, including the four spring flow protection measures, as well as the adequacy of the biological goals and objectives to protect the endangered species.
Chapter 2 of this report addresses the hydrologic modeling, reviewing both the updates to the MODFLOW model and the creation of a finite element model of the aquifer. It tackles the issues of how to represent recharge and conduits in the modeling and how to conduct uncertainty analysis. Chapter 3 describes the state of ecological modeling for Comal and San Marcos Springs, focusing on the fountain darter, submersed aquatic vegetation, Texas wild rice, and the Comal Springs riffle beetle. Chapter 4 evaluates the water quality monitoring program and the biomonitoring program, making recommendations for what should continue to be sampled and whether the biomonitoring program can provide the necessary data and information for the ecological modeling. Chapter 5 critiques the Applied Research Program, which is populated with projects intended to either inform the ecological modeling or fill knowledge gaps about the listed species. The final chapter tackles overarching issues, such as the need for
data management and integration within the HCP, performance monitoring of minimization and mitigation measures, and worst-case scenarios to be considered.
HYDROLOGIC MODELING
The primary objectives of the hydrologic modeling are to create a groundwater model that can reproduce known spring flows and then to use this model to predict (1) the effects of potential future hydrologic conditions (such as climate change and droughts) on spring flow, and (2) how management actions (like conservation measures) will affect water levels and spring flows.
The Edwards Aquifer’s unique hydrogeology, which is characterized by significant heterogeneity in both porosity and permeability and physical features such as conduits, faults, and barriers, complicates modeling efforts. There have been many efforts to characterize and model the Edwards Aquifer, most based on the popular MODFLOW code. The hydrologic modeling activities that are the subject of the Chapter 2 review include updates to the MODFLOW model, creation of a new finite element model of the aquifer, better aquifer characterization and delineation of boundary conditions, development of new methods for determining recharge, and uncertainty analysis.
The hydrologic modeling effort has shown continuous improvement in both the use of models and the incorporation of new data. The EAA is to be commended for its progress to date. Listed below are areas that merit further attention and recommendations for future work that will build upon the EAA’s strong foundation of modeling and data collection efforts.
The EAA could gain efficiency by moving toward a single model that incorporates the best concepts from existing modeling efforts. Continued development of “competing” models (i.e., having both a MODFLOW model and a finite element model) is inefficient and unnecessary and cannot be used for assessing model uncertainty. Any new model selected should have features that benefit and advance the conceptual model, such as telescoping meshes (to accommodate shorter time scales) and linear features for conduits and barriers.
Model uncertainty needs to be quantitatively assessed and presented in formal EAA documents. Quantifying model uncertainty increases a model’s defensibility and can provide a reasonable estimate of model error, which is important information when using a model for management decisions. Uncertainty has been mentioned in some of the EAA’s modeling reports but is not a standard feature in its documentation of modeling results, including
presentations to the Committee. Specific recommendations include 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; and having confidence intervals presented with all modeling results.
ECOLOGICAL MODELING
A major activity within the HCP is the development of new ecological models that will be able to predict species population metrics under a variety of potential future conditions. The primary threat to the ESA-listed species in the major springs of the Edwards Aquifer is the loss of habitat from reduced spring flows, which could occur as the combined result of fluctuating rainfall, regional pumping, and subsequent drawdown of the aquifer. Other threats include increased competition and predation from non-native species, direct or indirect habitat destruction or modification by humans (e.g., recreational activities), and other factors such as high nutrient loading and bank erosion that negatively affect water quality.
Three of the endangered species—the fountain darter, the Comal Springs riffle beetle, and Texas wild rice—have been designated as indicator species within the HCP and, along with submersed aquatic vegetation, are the initial targets of modeling efforts, including both habitat suitability analyses and predictive ecological models. In general, the approach to ecological modeling of combining field data, habitat suitability analyses, and a population dynamics model is appropriate and can support the management decisions that will need to be made as the HCP proceeds. There are, however, several aspects of the analyses that should be adjusted to ensure that robust conclusions are obtained.
The goal of the submersed aquatic vegetation modeling, which is in its early stages, should be clarified. Whether the goal is to simulate submersed aquatic vegetation biomass dynamics or to simulate habitat for the fountain darter model will affect how many models are needed and how each model is formulated and tested. Similarly, key issues about spatial resolution and whether to model individual species or a “generic” species depend on the goals of the modeling.
Given the absence of a planned ecological model for Texas wild rice, the current habitat suitability analysis should be treated as an hypothesis and tested for robustness throughout the San Marcos River. The EAA should consider designing minimization and mitigation measures for Texas wild rice in a manner to provide experimental analysis of the habitat suitability results. For example, the minimization and mitigation activities
could be used to test the validity of using water depth and velocity as the only predictive variables for optimal habitat for Texas wild rice. Similarly, one could use replicated reference and control areas to provide explicit tests of the efficacy of replanting and to quantify the roles of discharge, competition, and other factors that may limit Texas wild rice growth and survival.
The individual-based model for fountain darter is a scientifically sound approach for modeling population dynamics that will require extensive data for model formulation, calibration, and validation. Suggestions for improving the modeling effort include (1) hosting workshops at key times (to define the questions, formulate the model, and present preliminary results), (2) making clearer links between the monitoring data and the Applied Research projects and how both will be used to inform the modeling, and (3) engaging modelers with experience in developing similar individual-based models. Given the complexity inherent in the modeling effort, the habitat suitability analyses done for the fountain darter could act as a “back-up” to the ongoing individual-based modeling and provide additional quasi-independent results.
If the Comal Springs riffle beetle is to be an adequate indicator of some of the other ESA-listed species, it is critical to have a much deeper understanding of its spatial distribution, range of potential habitats, and natural history. Although the HCP has identified the beetle as a primary species for monitoring and calls the beetle an indicator of other species that are not being monitored, the degree to which it is a reliable indicator is presently not well understood nor has it been objectively tested.
It is recommended that as a top priority the EAA develop an ecosystem-based conceptual model, or a series of models of increasing resolution, that show how water quality and quantity, other biota, and restoration and mitigation activities are expected to interact with the indicator species, as well as with all covered species. Boxes in the conceptual model would represent targets of the monitoring program, while arrows linking the boxes would represent quantitative or empirically derived relationships between the boxes based on research. Such interactions for which too little data are available to establish empirical relationships could be targeted for monitoring and further research during the permit period.
MONITORING
The HCP requires the development and implementation of a monitoring plan to (1) evaluate compliance; (2) determine if progress is being made toward meeting the long-term biological goals and objectives; and (3)
provide scientific data and feedback information for the adaptive management process. This report focuses on the biomonitoring and water quality monitoring programs, two somewhat independent programs intended to provide the observational data needed to assess whether the HCP is meeting its goals of protecting the target species as well as collecting the ancillary biological community and water quality data required to identify plausible mechanisms for observed changes in the target species abundance or distribution.
The monitoring of physical, chemical, and biological characteristics of the Comal and San Marcos Spring and River systems has been ongoing since 2000 and is now even more comprehensive as a result of the HCP. While in general the Committee found the monitoring programs to be strong, it also identified areas for improvement.
The monitoring programs do not provide a clear mechanism to scale results to the entire spring and reach system because none of the sampling locations were selected using randomization procedures. Despite some sampling sites being labeled as “representative,” it is inappropriate to use observations derived from these sampling locations to make inferences about the entire river or spring systems. The term “index site” would more accurately describe these locations. Monitoring of index sites needs to continue in order to assess trends and build on existing databases. If the EAA finds it is necessary to provide system-wide estimates of population densities of target species rather than relying on trends at index sites, it will need to invoke special studies or conduct sampling using randomization techniques.
Enhanced sampling for nutrients is recommended. The presence of annual algal blooms and the importance of aquatic macrophytes in structuring fish and macroinvertebrate communities suggest that nutrient loading plays an important role in the spring and river systems. The current detection limits for soluble reactive phosphorus, NO3/NO2, and total nitrogen are so high that significant changes in nutrient concentrations could go undetected. If the detection limits for phosphorus species, NO3/NO2, and total nitrogen were reduced to 2, 10, and 50 micrograms/liter, respectively, by changing analytical methods, this would enable identification of nutrient concerns in both spring systems.
New quantitative sampling methods are needed for the Comal Springs riffle beetle to complement and improve upon the current method (the cotton lure approach). At the same time, a large-scale stratified random survey of the potential habitat available in both systems would provide more robust data on how flow variation and sedimentation affect the habitat and thus population numbers of the Comal Springs riffle beetle. The
comprehensive survey of beetle distribution proposed as part of the Applied Research Program should be given high priority.
APPLIED RESEARCH PROGRAM
Critical to the recovery and protection of all aquifer species is knowledge of the species-specific demography and ecology, including knowledge of natural population fluctuations. At the present time, there is considerably more knowledge about fountain darters and Texas wild rice than about the Comal Springs riffle beetle and most of the other ESA-listed species. The Applied Research Program is intended to fill knowledge gaps about the endangered species in the Comal and San Marcos systems, particularly under low flow conditions, and to provide 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 early on to be able to make well-informed decisions about the direction of the HCP in Year 7. Chapter 5 evaluates the projects that have been funded to date, most of which were useful for providing data and information to the ecological modeling efforts. The following paragraphs describe new study topics that should be considered for inclusion in the Applied Research Program.
Fountain Darter: Additional studies on fountain darter movement would be beneficial to the ecological modeling effort, preferably allowing for Lagrangian tracks to be estimated. A second set of special studies could confront the persistent lack of a relationship found between flow and fountain darter metrics. While the flow-triggered sampling is a good idea, these measurements could be further supported by studies that use lab and field measurements to ensure responses are recorded over a range of flows. A third issue is obtaining measurements related to individual fountain darter health that go beyond the densities and lengths of individuals measured in the current biomonitoring, such as variations on the classic condition index and non-lethal estimation of tissue composition.
Comal Springs Riffle Beetle: Most critical for the beetle is gathering information on life history, life cycle, and spatial distribution. This includes information on densities of both immature and adult life stages throughout the year, growth rates of the life stages, how many generations occur each year and if they are synchronous, how fast the life cycle proceeds, and how the life cycle and other life history attributes like fecundity might be affected by changing flow or sediment conditions. While generating such information is formidable in the short term, now is the time to establish such long-term research goals if a population model for the beetle is an objective of the HCP.
Submersed Aquatic Vegetation: New Applied Research projects for submersed aquatic vegetation should address the needs of the modeling
efforts by focusing on supplying data on submersed aquatic vegetation growth, dispersal, and recolonization for those species that are the best habitat for the fountain darter. Studies that could elucidate the interactions between submersed aquatic vegetation and the fountain darter would be particularly helpful.
Nutrients: There is an abundance of nutrients in the two spring systems, as indicated by the annual summer green algal blooms in the Upper Spring Run reach of the Comal River. Anecdotal evidence suggests that the blooms tend to accompany low flows and high temperatures. It would prove highly beneficial to have a better understanding of the nutrient budgets in the two spring systems. In addition to the physical impacts of low flow, there could be very important indirect effects of low flow on the overall productivity and food web dynamics of the spring and river ecosystems caused by nutrients.
Beyond considering new research topics, the Applied Research Program could be restructured to help to ensure that the limited funds available for the program target priority research needs to support the ecological modeling efforts and the success of the HCP more generally. The following structural modifications to further increase the usefulness and efficiency of the current program are recommended.
The Applied Research Program would benefit from a more transparent process for prioritizing and funding all Applied Research projects that includes stakeholder involvement, for example through the Science Committee, and peer review.
The Applied Research Program would benefit from greater competition and collaboration with outside scientific experts through open and widely disseminated solicitations for research. Increasing the diversity of thought, understanding, and perspective will serve to strengthen the HCP and increase the likelihood that project goals will be met.
The program should offer some longer (e.g., two- to five-year) projects in order to maximize interest and collaboration from the region’s leading researchers. Multiple-year project proposals can be awarded with the simple limitation that funding in subsequent years is contingent on funding availability, project needs, and project success.
OVERARCHING ISSUES
The EAA and other Permittees are at the beginning stages of implementing a complex HCP and are doing an excellent job in most respects. Nevertheless, there are a number of overarching concerns regarding the
implementation process that may hinder the later stages of the HCP, especially any future attempts to renew the HCP and the Edwards Aquifer Incidental Take Permit.
First, the HCP would benefit from more formal integration to enable clear explanation of the many sets of results emanating from the monitoring, modeling, and research efforts. Without greater attention to project integration, there is a danger that the large number of separate projects will not combine seamlessly into an overall science program. Several steps could be taken to enhance integration of the HCP. As mentioned previously, an overall conceptual model of the system including hydrological, climate, and biological community components could guide the development of quantitative modeling of sub-components, identify gaps in understanding, and provide context for understanding the responses of particular species of interest. A second way to achieve integration would be to develop a comprehensive data/information management system. Such a system would ensure both internal and external access to relevant data, facilitate data analyses and syntheses across multiple data types and sources, buffer against the potential turnover of key personnel, and increase transparency and communication across stakeholders. Finally, the EAA could convene an annual science meeting to discuss results, discover gaps in understanding, and help plan future activities. Ideally, such a meeting would include all project and contract scientists, other university and agency scientists who might be interested in becoming involved in future studies, and various stakeholder groups.
The second overarching issue is the need to monitor the performance of the many minimization and mitigation measures currently being implemented, including recreational control, removal of exotics, riparian restoration and bank stabilization, and replanting of Texas wild rice. Although these measures are not the primary focus on this report, the Committee feels it is critical for the EAA to commence performance monitoring as soon as possible.
Third, the Committee recommends that the EAA undertake more formal and rigorous statistical analyses of its laboratory and field data. Much of the data found in documents supporting the HCP do not include error bars or other measures that demonstrate the variability of the data or the uncertainty of model predictions. More formal statistical analysis, such as the incorporation of variance into estimated means and other summary statistics, would give additional credibility to the scientific basis of the HCP. There is significant opportunity for exploring the key field data sets and model results, both ecological and hydrological, with more advanced statistical methods than simple summary statistics and graphical plotting.
Finally, there is a prevailing assumption within the HCP that relevant legal and ecological conditions will remain relatively stable throughout the
Plan’s 15-year implementation period. However, this may not be the case and therefore the Committee recommends that the EAA begin to think now about possible worst case scenarios and their potential implications for both modeling and HCP implementation. On the modeling side, the Permittees should consider whether the models currently being developed rest on ecological assumptions that could be altered by a changing climate and, if so, whether potential or predicted alterations can themselves be incorporated into the model. On the implementation side, considering potential future changes now could allow the Permittees to develop contingency plans—ecological, political, or legal—for future “worst case” scenarios, building adaptability, flexibility, and resilience into the execution of the HCP. Examples of potential future “worst case” scenarios worth considering are:
- increased groundwater pumping from exempt/unregulated wells that undermines the HCP’s minimum flow requirements;
- drought conditions that exceed the drought of record from the 1950s;
- climate change impacts become significant faster than expected;
- high court affirmation of the Bragg constitutional takings decision; and
- subjugation to Aransas National Wildlife Refuge ESA issues.
These scenarios and others, along with their implications to the HCP, are described in detail in Chapter 6.
