Surface water-groundwater interaction issues in Texas texaswaterjournal.org An online, peer-reviewed journal published in cooperation with the Texas Water Resources Institute Volume 9 Number 1 | 2018 https://www.texaswaterjournal.org Volume 9, Number 1 2018 ISSN 2160-5319 Editorial Board Todd H. Votteler, Ph.D. Editor-in-Chief Collaborative Water Resolution LLC Kathy A. Alexander, Ph.D. Gabriel Collins, J.D. Center for Energy Studies Baker Institute for Public Policy Robert L. Gulley, Ph.D. Texas Comptroller of Public Accounts Robert E. Mace, Ph.D. Meadows Center for Water and the Environment Texas State University texaswaterjournal.org THE TEXAS WATER JOURNAL is an online, peer-reviewed journal devoted to the timely consideration of Texas water resources management, research, and policy issues. The journal provides in-depth analysis of Texas water resources management and policies from a multidisciplinary perspective that integrates science, engineering, law, planning, and other disciplines. It also provides updates on key state legislation and policy changes by Texas administrative agencies. For more information on TWJ as well as TWJ policies and submission guidelines, please visit texaswaterjournal.org. The Texas Water Journal is published in cooperation with the Texas Water Resources Institute, part of Texas A&M AgriLife Research, the Texas A&M AgriLife Extension Service, and the College of Agriculture and Life Sciences at Texas A&M University. Ken A. Rainwater, Ph.D. Texas Tech University Rosario Sanchez, Ph.D. Texas Water Resources Institute Managing Editor Kathy Wythe Texas Water Resources Institute Layout Editor Sarah Richardson Texas Water Resources Institute Staff Editor Kristina J. Trevino, Ph.D. Trinity University Cover photo: Sunrise over Coastal Bend Bays & Estuaries Program land. © 2017 John Reuthinger. See winning photos at WildlifeinFocus.org. http://WildlifeinFocus.org http://WildlifeinFocus.org https://www.texaswaterjournal.org https://www.texaswaterjournal.org Texas Water Resources Institute Texas Water Journal Volume 9, Number 1, December 17, 2018 Pages 129-149 Abstract: In Texas, surface water is owned and regulated by the State of Texas, whereas groundwater is owned by respective property owners under the rule of capture. Owners of surface water rights, issued by the state, and groundwater may use and sell their water as a private property right. The Texas Commission on Environmental Quality administers surface water rights, while groundwater conservation districts (where they exist) are primarily responsible for permitting groundwater use. This paper focuses on the complexity of both systems that are designed to manage water resources differently with specific emphasis on where surface water and groundwater interact. Surface water-groundwater interactions have contributed to disputes over the actual ownership and right to water. The available science and the limitations of the models currently used to make water avail- ability and permitting determinations are discussed, as are the investments in field data gathering and interpretation and model enhancements that can lead to better assessments of surface water-groundwater interactions and impacts. More complete science and enhanced models may also help reduce the timeline associated with the permitting of future water supply and use strategies. Keywords: surface water, groundwater, interaction, availability models, permitting decisions Surface water-groundwater interaction issues in Texas 1Principal Geoscientist at INTERA Incorporated. 2Deputy Executive Director of the Meadows Center for Water and the Environment and Professor of Practice of Geography at Texas State University and a former Senior Scientist and Deputy Executive Administrator at the Texas Water Development Board. 3Principal of RSAH2O, LLC and a former Chairman of the Texas Water Development Board and Commissioner of the Texas Commission on Environmental Quality *Corresponding author: syoung@intera.com Texas Water Journal, Volume 9, Number 1 Steven C. Young1*, Robert E. Mace2, and Carlos Rubinstein3 Citation: Young SC, Mace RE, Rubinstein C. 2018. Surface water-groundwater interaction issues in Texas. Texas Water Journal. 9(1):129-149. Available from: https://doi.org/10.21423/twj.v9i1.7084. © 2018 Steven C. Young, Robert E. Mace, Carlos Rubinstein. This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/ or visit the TWJ website. https://doi.org/10.21423/twj.v9i1.7084 https://creativecommons.org/licenses/by/4.0/ https://twj-ojs-tdl.tdl.org/twj/index.php/twj/about#licensing Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas130 Terms used in paper Acronyms Descriptive name BBASC basin and bay area stakeholder committee BFI baseflow index DORM Daily Operational Routing Model DFC(s) desired future condition(s) EAA Edwards Aquifer Authority ES Environmental Stewardship ESA Endangered Species Act GAM(s) groundwater availability model(s) GCD(s) groundwater conservation district(s) GMA(s) groundwater management area(s) IHA Indicators of Hydrologic Alteration LCRA Lower Colorado River Authority LCRB Lower Colorado River Basin MAG(s) modeled available groundwater(s) MBFIT Modified Base Flow Index with Threshold SCOTUS Supreme Court of the United States SW-GW surface water-groundwater TCEQ Texas Commission on Environmental Quality TIFP Texas In-stream Flow Program TPWD Texas Parks and Wildlife Department TWC Texas Water Code TWDB Texas Water Development Board USFWS U.S. Fish and Wildlife Service USGS U.S. Geological Survey WAM(s) water availability model(s) WRAP Water Rights Analysis Package INTRODUCTION The growing use of water resources and greater frequency of droughts, with associated impacts to streamflow, are placing a greater focus on groundwater and surface water interactions in Texas by state agencies (NAS 2005; Scanlon et al. 2005; TWDB 2016a; Toll et al. 2017; Young et al. 2017; Smith et al. 2015; Chowdhury et al. 2010). Among the regulatory issues affected by surface water-groundwater (SW-GW) interactions in Texas are managing water rights along a river, complying with the Endangered Species Act (ESA), implementing envi- ronmental flow recommendations, and obtaining bed and banks permits. A question central to all these regulatory issues is how to quantify the impacts of groundwater pumping on the availability of surface water. This question is at the center of several recent studies, conflicts and lawsuits in Texas involving the Rio Grande, San Saba, Colorado, and Brazos rivers. The situation on the San Saba River resulted, in part, in an inter- im charge for Texas House Natural Resources Committee (85th Legislative session) to evaluate “emerging issues in groundwater and surface-water interaction, in particular in areas of increas- ing competition for scarce resources”(Straus 2017). As shown by the recent events associated with pumping groundwater near the four aforementioned rivers, an emerging issue associated with SW-GW interactions is that groundwater permitting and availability must recognize a person’s owner- ship and property interest in water. Sound science is critical to ensuring such protection and determinations. To properly address questions of how groundwater pumping is affecting surface-water availability, there is a need to properly understand SW-GW interactions (NAS 2005). Several factors contribute to this lack of understanding, including an inad- equate number of field studies that address SW-GW interac- tions, the use of baseflow estimation techniques that do not Texas Water Journal, Volume 9, Number 1 131Surface water-groundwater interaction issues in Texas In some situations, the TCEQ may classify groundwater as “under the direct influence of surface water.” Groundwater classified as under the direct influence of surface water in Texas requires a higher level of treatment for a public water supply than does groundwater that is not under the direct influence of surface water. TWC Chapter 290, Subchapter D defines groundwater under the direct influence of surface water as: “Any water beneath the surface of the ground with: (A) significant occurrence of insects or other macroorgan- isms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium; (B) significant and relatively rapid shifts in water charac- teristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water conditions; or (C) site-specific characteristics including measurements of water quality parameters, well construction details, exist- ing geological attributes, and other features that are similar to groundwater sources that have been identified by the executive director as being under the direct influence of surface water.” The TCEQ definition above is based on U.S. Environmental Protection Agency regulation (40 CFR 141.2). Finally, the TWC defines groundwater as “…water perco- lating below the surface of the earth” (TWC §35.002(5) and §36.001(5)). However, stream underflow has been express- ly excluded from being considered groundwater because it is state water. This distinction is important because it grants the TCEQ the legal authority to restrict non-domestic pumping of groundwater near streams if groundwater is present in an underflow zone. OWNERSHIP AND REGULATION OF SURFACE WATER AND GROUNDWATER IN TEXAS Texas surface water law has evolved from the Riparian Doc- trine to the Prior Appropriation Doctrine. Surface water is owned by the State of Texas held in trust for the public (TWC §11.021, §11.0235). With passage of the Water Rights Adju- dication Act in 1967, Texas adopted a unified surface water permit system. Unless the purpose of use is domestic or live- stock (exempt uses that remain riparian), anyone wishing to use surface water must receive permission from the state in the form of a “water right.” The TCEQ is primarily responsible for granting surface water rights, which then become private property in and of themselves unless forfeited through nonuse. Texas groundwater law is rooted in the rule of capture. Tex- as landowners own the water beneath their property (TWC §36.002) and may use or sell the water as private property. The Texas Legislature, however, has authorized the establish- provide consistent estimates or consider bank flow, and model simulations that do not adequately reflect the physical process- es occurring in SW-GW interactions (Parsons 1999; Halford and Mayer 2000; HDR 2007; Mace et al. 2007; Asquith et al. 2005; Scanlon et al. 2005; Partington 2012; Young et al. 2017). The purpose of this paper is to (1) define key terms and identify statutes in the Texas Water Code (TWC) associated with SW-GW interactions, (2) summarize the role of SW-GW interactions in the management of water resources, (3) present key physical processes that occur in SW-GW interactions, (4) discuss the limitations of currently used techniques to estimate and model SW-GW interactions, and (5) present recommen- dations to improve the science in relation to SW-GW inter- actions in Texas. Although this paper is specific to Texas law, management issues, and case studies, the issues raised could be of benefit and application outside of the state for anyone con- sidering SW-GW interactions in their management decisions. DEFINITION OF GROUNDWATER AND SURFACE WATER The TWC does not define surface water specifically but rather makes the terms “surface water” and “state water” syn- onymous. TWC §11.021 defines state water as “The water of the ordinary flow, underflow, and tides of every flowing river, natural stream, and lake, and of every bay or arm of the Gulf of Mexico, and the storm water, floodwater, and rainwater of every river, natural stream, canyon, ravine, depression, and watershed in the state…” In addition to the surface water features identified in §11.021, the TWC also uses the term “watercourse.” The courts have described a watercourse as having (1) a defined bed and banks, (2) a current of water, and (3) a permanent source of supply (Domel v. City of Georgetown, Austin 1999). These criteria are crucial in determining if water is classifiable as state water. Generally, until water reaches a watercourse (where it becomes state water), it is classified as diffuse water. Diffuse water includes rainwater, snowmelt, and overland flow and is the property of the landowner until it joins a watercourse. Another water feature classified as state water is “underflow,” which is generally associated with the presence of subsur- face water within the bed and banks of a watercourse. Texas Commission on Environmental Quality (TCEQ) rule §297.1 defines a stream’s underflow as “[w]ater in sand, soil, and gravel below the bed of the watercourse, together with the water in the lateral extensions of the water-bearing material on each side of the surface channel, such that the surface flows are in con- tact with the subsurface flows, the latter flows being confined within a space reasonably defined and having a direction cor- responding to that of the surface flow” (30 Tex. Admin. Code §297.1(55)). Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas132 ment of groundwater conservation districts (GCDs), which generally have the authority to modify the rule of capture by promulgating rules for conserving, protecting, recharging, and preventing waste of underground water. TWC §36.0015 states that GCDs “are the State’s preferred method of groundwater management in order to protect property rights, balance the conservation and development of groundwater to meet the needs of this state, and use best available science in the con- servation and development of groundwater.” There are cur- rently 100 GCDs that cover about 70% of the area of Texas. GCDs operate though a board of directors, whose members are either elected or appointed, generally by elected officials, per the conditions established in the legislative act that created the district or TWC if the district was through petition. GCDs may choose to recognize SW-GW interaction through the adoptions of management goals to maintain springflow and/ or stream baseflow. The TCEQ cannot authorize or regulate groundwater pump- ing via permit, just as a GCD cannot regulate the permitting and diversion of surface water. Consequently, an inherent statutory conflict is created by having these separate regula- tory mechanisms, particularly as it relates to SW-GW inter- action. The differences in the regulatory agencies, technical disciplines, and ownership issues associated with surface water and groundwater have led to the development of programs to develop regulatory tools for evaluating groundwater availabil- ity and surface water availability but few tools for evaluating SW-GW interactions. SURFACE WATER-GROUNDWATER INTERACTION Traditionally, surface water and groundwater have been treated independently when managing these resources in Tex- as. However, it is well understood that these two resources are often hydrologically connected. In some instances, surface water serves as a source of flow that can change the chemistry and availability of groundwater. Conversely, groundwater can increase the flow volume and affect the chemistry of surface water. In some cases, the same stretch of river may lose flow to the aquifer in one season and gain flow from the aquifer in another season. As the demand for water and the need for new water supplies increase in Texas, understanding the hydrologic connection between surface water and groundwater becomes integral to developing appropriate legislation and strategies to effectively use and manage these two resources. Gaining and losing streams A stream that receives water emerging from a submerged spring or other groundwater seepage through its streambed is a gaining stream (Winter et al. 1998). A stream that loses water to groundwater by outflow through the steambed is called a losing stream (Winter et al. 1998). Figure 1 illustrates the dynamics of gaining and losing streams. A stream may always gain water from an aquifer (perennial streams) or always lose water to an aquifer (intermittent or ephemeral streams). The flow conditions in a stream might also vary over time and across space, such that it is characterized as both gaining and losing. The conditions that cause these variances can be natu- ral, such as flood events, or anthropogenic, such as pumping. An important metric for evaluating SW-GW interactions is the difference in elevation between the water table in an aqui- fer and the water level in a stream. For a gaining stream, the water-level elevation in the stream is lower than the water level in the immediate aquifer. Under these conditions, the aquifer discharges water to the stream, increasing the stream’s flow. For a losing stream, the water-level elevation in the stream is higher than the water-table elevation in the aquifer. Under these con- ditions, the stream recharges water to the aquifer. Groundwater contribution to a stream can originate from unconfined aquifers or from confined aquifers. For the case of an unconfined aquifer, groundwater flow typically exits an aquifer and flows to the stream as diffuse flow. In coastal aqui- Figure 1. Schematic showing groundwater flow toward a gaining stream (a) and groundwater flow away from a losing stream (b) (modified from Winter et al. 1998). Texas Water Journal, Volume 9, Number 1 133Surface water-groundwater interaction issues in Texas in the aquifer than in the stream. Figures 2B and 2C show the effects of a rainfall event on water levels in the stream, causing them to become temporarily higher than the water level in the aquifer that is in contact with the stream. During this time, stream water flows into the aquifer and is stored in the banks of the aquifer as bank storage. After the flood event has passed and the stream becomes a gaining stream again (see Figure 2D and 2E), the water held as bank storage returns to the stream and mixes with the water that originated from the aquifer. After bank flow has ceded, the stream and aquifer water levels eventually return to conditions typical for a gaining stream. Significant bank storage and flow occurs when (1) a stream reach is subject to stage increases, (2) bank materials have a high permeability, and (3) sufficient volumes of permeable bank material or alluvium provide storage (Rassam and Werner 2008). The abundance of high permeability alluvium will also promote the occurrence of underflow. In general, downstream reaches are more favorable to bank storage than headwater reaches (Kondolf et al. 1987) because they have greater drain- age areas that produce large flood peaks and are more likely to be flanked by alluvium with a large capacity to store water relative to streamflow. Kunkle (1962) showed, in some cases, annual discharge from a groundwater basin can be less than the annual discharge from bank storage. The identification and calculation of bank flow requires at a minimum measured water-level elevations and water quality parameters from a river gage and wells in the aquifer under- lying and adjacent to the stream. Figure 3 shows water levels measured in 2007 at a Colorado River gage and a water well located about 200 feet from the Colorado River (URS and Baer Engineering 2008). These data are from a monitoring program performed by the Lower Colorado River Authority (LCRA) to investigate SW-GW interactions near the City of Wharton from 2006 to 2008. Over that period, the groundwater level in the aquifer was higher than the stream water level in the Colorado River over 80% of the time, which means the Col- orado River was a gaining stream (see Figure 1) over 80% of the time. However, during multiple high stream stage events, the increase in stream water levels caused significant increas- es in the groundwater level that represent bank storage in the aquifer (as illustrated in Figures 2B and 2C). On several occa- sions, the bank storage became great enough to cause a reversal of groundwater flow direction 200 feet from the stream. Fol- lowing the peak stream stage and the accumulation of bank storage, bank flow (as illustrated in Figures 2D and 2E) occurs as water levels recede in both the aquifer and the stream until another high stage ensues. Although the data in Figure 3 can be used to demonstrate the occurrence of bank storage and bank flow to SW-GW interactions, additional information is needed to determine the amount of water transferred between the stream and the fers such as the Gulf Coast Aquifer System, the majority of groundwater contribution to streams occurs as diffuse flow. For the case of a confined aquifer, pressured groundwater flows through preferential flow pathways created by faults, fractures, and karstic features until it exits at a spring location and enters a stream. In the Texas Hill County, the confined section of the Edwards Aquifer produces some of the biggest springs in Tex- as. These springs include Barton Springs, San Marcos Springs, Comal Springs, Las Moras Springs, and San Felipe Springs. The Texas Water Development Board (TWDB) (2016a) made several key points regarding SW-GW interactions in Texas: • An estimated 9.3 million acre-feet of groundwater flows from major and minor aquifers to surface water in an average year. This represents about 30% of the average surface water flow in Texas. • Aquifer interactions with surface water vary regional- ly and within each aquifer. Between 14% and 72% of streamflow over aquifer outcrop areas is due to ground- water discharge from major and minor aquifers. • The largest groundwater contributions to surface water occur in East Texas, the Hill Country, and around major springs in West Texas. • The aquifer with the most groundwater discharge to sur- face water is the Gulf Coast Aquifer, with an estimated 3.8 million acre-feet per year. Besides indicating that SW-GW interactions can significant- ly affect streamflow, the TWDB (2016a) shows that local geol- ogy and meteorological conditions are important factors that affect SW-GW interactions. Baseflow and bank flow TCEQ Rule §297.1 defines baseflow as “[t]he portion of streamflow uninfluenced by recent rainfall or flood runoff and is comprised of springflow, seepage, discharge from artesian wells or other groundwater sources, and the delayed drainage of large lakes and swamps.” This definition implies that bank flow is not a part of baseflow. As discussed by Freeze and Cher- ry (1979), bank storage effects and bank flows can complicate the process of defining and determining baseflow. Bank storage refers to the variable amount of water stored temporarily in the stream banks during rising flood stage (Todd 1955). Bank flow is the release of bank storage back to the stream that occurs following high river stage. Despite being potentially important to characterizing SW-GW interactions, bank flow and bank storage are not recognized in TCEQ rules and are not consid- ered in the water balance simulated by water availability mod- els (WAMs) and groundwater availability models (GAMs). Bank flow is the flow of water into and out of the banks along a stream (Figure 2). Figure 2A shows water levels under conditions for a gaining stream where the water level is higher Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas134 aquifer. Among the additional information required to make such a determination are hydraulic properties of the aquifer and measurements of water quality parameters. The chemical data is used to partition flow based on mass-balance consider- ations. Numerous studies have successfully used geochemical analysis of stable isotopes, anions, and salinity to estimate base- flow (Boulton et al. 1999; Porter 2001; Oxtobee and Nova- kowki 2002; Brodie et al. 2005; SKM 2012; Scholl et al. 2015; Rhodes et al. 2017; Cook et al. 2018). The importance of bank storage to SW-GW interactions is difficult to assess in most Texas rivers because of the sophisticat- ed level of analysis and large quantity of data required to derive definitive answers. In order to thoroughly quantify bank stor- age effects, evaluations of flow exchange should include both Figure 2. Schematic showing groundwater flow toward a stream at sequential times. Water levels during average flow conditions at a gaining stream (A). Increase in stream elevation during a flooding event causes hydraulic gradient reversal at stream-aquifer interface. Streamflow enters aquifer and becomes bank storage in stream bank (B and C). Decrease in stream elevation after a flooding event. Bank storage flows back to the stream as water level in the streams lowers over time (D and E). Water levels in stream and aquifer return to conditions that existed prior to flood event (F). Texas Water Journal, Volume 9, Number 1 135Surface water-groundwater interaction issues in Texas calculations based on hydraulic data and geochemical data. One Texas river that has a relatively large amount of permeable bank material is the Brazos River. A recent study by Rhodes et al. (2017) that includes both hydraulic and geochemical analy- sis demonstrates that bank storage can be a significant compo- nent of groundwater flow to the Brazos River. During a four- month river stage recession following a high stage event, less than 4% of the water discharged from the subsurface resem- bled the chemical fingerprint of the alluvial aquifer. Instead, the chemistry of the discharged water closely resembled the high stage event river water. Rhodes et al. (2017) concluded that the Brazos River is well connected to rechargeable bank storage reservoirs but disconnected from the broader alluvial aquifer. SURFACE WATER AND GROUNDWATER AVAILABILITY IN TEXAS In 1997, Senate Bill 1 of the 75th Texas Legislature directed the TCEQ (then called the Texas Natural Resource Conserva- tion Commission) to develop WAMs for river basins in Texas. A WAM “is a computer based simulation program used to eval- uate the amount of surface water in a river or stream that would be available to existing or proposed water rights under speci- fied basin operations and hydrologic conditions”(HDR 2007). WAMs consist of two parts: the modeling program called the Water Rights Analysis Package (WRAP)( Wurbs 2001) and the text files that contain basin-specific information for the WRAP to process. WAMs do not explicitly simulate water fluxes asso- ciated with stream-aquifer interactions, but they can indirectly account for the effects of a losing stream through a channel loss function or a naturalized flow adjustment file (HDR 2007). As noted by HDR (2007), however, the majority of WAMs do not include channel losses because the losses are typically small relative to streamflows. The authors believe that a potentially more valuable surface water model for investigating SW-GW interactions than WAMs are flow-routing models for the stream basin. Flow-routing models solve hydrologic equations that describe how a pulse of water moves downstream. Flow-routing models calculate flow as a function of space and time using equations based on flow continuity and momentum. Two examples of routing models are the LCRA’s Daily Operational Routing Model (DORM) (Carron et al. 2010) and the Upper Rio Grande Water Opera- tion Model (Boroughs 2013). These and other routing models can be used to estimate SW-GW interactions by performing water budget calculations that account for all losses and gains along a stream reach except for those associated with SW-GW interaction. Data used by DORM for its water budget calcula- tions include hourly data from gaged tributaries, return flows, releases from Lake Travis, releases from Lady Bird Lake, and known diversions. Working with LCRA to find two- to four- week periods of stable low-flow conditions with high quality data, Young et al. (2017) found that DORM simulations pro- vided credible estimates of SW-GW interaction for low-flow periods in 2012, 2013, 2014, and 2015. Based on DORM results that were generally consistent with previous estimates Figure 3. Comparison of measured water levels in the Colorado River and in the Colorado River Alluvium near the City of Wharton in 2007 (from URS and Baer Engineering 2007). Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas136 of SW-GW interactions (Saunders 2009, 2012), Young et al. (2017) recommends that DORM simulations be incorporat- ed into field studies aimed at measuring SW-GW interaction along the Colorado River. In 2001, Senate Bill 2 tasked the TWDB with developing GAMs of all major and minor aquifers in Texas. The TWDB defines groundwater availability modeling as “the process of developing and using computer programs to estimate future trends in the amount of water available in an aquifer and is based on hydrogeologic principles, actual aquifer measure- ments, and guidance from persons with interest in the mod- els and the program” (TWDB 2016b). The goal of the GAM program “is to provide useful and timely information for determining groundwater availability for the citizens of Tex- as” (TWDB 2016b). GAMs are constructed using the family of USGS MODFLOW codes that simulate groundwater flow (McDonald and Harbaugh 1988; Harbaugh and McDonald 1996; Harbaugh et al. 2000; Harbaugh 2005; Niswonger et al. 2005; Panday et al. 2013). GAMs, in their current capacity, simulate water move- ment based on the physics of water flow and can simulate the exchange of water between aquifers and streams. Among the factors that limit the ability of GAMs to accurately simulate SW-GW interactions is that they were developed to address water issues at a relatively large spatial scale and are not readily suitable to simulate SW-GW interactions at a local scale of a few miles and less (Scanlon 2005; HDR 2007; Kelley et al. 2008). Another issue that limits GAMs’ capability for accurate- ly simulating SW-GW interaction at the local scale is that they use time periods of months to years; whereas, accurate mod- eling of SW-GW interaction will usually require time periods of hours to days (Scanlon 2005). Besides having the limita- tions associated with spatial and temporal scales that are large compared to scales that drive SW-GW interactions, GAMs are also limited because GAMs cannot simulate unsaturated flow, which can be an important process for accurate modeling of SW-GW interaction. Recognizing that these are GAM limita- tions and not necessarily a limitation of MODFLOW, as pack- ages to include unsaturated flow processes exist, highlights one of the ways to enhance GAMs, or a modification of a GAM, to improve simulations of SW-GW interactions. Despite the inherent limitations with GAMs for simulating SW-GW interaction at the local scale of a few miles, GAMs will not necessarily provide reasonable estimates of SW-GW interaction even at the regional scale unless considerable care is taken with its development. Specifically, the model calibra- tion process for a GAM is particularly important because of the wide range of factors affecting SW-GW interactions. These factors include how recharge, evapotranspiration, streamflow, stream channel geometry, stream-bed hydraulic properties, and runoff are represented in the model. Additionally, another major issue affecting GAM simulation of SW-GW interaction, discussed by Mace et al. (2007), is the vertical resolution (i.e., the layer thicknesses) of the groundwater model: “One of the difficulties in accurately representing surface water-groundwater interaction is the vertical resolution in the groundwater availability model. The interaction of a stream and an aquifer is an intimate affair that occurs locally on the order of feet to tens of feet. In many cases, the current groundwater availability models are too coarse, both laterally and vertically, to accurately represent surface water-ground- water interaction. The difference between a gaining stream and a losing stream can be the difference of a few feet of groundwater level change, especially for the aquifers along the Gulf Coast where there is not much topography.” The importance of vertical resolution (inclusion of shallow model layers) at the regional scale is twofold. One reason is that the vertical resolution affects a GAM’s capability to represent a shallow groundwater flow zone. This shallow flow zone is the primary conduit in the real physical aquifer system for much of the recharge that enter the groundwater system to move rel- atively quickly to discharge locations in the aquifer’s outcrop, which includes seeps, springs, and surface water bodies. A sec- ond reason is that the vertical resolution prevents deep pump- ing wells that are nearly hydraulically isolated from water table near ground surface from being represented in the same model layer that is a river or a lake. One of the first applications of shallow model layers to rep- resent a shallow, local flow system in a regional groundwater model was the Lower Colorado River Basin (LCRB) model (Young et al. 2009). The LCRB model sought to improve the accuracy of both recharge and SW-GW interaction by includ- ing a shallow and relatively thin model layer near the water table to represent the shallow groundwater flow system. The incorporation of the shallow groundwater layer was made with considerations toward improving how the model represents the aquifers and alluvium. The geology representation was guided by using maps of surface geology including alluvium developed by Barnes (1974). Figure 4 shows that the county-scale LCRB model provides a significantly better match to historical estimates of groundwa- ter contributions to the Colorado River than the regional-scale Central Gulf Coast GAM (Chowdhury et al. 2004). With regard to the source for pumped groundwater for Matagorda, Wharton, and Colorado counties from 1980 to 2000, the Cen- tral Gulf Coast GAM predicts that 66% is leakage from streams whereas the LCRB model predicts 71% is from recharge from precipitation (Young et al. 2009). The large differences in the source for the pumped groundwater illustrate that at a regional scale, model layering can have a significant effect on simulated SW-GW interactions. Among the GAMs that include a thin model layer near the water table to represent shallow ground- Texas Water Journal, Volume 9, Number 1 137Surface water-groundwater interaction issues in Texas water flow system are a GAM for the Yegua-Jackson Aquifer (Deeds et al. 2010), a GAM for the Northern Trinity and Woodbine Aquifers (Kelley et al. 2014), and a GAM for the central portion of the Sparta, Queen City, and Carrizo-Wilcox aquifers (Young et al. 2018). Independent studies funded by the TWDB (HDR 2007) and the TCEQ (Scanlon et al. 2005) have investigated the ability of models to predict SW-GW interactions. Both studies emphasized that there is a critical need for field data that can be used to develop appropriate conceptual models and guide- lines for developing GAMs to help standardize and improve the approaches used to simulate SW-GW interactions. Before significant improvements in simulating SW-GW interactions with GAMs and other groundwater can occur, additional field studies need to be conducted. Scanlon et al. (2005) recom- mended that additional field studies be performed that include (1) co-locating groundwater monitoring wells with stream gag- es, (2) characterizing stream morphology and aquifer hydraulic properties, (3) collecting water-level and water quality data, (4) evaluating streamflow gains and losses and aquifer bank storage and bank flow, (5) conducting aquifer tests near streams, and (6) evaluating the time it takes water to travel between streams and wells. STREAM HYDROGRAPHS Besides using models that simulate the movement of surface water or groundwater, SW-GW interactions can be estimated by using hydrograph-separation methods. Stream hydrographs show changes in measured water levels (that is, stream height or stage) at river gages as a function of time. Hydrograph-sep- aration methods (sometimes called baseflow separation) aim to distinguish a streamflow hydrograph into two components: 1) Quickflow – flow in direct response to a rainfall event including overland flow (runoff) and direct rainfall onto the stream surface (direct precipitation). 2) Baseflow – the steady flow derived from groundwater discharge to the stream and lateral movement in the soil profile (interflow). Many hydrograph-separation methods have been developed to estimate the baseflow and runoff components of stream- flow, and these methods have been implemented in a number of computer programs that facilitate the estimation process (Pettyjohn and Henning 1979; Nathan and McMahon 1990; Wahl and Wahl 1995; Sloto and Crouse 1996; Rutledge 1998; Arnold and Allen 1999; Eckhardt 2005; Lim et al. 2005; Pig- gott et al. 2005). Although each of the methods is based on formalized algorithms for identifying the baseflow component Figure 4. Comparison of measured baseflow along the Colorado River (Field Data) with simulated baseflow values from the LCRB groundwater model and from the Central Gulf Coast Groundwater Availability Model (data from Young et al. 2009; LCRB = Lower Colorado River Basin, GAM = groundwater availability model). The Field Data includes gain-loss studies performed with river gage data reported by Slade (2002) and Saunders (2006). Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas138 of total streamflow, they can differ substantially in their under- lying assumptions and degree of freedom in their application. Because of the different underlying assumptions with the dif- ferent methods, there are advantages to using more than one hydrograph-separation method to analyze a streamflow record to assess uncertainty. Hydrograph-separation methods have been widely used to estimate SW-GW interaction and recharge across watersheds in Texas (Young and Kelley 2006; Deeds et al. 2010; Scanlon et al. 2012; Kelley et al. 2014; Ewing et al. 2016; TWDB 2016a; Young et al. 2018). Most of these Texas studies have used either the Base-Flow Index (BFI) Program (Institute of Hydrology 1980; Wahl and Wahl 1995) or the Baseflow Program devel- oped for use with the Texas A&M University’s Soil and Water Assessment Tool (Nathan and Mahon 1990; Arnold and Allen 1999). A potential concern with these two methods and other hydrograph-separation techniques is that they do not explicitly account for discharge that did not originate from the ground- water basin (Scanlon et al. 2005) and thus will likely overesti- mate baseflow. Scanlon et al. (2005) identify these sources as in-stream detention and subsequent discharge of surface water, alluvium aquifer recharge such as bank storage/release follow- ing flood events, perched groundwater zones, or fractured zone recharge/discharge in the near subsurface. In addition to the type of hydrograph-separation programs used by hydrogeologists to identify groundwater contribution to a stream, there are other types of hydrograph-separation programs used by surface water hydrologists to identify flow regimes. These type of hydrograph separations are performed to support the Texas Instream Flow Program (TIFP). The pur- pose the TIFP is to perform scientific and engineering studies to determine flow conditions necessary for supporting a sound ecological environment in the river basins of Texas (TCEQ, TPWD, TWDB 2008). To identify flow regimes, surface water hydrologist use either the Indicators of Hydrologic Alteration (IHA) program (Richter et al. 1996) or the Modified Base Flow Index with Threshold (MBFIT) (Brandes et al. 2011) to class a portion of a hydrograph into one of four flow regimes: subsistence flow, baseflow, high flow pulses, or overbank flows. Figure 5A shows the results of a stream hydrograph analysis performed using the BFI program. The BFI program is used to partition a streamflow into a runoff component comprising diffuse surface water flow and a baseflow component compris- ing groundwater flow into a stream. Figure 5B shows results from applying IHA to the same stream hydrograph in Figure 5A to identify baseflow regimes and high flow pulse regimes. The application of the BFI and the IHA programs illustrate the different type of results produced by each program. Because the two programs use very different sets of underlying assump- tions, there is not a common set of information on which the two disciplines can rely to develop a shared understanding and quantification of SW-GW interactions. Figure 5. Analysis of a stream gage hydrograph by a surface water hydrologist using the IHA software (A) and by a groundwater hydrologist using the BFI software (B) (IHA = Indicators of Hydrologic Alteration; BFI = baseflow index). Texas Water Journal, Volume 9, Number 1 139Surface water-groundwater interaction issues in Texas Halford and Mayer (2000) share some of the same con- cerns that Scanlon et al. (2005) state regarding the reliability of the hydrograph-separation techniques without some type of third-party dataset or analysis to ground truth the estimat- ed groundwater contribution calculated from the hydrograph separation. Halford and Mayer (2000) question the accuracy of the hydrograph-separation technique when the underlying assumptions of the technique have not been validated. Based on their analysis of 14 studies in nine states, Halford and May- er (2000) say that: • “The recession-curve displacement method and other hydrograph-separation techniques are poor tools for esti- mating groundwater discharge or recharge when major assumptions of the methods are violated.” • “The identification of groundwater discharge in stream discharge records can be ambiguous because drainage from bank storage, wetlands, surface water bodies, soils, and snowpacks also decreases exponentially during the recession period.” USGS (2017) noted that an important limitation of the BFI program, as well as other hydrograph-separation meth- ods, is that “In general, the method [BFI program] interprets most regulated releases as baseflow. If the program is used for regulated streams, the effects of regulation must be carefully accounted for thorough manual adjustment of the program output.” Even when underlying assumptions of the baseflow separation methods are met, the applications of the methods can still be problematic. This situation is illustrated by results from Partington et al. (2012) who analyzed numerically sim- ulated river hydrographs with automated baseflow separation techniques. Partington et al. (2012) found that the automated baseflow separation underestimates the simulated baseflow by as much as 28% or overestimates it by up to 74% during rain- fall events. They also concluded that no separation method was clearly superior to the others, as the performance of the various methods varies with different soil types, antecedent moisture conditions, and rainfall events. Some of the concerns documented by Halford and Mayer (2000) and Scanlon et al. (2005) are confirmed by Young et al. (2018) who estimated baseflow from 35 stream gages in Groundwater Management Area 12. For the 35 stream gages, the average recharge rate across the watershed estimated using the BFI method and the program developed for use with the Texas A&M’s Soil and Water Assessment Tool was 2.70 inches and 3.78 inches, respectively, which is about a 140% differ- ence. Such a large difference is evidence that additional work is needed to vet and ground truth the applications of base- flow-separation techniques to quantify SW-GW interaction. In our opinion, TWDB (2016a) further illustrates the importance to vet and ground truth the approaches used for interpreting stream hydrographs in Texas. This study, prepared in response to House Bill 1232 of the 84th Texas Legislature, estimated the volume of flows from aquifers to surface water in Texas. TWDB (2016a) used the results from several U.S. Geo- logical Survey (USGS) studies (Wolock 2003a, 2003b; Wolock et al. 2004) to spatially distribute groundwater contributions to surface water for the outcrop areas of the major and minor aquifers. Wolock (2003a) analyzed hydrographs from approxi- mately 19,000 stream gages across the United States using the BFI program (Wahl and Wahl 1995). One output of the BFI program is the BFI Index, which is the average percentage that groundwater contributes to streamflow. Figure 6 shows the BFI values from Wolock (2003a) for the Lower Colorado River downstream of Tom Miller Dam in Austin. These nine values indicate that average annual groundwater contributions range from 40% to 65% of the total surface water flow in the Col- orado River. Among other SW-GW studies performed in the region are stream low-flow gain-loss studies by Saunders (2009, 2012). Results from these studies can be used to generate BFI values. The analysis of Saunders’ data produces BFI values that are up to four times smaller than those presented by Wolock (2003a) at some of the gages shown in Figure 6. Comparisons of studies involving SW-GW interaction that provide different water budgets show that the variability is not only caused by using different types of data over varying time periods but also by using different assumptions for interpreting the data. Among the assumptions that could be important to an analysis are those related to flow diversions, flow returns, regulated flows upstream, seeps from perched groundwater tables, pumping in or near the alluvium, alluvial recharge, and bank storage/bank flow. The need for well documented and vetted approaches for interpreting stream hydrographs is cited in previous studies funded by TWDB (HDR 2007; Young et al. 2017) and TCEQ (Scanlon et al. 2005) as an important and necessary step toward improving the understanding and modeling of SW-GW interaction in Texas. WATER RESOURCE MANAGEMENT DECISIONS AFFECTED BY SW-GW INTERACTION The TWC recognizes that surface water and groundwater resources are hydrologically connected, at least locally, and requires that regulatory authorities consider this when issu- ing permits. TWC §36.113(d)(2) requires that GCDs, when evaluating groundwater permits, consider whether “…the pro- posed use of water unreasonably affects existing groundwater and surface water resources or existing permit holders…” Sim- ilarly, TWC §11.151 states “in considering an application for a permit to store, take, or divert surface water, the commission [TCEQ] shall consider the effects, if any, on groundwater or groundwater recharge.” Statute recognizes the potential inter- Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas140 connectivity between groundwater and surface water but (1) doesn’t specify what level of interaction would spark action on a permit, (2) doesn’t require any action by the regulating body, and (3) doesn’t coordinate the regulatory realms of TCEQ from the surface water perspective or GCDs from the ground- water perspective. Pumping near streams In Texas, there are thousands of shallow wells with depths less than 100 feet that are located near streams. Some of these wells, and those located in the river alluvium within a few hun- dred feet of the river, pump sufficient water to impact the flow between the stream and the aquifer. Historically, there have been relatively few cases where regulators curtailed pumping. The general lack of action by parties affected is likely the result of a combination of several factors including (1) the lack of clarity in the TWC with regard to how to characterize under- flow and how to assess pumping impacts, (2) the dearth of field measurements characterizing SW-GW interactions, (3) the absence of a demonstrated and standardized approach for analyzing stream hydrographs, (4) the reluctance of GCDs to require well owners to meter and report water use, and (5) the inaccuracies associated with many historical gain/loss studies on stream reaches and the inability of WAMs and GAMs to evaluate SW-GW interactions. The drought-induced periods of lower surface water avail- ability during the last decade have created conditions such that affected parties or stakeholders have requested regulatory assis- tance to protect state waters from adverse impacts caused by groundwater pumping. This has occurred on the Rio Grande, San Saba, Brazos, and Colorado rivers. Rio Grande in New Mexico In January 2013, Texas submitted a complaint to the Supreme Court of the United States (SCOTUS) alleging that New Mex- ico was in violation of the 1938 Rio Grande Compact. Specifi- cally, Texas alleged that New Mexico had violated the Compact by allowing the diversion of surface water through the pump- ing of groundwater that is hydrologically connected to the Rio Grande, thereby diminishing Texas’ ability to obtain the water the Compact apportioned to it. The New Mexico wells, which are estimated to number 3,000, pump as much as 270,000 acre-feet/year (TLO 2018). In addition, New Mexico has Figure 6. Baseflow index (BFI) from Wolock (2003a) for stream gages on the lower reach of the Colorado River. The BFI figures are percentages of groundwater contribution to streamflow. Texas Water Journal, Volume 9, Number 1 141Surface water-groundwater interaction issues in Texas permitted wells that will facilitate additional water use in the future. In January 2017, New Mexico requested that SCOTUS dismiss the complaint from Texas and dismiss a request from the United States to intervene as a party to the litigation. The Special Master appointed by the Supreme Court on this case ruled against New Mexico’s motion to dismiss Texas’ complaint and to hear oral arguments for the United States complaint. In early 2018, SCOTUS heard arguments by the United States to intervene as a party and to essentially make the same claims as Texas. In March 2018, the SCOTUS ruled that the Unit- ed States can be a party to the litigation. Litigation will likely proceed well into 2019 to discovery, motions, and eventually a hearing of the merits before the Special Master. The Special Master will then make recommendations to SCOTUS on the merits of the case (SCOTUS 2013). San Saba River Since 2011, the TCEQ has received complaints alleging shal- low groundwater wells are being used to pump surface water in the form of underflow from the San Saba River. The area iden- tified is a 40-mile reach between Menard and Brady (House Committee on Natural Resources 2018; Sadasivam 2017; 2018), where numerous wells within one mile of the river are completed in the alluvial deposits, which are believed to be a lateral extension of the river. Before 2000, the San Saba River was never known to cease flowing—not even during the record drought of the 1950s. From July to October in six of the past 15 years, and for every summer from 2011 to 2015 (House Committee on Natural Resources 2018; Sadasivam 2017; 2018), the river has gone dry along the 40-mile reach. In 2015, TCEQ Investigation Report Number 1254241 (TCEQ 2015) presented findings from its hydrogeological investigation and determined that some of the groundwater wells were illegally capturing state waters and that, for future pumping to contin- ue, the well owners needed to obtain the appropriate surface water rights. In May 2018, the Texas House Natural Resources Committee conducted a public hearing in Brady, Texas that included both local and statewide perspectives on issues relat- ed to SW-GW interactions. During the hearing, arguments were heard from upstream users that natural climate changes and decreased springflows are reasons for the low surface water flows whereas the downstream users claim that wells drilled close to the rivers are pumping the San Saba dry. Among the factors that could affect future actions is the threat of federal regulation. The San Saba is home to five species of mussels that the U.S. Fish and Wildlife Service (USFWS) is considering list- ing as endangered. If any one of those mussel species is found to be endangered, it could mean restrictions on water use from the San Saba. Brazos River In 2009, surface water rights holders in the Brazos River Basin were subject to the first of several calls from the Dow Chemical Company to exercise its senior priority water right. These water calls sparked a series of water diversion curtailments and associated actions that led the TCEQ to, in response to a petition from affected water right holders, establish a water- master program to regulate diversion from the Brazos River starting in 2015. Curtailments have heightened awareness that groundwater pumping in the Brazos River Alluvium could be affecting surface water availability. Within Robertson, Brazos, and Burleson counties, the GCDs have issued permits totaling more than 130,000 acre-feet/year, and the TWDB has reported pumping greater than 100,000 acre-feet/year for several years in the Brazos River Alluvium. Recently, the TWDB (Wade et al. 2017) used the Brazos River Alluvium GAM (Ewing and Jigmond 2016) to establish 210,536 acre-feet/year as the min- imum modeled available groundwater (MAG) for Groundwa- ter Management Area (GMA) 12 between 2013 and 2070. The concern that groundwater pumping could affect surface water availability can be investigated by evaluating the water bud- get for the TWDB GAM simulations (Wade et al. 2017) and additional GAM simulations that involved no pumping. The joint analysis of these GAM simulations indicate that nearly all of the groundwater pumped from the Brazos River Alluvium wells originates from the Brazos River. Colorado River During the first joint planning cycle, Environmental Stew- ardship (ES) petitioned GMA 12 (ES 2011) to argue that the desired future conditions (DFCs) did not adequately consid- er SW-GW relationships and did not include protection for the Colorado River, Brazos River, and associated streams and springs. During the second joint planning cycle, ES (2016) presented results from GAM and WAM simulations to argue that future groundwater pumping would lead to declines in Colorado River flow to impact over 1,100 water rights. ES (2016) stated: “Critical environmental flow standards for the Colorado and Brazos rivers are threatened by groundwater pumping and must be considered and mitigated in establishing DFCs for aquifers that impact the Colorado and Brazos rivers and their tributaries.” “There are logical arguments and credible evidence that the groundwater pumping in the proposed DFCs will have an adverse impact on surface water permits making it proper that the impact on surface water rights be considered under Section 36.108(c)(7).” Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas142 In finding that the GMA 12’s DFCs were reasonable and GMA 12 did not need to account for SW-GW interactions, the TWDB (2012) stated the following: 1. “Senate Bill 3 does not place the responsibilities dis- cussed by Environmental Stewardship on the Districts. Before granting or denying a permit, a district must consider, among other things, whether ‘the proposed use of water unreasonably affects existing groundwater and surface water resources or existing permit holders.’ But that requirement is part of the permitting process; there is no explicit requirement in the statutes under which this petition was brought for the Districts to consider impacts on spring flow and other interactions between groundwater and surface water.” 2. “A number of factors affect instream flow and outflows from the Colorado and Brazos rivers and technical work remains to be done to better monitor, analyzed, and manage that interaction.”...“But, the issue at hand is whether the DFCs are reasonable as expressions of the desired future conditions of the aquifers.” An overarching concern expressed by ES (2011) is that GMA 12 did not use the science and technology necessary and appro- priate to simulate SW-GW impacts and evaluate groundwa- ter pumping impacts on streamflows. During the second joint planning session, ES (2016) maintained that the DFCs are not protective of the environment and recognized that the current- ly adopted DFCs are the current legal standard and, as such, should not be significantly changed until the GAM has been improved and better data are available to assess SW-GW inter- actions. To help correct this situation, ES, the LCRA, the Bra- zos River Authority, and the GCDs in GMA 12 have worked with the TWDB to update the GAM for the central portion of the Queen City, Sparta, and Carrizo-Wilcox aquifers (Young et al. 2018), which includes improved SW-GW interactions. The GAM update included several modifications to better represent a shallow groundwater flow system. One of these modifications was to explicitly represent the Colorado River Alluvium and the Brazos River Alluvium as independent hydro- stratigraphic entities with thicknesses and hydraulic proper- ties based on hydrogeological studies and with pumping rates based on wells screened across the alluviums. Another modifi- cation was to represent aquifers using two model layers instead of a single model layer where they outcrop and receive recharge from rainfall. In addition, the GAM grid spacing in the vicinity of the Colorado River and Brazos River was changed from 1 mile by 1 mile to as small as 0.25 mile by 0.25 mile in order to more accurately represent well locations and the location and bathymetries of the Colorado and Brazos rivers. Rio Grande at El Paso While pumping near El Paso has not recently been a concern for regulatory agencies with regard to SW-GW interactions, it has been historically and may likely be in the future. In the first half of the 1900s, estimated pumping from deep wells in the El Paso area increased from about 2,200 acre-feet/year in 1910 to about 31,000 acre-feet/year in 1953 (Knowles and Kennedy 1956). This caused a reversal of flow between the Rio Grande Alluvium and the deeper aquifers. Hutchison (2006) noted that in the El Paso area, groundwater flow was general- ly toward the alluvium until about 1940, then away from the alluvium after 1960. Hutchison (2006), using the groundwa- ter model developed by Heywood and Yager (2003), showed groundwater pumping in the El Paso area caused a switch from an overall flow of groundwater to surface water of about 3,000 acre-feet/year to 5,000 acre-feet/year before 1925 to an overall flow of surface water to groundwater after 1925. Over the last 20 years, the net losses from the Rio Grande have stabilized at about 33,000 acre-feet/year (Mace et al. 2007). With regard to the reported SW-GW interaction for the Rio Grande at El Paso, it is important to recognize that these fluxes contain biases introduced by the uncertainties associated with using regional models. Bed and bank permits, environmental flows, endangered species and desired future conditions Besides surface water rights, other regulatory issues that could be affected by SW-GW interactions are environmental flows, habitat for endangered species, bed and bank permits, and desired future conditions. Environmental flows Senate Bill 2 passed into law by the 77th Texas Legislature in 2001 established the TIFP. TIFP is jointly administered by the Texas Parks and Wildlife Department (TPWD), the TCEQ, and the TWDB in collaboration as appropriate with other entities. The goal of the TIFP is to identify flow regimes (quantity and timing of flow) that are adequate to maintain an ecologically sound environment, conserving fish and wild- life resources while also providing sustained benefits for other human uses of water resources. One of the objectives of the instream flow program is to mimic the natural flow regime as closely as possible. Streamflow requirements (standards) for particular locations in specific stream systems are defined in terms of flow regimes. TWC §11.002.16 defines an environmental flow regime as “quantities that reflect seasonal and yearly fluctuations that typically would vary geographically, by specific location in a Texas Water Journal, Volume 9, Number 1 143Surface water-groundwater interaction issues in Texas watershed, and that are shown to be adequate to support a sound ecological environment and to maintain the productivi- ty, extent, and persistence of key aquatic habitats in and along the affected water bodies.” The development of an instream flow regime includes four components: subsistence flows, base- flows, within-bank high flow pulses, and overbank high pulse flows. For some streams, SW-GW interactions can become an important process that impacts the quantity and quality of streamflow during subsistence flows or baseflows. Subsistence flows occur during drought or very dry conditions. The prima- ry objective of subsistence flow standards is to maintain tolera- ble water quality conditions to provide minimal aquatic habitat space for the survival of aquatic organisms. Baseflows represent the range of average or normal flow conditions without the effects of recent rainfall. A primary objective of baseflow stan- dards is to provide adequate habitat for the support of diverse, native aquatic communities and maintain groundwater levels to support riparian vegetation. Endangered Species Act The ESA took effect in 1973. Its purpose is to conserve and recover listed endangered species and the ecosystems upon which they depend. SW-GW interactions are potentially important to the ESA in the execution of ESA’s Section 9— the taking provision. This section makes it a felony to “take” a threatened or endangered species without specific autho- rization from the USFWS. The ESA provides for both civil and criminal prosecution for illegal “takes.” The U.S. Supreme Court has expanded a “take” to include activities that disrupt the habitat of the threatened or endangered species or interfere with usual feeding and breeding activity. Species in Texas that have protection under the ESA are listed in the Texas Parks and Wildlife Code 68.002 and the Texas Administrative Code. The aquatic animals under ESA protection includes birds, fish, and amphibians. The Texas hornshell mussel is under ESA protec- tion in Texas (Federal Register 2018) and 11 other freshwater mussel species are currently under review by the USFWS for ESA listing (Ingram 2017). As a result of legal threats of a federal takeover of the Edwards Aquifer under the ESA, the Texas Legislature adopted the Edwards Aquifer Authority (EAA) Act in 1993 (Votteler 1998). The EAA was created to preserve the Edwards Aqui- fer while protecting threatened and endangered species in the aquifer-fed Comal and San Marcos springs. The creation of the EAA clearly demonstrates that SW-GW interactions can be important to maintaining habitat for endangered species. The ESA was also a key component of lawsuits involving the deaths of an unknown number of whooping cranes in Aransas Bay during the drought of 2008 and 2009 (USCA 2014; Vot- teler 2017). Plaintiffs argued that the deaths were indirectly a result of insufficient freshwater flows into Aransas Bay caused by diversion of water, authorized under water rights issued by the TCEQ, from the San Antonio and Guadalupe river basins. An initial court ruling by a Corpus Christi district judge stat- ed that the ESA had been violated by TCEQ’s administration of water rights, but a later ruling by the 5th Circuit Court of Appeal in 2014 stated that the TCEQ did not violate the ESA based on the narrow issue of proximate cause. Proximate cause is a legal concept providing that a person should only be held liable for that sequence if the outcome would have been rea- sonably foreseeable. Despite the 5th Circuit ruling exonerating the TCEQ of violating the ESA, the ruling confirms that ESA considerations need to be properly evaluated as part of water resource planning. Bed and banks permits TWC §11.042 and TCEQ Rule §295 allow the bank and bed of any flowing natural stream in Texas to convey water from the place of storage or discharge to the place of use or diversion. This can include wastewater discharges that are derived from a groundwater source where ownership may be maintained. A bed and bank permit requires the applicant to indicate the source, amount, and rates of discharge and diversion (TCEQ 2017). This information is necessary for the agency to calcu- late conveyance losses that may result from the bed and banks transfer. Per TCEQ §295.114(b)(6) conveyance losses include the loss to transportation, evaporation, seepage, channel, or other associated carriage losses from the point of discharge to the point of diversion. SW-GW interactions are important to conveyance losses where streams lose flow to the adjacent aqui- fer. Such losses would occur where the stream stage is at a high- er elevation than the water table and the amount of conveyance losses would depend on the geometry of the stream channel, the hydraulic gradient away from the stream, the hydraulic properties of the streambed, and the hydraulic properties of the aquifer. Desired future conditions House Bill 1763 of the 79th Texas Legislature requires joint planning among GCDs in a GMA to establish DFCs every five years. TWDB rules define DFCs as “[t]he desired, quanti- fied condition of groundwater resources (such as water levels, water quality, spring flows, or volumes) at a specified time or times in the future or in perpetuity…” TWC §36.1008 (2) (d)(4) requires that, as part of the process for setting DFCs, GMAs consider “environmental impacts, including impacts on spring flow and other interactions between groundwater Texas Water Journal, Volume 9, Number 1 Surface water-groundwater interaction issues in Texas144 and surface water” among other factors. GMAs have different interpretations of what “consider” means, which have general- ly been informed by overall management goals. For example, the GCDs in GMA 9 have developed a DFC in the Edwards Group of the Edwards-Trinity (Plateau) Aquifer that “provides maximum, reasonable and achievable protection for springs and baseflow to creeks and rivers (GMA 9 et al. 2016). Other GMAs have chosen DFCs that do not maintain baseflow and springflow. In GMA 12, GCDs, river authorities, and the Colorado-La- vaca Basin and Bay Area Stakeholder Committee (BBASC) co-funded work on the Central Sparta-Queen City-Car- rizo-Wilcox GAM to improve the capability of the GAM to simulate SW-GW interactions. The improved capability is pri- marily achieved by creating a shallow groundwater flow zone in the aquifer outcrops, through the addition of model lay- ers, which interacts with streams independently of the deeper groundwater flow zone. To help address their concerns with improving the management of the shallow groundwater flow system, the GCDs in GMA 13 have adopted a DFC that lim- its drawdown in the outcrop of the Carrizo-Wilcox Aquifer (Hutchison 2017). DEVELOPING BETTER SCIENCE A number of activities could be accomplished to improve the science—and thus the regulatory tools—for quantifying SW-GW interactions. Conduct field studies Lack of field data is perhaps the greatest obstacle to improv- ing the capability of GAMs to simulate SW-GW interactions, as data are required to develop and validate approaches for modeling this interaction. Field studies are lacking because they are relatively expensive and no state programs currently mandate these studies. As part of a TCEQ study concerning SW-GW interactions, Scanlon et al. (2005) recommended that future studies include (1) co-locating groundwater monitoring wells with stream gages, (2) evaluating streamflow gains and losses, (3) evaluating stream channel morphology, (4) conduct- ing aquifer tests near streams, and (5) evaluating the time it takes water to travel between streams and wells. An important aspect of any field study is that it collects the necessary information to support the development and testing of models that can be used by state agencies, river authorities, private or public utilities, and hydrogeological consultants to simulate SW-GW interactions. Specifically, field studies should be evaluated in light of anticipated statutory issues that could be before the Texas Legislature in future sessions. Such stud- ies should include the measurements of water levels and water quality parameters, the evaluation of stream hydrographs, the quantification of bank storage and bank flow, and the model- ing of SW-GW interactions. Vet approaches for calculating baseflow using hydrograph separation Because of the wide range of conditions that exists along riv- ers and the relatively simple algorithms used by most hydro- graph-separation techniques to estimate baseflow, there is con- siderable opportunity in the analysis for introduction of error into the estimate for baseflow. As such, when estimates of base- flow are important to understanding SW-GW interactions, the baseflow estimate should be properly vetted and uncertainties should be identified and quantified. The vetting process should include a thorough discussion and analysis of factors that could affect the application such as return flows, diversions, dam flows, groundwater pumping, and bank storage. This discus- sion should quantify, to the extent possible, the potential for each of these factors to impact the stream hydrograph and to introduce uncertainty into the calculated baseflow. The analysis should include multiple and even alternative methods for esti- mating baseflow in order to help account for the uncertainty associated with any one technique and the sensitivity of the calculated baseflow to the actual mechanics used to implement a particular technique. Update and improve groundwater availability models GAMs were originally designed to address large region- al-scale groundwater issues and provide information to region- al water planning groups and for GCD management plans. Since the start of joint planning, there has been increased inter- est on the part of GCDs and other stakeholders to use GAMs to address groundwater management issues at the local scale. Among the reasons for the expanded interest are that GAMs are generally considered to represent the best available science, and the prolonged periods of low surface water availability in 2009 and 2011 created additional interest in using groundwa- ter as a water supply. The application of GAMs to evaluate the impacts of specific well fields usually requires discretization and additional field data to better represent site conditions. Such modifications increase the costs for developing a GAM and can complicate its use in regional planning. Given that GAMs are increasingly being used for much more than what the original TWDB GAM program intended, we make two recommendations to improve the GAMs. The first is to evaluate whether the mission of the GAM program should be modified to better address issues associated with SW-GW interactions. The second is to develop more standardization among the GAMs, where appropriate, for representing inter- actions that occur in aquifer outcrops such as recharge, evapo- transpiration, and SW-GW interactions. Along with this stan- Texas Water Journal, Volume 9, Number 1 145Surface water-groundwater interaction issues in Texas dardization comes the case-by-case analysis of which analytical and numerical methods best represent SW-GW interaction and whether these representations can be accurately included in appropriately-scaled GAMs. The better science derived from WAMs and GAMs as well as increased capabilities may result in less contested issues relative to water permitting activities. Develop science to better define baseflow, bank flow, underflow Among the key needs for improving the regulation of SW-GW interactions are the science and data necessary to define the terms used to characterize SW-GW interactions. These terms include baseflow, bank flow, and underflow. There are two significant technical problems associated with defining these terms. The first problem is that these three terms define quantifies that are transient and spatially variable. The second problem is the lack of science to demonstrate how to appro- priately accommodate temporal and spatial variability into the measurement of each term. Because of these two technical problems, regulatory agencies called upon to mitigate disputes involving SW-GW interactions may not have, or in most cases do not have, sufficient information to make appropriate regu- latory distinctions and determinations. With respect to developing a science program to better char- acterize SW-GW interactions, there are two important consid- erations. One consideration is that the environmental condi- tions, which include geology, hydrogeology, and meteorology, have a significant impact of SW-GW interactions. As a result, there is no need to study every stream because streams with similar environmental conditions should have similar type of SW-GW interactions. A second consideration is that because SW-GW interactions are not equally important across Texas, a science program should prioritize the critical areas for study based in part on their environmental conditions. CONCLUSIONS SW-GW interactions can be important for managing water rights along a river, complying with the ESA, implementing environmental flow recommendations, and obtaining bed and banks permits. A key issue to these regulatory and man- agement concerns is how to quantify the exchange of water between streams and aquifers and to what extent does ground- water pumping impact this exchange and the availability of surface water. Currently, Texas does not possess a sufficient understanding of SW-GW interactions to readily address these concerns at the granularity necessary to facilitate permitting determinations. The uncertainties associated with quantifying SW-GW interactions have contributed to disputes regarding actual own- ership and rights to water. Locations where these disputes have recently occurred or are occurring include the Rio Grande, the San Saba River, the Colorado River, and the Brazos River. To help effectively integrate, regulate, and manage surface water and groundwater resources in Texas, recommendations include conducting field studies focused on quantifying SW-GW inter- actions, performing additional vetting and ground truthing on hydrograph-separation techniques, improving the capability of GAMs to simulate SW-GW interactions, and developing the science and tools necessary to define and quantify underflow, bank flow, and baseflow. Communication and cooperation among river authorities, GCDs, the TCEQ and TWDB must also be improved. Such cooperative efforts recently occurred while updating the GMA 12 Carrizo-Wilcox GAM, for which appreciable funding was contributed by the LCRA and Brazos River Authority and by the Post Oak Savannah GCD and Brazos Valley GCD to spe- cifically address SW-GW interactions in the GAM. 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