Drought has long posed an existential threat to society. Engineering and technological advancements have enabled the development of complex, interconnected water supply systems that buffer societies from the impacts of drought, enabling growth and prosperity. However, increasing water demand from population growth and economic development, combined with more extreme and prolonged droughts due to climate change, poses significant challenges for governments in the 21st century. Improved understanding of the cascading multisectoral impacts and adaptive responses resulting from extreme drought can aid in adaptive planning and highlight key processes in modeling drought impacts. The record drought spanning 2008 to 2015 in the Colorado Basin in the state of Texas, United States, serves as an outstanding illustration to assess multisectoral impacts and responses to severe, multi-year drought. The basin faces similar water security challenges to those across the western US, such as groundwater depletion and sustainability, resource competition between agriculture and growing urban populations, limited options for additional reservoir expansion, and the heightened risk of more severe and frequent droughts due to climate change. By analyzing rich, high-quality data sourced from nine different local, state, and federal sources, we demonstrate that characterizing regional multisector dynamics is crucial to predicting and understanding future vulnerability and possible approaches to reduce impacts to human and natural systems in the face of extreme drought conditions. This review reveals that, despite the severe hydrometeorological conditions of the drought, the region's advanced economy and existing water infrastructure effectively mitigated economic and societal impacts.

The karstic Trinity Aquifers of central Texas provide baseflow to streams and are used extensively as water supplies for domestic, municipal, agricultural, and industrial purposes. Rapid increases in population in the area are placing significant demands on the aquifers in an area that has very limited surface water supplies and is prone to significant droughts. Droughts occur frequently in this area and current levels of pumping in the Trinity Aquifer have resulted in both the capture of springflow in karstic areas, resulting in a major spring ceasing flow during drought and groundwater mining in other less karstic areas of the Trinity. The most significant historical drought in central Texas occurred in the 1950s and lasted for up to 10 years. Tree-ring data show that even more significant droughts have occurred over the past thousand years. Predictions for changes to precipitation due to climate change are for more extensive flooding and more severe drought. Long-term trends in water levels are downward with only limited recovery during very wet periods. An understanding of how these aquifer systems function is key to proper aquifer management which is likely to involve limits to pumping during average conditions and more severe reductions during periods of drought and incorporating alternative water supplies such as rainwater harvesting, aquifer storage and recovery, and perhaps importation of water from distant sources.

could address the origin of groundwater, and intercon-nections between different hydrostratigraphic units. The differences in TDS, SO 42– , and NO 3– concentrations between Edwards and Trinity Aquifers are considered statistically significant. The isotopic tracers (δ 18 O and δD; 3 H and 14 C; 87 Sr/ 86 Sr) provide information on the origin of recharge, residence time of groundwater, and mineral-solution reactions in soils/aquifer rocks in these formations. Hydraulic head data further support limited vertical communication between the aquifers. This study indicates that vertical inter-aquifer flow between the Ed - wards and Trinity Aquifers is limited in the study area. These implications support independent groundwater management of these two essential aquifer systems in Central Texas.

Groundwater availability from the Edwards and Middle Trinity Aquifers in the study area is generally limited. However, the Lower Trinity Aquifer within the study area is untested because of the aquifer depths (drilling costs), assumed low yields and poor water quality. This study provides an initial assessment of groundwater availability in a Lower Trinity well field based on an aquifer test, geochemistry, and analytical modeling. The well field contains three wells that range in total depths from 1,505 to 1,620 ft below ground surface. Depths to static water levels in the wells range from 140 to 150 ft below ground surface. Pumping of two wells occurred at 170 gpm for 96 hrs and resulted in maximum drawdowns of 320 to 460 ft in the pumping wells. To estimate aquifer parameters, continuous water-level data were collected with transducers and fit to Theis and Cooper-Jacob analytical solutions using Aqtesolv software. Results of transmissivity from observation wells ranged from 266 to 394 ft2/day and storativity of 2.34E-05 to 6.75E-05. These parameters are similar to published values of Lower Trinity wells in the Hill Country to the west of the study area. Groundwater specific conductance was monitored throughout the test and corroborated groundwater samples analyzed for total dissolved solids with values of about 560 mg/L. Preliminary ion geochemistry indicates Ca-HCO3 water with tritium (-0.05 TU) and carbon-14 (0.006 PMC) isotopes, indicating very old water. Combined use of the Cooper-Jacob solution, distance-drawdown equation, and parameters from the aquifer test forecasted drawdown from the well field with pumping at 300 gpm (158 MGY) over a 30-year period. Preliminary results indicate an estimated drawdown up to 100 ft at 2 miles distance from the well field. However, given the deeply confined and compartmentalized system, management will need to consider an acceptable long-term managed depletion of the aquifer. The nearby Lower Trinity Aquifer of Western Travis County offers a cautionary example of groundwater mining. This study suggests that the water quality and yield of the Lower Trinity Aquifer may be an alternative groundwater supply for the study areas. More groundwater studies and modeling are needed to evaluate long-term drawdown effects for the study area. In addition, the aquifer may have potential for aquifer storage and recovery activities.

The Barton Springs/Edwards Aquifer Conservation District (BSEACD), in cooperation with Travis County, compiled existing and new hydrogeologic data to evaluate groundwater resources in southwestern Travis County (SWTC). Analysis and interpretation of these data provide the foundation for hydrogeologic evaluations presented in the Hydrogeologic Atlas of Southwest Travis County, Central Texas (Hunt et al., 2020), which refines the area’s hydrogeologic framework and conceptual model, establishes current aquifer conditions, and estimates groundwater use in SWTC. Authors of the Atlas created a geodatabase to provide a single repository for the source data of the study. The geodatabase may help provide baseline data for future groundwater studies of the region. An accompanying report (Cockrell et al., 2020) describes the development of the geodatabase, documents data sources, and describes the data analyses performed.

Hays County is experiencing some of the highest growth rates in the country, and groundwater is the primary option for water in the county. One spring in the study area, Jacob’s Well, experiences periodic cessation of flow due to a combination of drought and pumping from nearby water supply wells. Jacob’s Well is important for ecological and water resources of the region. A springshed delineation study was conducted at the request of the local groundwater conservation district to protect flow at Jacob’s Well from excessive pumping. Much of the flow to Jacob’s Well emanates from a large phreatic cave passage with over a mile of surveyed passages in the Cow Creek Limestone of the Middle Trinity Aquifer. The aquifer has varying degrees of karst development with both conduit, fracture, and diffuse flow components. Many methods have been derived to set protection areas for water-supply wells, springs, and aquifers in karst settings. In this study we relied upon geologic structure maps, potentiometric maps, hydrographs, aquifer tests, dye traces, and a variety of water-balance analyses. Ultimately, the data indicated that the Jacob’s Well springshed boundaries were best explained by the watershed boundaries for Cypress Creek, upgradient of the spring. This information was used to delineate a groundwater management zone recommended to the local groundwater conservation district and a local stakeholder group. The next step is to develop groundwater management rules and other strategies for the protection of springflow. This springshed delineation could also be used to protect the aquifer and Jacob’s Well from sources of contamination.

ABSTRACT Multiport monitor wells have been used by the Barton Springs/Edwards Aquifer Conservation District (BSEACD) to study complex, multilayer, and stacked aquifers in central Texas. Much of the data from water wells that are used for hydrogeological studies are of limited use owing to the thickness of the aquifers, vertical variation in hydraulic properties, and the often-uncertain completion of the wells. To address these concerns, hydrogeologists and engineers have employed various methods, such as installation of nested wells, multilevel completions in a single borehole, and multiport wells. The BSEACD has used multiport wells to determine vertical variations in an aquifer and the hydraulic relationships between stacked aquifers. With multiport wells, properties such as hydraulic head, temperature, hydraulic conductivity, and water quality of discrete units within an aquifer can be determined. The use of multiport wells has shown how portions of the Upper Trinity lithologic units are hydraulically connected to the overlying Edwards lithologic units, and how the Edwards Aquifer is hydraulically isolated from the Middle and Lower Trinity Aquifers.

ABSTRACT The Edwards aquifers are typically faulted, karstified, and transmissive. Water quality is generally excellent; the hydrochemical facies is mostly a calcium bicarbonate water with total dissolved solids (TDS) <500–1000 mg/L. Exceptions to this result from both natural and anthropogenic factors. In the Edwards Plateau, mixing of the formation water with underlying water from the Trinity aquifers or Permian rocks increases salinity to the west. Along the Balcones fault zone, the southern and eastern borders of the Edwards (Balcones Fault Zone) Aquifer are demarcated by a bad-water line where salinity rises to over 1000 mg/L. Detailed studies show that this line is a band, because salinities in the aquifer are not uniform with depth. The bad-water (or saline-water) zone is relatively stable over time, and six hydrochemical facies were identified, which are created by different combinations of dissolution of evaporite and other minerals, mixing with basinal brines, dedolomitization, and cross-formational flow from underlying formations. Flow in this zone is restricted, the waters are reducing, and recent studies suggest that microbes play important chemical and physical roles. The bad-water zone has sufficient water in storage and sufficient permeability so that desalination could be a future water-source option.

The Barton Springs segment of the Edwards (Balcones Fault Zone) Aquifer is a prolific karst aquifer system containing the fourth largest spring in Texas, Barton Springs. The Barton Springs segment of the Edwards Aquifer supplies drinking water for similar to 60,000 people, provides habitat for federally listed endangered salamanders, and sustains the iconic recreational Barton Springs pool. The aquifer is composed of Lower Cretaceous carbonate strata with porosity and permeability controlled by depositional facies, diagenesis, structure, and karstification creating a triple permeability system (matrix, fractures, and conduits). Groundwater flow is rapid within an integrated network of conduits discharging at the springs. Upgradient watersheds provide runoff to the recharge zone, and the majority of recharge occurs in the streams crossing the recharge zone. The remainder is direct recharge from precipitation and other minor sources (inflows from Trinity Group aquifers, the San Antonio segment, the bad-water zone, and anthropogenic sources). The long-term estimated mean water budget is 68 ft(3)/s (1.93 m(3)/s). The Barton Springs/Edwards Aquifer Conservation District developed rules to preserve groundwater supplies and maximize spring flow rates by preserving at least 6.5 ft(3)/s (0.18 m(3)/s) of spring flow during extreme drought. A paradox of the Barton Springs segment of the Edwards Aquifer is that rapid recharge allows the Barton Springs segment of the aquifer to be sustainable long term, but the aquifer is vulnerable and limited in droughts. The karstic nature of the aquifer makes the Barton Springs segment vulnerable to a variety of natural and anthropogenic contaminants. Future challenges will include maintaining the sustainability of the aquifer, considering climate change, population growth, and related land-use changes.

Increased demand for groundwater in central Hays County is prompting studies to evaluate the availability of groundwater in the Trinity Aquifers of central Texas. These aquifers, consisting mostly of limestone, dolomite, and marl, exhibit varying degrees of karstification. Near the surface, karst features such as caves and sinkholes are evident, but are widely scattered. Even at depths greater than 400 m (1,300 ft), units that are mostly limestone show some degree of karstification where dissolution along fractures has caused development of conduits. Studies are being conducted to better understand the horizontal and vertical flow components of the Trinity Aquifers. These studies involve aquifer testing, groundwater geochemistry, geologic and structural mapping, flow-loss/gain measurements in streams, hydraulic head measurements, dye tracing, and installation of multiport monitor wells. The Middle Trinity Aquifer meets the definition of a karst aquifer due to its conduit permeability within soluble rocks. However, the same aquifer has contrasting properties that are separated by the complex Tom Creek Fault Zone. The westerly Hill Country Middle Trinity Aquifer is a shallow karst aquifer system characterized by rapid conduit flow and active surface and groundwater interactions. In this area, Middle Trinity units are situated at or near the surface. To the east, the Balcones Fault Zone Middle Trinity Aquifer is a deeply confined karst aquifer system with more limited conduit development, slower groundwater flow, and no direct surface-groundwater interactions. In this area, Middle Trinity units are encountered at depths of 150 m (500 ft) or greater. The results of this study will influence future hydrogeologic and resource evaluations and modeling of the Middle Trinity Aquifer.

Jacob’s Well (JWS) is a karst spring issuing from the Middle Trinity Aquifer in central Texas, sustaining habitat and recreational resources along Cypress Creek and the Wimberley Valley area. Historically JWS was perennial and contributed up to 25% of the baseflows to the Blanco River, even during the 1950s drought of record. The Blanco River provides allogenic recharge to the Edwards Aquifer downstream. Over the past 15 years, increased pumping from the Trinity Aquifers has resulted in capture of JWS springflow and ultimately cessation of flow during drought. This study characterizes the hydrogeology of JWS and helps constrain the sustainable yield of the JWS springshed, defined here as maintaining measurable flow during severe drought conditions. The JWS springshed is estimated by integrating hydrologic data with methods published by Lanini et al., 2016 and Bonacci and Andric, 2015. Estimated average annual effective recharge is about 30% of rainfall. During the drought of record, JWS flow was measured at 2.6 cubic feet per second (cfs) in March 1955 and estimated as low as 0.2 cfs in August 1955. Monthly average springflow since 2005 is 8.8 cfs. Four large-scale production wells in the JWS springshed pump a monthly average of 0.3 cfs. Numerous residential wells (n=650) pump an estimated 0.2 cfs. Thus, the minimum springflow during severe drought conditions of 0.2 cfs is exceeded by total monthly average pumping of 0.5 cfs. The result is cessation of springflow during recent droughts that are less severe than the 1950s drought of record. Reducing pumping by up to 90% within the springshed during drought could result in continuous springflow and a sustainable yield. Although this much reduction in pumping would be unrealistic, approaches to achieving reductions could include a special management zone that can focus on demand reduction through conservation, education, additional drought curtailments, and other regulatory rules. Alternative supplies could include rainwater, the Lower Trinity Aquifer, aquifer storage and recovery, and temporary interconnections to other water sources outside the springshed. The sustainable yield of JWS Springshed is a worthy long-term goal, but may be difficult to achieve; thus preservation of flow may need to focus on increasing the percentiles of flow during all conditions for JWS.

The Edwards Aquifer is a prolific karst aquifer system in Central Texas that provides drinking water to about 2 million people. Because a significant portion of the water recharging the Barton Springs segment of the Edwards Aquifer enters the subsurface through caves and enlarged fractures in the bed of Onion Creek, the presence of nonpoint source pollution in storm water flowing in Onion Creek can have a direct impact on water quality in the Barton Springs segment of the Edwards Aquifer. To address this concern, the Barton Springs/Edwards Aquifer Conservation District constructed a concrete vault over the entrance to Antioch Cave in the bed of Onion Creek. This structure was designed to prevent entry into the cave of contaminated storm water by closure of two valves on the vault during storm events. When the storm water passes, the valves open and allow the cleaner baseflow water to enter the cave. Results of water-quality sampling at Antioch indicate that the system is capable of significant reduction of nonpoint source pollution entering the aquifer through Antioch Cave. Over a period in 2010 that included five storm events, approximately 1105 kg (2436 lbs) of nitrogen from nitrate/nitrite, 134 kg (295 lbs) of total phosphorus, and 86,385 kg (190,480 lbs) of sediment were prevented from entering Antioch Cave. This amount of sediment is equivalent to about eight dump-truck loads that are prevented from entering the aquifer.