Climate Change and Groundwater: Planning and Adaptations for a Changing and Uncertain Future

There is now little doubt in the scientific community that the Earth’s climate is changing at an accelerating rate and that rising global temperatures are due largely to increasing atmospheric concentrations of greenhouse gases (GHGs). Changes in temperature and precipitation will potentially affect local water demands and supplies, with the degrees of impacts depending on the direction, magnitude, and type of climate change and local circumstances, including existing climate conditions, the amount and types of water use (e.g., domestic, agricultural, and industrial), and the water sources exploited. The foundation of climate change predictions is general circulation models (GCMs), which are three-dimensional models that include the entire planet and simulate all major processes that impact climate, including sea ice and evapotranspiration over land. Adaptation options for climate change are essentially the same as those available for dealing with water scarcity in general. Changes in the local supply of water can be addressed through either management of demands or changes in the water sources utilized. Groundwater depletion can be reduced through decreasing extractions by either conservation (curtailing some water uses or increasing water use efficiency) or transitioning to alternative water sources (e.g., reclaimed water reuse and desalination).

Climate impacts groundwater quantity mainly through its influences on recharge and the demand for groundwater. Precipitation either infiltrates into the soil, runs off, or ponds on land surface and is lost to subsequent evaporation. Infiltrated water may either percolate to the water table and become aquifer recharge, be held in the soil under capillary pressure as soil moisture, be lost to evaporation from the soil or transpiration by vegetation (referred to collectively as evapotranspiration), or may flow in the unsaturated zone and later discharge to surface waters or land surface (interflow). Groundwater recharge rates are a complex function of both the intensity and duration of rainfall events, soil moisture content, and land surface and soil properties that control initial and equilibrium infiltration rates. The relationships between mean annual rainfall, available water, and aquifer recharge are not linear. Climate change can impact the demand side of the aquifer budgets by increasing the demand for water in general or by reducing the amount of available surface water prompting a shift toward more groundwater pumping

Data on mean global surface temperature relative to the 1951–1980 average indicate an increase of 0.98 °C (1.76 °F) by 2019 with a clear upward trend since the late 1970s and nineteen of the twenty warmest years having occurred since 2001. The IPCC concluded that there is low confidence that average precipitation over global land areas had changed between 1901 and 1951 and a medium confidence that it had changed afterwards. Global mean sea level had risen by about 0.23 m (9 in) between 1880 and 2019, with the rate of rise having increased since the middle 1990s and is reported to have been 3.6 mm/yr over the period 2006–2015. The foundation for climate change projections is general circulation models (GCMs). The most recent IPCC reports have been supported by the Coupled Model Intercomparison Project (CMIP), an international collaboration of climate modeling groups in which simulations were run with different GCMs using the same set of future emissions scenarios. CMIP3 and the IPCC AR4 used a suite of SRES (Special Report on Emissions Scenarios) scenarios. CMIP5 and IPCC AR5 used four Representative Concentration Pathways (RCPs).

The Intergovernmental Panel on Climate Change (IPCC) reports are a primary source of climate change data. The peer-review process for IPCC assessments is extensive and the reports are generally taken in the scientific community as objective and authoritative. Within the climate change skeptic or contrarian community, the charge is frequently made that the IPCC reports are exaggerated and overly pessimistic, where on the other side of the spectrum, some scientists claim that the consensus building goal of the IPCC results in a tendency to underplay climate change impacts to avoid appearing too alarmist. The global climate will experience continued warming caused by past anthropogenic emissions as well as from additional future anthropogenic emissions. The CMIP5-based projections in the IPCC Fifth Assessment show that the Mediterranean Basin and North Africa, southern Africa, the Caribbean and Central America, Mexico and the southwestern United States, and much of Australia will likely become drier. Areas simulated to experience greater annual average precipitation include the north and south polar and boreal regions, most of the eastern and central United States and Canada, and South and East Asia.

Groundwater recharge is impacted by climate change mainly through available water (precipitation—evapotranspiration). Local groundwater recharge may be controlled to a greater degree by the seasonality, intensity, and duration of rainfall events, and soil and surficial rock properties. Land use and land cover changes, either natural or anthropogenic, can also impact recharge rates. Modeling climate change impacts on groundwater has often been performed using the “top-down” approach starting with greenhouse gas (GHG) emissions scenarios and GCMs. The GCM simulation results are then downscaled to provide greater geographic spatial resolution. Precipitation and temperature data are next inputted into a hydrologic model that partitions precipitation between evapotranspiration (ET), runoff, and aquifer recharge. Finally, recharge data are inputted into a groundwater flow model, which is used to evaluate changes in aquifer water levels and discharge. Solute-transport modeling is performed to simulate salinity changes. The top-down approach tends to not be practicable in the applied water management realm because of very high technical (and thus financial) resources requirements and its chain of uncertainty. Published modeling studies of the impacts of climate change on groundwater recharge provide insights on the range of available approaches, limitations of modeling strategies, and the potential extent of climate impacts.

Rising sea levels pose a direct threat to coastal communities through permanent inundation of low-elevation areas and greater and more widespread temporary inundation from possibly more intense storms and king tides atop a higher sea level baseline. Sea level rise will also cause higher groundwater levels in coastal areas causing inundation of low-lying areas and impacts to shallowly buried infrastructure. The risks posed by rising sea levels are compounded by the large and growing populations of coastal urban areas. Areas vulnerable to direct marine inundation and groundwater inundation sea level are most commonly screened using the hydrostatic or “bathtub” approach, which assumes that sea level and the water table rises at the same rate everywhere in a coastal study area, or more accurate dynamic and groundwater modeling. Sea level rise will contribute to the salinization of coastal aquifers through more frequent and greater extent overwash events and saline water intrusion, with the later evaluated through density-dependent numerical groundwater modeling.

Small, low-lying islands are particularly vulnerable to global climate change and sea level rise with the principal impacts falling into three main categories: shoreline erosion, inundation and flooding, and saline water intrusion into surficial freshwater aquifers. Fresh groundwater on small islands occurs primarily as freshwater lenses, floating atop saline groundwater, whose natural size depends on island area, shape, and topography, aquifer properties (transmissivity), and rainfall. Freshwater lenses are limited resources that are vulnerable to salinization from over exploitation and from storm overwash. The sustainability of freshwater lenses depends on maintaining a balance between recharge, captured discharge, and pumping. Decreases in recharge during drought periods can result is a profound shrinkage of freshwater lenses. Anthropogenic activities can impact freshwater lenses through induced changes in recharge rates caused by increased impervious covers. Management of freshwater lenses can be improved through improved well and wellfield design, monitoring of aquifer water levels and salinity, controls on groundwater pumping, implementation of land use practices to reduce the risk of anthropogenic contamination, and managed aquifer recharge.

Climate change will impact physical infrastructure, including water, wastewater, transportation, energy, residential, commercial, and industrial facilities, in many manners. Most basically, communities and facilities located in low-lying coastal areas will become increasingly vulnerable to inundation from rising sea levels, initially during storm and extreme tidal events, and eventually permanently as mean sea level approaches land surface. Heavy precipitation events and associated flooding will also impact infrastructure in low-lying coastal and riverine (flood plain) areas. Coastal flooding can be caused by direct ingress of seawater, through rising groundwater levels (i.e., groundwater inundation; GWI), and in some instances backflow through stormwater drain systems. GWI can be more difficult to manage than direct marine flooding because conventional physical barriers (e.g., sea walls, dikes, and levees), depending on their construction and local hydrogeology, may be largely ineffective. Climate change impacts could accelerate physical degradation of the built environment, and increase operation, maintenance, engineering design, and construction costs to support long-lived public water, wastewater, road, and other systems.

Adaptation was defined by the IPCC as “the process of adjustment to actual or expected climate and its effects, in order to moderate harm or exploit beneficial opportunities.” Adaptation to climate change is undertaken privately or publicly, and by individuals, informal groups of people, governmental agencies, and businesses. Adaptation can be undertaken by individuals for their own benefit or it can be made up of actions by governments and public bodies to protect their citizens. Planned adaptation to climate change starts with a recognition of vulnerabilities to climate change. A commonly used model for climate change vulnerability assessments is the risk-hazard framework, which can be defined broadly as the processes of estimating both the probability of occurrence of adverse impacts to systems or assets and the probable magnitude of adverse impacts. A decision is then made as to whether the risk climate change poses to each identified vulnerable asset is acceptable or not. The next step in the process is consideration of various strategies that could employed to reduce the risks to vulnerable assets. The capacity to adapt to climate change is a function of available societal, economical, and technical resources, and the ability and willingness of societies to effectively respond.

Countries and communities exposed to climate change will have to adapt to increased flooding and other direct physical threats by either retreat, accommodation, or protection. The principal threats to groundwater supplies from climate change are (1) increased aridity decreasing recharge and thus the amount of water that can be sustainably extracted, (2) deceases in annual average precipitation decreasing surface water supplies causing additional demands on groundwater, (3) changes in the timing and form of precipitation (e.g., decreases in annual snowpacks) that could decrease recharge, (4) increased temperatures and decreases in precipitation may drive additional demands for water in general, and (5) increased risk of salinization. Adaptations to the negative consequences of climate change on groundwater supplies are essentially the same options available to address water scarcity in general. Decreasing groundwater supplies and reliability may be addressed by some combination of demand management, development of alterative water supplies (e.g., desalination, wastewater reuse, rainwater harvesting), and optimization of existing supplies (e.g., managed aquifer recharge, conjunctive use, and community water system interconnections, coordination, and consolidations).

Conjunctive use is usually defined as the coordinated use of surface water and groundwater but can be more broadly defined as the coordinated use of all available water resources with the objective of optimizing overall water management. A basic principal of conjunctive use is that surface water should be used when available with groundwater use curtailed to allow aquifers to recharge and groundwater levels to recover. Managed aquifer recharge (MAR) plays an important role in many conjunctive use schemes by increasing the supply of groundwater available to stabilize water supplies. The ability to implement conjunctive use and its effectiveness depends upon local hydrological and hydrogeological conditions and the extent to which local water management institutional frameworks and practices are congruent with the practice. Key governance issues are protection of those who invest in facilities and store water, and a fair method to distribute costs among those who benefit from the systems. Summaries are provided of conjugate use in Southern California, Arizona, and Florida.

Key practical issues concerning adaptation to climate change with respect to groundwater (and water in general) are who actually makes water supply decisions, the role of government in water supply planning, how decisions are made, to what extent and how is climate change information received and acted upon, and the time frame used for planning and capital investments. Water management decisions in the water sector still largely fall within the realm of water users and suppliers, who are responsible for finding means to meet their own water needs (and those of their customers), operating under the purview of governmental agencies that have regulatory authority over water use. Governmental agencies also perform or sponsor research, provide educational services, and may build and operate large-scale water infrastructure. The degree to which climate change adaptation is a consideration of a water utility or other public or private organization depends upon organizational priorities, which are largely driven by organizational leadership engagement. Projected anthropogenic climate change impacts with respect to groundwater for the common 20 to 50-year planning periods are likely small relative to natural climate variability and increases in demands due to population growth.

The impacts of climate change on water resources will vary between regions. Some areas are projected to get drier, whereas others will see little change in precipitation or may benefit from greater rainfall. Whereas climate models consistently predict higher temperatures in the future, there is much greater variation in both the direction and magnitude of projected local changes in precipitation. The Coupled Model Intercomparison Project 5 (CMIP5) ensemble modeling results presented in the IPCC Fifth Assessment Report most consistently project decreases in mean annual precipitation in the Mediterranean region, the southwestern United States, Central America, and southern Africa. The vulnerability of groundwater resources to climate change in some areas that have been flagged as climate change hot spots or otherwise have a high dependence on groundwater is examined. Adaptation options available and implemented to date are considered.

Adaptation to the impacts of climate change on water resources will involve local decisions made by water users and suppliers. In the case of large users and water suppliers, planning is performed by in-house technical staff, often supported by external consultants, and involves estimation of future demands and identification of a source or combination of sources that can most reliably and least expensively provide their required water. There are compelling arguments that measures for adapting to climate change should reduce reliance on specific climate projections and instead focus on a range of plausible futures. The “top-down” modeling approach, which starts with climate change modeling and proceeds to downscaling and hydrological and groundwater modeling tends to have large uncertainties and not provide actionable projections. The “bottom-up” or sensitivity analysis-based approach is more practical from an applied perspective. A suite of potential climate change scenarios for groundwater and surface water resources is used to evaluate the sensitivity of a supply system to a range of changes. In many groundwater-dependent areas in developed countries, existing groundwater flow models developed for the management of resources under current climate conditions can be utilized to assess the impacts of climate changes.