5.4 Methods for Investigating Global Change Beneath the Surface
Green et al. (
2011) explored and reviewed
a range of techniques for exploring subsurface effects of climate change, which are summarized here. Methods available to detect temporal changes in groundwater
quantity
and quality
are numerous and range markedly in observation scale and “directness” of observation. The most direct, but also smallest-scale observations are obtained from head measurements in piezometers and water quality
measurements of water samples obtained in wells. While in-situ measurements arguably provide the most accurate and reliable measures to detect change, spatial variability
and transfer of information
across scales (i.e., scaling) must be considered. Moreover, observation networks do not exist across large parts of the globe, and installing and maintaining measurement systems is expensive and labor intensive. To evaluate temporal trends at regional to global scales and to study their relationship to change in regional to global climate and human
activities, studies of extensive data sets (monitoring
networks) of such “point-data” are required. Hydroclimatically similar regions can be explored using a global database of historical climate data. Similarity between historical climates in different regions is a necessary starting point but may not be sufficient to constitute analogous climate change scenarios.
Most hydrogeophysical methods have the advantage that they allow detection of change over larger volumes of the subsurface, but at the expense of detail, notably regarding water chemistry. Remote sensing of systematic change in the recent past and future across the globe has limited ability to “see” watershed-scale groundwater. The major benefit of remote sensing technologies is their ability to access spatial information in remote areas where in-situ monitoring is sparse or non-existent. Furthermore, conjunctive use of well data, hydrogeophysics and remote sensing is essential.
5.4.1 Age Dating and Chemical Proxies
Tracer methods are standard tools of hydrologists to obtain constraints on the age of groundwater
and on the processes and conditions experienced during recharge
and upon transit in the groundwater system (Clark and Fritz
1997; Cook and Herczeg
2000; Kooi
2008b; Loosli et al.
2001; Plummer
1993). Age dating refers to methods that aim to constrain the timing of recharge, often via the time since recharge. Groundwater
ages can be estimated using radioactive isotopes with well
-known, stable source concentrations (e.g.,
14C), radioactive isotopes with variable
source concentration and a daughter isotope that can be fairly uniquely linked to the mother species (e.g.,
3H/
3He), or conservative chemical
species which exhibit negligible decay and which have a well-known, systematically changing source concentration (e.g.,
85Kr, CFC’s, SF
6).
These “direct methods” of age dating, in principle, allow construction of a continuous record of water age with distance along a flow path, thereby potentially revealing temporal changes in recharge. Accuracy of age-dating methods covering time scales of 100–500 years is low, making temporal changes in this age-range difficult to resolve.
Several “indirect” age-dating methods provide additional useful constraints on groundwater age. These methods generally determine whether a water sample is recharged before or after a known event. An absolute age of a water sample can only be calculated when the sample corresponds to a distinct event marker. The nuclear bomb test peaks in 3H, 14C and 36Cl are key examples. These indirect methods are most useful to study spatial variability in groundwater flow systems.
Several chemical
proxies are used to trace changes in groundwater
flow and changes in recharge
conditions associated with climate change and surface environmental
change in general. Key proxies are the stable isotopes of water (Clark and Fritz
1997) and noble gases dissolved in groundwater (Porcelli et al.
2002; Stute and Schlosser
1993). Also, chloride content of groundwater and, in particular in vertical SWC profiles collected in thick vadose zones
in desert
areas, have been exploited to infer changes in recharge conditions (e.g. Edmunds and Tyler
2002). Although noble gases have been applied primarily in paleohydrological reconstructions of long time scales (Kooi
2008a), they should also provide valuable constraints regarding changes in groundwater systems on timescales of decades to centuries.
5.4.2 Hydrogeophysical Techniques
Three hydrogeophysical methods are particularly relevant to the study of groundwater
and the changes that arise from climate variability
and change:
1.
electrical/electromagnetic methods,
2.
subsurface temperature logging, and
3.
land-based gravity surveying.
A wide range of electrical/electromagnetic imaging and logging methods can be used to study groundwater systems and their responses to climate-related phenomena. This group of methods includes spontaneous/self potential (SP), electrical resistivity, induced polarisation (IP), a range of time and frequency domain electromagnetic methods, and ground-penetrating radar (GPR). Their advantage over point sampling is that large areas can be covered either in land-based surveys or airborne surveys. Borehole logging methods can be used in a similar fashion to provide vertical profiles of these properties with depth and to constrain survey data.
Perhaps the most common application of these methods is to studies of saline water in aquifers
(Dent
2007). Climate change is expected to result in higher sea levels, posing an even greater threat to coastal aquifers
. Thus, these hydrogeophysical methods are ideally suited for monitoring
changes in groundwater
salinity
over large coastal areas due to the effects of sea level rise. These techniques may prove invaluable for detecting changes in salinity over broad agricultural areas.
Subsurface temperature can be used to reconstruct climate change and land cover change, because the signal of surface temperature change is preserved in subsurface environment (e.g., Chapman et al.
1992; Davis et al.
2010; González-Rouco et al.
2009). Changes in surface temperature associated with changes in air temperature (Smerdon et al.
2009) can propagate into the subsurface, and can be detected by measuring ground temperatures up to several hundred meters deep (Beltrami and Mareschal
1995; Čermák et al.
1992). Temperature-depth profiles collected in boreholes
can reveal and be used to help reconstruct the surface temperature changes due to climate change and land cover change during a few to several hundred years (Beltrami
2002; Huang et al.
2000; Roy et al.
2002). Effects of global warming on subsurface temperature subsequently affect the ecology
and water quality
.
Land-based gravity measurements have been used to detect changes in groundwater
storage
. Pool and Eychaner (
1995) observed that measured gravity changes of about 13 microGal represented storage changes of about 0.30 m of water. Gravity meters are now sufficiently accurate to measure variations of about 2 microGal, and finer instrumental precision with temporal averaging. Gravity measurements have also been used to detect the changes in groundwater storage
in situ (gravity profiling) and using the GRACE satellite data as discussed in the next section.
5.4.3 Remote Sensing of Space-Time Trends
Satellite remote sensing (RS) represents the most powerful method for detection and monitoring of environmental and climate change on a global scale. However, capabilities of RS to “look below the ground surface” and to detect properties that directly bear on groundwater conditions are extremely limited. Notable exceptions to this are satellite-based observations of the gravity field associated with changes in groundwater storage.
Remote sensing and earth observation technologies provide an important means of collecting groundwater-related data on a regional scale and to assess the state of the resource. Satellite remote sensing, despite drawbacks of temporal frequency and estimation errors, offers the advantages of global coverage, availability of data, metadata, error statistics, and the ability to provide meaningful spatial averages.
Aerial thermal infrared imaging is being used for mapping groundwater
discharge
zones in estuaries, rivers and oceans. Peterson et al. (
2009) used aerial thermal infrared imaging to reveal that submarine groundwater discharge (SGD) along the western coast of the Big Island of Hawaii is often focused as point-source discharges that create buoyant groundwater plumes that mix into the coastal ocean.
Landsat, the Moderate-resolution Imaging Spectroradiometer (MODIS), the Advanced Very High Resolution Radiometer (AVHRR), and certain other instruments can resolve the location and type of vegetation, which can be used to infer a shallow water table. Altimetry measurements and Interferometric Synthetic Aperture Radar (InSAR) over time can show where subsidence is occurring, which is often an indicator of groundwater depletion. Microwave radar and radiometry measurements can be used to estimate snow and surface soil water, which further constrain groundwater assessments.
Perhaps the most valuable remote sensing technology for groundwater
investigations is satellite gravimetry employed by the Gravity Recovery and Climate Experiment (GRACE) – a satellite gravimetry technology that may be used to assess groundwater storage
changes. Since its launch in 2002, the GRACE satellites have been employed to detect tiny temporal changes in the gravity field of the Earth (Ramillien et al.
2008). Temporal changes in measured gravity are primarily caused by changes in total water (mass) storage (TWS) in the atmosphere, ocean and at and below the surface of the continents. GRACE is being used to generate time series of total terrestrial water variations (Tapley et al.
2004), which can be used to assess groundwater storage changes. Wahr et al. (
2006) presented the first technique for deriving terrestrial water storage variations from global gravity field solutions delivered by GRACE. Rodell and Famiglietti (
2002) showed in a pre-GRACE-launch study that interannual variations and trends in the High Plains aquifer
water storage would be detectable by GRACE, pointing to new opportunities for groundwater remote sensing. Rodell et al. (
2007) developed time series of groundwater storage variations averaged over the Mississippi River basin
and its four major sub-basins using in situ data, and used these to evaluate GRACE-based estimates in which SWC and snow water equivalent fields output
from a sophisticated land surface model were used to isolate groundwater from the GRACE terrestrial water storage
data. At the smaller spatial scale of Illinois (145,000 km
2), Swenson et al. (
2006) showed that GRACE captures the signal of changes in total water storage very well
, while Yeh et al. (
2006) showed that GRACE-based estimates of groundwater storage
variations compared well with borehole
observations on seasonal timescales. Swenson et al. (
2008) used Oklahoma Mesonet data and local groundwater level observations to further refine methods to remove the SWC signal from the total water storage
change signal recorded by GRACE.
Post-launch studies using GRACE data have demonstrated that when combined with ancillary measurements of surface water
and SWC, GRACE is capable of monitoring
changes in groundwater
storage
with reasonable accuracy (temporal resolution 10 days to monthly, spatial resolution 400–500 km, mass change ~9 mm water equivalent). Syed et al. (
2008) also found agreement between the storage changes estimated by GRACE and the Global Land Data Assimilation System (GLDAS), where GLDAS was used to disaggregate terrestrial water storage
between soil, vegetation
canopy and snow.
The need to better quantify potential changes in the water cycle associated with climate change (GEWEX
1; WATCH program
2) has provided a major stimulus for improvement of techniques to monitor key variables
and components of the hydrological cycle
using space-based platforms. Advances and new developments in monitoring
of soil moisture (de Jeu et al.
2008; Liu et al.
2009), precipitation
, and evapotranspiration
(Anderson and Kustas
2008; Kalma et al.
2008) provide crucial elements to help constrain space-time trends in groundwater
recharge
. Future research will undoubtedly focus on the further integration
of these multi-platform and multi-parameter observations, including GRACE data, in extensive hydrological models
. Recent dedicated hydrological missions for improved monitoring of soil moisture (2009: SMOS/ESA; 2011: SMAP/NASA) and precipitation
(2012: GPM/NASA) enhance RS capabilities of groundwater resources
assessment.
The monthly temporal resolution of GRACE is an issue for many applications, but it should be sufficient for regional groundwater
assessments. To address such scale issues, Zaitchik et al. (
2008) used an advanced data assimilation approach to incorporate GRACE data into a land surface model, and hence merge them with other datasets
and our knowledge of physical processes as represented in the model. In simulations
over the Mississippi River basin
, the GRACE-assimilation groundwater storage
output fit observations better than output from the open loop, and they were of much higher spatial and temporal resolution than GRACE alone. Yamamoto et al. (
2008) reported the larger difference, in particular at low latitude regions, between current terrestrial water models
of global river basins
and GRACE data. This technique may be the key to maximising the value
of GRACE data for groundwater resources
studies (e.g., Fukuda et al.
2009).
5.5 Assessments of Subsurface Hydrology: Numerical Simulations
Mathematical groundwater models play a central role, both for interpreting and integrating data and for generating general insight to the response of groundwater systems to climate change and other forcings on multiple spatial and temporal scales. While observations are essential to explore and document subsurface global change, numerical models provide key tools, not only to assist in developing a process-based understanding of observed changes (i.e., hindcasting), but also predict the future response of the subsurface parameters to climate change, land-use change and water management scenarios (forecasting). Distributed groundwater models simulate flow in the subsurface, both in saturated and unsaturated conditions, as well as for porous and fractured media. Specialised codes are used to simulate chemical processes, such as solute transport and reactions, heat transport, and density-dependent flow (e.g., for coastal regions). In addition to groundwater models, which form the basis for groundwater assessment, other potential models include coupled land surface-atmospheric models, biogeochemical models, surface-water hydrological models, coupled surface-water/groundwater models, and coupled land surface and variable-saturated groundwater models.
Process-based continental or global-scale hydrological models
are rare. Thus, most studies develop watershed or smaller scale models, which are better constrained by available data and, thus, more easily calibrated. However, there remain challenges for coupling GCM predictions with hydrological models (Scibek and Allen
2006b; Toews and Allen
2009; Xu
1999), including issues discussed in the section Global Climate Projection.
The appropriate level of model complexity
for a given problem may remain subjective, but some level of process interaction
within the plant-soil-groundwater
-atmospheric system must be present. Tietjen et al. (
2009) made a case for at least two soil layers in a soil-vegetation
model that simulated soil-water
dynamics under different climatic conditions. Others have applied relatively complex, spatially distributed subsurface models
and coupled surface-groundwater models (Goderniaux et al.
2009; Hunt et al.
2013; van Roosmalen et al.
2007,
2009).
Numerical model-based studies continue to improve, but for the most part, the approaches are similar to the limited examples given above and more comprehensive case studies discussed by Green et al. (
2011). Models
used to predict terrestrial and subsurface effects of climate change must incorporate appropriate processes and their interactions in space and time. Integration
studies encompassing changes in human
or socio-economic scenarios (apart from emissions
scenarios), such as land use
and water demand
are generally lacking (Holman
2006).
5.7 Adapting to Climate Change: Integrated GroundwaterManagement
Climate adaptation measures are developed to cope with the consequences of a changing climate and reduce future risks. Adaptation encompasses both national and regional strategies as well as practical measures taken at all political levels and by individuals.
In many parts of the world, groundwater
is crucial to sustainable
development through provision of low-cost, reliable and high-quality
water supplies. About 70 % of drinking water
in the European Union
, 80 % of rural water supply
in sub-Saharan Africa and 60 % of agricultural irrigation
in India
depend on groundwater (IAH
2006). Groundwater
also sustains ecosystems and landscapes in humid regions in supporting wetlands and riparian
areas, and also supports unique aquatic ecosystems in more arid regions and in coastal environments. The largely hidden nature of groundwater means that development is often untallied and thus uncontrolled and not incorporated into overall water resource
management
, resulting in over-exploitation
and contamination
. Thus, even without considering climate change, sustainable management of groundwater is a major challenge. Groundwater is a widely distributed resource responding at basin
scales, and local stakeholders (e.g., municipalities, industrial enterprises and farmers
) are influenced by national policies determining land and water use. In general, governance
systems, resource policies, innovation incentives, data collection and information
provision need to relate to a wide range of scales (see Chap.
6), with different adaptive management
approaches in rural and urban environments (IAH
2006).
Climate change challenges the traditional assumption that past hydrological experience provides a good guide to future conditions. In times of surface-water shortages during droughts, a typical response is for groundwater resources to be abstracted as an emergency supply. Under conditions of climate change, this response could be unsustainable, especially in areas expected to experience an increase in drought frequency and duration. Also, rising sea levels under climate change will further threaten coastal freshwater aquifers, especially those already experiencing salinisation due to over-exploitation.
Alley (
2006) suggested that the effects of discharge
and groundwater
development often take many years to become evident. Thus, government
s tend to neglect the data collection and analysis needed to support informed groundwater management
until problems materialize. This type of reactionary stance to groundwater management
is flawed because, although some groundwater systems are renewable, many groundwater resources
contain “fossil” groundwater and thus are nonrenewable
natural resources on human
time scales. For example, the groundwater that is removed from storage
in many arid and semiarid regions was recharged during wetter periods under paleoclimate conditions (Alley et al.
2002).
Adaptation
approaches can be preventative or reactive and apply to natural and social systems. Ensuring the sustainability
of investments in groundwater
resources
planning
and development, over the entire lifetime of a scheme and taking explicit account of changing climate, is referred to as
climate proofing (CEC
2007). At a minimum, and in the absence of reliable projections of future changes in hydrological variables, adaptation
processes and methods can be implemented, such as improved water use efficiency
and water demand
management
, offering no-regrets options to cope with climate change.
The Netherlands are investing in “climate proofing
” (Kabat et al.
2005) that uses hard infrastructure and softer measures, such as insurance schemes or evacuation planning
, to reduce the risks of climate change and hydrologic variability
to a quantifiable level that is acceptable by the society or economy. The Netherlands and the rest of the world’s coastal delta regions are vulnerable to climate change and sea-level rise. Rather than coping with extreme climatic events, as people from all over the world have done over human
history, climate proofing
is a proactive approach to develop precautionary measures to address the low-probability but high-magnitude hydroclimatologic events forecasted under climate change and variability (Kabat et al.
2005). Climate proofing should be driven by opportunities for technological, institutional, and societal innovations, rather than by the fear of climate-change induced threats. The climate-proofing approach could be used by water-resource
scientists, engineers, and managers to develop forward-thinking, innovative solutions and precautionary measures for a range of probable hydroclimatic events under future climate change. The discredited stationarity of hydroclimatology (Milly et al.
2008) may promote innovation and suitable precautionary measures to protect the sustainability
of groundwater
resources
under projected hydroclimatic regimes. Thus the process of adaptation
to climate change must itself be adaptive
over time.
Potential adaptive
responses include some combination of technological (e.g., deepening of existing boreholes
), behavioral (e.g., altered groundwater
use
), managerial (e.g., altered farm irrigation
practices), and policy
oriented (e.g., groundwater abstractions licensing regulations
) approaches. The IPCC (
2007a) argued that while most technologies
and strategies are studied and developed in certain countries, the effectiveness of various options to substantially reduce risks for vulnerable water-stressed areas is not yet known, particularly at higher levels of warming and related impacts. Shah (
2009) noted an indirect feedback
of pumping
on climate change due to energy use and associated carbon emissions
. This is one obvious example of the interactions between potential groundwater-atmosphere feedbacks and adaptation
to global change
that must be considered.
For integrated water resources management
, two types of decisions deal with: (1) new investments, and (2) the operation and maintenance of existing systems. Information
is needed about future water availability
and demand
, both of which are affected by climate change at the river-basin
scale (Ballentine and Stakhiv
1993). As explained by the IPCC (
2008), supply-side options generally involve increases in storage
capacity or water abstraction. Demand
-side adaptation
options rely on the combined actions of individuals (industry users, farmers
and individual consumers) and may be less reliable. Some options, such as those incurring increased pumping
and treatment costs, may be inconsistent with climate change mitigation
measures because they involve high energy consumption.
One of the major challenges facing water resources managers
is coping with climate change uncertainty
, particularly where expensive investment in infrastructure such as well
-field design, construction and testing and laying of pipelines is required (Brekke et al.
2004; Taylor et al.
2013). Dessai and Hulme (
2007) discussed this challenge and related questions, including: To what amount of uncertainty in climate change should we adapt? Are robust adaptation
options socially, environmentally and economically acceptable and how do climate change uncertainties compare with other uncertainties such as changes in demand
? The answers to these questions leading to robust adaptation
decisions will require the development of probability distributions of specified outcomes (Wilby and Harris
2006) and negotiation
between decision-makers
and stakeholders involved in the adaptation process (Dessai and Hulme
2007). For lower income countries, availability
of resources and building adaptive
capacity are particularly important in order to meet water shortages and salinisation of fresh waters.
Examples of current adaptation
to observed and anticipated climate change in the management
of groundwater
resources
are few, with groundwater typically considered as part of an integrated water-supply system. Here, three examples serve to highlight the difference in approach in technically-advanced and developing country contexts. The ability of California
’s water supply
system to adapt to long-term climate and demographic changes was examined by Tanaka et al. (
2006) using a state
-wide economic-engineering optimisation model of water supply management and considering two climate warming scenarios for the year 2100. However, recent drought conditions
3 raised concerns
regarding long-standing issues of groundwater quality
and management in California. Even so, the prediction by Tanaka et al. (
2006) that California’s water supply system appears physically capable of adapting to significant changes in climate and population may remain valid, albeit at significant cost. Such adaptations would entail large changes in the operation of California’s large groundwater storage
capacity, significant transfers of water among water user
s and some adoption
of new technologies
. In the Sacramento Valley, California
, Purkey et al. (
2007) used four climate time series to simulate agricultural water management with adaptation
in terms of improvements in irrigation
efficiency and shifts in cropping patterns during dry periods leading to lower overall water demands in the agricultural sector with associated reductions in groundwater pumping
and increases in surface-water allocation
s to other water use sectors. Land-use adaptation
to projected climate change may include management changes within land-use classes (e.g., alternative
crop rotations) or changes in land classification (e.g., converting annual cropping systems to perennial grasslands or forests
). Soil and water conservation programs already encourage some of these types of land-use
changes.
A similar technological approach to that demonstrated for California
is presented for the Mediterranean
region of Europe
. This region is experiencing rapid social and environmental
changes with increasing water scarcity problems that will worsen with climate change. Iglesias et al. (
2007) found that these pressures are heterogeneous across the region or water use sectors and adaptation
strategies to cope with water scarcity include technology, use of strategic groundwater
and better management
based on preparedness rather than a crisis approach. Iglesias et al. (
2007) also promoted the importance of local management at the basin
level but with the potential benefits dependent on the appropriate multi-institutional and multi-stakeholder
coordination.
In contrast to the examples from North America and Europe
, Ojo et al. (
2003) discussed the downward trends in rainfall and groundwater
levels, and increases in water deficits and drought events affecting water resources availability in West Africa. There, the response strategies needed to adapt to climate change emphasize the need for water supply
-demand
adaptations. The mechanisms needed to implement adaptation measures include: building the capacity and manpower of water institutions
in the region for hydro-climatological data collection and monitoring
; the public participation
and involvement of stakeholders; and the establishment of both national and regional cooperation
.
Furthermore, water resources management
has a clear association with many other policy
areas such as energy, land use
and nature conservation. In this context, groundwater
is part of an emerging integrated water resources management approach that recognises society’s views, reshapes planning
processes, coordinates
land and water resources management, recognises water quantity
and quality
linkages, manages surface-water and groundwater resources
conjunctively, and protects and restores natural systems while considering of climate change. Also, biofuel production has implications for groundwater recharge
quantity
and quality (IPCC
2008).
In summary, groundwater resources stored in aquifers can be managed given reasonable scientific knowledge, adequate monitoring and sustained political commitment and provision of institutional arrangements. Although there is no single approach to relieving pressures on groundwater resources, incremental improvements in resource management and protection can be achieved now and in the future under climate change. Sustainable management of groundwater will only be possible by approaching adaptation through the effective engagement of individuals and stakeholders at community, local government and national policy levels. Adaptative decision processes in the face of global change should be addressed even to improve management and decision making in an otherwise unchanging world. That is, natural and human-induced variability under historical conditions will be better quantified and managed using new scientific advances gained under the auspices of global change research, making such work a “win-win” proposition.