Download challenges for future sustainable water resources management in

CHALLENGES FOR FUTURE SUSTAINABLE WATER RESOURCES
MANAGEMENT IN THE FACE OF CLIMATE CHANGE
Manfred Koch
Department of Geotechnology and Geohydraulics
University of Kassel, Germany
Abstract
Human-induced effects on water resources are observed in many parts of the world.
These may include enhanced seawater intrusion, reduced aquifer storage, land subsidence, the
diminishment of base flow in rivers and streams, and increased potential for contamination.
The situation is further exacerbated by climate variability and predicted climate change - manmade or naturally occurring - which affect the various components of the hydrological cycle
in ways yet not well understood and which are most likely to wreak havoc for water resources
availability and sustainability or increase their vulnerability in many regions of the world.
While there have been many studies and projects in recent years devoted to the investigation
of the effects of climate change on surface water resources, those devoted to the impacts on
groundwater are much less numerous and have only come to the fore more recently. This as
a consequence from the understanding that groundwater will be pivotal to sustainable water
supplies, because of its capacity to balance large variations in precipitation and demand in the
wake of climate variability or permanent change.
Here we discuss some of the concepts and approaches to that regard, with the emphasis on the
issues of numerical modeling which is indispensable for future predictions of water resources
systems as meteorological inputs and water needs will change. In principle this requires a
fully integrated modeling approach for flow and transport across the various compartment of
the hydrosphere, with the groundwater aquifer as the receiving “end-member” in the chain.
However, this is still a particularly challenging task, as these computational models must
resolve all the fundamental physical processes – each by itself acting on a completely
different spatial and temporal scale in the corresponding sections of the hydrological cycle.
One of the most commonly used approach to circumvent some of these computational
burdens and to cut down the problem into “pieces” is known under as “downscaling” which
consists in extrapolating coarse-grid predictions from Global Climate Models (GCMs) to a
finer scale required for the hydro-climatic assessment surface- and/or groundwater models.
There is still an active debate to the pro and cons of the various downscaling methods in use.
.
1. WATER RESOURCES IN THE FACE OF CLIMATE CHANGE
The “blue planet” Earth appears to have plenty of water. However, only a tiny fraction is
potentially available for human use. In fact, fresh water accounts only for about 3.5% of the
total water resources in the world. Of this 3.5%, about 50% is locked up in the polar ice caps
- and also not directly available -. Of the other 50%, the large bulk (46%) is stored as
groundwater and only 3% resides on the earth’s surface in lakes and rivers. As the former is
often neither accessible nor cost- effective to recover, this leaves only the named small
portion of surface water for direct human use. The global water situation becomes even more
awkward if one keeps in mind that surface- and groundwater – both being part of the
hydrological cycle - is constantly renewed by input from atmospheric water, i.e. net
precipitation. This means that for a long-term “sustainable” exploitation of water resources –
where water withdrawal does not exceed the rate of net atmospheric recharge (Alley et al.,
1999) - only the effective amount of water shuffled through the atmosphere - about 1/10 of
the total surface water resources above with a residence time of about two weeks - is
eventually available for supporting biological life on earth.
Although the above statements and facts may already picture a globally precarious water
situation in general, there is more reason for concern on the regional or local scale in many
parts of the world where natural or human-induced detrimental impacts on water resources
availability and quality are continuously posing a threat to the overall environment,
ecosystems and, not to the least, to the development of local economies. While it is fair to say
that most of these pressures on the environment and water resources, in particular, are due to
the tremendous increase in population on the earth in recent decades - often in regions and
countries where water has always been scarce per se - the situation is being exacerbated since
the middle of the last century by adverse changes of the global and local climate, with
corresponding alterations in the hydrological cycle.
Whether these climate changes are a reflection of what is called man-made “Global Warming
or Global Change”, or just a “kink” in the natural variability of climate or hydrological
systems over the geological time scale, that are known to act over a wide range of temporal
scales (cf. Markovic and Koch, 2007; Koch and Markovic, 2008) is another story. The Fourth
Assessment Report (AR4) of the IPCC (2007), the most comprehensive and up-to-date
scientific assessment of this issue, states with “very high confidence” that human activities,
such as fossil fuel burning and deforestation, have already altered the global climate in an
irrevocable way. During the 20th century alone, the global average surface temperature rose by
about 0.6°C and global precipitation over land increased by 2%. As for the 21st century, the
IPCC AR4 projects that the global average temperature will rise another 2 to 5°C by 2100,
depending on the assumed increases of the atmospheric concentrations of greenhouse gases
(as specified in the IPCC SRES-scenarios reports). This temperature increase will eventually
result in continued rises of sea levels and overall rainfall, changes in rainfall patterns and
timing, decline in snow cover, and land and sea ice extent. Thus, the Earth may experience a
faster rate of climate change in the 21st century than seen over the last 10,000 years.
It is now commonly accepted that climate change – via its effect on the hydrological cyclewill have huge impacts on the spatial and temporal distribution of the available water
resources within the surface and subsurface compartments of the hydrosphere. These impacts
can go either way, i.e. may lead to transient increases (flooding) but, more often, decreases of
water resources, entailing reduced groundwater recharge and storage, land subsidence, the
diminishment of base flow in rivers and streams and seawater intrusion in coastal regions
(Arlai and Koch, 2006) (Fig. 1). Consequently, numerous efforts are presently being made by
governments and water authorities in many countries of the world to develop water resources
planning-strategies for mitigation and adaptation to that threat. This, in the view that many
large-scale water resource projects, such as reservoirs, distribution systems, groundwater
recharge facilities and desalinization systems can take many years to plan and to construct.
Shifting the location of and adapting agricultural activities may also require large lead times.
Fig. 1: Illustration of some effects of climate change on surface- and groundwater resources
As mentioned above, water for human, industrial or agricultural use is tapped either from
surface- water- (rivers, lakes, or man-made reservoirs) or from groundwater storage systems,
whereby the proportions may vary significantly from country to country or even for different
watershed basins. In fact, as the construction of new surface water reservoirs and damns is
getting more and more hampered by land-use restrictions and other ecological concerns,
groundwater aquifers commonly offer the only available source for new water development
projects. As a consequence, large regions of the world are starting to become heavily
dependent upon groundwater for domestic water and agricultural irrigation - such as in
Thailand where conjunctive use patterns are being investigated as a viable alternative for rice
cultivation (Bejranonda et al., 2006; 2007) - and there is an urgent need to investigate the
possible impacts of possible climate change and modified climate variability on these
groundwater resources und to make sure that they can be sustained in the long run under
climatically averse conditions. From a regional or continental perspective, our understanding
of climate variability and change impacts on groundwater resources - related to availability,
vulnerability and sustainability of freshwater - remains still limited (Alley et al., 1999).
However, whereas numerous studies and projects in recent years have been devoted to the
investigation of possible detrimental effects of climate change on surface water resources in
many regions of the world (e.g. GLOWA in Germany, PRUDENCE in Europe), there is still
a dearth of investigations devoted to groundwater systems (Loaiciga et al., 2000; Loaiciga,
2003; Allen et al., 2004). In fact, surficial aquifers which supply much of the flow to streams,
lakes, wetlands, and springs are the part of the groundwater system most sensitive to climate
change; yet, limited attention has been directed at determining the possible effects of climate
change on shallow aquifers and their interaction with surface water. The UNESCO, becoming
aware of the urgency for alleviating this precarious situation, has now set up the special
international program GRAPHIC (Groundwater Resources Assessment under the Pressures of
Humanity and Climate Change) which, so far, has focused on studying the impacts of climate
change on groundwater systems in mainly, developing countries, namely, in Africa which has
been particularly suffering from declines of its water resources in recent times.
2. SYSTEM ANALYSIS OF SURFACE - AND GROUNDWATER RESERVOIRS
2.1 Basics of water budget analysis and implications on sustainable water management
In order to understand the impact of climate variability on the behavior of the surface- and
groundwater reservoirs, to better grasp the notion of sustainability or its counterpart,
vulnerability, and to set the tone for the later-discussed modeling approaches, some basic
concepts of water budget analysis need to be presented. To that avail one must define the
appropriate system with its boundaries and write the fundamental balance equation between
the fluxes into (Qin) and out of (Qout) the system and the temporal change in storage dS/dt :
dS / dt = Qin - Qout
(1a)
Applied to the total surface- subsurface reservoir of the hydrological cycle (Fig. 2) this gives
dST / dt = P + Qswi – ET – Qswi
(1b)
where P, effective precipitation; Qswi, inflow into the system; ET, evapotranspiration; Qswi,
outflow out of the system.
Cutting down this integrated surface- subsurface water compartment of the hydrological cycle
into three subsystems, namely, (1) the surface water system, (2) the subsurface vadose
(unsaturated) zone system and, (3) the groundwater zone (aquifer) system proper, one gets
dSS / dt = P – ET – O – I
(2a)
dSV / dt = I – R
(2b)
dSG / dt = R + Gin – Gout - Qpump
(2c)
where SS / dt , dSV/ dt and dSG/ dt are the rates of change in the three subsystems; O,
overland flow, I, infiltration, R, groundwater recharge; Gin , groundwater inflow; Gout ,
groundwater outflow; and Qpump, net groundwater withdrawal through pumping.
Fig 2. Left panel: the hydrological cycle; right panel: notations of fluxes for budget analysis
Eqs. (2a), (2b) and (2c) are coupled through the (usually unknown!) fluxes at the system
interfaces, in particular the infiltration I and the groundwater recharge R. The only way to
overcome this intricacy in a general water budget analyses would be the use of a fully coupled
model hat simulates surface-water flow (Eq. 2a), vadose zone flow (Eq. 2b) and groundwater
flow (Eq. 2c). As will be discussed in the model section, such fully coupled models - which
also must be able to properly simulate the different physical processes governing the flow in
these three different hydrological storage compartments - are still in their infancy stage and
under scientific development. For a simplified way out of this dilemma, so-called semicoupled surface- groundwater models are often used for conjunctive water use applications
(Bejranonda et al., 2007). The situation becomes even more intricate when impacts of climate
change on the water fluxes in the surface and subsurface compartments of the hydrological
cycle are to be simulated. In this case an atmosphere-ocean coupled general circulation model
(AOGCM) or, more simply, a global circulation model (GCM) must be used first to compute
the meteorological input variable precipitation P – and indirectly, the temperature - which is
then employed (after a complex process, called “downscaling”, see Section 3) as the driving
flux in Eq. (2a), respectively, the appropriate surface water hydrological model.
The equations defined above provide the basics for the understanding of sustainable groundand surface water management (Alley et al, 1999), at least with regard to water flow quantity,
though not necessarily with regard to water quality - as indicated by Arlai and Koch (2007)
for the heavily stressed Bangkok Aquifer where vertical saltwater intrusion of lead to a
significant deterioration of the groundwater quality -. Since the water taken out of the water
system and used for consumption – the abstraction - must come from somewhere, long-term
“safe yields” can only be achieved if the total net storage change is zero, i.e. the total inflow
must be equal to the total outflow + abstractions.
As for sustainable groundwater resources management, - which is the emphasis of the present
paper - assuming a certain amount of groundwater abstraction by pumping to satisfy
particular water availability needs, the most important parameter to assess is then the amount
of recharge R (Eq. 2c) entering the aquifer either directly from atop as natural seepage,
through stream beds under hydraulically favorable conditions, or through artificial recharge as
return flow from irrigation canals or pumped infiltration galleries.
In fact, artificial
groundwater recharge, also known as Aquifer Storage and Recovery (ASR) (using mostly
reclaimed wastewater) has nowadays become one of the most effective approaches in many
regions of the world, namely, in the dry southwestern states of the US, to even out
groundwater resource deficiencies owing to either excessive demand fluctuations (outflow
increase) and/or recent climate variations (natural inflow decrease) (cf. Ponce et al., 1999).
Because of the inextricable linkage of the groundwater aquifer as the receiving end-member
in the hydrological cycle to the upper surface water reservoirs (Fig. 2), groundwater
management strategies under external stresses needs to involve consideration of surface
water resources as well. For example, pumping water from aquifers that are hydraulically
connected with surface-water bodies can have a significant effect on those bodies by reducing
groundwater discharges to surface water and possibly causing outflow from those bodies into
the groundwater system. It follows that groundwater resources assessment in terms of
sustainability and future forecasting under the various pressures of humanity and climate
change can only be undertaken properly in an integrated manner within the context of the
complete hydrologic system (cf. Alley et al., 1999).
To that regard it is interesting to note that the European Water Framework Directive
increasingly recognizes that groundwater cannot be considered isolated from the regional
hydrological cycle and the surface waters, but needs to be managed holistically. This idea is
executed as part of Integrated Water Resources Management (IWRM) whereby all activities
within a river basin must be managed sustainably in a coherent manner in order to maximize
the economic and social benefits while preserving and/or restoring freshwater ecosystems.
2.1 Scales and variability of climate and hydrological systems
In order to understand the impact of climate variability (change) on the behavior of the
surface and subsurface water compartments of the hydrological cycle (Fig. 2), one must heed
the notion of the spatial and temporal (residence time) hydrological scale of the system
considered. Thus every-day experience tells us that surface-water systems such as river and
lakes respond much faster to short-tern climate fluctuations than groundwater systems, the
latter being only affected by long-term climate variations. In fact, this is another manifestation
of the “skin-effect” which pertains to the low-frequency filtering of the external climate signal
(i.e. effective precipitation or temperature) by the subsurface (Koch and Cekirge, 1996; Koch
and Markovic, 2007). Particularly, only the long-period oscillations penetrate deeper into the
aquifer, whereas the short-period fluctuations stay in the upper "skin" of the system.
Mathematically, the skin-effect is a consequence of the parabolic nature of the diffusion-like
partial differential equation that governs fluid flow within the groundwater system.
Studies of the variability of precipitation and shallow groundwater level time-series in
Florida by Koch and Cekirge (1996) and in the Elbe basin in Germany by Koch and Markovic
(2008) indicate variability scales in these two regions that are partially coincident with
known climate cycles, such as the El Niño/Southern Oscillation (ENSO) (2–6 yr) and the
North Atlantic Oscillation (NAO) (7–11 yr), respectively. These results show the importance
of inter-annual to inter-decadal climate variability on groundwater recharge estimates and
provide a better understanding of the climate-induced transients.
In any case, the overall consequence of this scale-phenomenon is that ground-water systems
respond more slowly to short-term variability in climate conditions than do surface-water
systems. While this may be beneficial when pumping from a groundwater aquifer during
times of low surficial recharge and extended drought seasons - as often done in conjunctive
water use policies (Bejranonda et al., 2006) - since the deeper aquifer is able to “buffer”
recharge deficiencies over a longer time – in fact, chemical and isotopic analyses of
groundwater have been used in some cases to reconstruct paleo-climatic conditions even
thousands of years ago -, while still providing sustainable yields, there are two sides of the
medal to this. If the aquifer has been over-drafted for a period of time too long below the
“safe yield” - as has been the case for the Bangkok multi-layer aquifer (Koch and Arlai,
2007) – it will need also a much longer time than a surface water reservoir to replenish again
during “hydrologic benevolent” climate periods. All this makes groundwater systems much
harder to manage than surface water systems in the face of climate change and requires a
longer-term policy-approach for water authorities to realize and to set up mitigation strategies
(Arlai et al., 2006). Nevertheless, as it becomes clear that groundwater will be pivotal to
sustainable water supplies because of its capacity to balance large fluctuation in atmospheric
input (effective precipitation) and human-driven demand, the urgency for action is imminent.
3. MODELING THE IMPACT OF CLIMATE CHANGE ON SURFACE- AND
GROUNDWATER RESOURCES
3.1 General issues related to hydro-climate modeling
Mathematical or numerical models have for a couple of decades been applied on a routine
basis for making hydrological predictions to support water resources management decisions.
In recent years hydrological models, - surface-water, groundwater models, or nowadays also
integrated (coupled) surface-groundwater models (see below) - are increasingly also being
used for making climate change impact- or hydro-climatic forecasts (Lettenmaier et al, 1994;
Whitfield and Cannon, 2000; Chiew and McMahon, 2002; Loaiciga, 2003; Wilby and Harris,
2006). To that avail the hydrological model employed is coupled sequentially to a global
climate model (GCM), whereby the latter provides the input forcing parameters (e.g.
precipitation and temperature) to the former. As most of these hydro-climatic studies are
performed on the regional or local scale, one is faced with another fundamental problem,
namely, how to link the different scales - the temporal, but more so, the spatial ones - the
GCM’s and the hydrological models are encompassing by virtue of their different theoretical
formulation, but also of present-day computational limits. The buzz-word threading through
the hydro-climatic literature to deal with this most fundamental issue is known under
“downscaling” and some of its basic concepts will be discussed further down.
As mentioned earlier, while there have been umpteen of hydro-climate modeling studies
dealing with the impacts of a range of climate change scenarios on future precipitation,
evaporation and temperature in various catchment basins across the world and to use these
variables to drive surface water models for quantity as well as quality of streamflow, those
related to groundwater aquifers are more scarce (Loaiciga et al., 2000; Loaiciga, 2003; Allen
et al., 2004; Scibek and Allen, 2006; Green et al., 2007, to name a few). Not to the least this
is due to the fact that the understanding of the physical processes that govern the downward
continuation of the relevant climatic surface signals into the subsurface groundwater reservoir
is far from being understood. Thus, being practically the slowest reacting end-member of the
hydrologic cycle, groundwater will be affected by climate variability and change by the nature
of the recharge, the kinds of interaction with the surface water system, and possibly changes
in water use demand only after a certain time, often too late to react appropriately.
Many of the groundwater responses to the climate impact signals precipitation and
temperature are most likely to be nonlinear. Water table levels of unconfined aquifers will be
impacted first and fluctuating the most. Decreases in groundwater recharge may not only
affect water supply, but also the water quality due to, for example, saltwater intrusion in
coastal aquifers, as appears to be the case for the Bangkok aquifer system (Arlai et al, 2006).
One problem is that many of the parameters in the hydro-climatic model equations cannot be
directly measured with meaningful precision and must be estimated by trial and error during
the modeling job. This holds for the hydrological models where specific yield, precipitation,
potential evaporation, groundwater levels and stream flows must be known as a function of time to be
able to run the model in transient mode and, even more so, for the driving climate model where there
is often a lack of adequate field data to validate its predictive power. Other open questions are (1)
how do climate-dependent factors change when climate changes? and (2) to which extent are
the process descriptions in the present hydrological models still valid in a changing climate?
3.2 From global to local scale: downscaling from the climate to the hydrological model.
Projections of climate change are based on global circulation models (GCM) or more
specifically Atmosphere-Ocean General Circulation Models (AOGCMs) which simulate the
atmospheric and oceanic processes across the globe and solve the corresponding partial
differential equations for mass and momentum on a three-dimensional grid in space and time.
Although the exponential growth of computer power has led to tremendous improvements in
the resolution of these models over the last two decades - since the publication of the first
assessment report (FAR) of the IPCC in 1990 to the last (fourth) one (AR4) (IPCC, 2007),
some of the models have now a horizontal resolution of about 2-1o (T101) and a division of
the atmosphere and the ocean in 30-50 vertical layers -, for routine hydrological impact
studies on the regional and local catchment scale, even this enormous resolution is often not
sufficient enough. Therefore these climate models only show averaged conditions, discarding
individual hydrology events that operate at smaller scales and more locally. While the future
may remedy this, for the hydrological community unsatisfactory situation with the advent of
still finer AOCGM’s, the present-day hydrological impact scenarios are left to the use of a
technique called downscaling (Wilby and Wigley, 2000) which consists essentially in the
conversion of output from a coarse-grid GCM, namely temperature and precipitation, onto a
fine grid suitable for hydrological simulation of the surface and/or subsurface water flow
processes under question. Fig. 3 illustrates the most salient features of downscaling process,
at it would be used for driving a coupled surface-water (HSPF or SWAT, Neitsch et al. 2005)
/groundwater model (MODFLOW-96, McDonald and Harbaugh, 1988).
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Fig.3. Downscaling a global
Fig. 3. Downscaling from a global climate model to a local coupled surface/groundwater
model with a conceptual application to a catchment in the Rayong province, Thailand.
Although the last decade has seen a huge amount of research devoted to the development and
analysis of various downscaling techniques (e.g. Wilby and Wigley,, 1997; 2000) the jury is
still out about which method is the most suitable in a particular hydro-climatic impact study
and it has been often recommended to use different downscaling techniques in parallel. In
any case, the proverb “you get what you pay for“ appears here to be highly applicable when
judging the various downscaling methods proposed. These can be classified as follows:
1). Dynamical Downscaling
Dynamical downscaling is practically nothing else than a continuation of coarse-gridded
GCM modeling on a nested higher resolution Regional Climate Model (RCM) (Wilby and
Wigley, 2000). RCM’s use the GCM to define time-varying atmospheric boundary conditions
around a finite domain from which the physical dynamics of the atmosphere are modelled,
using horizontal grid spacing of about 10 – 50 km. Numerous RCM’s have been developed
and applied to climate studies for specific regions and countries in the world, such as the
Canadian Regional Climate Model (CRCM), the MM5 and RegCM3 models for the US, the
REMO model for Germany and PRECIS from the Hadley Center, UK. While the RCM’s are
physically consistent, likewise to the GCM’s, and do better represent mesoscale phenomena
and weather extremes, some are suitable to use only with their “mother” GCM to handle the
enormous amount of input and boundary data in a comfortable manner. In any case, since
RCM’s, as GCM’s, are computationally extremely demanding, their regular application in
routine hydro-climate impact scenarios studies is still somewhat limited.
2). Statistical downscaling
Statistical downscaling techniques (Wilby et al., 2002 ) combine existing and past
empirical knowledge to address the disparity between coarse spatial scales of GCMs and
point meteorological observations. This methodology uses a statistically-based regression or
transfer model to determine a relationship between large-scale climate variables (the
predictors) and observed regional or local climate variables (the predictands). The derived or
calibrated regression model is then applied on similar predictors from GCM future climate
scenarios simulations to estimate the corresponding local or regional future climate
characteristics. Obviously, it is assumed that these relationships remain also valid in the
future; an issue which is still raising some concern.
Fig. 4. Statistical downscaling method using
the method of transfer function. Blue arrows
denote the calibration procedure for existing
predictors and predictands, red arrows the
procedure to be used when using the GCM
in a future climate impact study.
(adapted from
http://www.cics.uvic.ca/scenarios/index.cgi)
Another variant of the class of downscaling techniques is stochastic weather generation,
whereby parameters of a conventional weather generator are modified and scaled in direct
proportion to corresponding parameters in GCM model climate scenario to generate local
future climate data. While weather generators are computationally very expedient allowing
them to generate numerous Monte-Carlo realizations of possible climate scenarios, they are
not easy to calibrate on existing data. One of the most widely used weather generator in this
kind of studies has been the LARS-WG (Semenov et al., 1998).
4. INNOVATIVE APPROACHES TO MITIGATE THE IMPACT OF CLIMATE
CHANGE ON GROUNDWATER RESOURCES
In addition to the sophisticated hydro-climate modeling techniques discussed previously
to forecast possible impacts of climate change on water resources and groundwater, in
particular, a variety of innovative practical approaches have been undertaken to enhance the
sustainability of ground-water resources over short and intermediate time periods of increase
demand or availability. These approaches typically involve some combination of the use of
aquifers as storage reservoirs called Aquifer Storage and Recovery (ASR) (Allen et al., 1999),
artificial recharge of water through wells or surface spreading, and use of recycled or
reclaimed water – mostly favored in California -, and the conjunctive use of surface water and
groundwater as in some irrigation projects in Thailand (Bejranonda et al., 2006), all for the
purpose of weathering possibly climate-change-induced long periods of droughts. The
application of such methods lead to scientific questions about the extent and nature of
groundwater and surface-water interactions, geochemical effects of mixing water from
different sources (Arlai and Koch, 2007), the production and degradation of by-products from
injection of treated water, and other issues, the understanding of which usually requires some
kind of surface/groundwater flow and transport modeling (Bejranonda et al., 2007).
5. CONCLUSIONS
The effects of potential long-term changes in climate, caused either by man-made adverse
activities or by natural climate variability of internal or external origins on water resources,
namely, groundwater - which is most likely become the major source of water for human
consumption, as surface water resources are on the demise – has been discussed with regard
to its availability, sustainability and its counterpart, vulnerability. Climate change could affect
groundwater sustainability in several ways, including (1) changes in groundwater recharge
resulting from seasonal and decadal changes in precipitation and temperature, (2) more severe
and longer lasting droughts, (3) changes in evapotranspiration due to changes in temperature
and vegetation, (4) possible increased demands for ground water as a backup source of water
supply or for further economical (agricultural) development, and (5) sea water intrusion in
low-lying coastal areas due to rising sea levels and reduced groundwater recharge that may
lead a deterioration of the groundwater quality there. This appears to be situation for the
Bangkok coastal aquifer system (cf. Arlai et al., 2007).
Because groundwater systems tend to respond much more slowly to long-term variability in
climate conditions than surface-water systems, their management requires special long-term
consideration and ahead-planning. As the effects of ground-water development may require
many years to become evident, there may be an unfortunate tendency to forego the data
collection and analysis that is needed to support informed decision making until well after the
problems have materialized and the damage is irreversible.
Coupled hydro-climate models are needed to predict climate one or more decades ahead into
the future to assist in rational planning of water resource systems as water needs change. It is
important that these models predict trends at the decadal time scale, but also provide an
indication of the permanence of these changes to distinguish permanent changes from rather
temporary excursions from the present-day climate. In fact this is still a moot point. Although
model forecasts of future climate events are still imprecise, they nevertheless may represent
the best available decision-making information at the present time.
Some challenges and further research needs to better understand the effects of climate
variability and change on future water resources availability may be summarized as follows:
• evaluating and improving global climate models in terms of the most critical parameters for
hydrology, such as extremes of precipitation, and evapotranspiration incorporating humidity,
cloudiness, and radiation;
• better representation of yearly- to decadal-scale climate variability in global climate models
through representations of driving mechanisms such as El Nino, the Inter-decadal Pacific
Oscillation for the Pacific region and the Northern Atlantic Oscillation (NAO) and the Arctic
Oscillation for the northern Atlantic hemisphere (Marcovic and Koch, 2005);
• reducing uncertainty in climate projections through further research into methods of
determining and narrowing uncertainty for particular applications such as hydrology;
• studies to separate anthropogenic-induced changes from natural climate change and
variability by also including high resolution paleo-records of hydrological and climatological
parameters and analyzing them with modern methods of stochastic time series analysis (cf.
Koch and Markovic, 2007; 2008);
• further development and application of downscaling methods that represent climate at the
relatively fine spatial and temporal scales of landscape hydrology;
• better representation of the continental physical hydrology in global climate models to
simulate the interactions between climate and hydrology;
• better understanding of large-scale physical hydrology and its effects on the subsurface
recharge process (Allen et al., 2004; Scibek and Allen, 2006) to be able to project future
hydrological behavior for unprecedented climate conditions;
• improving the understanding of the interactions of groundwater with land and surface
water resources by developing better integrated surface water/groundwater models;
• get a better hold on the importance of feedbacks through vegetation on hydrology and how
these might change under future climate and CO2 concentrations (Green et al., 2007);
• increased use of remotely sensed data for climatological and hydrological applications as
from the very promising GRACE earth satellite project (Rodell and Faminglietti, 2002).
In conclusion, proper consideration of climate variability and change will be a key – at
present still underemphasized - factor in ensuring the sustainability and proper management
of water resources and groundwater in particular. The achievement of this goal will require
more collaboration across the fields of climatology and hydrology, as more reliable methods
for water planning that provide planning certainty for water users under the impact of climate
change must be developed.
6. ACKNOWLEDGEMENTS
The author thanks cordially Prof. Dr.-Ing. Phatcharasak Arlai from the Research Unit for
Sustainable Water Resources and Environmental Management at the Program of Civil and
Environmental Engineering, Faculty of Science and Technology, Nakhon Pathon Rajabhat
University, Thailand, for the initiation of this review and the possibility of presenting it as a
keynote speaker at the 2008 NPRU Academic Conference in Nakhon Pathom.
7. REFERENCES
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