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 - man-

made 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 cycle-

will 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 semi-

coupled 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 ground-

and 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?

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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).

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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