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  • 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 earths 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 0.6C 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 5C 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...

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