Over the past decade—and increasingly since the publication of the Second Assessment Report (SAR) (Arnell et al., 1996; Kaczmarek, 1996)—there have been many studies into climate change effects on hydrology and water resources (see the online bibliography described by Chalecki and Gleick, 1999), some coordinated into national programs of research (as in the U.S. National Assessment) and some undertaken on behalf of water management agencies. There are still many gaps and unknowns, however. The bulk of this chapter assesses current understanding of the impacts of climate change on water resources and implications for adaptation. This section highlights significant developments in three key areas since the SAR: methodological advances, increasing recognition of the effect of climate variability, and early attempts at adaptation to climate change.

The impacts of climate change on hydrology usually are estimated by defining scenarios for changes in climatic inputs to a hydrological model from the output of general circulation models (GCMs). The three key developments here are constructing scenarios that are suitable for hydrological impact assessments, developing and using realistic hydrological models, and understanding better the linkages and feedbacks between climate and hydrological systems.

The heart of the scenario “problem” lies in the scale mismatch between global climate models (data generally provided on a monthly time step at a spatial resolution of several tens of thousands of square kilometers) and catchment hydrological models (which require data on at least daily scales and at a resolution of perhaps a few square kilometers). A variety of “downscaling” techniques have been developed (Wilby and Wigley, 1997) and used in hydrological studies. These techniques range from simple interpolation of climate model output (as used in the U.S. National Assessment; Felzer and Heard, 1999), through the use of empirical/statistical relationships between catchment and regional climate (e.g., Crane and Hewitson, 1998; Wilby et al., 1998, 1999), to the use of nested regional climate models (e.g., Christensen and Christensen, 1998); all, however, depend on the quality of simulation of the driving global model, and the relative costs and benefits of each approach have yet to be ascertained. Studies also have looked at techniques for generating stochastically climate data at the catchment scale (Wilby et al., 1998, 1999). In principle, it is possible to explore the effects of changing temporal patterns with stochastic climate data, but in practice the credibility of such assessments will be strongly influenced by the ability of the stochastic model to simulate present temporal patterns realistically.

Considerable effort has been expended on developing improved hydrological models for estimating the effects of climate change. Improved models have been developed to simulate water quantity and quality, with a focus on realistic representation of the physical processes involved. These models often have been developed to be of general applicability, with no locally calibrated parameters, and are increasingly using remotely sensed data as input. Although different hydrological models can give different values of streamflow for a given input (as shown, for example, by Boorman and Sefton, 1997; Arnell, 1999a), the greatest uncertainties in the effects of climate on streamflow arise from uncertainties in climate change scenarios, as long as a conceptually sound hydrological model is used. In estimating impacts on groundwater recharge, water quality, or flooding, however, translation of climate into response is less well understood, and additional uncertainty is introduced. In this area, there have been some reductions in uncertainty since the SAR as models have been improved and more studies conducted (see Sections 4.3.8 and 4.3.10). The actual impacts on water resources—such as water supply, power generation, navigation, and so forth—depend not only on the estimated hydrological change but also on changes in demand for the resource and assumed responses of water resources managers. Since the SAR, there have been a few studies that have summarized potential response strategies and assessed how water managers might respond in practice (see Section 4.6).

There also have been considerable advances since the SAR in the understanding of relationships between hydrological processes at the land surface and processes within the atmosphere above. These advances have come about largely through major field measurement and modeling projects in different geographical environments [including the First ISLSCP Field Experiment (FIFE), LAMBADA, HAPEX-Sahel, and NOPEX; see www.gewex.com], coordinated research programs (such as those through the International Geosphere-Biosphere Programme (IGBP; see www.igbp.se) and large-scale coupled hydrology-climate modeling projects [including GEWEX Continental-Scale International Project (GCIP), Baltic Sea Experiment (BALTEX), and GEWEX Asian Monsoon Experiment (GAME); see www.gewex.com/projects.html]. The ultimate aim of such studies often is to lead to improved assessments of the hydrological effects of climate change through the use of coupled climate-hydrology models; thus far, however, the benefits to impact assessments have been indirect, through improvements to the parameterizations of climate models. A few studies have used coupled climate-hydrology models to forecast streamflow (e.g., Miller and Kim, 1996), and some have begun to use them to estimate effects of changing climate on streamflow (e.g., Miller and Kim, 2000).

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