The preceding sections have assessed the potential effect of climate change on river flows, groundwater recharge and other biophysical components of the water resource base, and demands for that resource. The consequences, or impacts, of such changes on risk or resource reliability depend not only on the biophysical changes in streamflow, recharge, sea-level rise, and water quality but also on the characteristics of the water management system. This section considers what possible changes in hydrology and demand will mean for water supply, flood risk, power generation, navigation, pollution control, recreation, habitats, and ecosystems services in the absence of planned adaptation to climate change. In practice, of course, the actual impacts of climate change will be rather different because water managers will make incremental or autonomous adaptations to changealbeit on the basis of imperfect knowledgeand the impact of change will be a function of adaptation costs and residual impacts. However, very few studies have incorporated deliberate adaptation strategies (Alexandrov, 1998, is one), and studies that do not consider adaptation provide a base case for assessing the magnitude of the climate change problem. More significant, some studies have not accounted for nonclimatic changes in the way water resources are managed or systems are operated and have applied the future climate to the present management system. This is unrealistic, but the extent of adaptation by many water managers is uncertain. It is important to assess the effect of climate change by, say, the 2050s in the context of the water management system that would exist by then in the absence of climate changeconsidering, for example, changes in demand or legislative requirements.
The sensitivity of a water resource system to climate change is a function of several physical features and, importantly, societal characteristics. Physical features that are associated with maximum sensitivity include:
Societal characteristics that maximize susceptibility to climate change include:
This section first considers the global-scale implications of climate change on broad measures of water resources then assesses in more detail potential impacts on defined systems.
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