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 change—albeit on the basis of imperfect knowledge—and 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 change—considering, 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:

• A current hydrological and climatic regime that is marginal for agriculture and livestock
• Highly seasonal hydrology as a result of either seasonal precipitation or dependence on snowmelt
• High rates of sedimentation of reservoir storage
• Topography and land-use patterns that promote soil erosion and flash flooding conditions
• Lack of variety in climatic conditions across the territory of the national state, leading to inability to relocate activities in response to climate change.

Societal characteristics that maximize susceptibility to climate change include:

• Poverty and low income levels, which prevent long-term planning and provisioning at the household level
• Lack of water control infrastructures
• Lack of maintenance and deterioration of existing infrastructure
• Lack of human capital skills for system planning and management
• Lack of appropriate, empowered institutions
• Absence of appropriate land-use planning
• High population densities and other factors that inhibit population mobility
• Increasing demand for water because of rapid population growth
• Conservative attitudes toward risk [unwillingness to live with some risks as a tradeoff against more goods and services (risk aversion)]
• Lack of formal links among the various parties involved in water management.

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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