The availability of water for human and ecosystem use depends on two main factors: (1) the climate-driven global water cycle and (2) society's ability to manage, store, and conserve water resources. Each of these factors is complex, as is their interaction. Water cycling—which includes evaporation and transpiration, precipitation, and both surface runoff and groundwater movement—determines how freshwater flows and how it interacts with other processes. Precipitation amounts, intensity, geographic distribu
10 For additional discussion and references, see Chapter 8 in Part II of the report.
tion, and other characteristics matter for water management, and all are affected by both short-term climate variability and long-term climate change. Likewise, soils, topography, land cover, precipitation intensity, and other variables influence how much precipitation can be stored for use. Other variables such as level of consumption, pollution, conservation, pricing, distribution, and land use changes are also important for water management decisions. These complex processes prevent any easy conclusions about regional water supplies based solely on climate model-driven projections. Nonetheless, historical and current changes in some variables are becoming clear.
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