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American water rights challenge existing uses in the West (see Southwest region)."' By changing the existing patterns of precipitation and runoff, climate change will add another stress to existing problems. Changing, water detriands Water demands are expected to change with in- creased temperatures. Evaporation is projected to increase over most of the United States as tempera- tures rise. Higher temperatures and longer dry peri- ods are expected to lead to increased water demand for irrigation. This may be partially offset by more efficient use of water by plants due to rising atmo- spheric carbon dioxide. Higher temperatures are projected to increase cooling water withdrawals by electrical generating stations. In addition, greater cooling requirements in summer will increase elec- tricity use, which in turn will require more cooling water for power plants. Industrial and municipal demands are expected to increase slightly. 146 II ° ° °Ilhm� IIpast c(Ninn tuiiry is no II loiru�g(Nir urv�au� oirui,au�lll ik� )gui iii dk� to autc,ir uin,a ini, in(: :ruriut. Water planning and management have been based on historical fluctuations in records of stream flows, lake levels, precipitation, temperature, and water demands. All aspects of water management including reservoir sizing, reservoir flood operations, maximum urban stormwater runoff amounts, and projected water demands have been based on these records. Water managers have proven adept at balancing supplies and demand through the significant climate variability of the past century. 14' Because climate change will significantly modify many aspects of the water cycle, the assumption of an unchanging climate is no longer appropriate for many aspects of water planning. Past assumptions derived from the historical record about supply and demand will need to be revisited for existing and proposed water projects. 142,151,174 Drought studies that consider the past 1,200 years indicate that in the West, the last Water II'' �esrluir °ceem century was significantly wetter than most other centuries. Multi - decade "megadroughts" in the years 900 to 1300 were substantially worse than the worst droughts of the last century, including the Dust Bowl era. The causes of these events are only partially known; if they were to reoccur, they would clearly stress water management, even in the absence of climate change (see figure below). 97,149,189 The intersection of substantial changes in the water cycle with multiple stresses such as population growth and competition for water supplies means that water planning will be doubly challenging. The ability to modify operational rules and water allocations is likely to be critical for the protection of infrastructure, for public safety, to ensure reli- ability of water delivery, and to protect the environ- ment. There are, however, many institutional and legal barriers to such changes in both the short and long term.190 Four examples: • The allocation of the water in many interstate rivers is governed by compacts, international treaties, federal laws, court decrees, and other agreements that are difficult to modify. Reservoir operations are governed by "rule curves" that require a certain amount of space to be saved in a reservoir at certain times of Long Term Aridity Changes in the West S a 4 VOY I 0 $00 91 1000 1100 1200 1300 1400 1600 1600 7700 7300 1 0O 2 Year Cook et € l 189 The black line shows the percentage of the area affected by drought (Palmer Drought Severity Index less than —1) in the West over the past 1,200 years. The red line indicates the average drought area in the years 900 to 1300. The blue horizontal line in the yellow box indicates the average during the period from 1900 to 2000, illustrating that the most recent period, during which population and water infrastructure grew rapidly in the West, was wetter than the long -term average (thin horizontal black line).189 Droughts shown in the period 1100 -1300 significantly exceed those that have occurred over the past 100 years. 49