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accounted for 61 percent of all presidential disaster declarations in the decade starting in April 1974 (Riebsame et al., 1986).

CHANGES IN VULNERABILITY TO HYDROLOGIC VARIABILITY

Factors Tending to Reduce Vulnerability

The susceptibility of the West's economy to hydrologic extremes has changed over time. A decline in the economy's dependence on water and an increase in the control over supplies have tended to make the West less sensitive to changes in water supplies. Water's influence on economic development generally weakened during the last century. Development of steam engines, internal combustion motors, and electricity generation and transmission reduced the significance of on-site water power. Expansion of railroads, highways, and air transport diminished the importance of water-based transport. Water intensive industries such as irrigated agriculture declined in relative importance, and industries in general learned to prosper with less water (National Water Commission, 1973).

The tremendous expansion of the infrastructure to store and transport water and to tap ground water supplies also has tended to reduce the susceptibility of the nation's economy to climate variability. More than 63,000 dams with 869 million acre-feet of storage are included in the 1982 inventory of the nation's dams. More than three-fourths of these dams and two-thirds of the storage were completed since 1945 (U.S. Army Corps of Engineers, 1982).1 About 47 percent of these dams and 55 percent of the storage are in the 17 western states, giving the region considerable capacity to prevent floods and to supply water during drought. Ground water also provides an important buffer against fluctuations in surface supplies in many areas of the West. Ground water use was essentially limited to areas with low pumping depths or artesian pressure until technological advances in the 1930s made it feasible to pump water from much greater depths. Water stored within deeper aquifers is less susceptible to climate variations, but the economies of some areas have become dependent on the use of nonrenewable ground water supplies.

Factors Tending to Increase Vulnerability

Countering these changes are several trends tending to make the



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