West of the one hundredth meridian, the average annual rainfall is generally less than twenty inches except for higher mountainous areas and the humid area west of the Cascade range in western Washington and Oregon and an area west of the Coast range in northwestern California. With less than twenty inches of rainfall, irrigation and dry-farming methods are required to supplement and conserve the native moisture if successful crops are to be produced. At twenty inches of rainfall successful agriculture is subject to extreme hazards from frequent droughts that call to mind the tragedy of crop failure and dust storms. In the humid Pacific Northwest the dry summer climate introduces another variable limiting successful agricultural production without supplementary water.
American institutional arrangements, sustenance patterns and resource policies were conceived in humid England and developed in the humid regions of the United States. However, the general aridity of the West stands in marked contrast to the humidity that prevailed in the physical environment where American social institutions and traditions were formed. This alteration of the physical environment has caused an important shift in the balance of human ecology requiring significant modification in institutional arrangements and social policy, especially in regard to the control and development of natural resources.
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