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We examine the role of irrigation in explaining U. S. agricultural gains post-1940. Specifically, we analyze how productivity and farm values changed in the western United States as a result of technological and policy changes that expanded access to ground and surface water. To statistically identify the effects, we compare counties based on their potential access to irrigation water defined by physical characteristics. We find areas with access to large streams and/or groundwater increase crop production relative to areas with only small streams by 19 billion annually, equivalent to 90 percent of the total annual increase in the western United States after 1940.
Edwards et al. (Fri,) studied this question.