Horses and Heavy Rain: Did Technology-Driven Land-Use Change Alter Regional Hydroclimate over the Central United States?

Agricultural operations in the central U.S. used work animals such as horses from European settlement until the mid-20th century. Work animals were gradually replaced by motorized equipment over the period from about 1925 through 1960. With greatly reduced need to grow oats for horse feed, about 95% of the land that had been used for producing oats (totaling 19 million hectares) was converted to more profitable crops such as soybeans and maize. In the same region the mid- to late-20th century saw a pronounced shift of the precipitation intensity spectrum toward heavier events. Is there a relationship between this technology-driven land use change and the observed shift in regional hydroclimate? We explore this possibility through multi-decadal simulations over the central U.S. using the WRF-ARW regional climate model coupled with the Community Land Model (CLM 4.5). Cropland planted in maize, soybean, winter wheat, small grains (including oats and spring wheat), and other C3 and C4 crops was reconstructed on a decade by decade basis from 1940-2010 using county-level crop data. These crop distributions were used as land surface boundary conditions for two multi-decadal regional climate simulations, one with 1940s land use and another with modern (circa 2010) land use. Modern land use produced a shift in the simulated daily precipitation intensity spectrum toward heavy events, with higher frequencies of heavy precipitation amounts and lower frequencies of light amounts compared to 1940s land use. These results suggest that replacement of work animals by mechanized transport led to land use changes that may have contributed about 20% of the observed increase in the frequency of intense precipitation over the central United States.