The impact of summer rainfall on the temperature gradient along the United States-Mexico border

The international border running through the Sonoran Desert in southern Arizona and northern Sonora is marked by a sharp discontinuity in albedo and grass cover, due to long-term, severe overgrazing of the Mexican lands. The Mexican side of the border has higher surface and air temperatures, associated with differential evapotranspiration rates rather than with albedo changes along the border. On a seasonal time scale, the temperature gradient increases with higher moisture levels, probably due to a vegetative response on the United States' side of the border; at the daily level, the gradient in maximum temperature decreases after a rain event as evaporation rates equalize between the countries. The results suggest that temperature differences between vegetated and overgrazed landscapes in arid areas are highly dependent upon the amount of moisture available for evapotranspiration. -from Author