I mentioned this paper last week [h/t to Faisal Hossain] but want to discuss further today. The paper is
DeAngelis, A., F. Dominguez, Y. Fan, A. Robock, M. D. Kustu, and D. Robinson (2010), Evidence of enhanced precipitation due to irrigation over the Great Plains of the United States, J. Geophys. Res., 115, D15115, doi:10.1029/2010JD013892.
“At the end of World War II, there was a rapid increase in irrigation over the Ogallala Aquifer in the Great Plains of the United States via groundwater withdrawal, and we hypothesize that this disruption of the local hydrological cycle has enhanced the regional precipitation. We examined station and gridded precipitation observations for the warm season months over and downwind of the Ogallala over the 20th century. Increases in precipitation of 15–30% were detected during July from the easternmost part of the aquifer to as far downwind as Indiana. The timing (1940s, July) and spatial pattern of the precipitation increase are consistent with the history of Ogallala irrigation and mechanisms by which increases in evapotranspiration can affect convection. Additionally, we conducted a vapor tracking analysis and found that evapotranspiration over the Ogallala Aquifer contributes to downwind precipitation and that the contribution is greater when the evapotranspiration is higher. This makes it hydrologically possible that the irrigation development was associated with the observed precipitation increases. Finally, there is no clear evidence that atmospheric circulation changes or modes of internal climate variability increased the July precipitation. Further analysis of the influence of Ogallala irrigation on precipitation will include the controlled analysis of climate model simulations that explicitly include irrigation.”
Text from the paper includes
“The effect of this human alteration of the natural water cycle on regional precipitation over this area is the subject of this study. We hypothesize that the increase in irrigation over the 20th century resulted in a detectable enhancement of precipitation over the Great Plains. An analysis of long‐term precipitation observations and simulations is combined with wind observations and vapor transport analysis to search for the link between irrigation and increases in precipitation over the region.
The mechanisms linking increased irrigation and enhancement of precipitation are most likely related to the effects of increased ET on precipitable water and convection over this region. The possibility of convection being influenced by irrigation is supported by the fact that most irrigation over the Ogallala occurs in July and August (Figure 1c) when more than 80% of precipitation originates from thunderstorms [Changnon, 2001]. Convection is associated with the convective available potential energy (CAPE) of the atmosphere, which increases with warmer and moister lower tropospheric conditions. Higher values of CAPE make convection more likely when synoptic conditions are favorable for convection, or can be the difference between convection and no convection if synoptic conditions are borderline favorable [Barnston and Schickedanz, 1984; De Ridder and Gallée, 1998]. It follows that if irrigation influences lower troposphere temperature and moisture, it will impact CAPE and therefore convective precipitation.”
This excellent research contribution illustrates why fossil water irrigation introduces another additional human climate forcing that was not assessed in the 2007 IPCC reports.