Rainfall increase linked to aquifer activity

A study from 2010 in the Journal of Geophysical Research: Atmospheres found that July rainfall has increased by 15 to 30 percent from the eastern part of the Ogallala aquifer up to Indiana. This is a statistical observation, not proof of causation.

Evapotranspiration and rainfall connection

The study suggests that evapotranspiration from the Ogallala aquifer contributes to downwind rains, based on a vapor tracking analysis. No clear evidence links this increase to a natural change in atmospheric circulation.

Historical data and rainfall patterns

A century of data reviewed by DeAngelis' team examined station records and gridded precipitation data for hot months throughout the 20th century, above and downwind of the Ogallala. The signal emerges in July: between 15 and 30 percent more rainfall, from the east of the aquifer up to Indiana.

Timing and irrigation development

The timing aligns with the history of irrigation. At the end of World War II, pumping irrigation rapidly developed above the Ogallala, and the increase in July rainfall dates back to the 1940s. July also falls in the season when irrigation is highest.

Geographic pattern and irrigation

The geographic pattern of the increase, stretching downwind from the east of the aquifer, is consistent with this. Economists have compared counties above the Ogallala to similar neighboring counties. Access to groundwater has increased the intensity of irrigation and, initially, reduced the impact of droughts.

Water movement and vapor tracking

The vapor path, from aquifer to cloud, underground, the water of the aquifer advances west to east at about 1 foot per day, or about 30 centimeters. Once evaporated, it travels at the rate of the winds.

Evapotranspiration and atmospheric effects

The authors suggest that increased evapotranspiration humidifies the atmosphere and promotes convection, these upward movements of air that form thunderstorm clouds. Around a pivot that turns in the middle of summer, the act seems trivial; at the scale of an entire region, it could weigh on the rains.

Verification through vapor tracking

To verify that the idea holds physically, researchers conducted a vapor tracking analysis. It shows that evapotranspiration above the Ogallala contributes to downwind rains, and the more this evapotranspiration is high, the more it contributes.

Simulations and irrigation impact

In 2019, Yang and his colleagues ran simulations with and without irrigation (Journal of Geophysical Research: Atmospheres, volume 124). The summary relayed by the ARM program indicates that precipitation increases mainly in the Midwest and Southeast United States, downwind of the irrigated area.

Observational study and model limitations

The 2010 study is observational: it associates an increase in rainfall with a period and a geography, without isolating the cause. The 2019 simulations go in the same direction, but a model first shows what happens in the model.

Significance of the Ogallala aquifer

The aquifer is not a detail of the landscape. About 27 percent of irrigated land in the United States is above it, and it provides about 30 percent of the groundwater used for irrigation in the country.

Depletion and future projections

Since 1950, irrigation has reduced its saturated volume by about 9 percent, according to estimates. The U.S. Department of Agriculture notes that nothing has compensated for withdrawals since the postwar agricultural boom.

Drinking water and future challenges

The aquifer also supplies drinking water for 82 percent of the 2.3 million people counted in 1990 in the study area. Once depleted, it would take more than 6,000 years to replenish through rainfall.

Climate projections and regional impacts

Climate projections complicate the equation further: in many models, the region served by the aquifer becomes warmer and drier as the aquifer level drops. In the north, in Nebraska, rainfall should rather increase, while the south could receive less than the current 16 inches annually, or about 406 mm.