Agriculture and irrigation affect rainfall in the U.S. Midwest

According to a study, the interaction between groundwater resources, crops and irrigation impacts the rate of rainfall recycling with a direct effect on agricultural management and water availability

by Matteo Cavallito

 

Agricultural practices and irrigation could have significantly affected rainfall dynamics in the U.S. Midwest, a key region for food production in the country. The hypothesis is contained in research published in the journal Proceedings of the National Academy of Sciences that focuses on the phenomenon of rainfall recycling, the process by which moisture released into the atmosphere by plants, soil, lakes and other landscape features is returned to the soil as rain.

Agriculture, recycling and rainfall

The group of scientists, led by Zhe Zhang, a researcher at the National Center for Atmospheric Research (NCAR) of the National Science Foundation (NSF) in Boulder, Colorado, focused on the so-called Corn Belt, the agricultural region located in the west-central U.S.

“The US Corn Belt, a major foodbasket in the world, has undergone vast agricultural expansions through history, which have profoundly transformed the landscape and water cycle,” the study says.

However, “previous modeling studies with simplifying assumptions or missing physical processes have struggled to accurately quantify the impact of land surface on precipitation recycling, a key process where evapotranspiration (ET) contributes to precipitation in the same region.” The research therefore aimed to quantify the impact of agricultural activities on recycling by highlighting their effect on precipitation during the growing season of crops.

Recycling rate reaches 18 percent

Using high-resolution atmospheric simulation models, scientists could analyze the interaction between groundwater, crop growth and irrigation. The researchers, in detail, compared simulations that included all of these factors with others that were partially devoid of them. The study looked at three specific years: 2010, which was wetter than usual; 2011, which had average rainfall; and 2012, a dry year.

“The increase in precipitation is attributed to a significant enhancement of the precipitation recycling ratio from 14 to 18%,” the research notes.

This result is related to a combination of surface groundwater, corn plants releasing moisture into the atmosphere and evaporation from irrigation systems. Without these factors, the recycling rate would have been only 14 percent, or 30 percent less. The researchers observed a higher fraction of recycled precipitation in 2012, when moisture from other regions, such as the Gulf of Mexico, was more limited. In general, the research notes, the phenomenon of recycled rainfall “is stronger in a dry year than normal and wet years, depending on both large-scale moisture transport and local evapotranspiration.”

Water and food security

The research provides new information on a multifaceted phenomenon, the authors explain. “This research shows how agricultural practices can modify regional climate, with implications for food and water security,” explained Zhe Zhang, the study’s lead author. “In an agricultural region like the U.S. Corn Belt where rainfall is critical, it’s important for both farmers and water resource managers to understand where the rain comes from.”

The researchers announced that they will now conduct further studies to investigate how changes in rainfall may affect agricultural productivity. Understanding rainfall recycling, they add, not only helps improve climate predictions, but also provides useful insights for planting strategies and water resource management.