Rain falls on a crop field, and the plants use it to grow. But grass, shrubs or trees would need much of that rain too if the land had never been farmed.

Researchers in Chile developed a new way to calculate how much additional water farms and tree plantations use compared with natural vegetation.

The usual water footprint estimate was about 2.7 times as large as their revised total, and the entire gap came from how they counted rain. Doctoral student Steven Hidalgo-Toapanta and Patricio Neumann-Langdon of the Universidad del Bío-Bío in Chillán, Chile, worked with Pasiano Rivas-García of Mexico’s Universidad Autónoma de Nuevo León.

They based their estimates on weather records, crop data and a national farm census covering 2021 and 2022, rather than measurements in the field. “Rainfall would still be used by vegetation without agriculture or forestry,” said Neumann-Langdon. “Our approach asks how much human land use actually changes natural water flows, rather than counting all water passing through crops.”

Water footprints tally the water crops use. Blue water comes from rivers or groundwater for irrigation, while green water is rain that soaks into the soil and plants then use. The standard method, described in the Water Footprint Network’s 2011 manual, includes all that green water in the crop’s total. Yet a 2014 study in northern Europe found that tree plantations and native forests could need similar amounts of rain. The Chilean team compared 100 crops, including wheat, rice, cherries, pine and eucalyptus, with well-watered grass standing in for wild vegetation.

They repeated the comparison each month in 149 municipalities across five regions of south-central Chile. Only the rain a crop used beyond what the grass would use counted as water taken from nature. Under natural vegetation, that extra water would have flowed into streams or soaked into the ground.

Across the study area, farming and forestry took an estimated 1.3 trillion gallons (5 billion cubic meters) of water from nature each year. That could fill roughly 2 million Olympic swimming pools. Most of this water came from irrigation. Rain accounted for 77% of the standard estimate, but its share fell to 39% under the new method. For cereals such as wheat and corn, along with the other field crops studied, the extra rainwater use fell to zero.