Making the Most of Water

Every person on the planet needs food to live. And as the world’s population grows, so does the size of that demand. But producing food involves more than just planting seeds. Agriculture requires water, and quite a lot of it. How can we make sure Earth’s finite supply of water is best apportioned to grow the growing amount of food we’ll need?

Scientists know how many calories a person needs to consume, and how many people need to eat those calories. What if we could predict precisely how much water is needed to produce one calorie of food? And what if we could use exactly that quantity of water to produce it?

Kate Brauman, a postdoctoral fellow in the Institute on the Environment’s Global Landscapes Initiative, hopes to answer those questions. By comparing global maps of precipitation, managed irrigation and weather patterns with the yields of 16 common crops, Brauman is learning just how much water is required to grow different kinds of food in different places.

GLI combines statistical information from various sources to create global maps of crop yields. “Take satellite imagery, which tells you where cropland is but not what’s being grown there, and crop census data, which tells you what’s being grown in a country but not where,” Brauman says. “We’re bringing these together to spit out maps that say, this is what is being grown where.”

Brauman’s role in the initiative is to add a water element by comparing the global crop yield data with water usage data. Our instincts tell us, Brauman explains, that drier locations need more water and wetter locations need less water to produce the same yields. To quantify this, she has divided the planet into 25 climate “bins,” based on average annual precipitation and a measure of average annual evapotranspiration (movement of water from plants into the air). Places with similar climates fall into the same bins, allowing Brauman to compare crop yield data from far-flung locations.

Brauman uses this approach to map locations of irrigated crops, the quantity of water added, and each crop’s yield. The maps can show “yield gaps”—locations where more crops could be grown. So far, Brauman says, irrigation management appears to play as big a role as climate in crop water use.

Brauman’s research leads to more questions. For example, if farmers use precisely the needed amount of water to grow a crop, will the soil remain healthy? Brauman points out that plants take only the water and nutrients they need from soil, leaving salts behind. Here in Minnesota, rain and snowmelt flush salts out of the soil every year. But in a drier climate, such “perfect irrigation” could render the soil unusable.

Many questions and details remain, but if scientists can figure out how to best use our limited global water supply without sacrificing water quality, they can increase the likelihood of feeding a growing population for years to come.

ASHLEY KUEHL is a freelance writer from Minneapolis. She has written about sustainability and the environment for Twin Cities Daily Planet and Momentum.