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A rainy drought

Speed read
  • Climate change will increase drought, despite consistent rainfall
  • Warming and seasonal shift ramp up evaporation, lowering the water in soil
  • This will impact planting seasons and potentially reduce agricultural output

Much of what you think you know about droughts is probably wrong.

Unless you study hydrological systems, you may think that droughts are caused by a lack of rain. Water stops falling from the sky, and as a result the land dries up. Crops fail, fires kindle more easily, and people starve.

<strong>Predicting future water supply.</strong> Researchers from Indiana University recently modeled the potential effects of climate change on the Wabash River. Courtesy Quinn Thomson. <a href='https://creativecommons.org/licenses/by-sa/3.0/deed.en'>(CC BY-SA 3.0)</a>While this is true of many water scarcities, climate change will most likely bring a different kind of aridity to the Wabash River Basin that occupies most of Indiana. In a few decades, Hoosiers will likely experience rain-filled droughts.

This prediction is based on the work performed by Indiana University (IU) Professor Chen Zhu and his post-doctoral colleague Dr. Jennifer Dierauer. Their recent paper relies on a hydrological modeling tool that uses climate change predictions to better understand Indiana’s future water supply. Dierauer and Zhu found that despite rainfall amounts similar to current levels, Indiana’s soil won’t have the same moisture content.

Different droughts

To understand this, it’s important to know that there are different kinds of droughts. Meteorological drought is the kind most people recognize, where the sky produces less rain than the area usually receives. Hydrological and soil moisture droughts are a little different.

<strong>Even if rainfall remains steady,</strong> earlier spring warming and higher annual temperatures would lead to increased evaporation and less available water overall--a phenomenon known as hydrological drought.“So, you have runoff and the runoff goes to the stream and then goes to the rivers,” says Dierauer. “In hydrological drought, we might not have much reduction in rainfall, but you still see less water in your streams and rivers or reservoirs that are available for you to use.”

To help explain why the same amount of rain could result in less moisture, Zhu suggests imagining you have scooped up a fistful of beach sand. “In your fist the sand has grains and between the grains is a void,” he says. “That void is occupied by both air and water, and that's what we call soil moisture.”

Both hydrological and soil moisture droughts can be caused by a lack of rainfall, but other factors come into play, such as rising temperatures. As Dierauer points out, the global warming associated with climate change is going to alter water levels in the Wabash River Basin.

“By the 2080's, in a worst-case carbon emission scenario, we could see 10 degrees Fahrenheit increase of temperature in the summer,” says Dierauer. “One of the big factors is evaporation. The water falling down from the sky will evaporate back to the sky again because of the heat from sunshine. A small increase in evaporation will mean a reduction in the water available in streams, rivers, and in the soil.”

<strong>The Wabash River runs for 503 miles</strong> from its headwaters near Ohio’s Western border, through most of  Indiana to southern Illinois. Courtesy Kmusser. <a href='https://creativecommons.org/licenses/by-sa/3.0/deed.en'>(CC BY-SA 3.0)</a>That said, hotter summers leading to increased evaporation isn’t the only aspect of climate change we need to worry about. Dierauer also mentions seasonal shift, which will bring about spring temperatures earlier than normal for the region.

Both of these factors combined leads to less water in rivers, streams, and the soil, which will have a major impact on agriculture. The Wabash River Basin covers nearly two-thirds of Indiana, and nearly three-quarters of this watershed is devoted to agricultural purposes. If these predictions prove true, one of America’s most important breadbasket states won’t be able to produce food like it has in the past.

The right tool for the job

Dierauer and Zhu’s work relied on the Soil and Water Assessment Tool (SWAT) hydrological model. Built by the US Department of Agriculture, SWAT is used to simulate surface and groundwater in watersheds and river basins. Zhu felt lucky to have it.

“When you use software from a federal government agency, you have the benefit that it's documented, and it has quality control and assurance,” says Zhu. 

Zhu describes SWAT like a budgeting tool. You get a certain amount of money in your paycheck, but you also have to spend some of it on food, rent, and utilities. If you spend more than you make, you have a deficit. If you spend less, you have a surplus. 

<strong>Where food comes from.</strong> Nearly three-quarters of land in the Wabash River Basin is devoted to agriculture. Reduced water could negatively impact crops in this important breadbasket state.“For SWAT there is in some sense a differential equation, but the basic equation is a mass balance, like money accounting,” says Zhu. “How much water is coming into the Wabash? Where does the water go? But in the end, it should balance.”

Like most scientists, Zhu wasn’t able to do all of this on his own. XSEDE (The Extreme Science and Engineering Discovery Environment) provided computational resources, and the Cyberinfrastructure Integration Research Center helped him create the FutureWater gateway. This web-based tool allows the general public, as well as government, industry, and environmental stakeholders to explore modeling forecasts for the Wabash River Basin.

Zhu is quick to point out that he’s a scientist and not someone out to make a political statement about climate change. However, he does think this kind of work can bring people together.

“We’re just getting the best data and the science together to create a forecast,” says Zhu. “Water issues always demand quantitative answers. For example, how millions of gallons you need or are short of. Our model results provide a basis for planning and adaptation discussion. We are lucky we have access to IU's formidable Big Red series supercomputers, which makes this type of modeling possible.”

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