Flooding along the Missouri River on the Iowa-Nebraska border in 2011.
US Army Corps of Engineers
When the twin GRACE satellites were launched in 2002, the casual observer might have been underwhelmed by their mission—to make precise measurements of Earth’s gravity. They’ve proved, however, to be unbelievably useful Swiss Army knives of geoscience, measuring everything from the loss of ice from Greenland and Antarctica to groundwater depletion in California. Now, the GRACE that keeps on giving has been shown to improve warnings ahead of major floods in some areas.
For many, floods seem to show up suddenly and then overstay their welcome. In some situations, they can be incredibly damaging and often quite dangerous. But a lot more goes into determining the size of a flood than just the amount of rain that falls, and that data provides the key to better forecasting of flood risk.
When looking at graphs of streamflow—the volume of water traveling downstream per second—for a river or stream, hydrologists can identify two kinds of behavior. There’s the consistent base flow, supplied mainly by groundwater entering the river, and the temporarily higher flow that follows rain storms.
The way that rainwater makes it into the river depends on the conditions the raindrops find when they hit the ground. There are limits to how much water can infiltrate the soil, and intense rainfall can exceed the ground’s capacity to soak it up. You can hit that limit sooner if the soil is already sopping wet from previous rainfall, or if the groundwater table is close to the surface. Water might not infiltrate much if the ground is frozen during the early spring—that’s also a risky time because a little warmth and rain can melt the remaining snowpack in a hurry.
And then there are the changes humans have made. The more an area is paved and drained by storm sewers, the more surface runoff heads directly to the river channel. Humans also purposely try to manage floodwater through levees (which make the flooding worse in nearby areas that don’t have levees), dams, and spillways.
Warnings of flood risk mainly rely on weather forecasts, meaning they can only look a few days ahead. That situation could be improved if we had good information on the conditions throughout the watershed that funnels water into a river. It’s not easy to get complete measurements of groundwater levels, snow, and soil moisture, though.
Enter GRACE. These satellites measure gravity so sensitively as they pass over an area that they can measure changes in the amount of water in it. Knowing these conditions accurately lets researchers know far ahead of time whether a storm could come along and cause serious flooding. Combined with the GRACE information, short-term flood predictions based on the weather forecast could be more precise.
To test whether this actually works, three researchers from the University of California Irvine, led by JT Reager, used GRACE measurements from times prior to the incredible Missouri River floods in the early summer of 2011. A 500-year flood occurred along the river when large rainstorms hit an area primed by record snowmelt and high groundwater levels. The researchers analyzed the mathematical relationship between the GRACE data and flow in the Missouri River near St. Joseph, Missouri. The relationship was exponential—as the volume of GRACE-measured water in the watershed becomes large, flow in the river increases rapidly.
The researchers then used that exponential relationship to forecast average flows in the river for the following months and compared those forecasts to the actual measured flows. Even forecasting six months into the future, they were able to predict high streamflow in mid-2011, although their forecast underpredicted the actual average streamflow by about 35 percent until the forecast was only one month away.
In fact, the method was able to forecast large streamflows near the 99thpercentile of historic observations as much as eleven months into the future—performance that forecasts based only on soil moisture or snowpack data couldn’t match. That means that, almost a year in advance, this technique could in certain situations tell you that flood risk will be high.
It can’t do this for every type of flood, though. The researchers applied the same technique to the Columbia River flood that also took place in May and June of 2011, as well as the Indus River flooding in 2010. The GRACE-enabled forecast only predicted high flows in the Columbia about three months ahead, and it didn’t help at all for the Indus. That’s because watershed conditions were less important for these floods, which was especially the case for the Indus flooding in Pakistan. The floods were caused by torrential monsoon rains that dwarfed any contribution from snowmelt or sustained high groundwater levels.
Even so, this is clearly yet another application of the GRACE satellites that could give many regions a useful heads-up before periods of risk for very large floods.
Scott K. Johnson / Scott is a hydrogeologist and educator. He joined the Ars science crew in 2011.
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