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Shortly after Hurricane Harvey unleashed its flooding on Houston, we a remarkable observation shared by a scientist on the weight of all that floodwater had measurably depressed the Earth’s crust. This week, a more detailed study of that observation was published in the journal Science Advances.
A team of researchers led by of NASA’s Jet Propulsion Laboratory extended its analysis to the weeks after the hurricane and found that the network of sensitive GPS sensors could actually track the volume of floodwater as it receded.
While bedrock is commonly considered representative of concepts like “firm” and “unmovable,” it has some compressibility when the forces are big enough. This “elastic” behavior explains how the land surface around Houston could sag slightly under the weight of Harvey’s prodigious rainfall.
It also offers another way to measure the storage of water on land. Efforts to track water levels in streams, reservoirs, and wells have been around for a long time, but satellites have added new options. The GRACE satellite pair, for example, has detected changes in water storage by measuring changes in mass due to its gravitational pull. While this is an incredible tool for monitoring something like a California drought, the resolution is too coarse to tell you much about flooding in Houston. But if you have enough GPS sensors around, they can provide a spatial resolution that’s much finer.
As Milliner shared immediately after the hurricane, careful processing of GPS data around the Space City revealed vertical land surface movement of as much as 21 millimeters (almost an inch). Viewed over time, you can actually see the depressed area migrate with the movement of the hurricane.
Just as you can infer the thrust force of a rocket engine based on the rocket’s acceleration, you can flip these GPS measurements of vertical land movement into accurate estimates of the volume of water supplying the added weight. Doing this, the researchers calculated that Harvey dropped about 95 cubic kilometers (23 cubic miles) of water on the Gulf Coast. Around two-thirds of it quickly drained back into the ocean at a rate of about eight cubic kilometers per day over the first week. That’s far more water than was captured by stations measuring river flow, showing that much of the water either did not drain through major channels or entered those channels downstream of the last measuring station.
Almost 26 cubic kilometers of floodwater was temporarily stored on land, evaporating (or transpiring through plant leaves) much more slowly at about one cubic kilometer per day. It took five weeks for the GPS measurements to return to their pre-Harvey state, bouncing back as the land was relieved of the excess water’s weight.
These numbers are interesting to think about, but the researchers point out that they could also be extremely useful. A better understanding of where the floodwater around Houston went could help aid the effort to improve the city’s drainage system, for one. But the researchers also think GPS data could be used in near-real time to improve local flood forecasts.
To test this, the researchers ran their analysis again as if it was August 2017 and they were receiving the data one day at a time. They found that it worked just as well, providing additional data that might have been useful in the midst of the disaster. It wouldn’t have been quite this easy, though, as the GPS data needed some initial processing to extract the floodwater signal from the noise. Doing that on the fly would have raised the degree of difficulty.
Still, the researchers say, their findings gives us another good reason to maintain GPS networks as good as the one around Houston, in case we want to try.
Science Advances, 2018. DOI: ().