New study tracks hurricane harvey stormwater with GPS

After the storm passed, where did all that water go?

Flooding in Otey, Texas, following Hurricane Harvey in 2017. This is one of a series of images captured by NOAA's National Geodetic Survey in the aftermath of the storm. Image Credit: NOAA
Flooding in Otey, Texas, following Hurricane Harvey in 2017. This is one of a series of images captured by NOAA's National Geodetic Survey in the aftermath of the storm. Image Credit: NOAA

Hurricane Harvey dumped in excess of 5 feet (1.5 meters) of water on southeast Texas in late August 2017, making it the wettest recorded hurricane in U.S. history. Be that as it may, after the storm passed, where did everything that water go?

In another, NASA-led study thinks about, researchers utilized Global Positioning System (GPS) data to answer that question and to track not just where Harvey’s stormwater ended upland, yet in addition to what extent it took to dissipate.

The 30 percent of the water that was put away ashore at that point step by step dispersed over a time of around five weeks, likely through evapotranspiration, groundwater runoff into nearby rivers and the replenishment of aquifers.

First author Chris Milliner of NASA’s Jet Propulsion Laboratory in Pasadena, California said, “We determined that in the first eight days post-landfall, 30 percent of Harvey’s stormwater was captured or stored on land — most as standing water that sits on the surface. Around 60 percent was lost or drained into the ocean and Galveston Bay over the first few days after the storm, and the remaining 10 percent was lost via evapotranspiration or a combination of evaporation and plant transpiration.”

Animation shows changes of GPS vertical positions post-Harvey over time. The warmer colors represent areas where the surface is depressed from the weight of Harvey's water. The red line is Harvey's actual path. GPS measured the subsidence, or surface depression, around Houston (center of the image) to be just under an inch (22 mm). The pattern of subsidence also migrated across the Gulf Coast toward western Louisiana over a seven-day period coinciding with the position of the hurricane. Image Credit: NASA / JPL-Caltech
Animation shows changes of GPS vertical positions post-Harvey over time. The warmer colors represent areas where the surface is depressed from the weight of Harvey’s water. The red line is Harvey’s actual path. GPS measured the subsidence, or surface depression, around Houston (center of the image) to be just under an inch (22 mm). The pattern of subsidence also migrated across the Gulf Coast toward western Louisiana over a seven-day period coinciding with the position of the hurricane. Image Credit: NASA / JPL-Caltech

Working:

The GPS system enables scientists to estimate changes in Earth’s surface elevation to the precision. Scientists used daily elevation measurements from about 220 of these ground stations, from western Texas to Louisiana, to track changes in the amount of stormwater on land after the hurricane.

Milliner said, “When you sit on a mattress, your weight depresses its surface. Earth’s crust is also elastic and behaves in a similar way under the weight of water. GPS is measuring the amount of subsidence (or depression), which tells you how much water mass must be pressing on the surface and where that water is distributed.”

Scientists observed that in the initial a few days after Hurricane Harvey, the land around Houston brought down in elevation by as much as 20 millimeters. The GPS data likewise followed a reasonable example of land subsidence that moved over the Gulf Coast over a seven-day time span, predictable with the situation of Hurricane Harvey.

This animation shows the elastic response of Earth to increasing and decreasing a water load on the surface. The amount of depression corresponds with the expected depression the surface underwent due to the weight of Hurricane Harvey's stormwater. Image Credit: NASA / JPL-Caltech
This animation shows the elastic response of Earth to increasing and decreasing a water load on the surface. The amount of depression corresponds with the expected depression the surface underwent due to the weight of Hurricane Harvey’s stormwater. Image Credit: NASA / JPL-Caltech

Following this underlying area subsidence, estimations from GPS stations found that Earth’s surface bit by bit ascended back up, demonstrating water was draining and evaporating out of land – similarly as a mattress behaves when you gradually stand up and expel your weight from it.

In order to identify Earth’s mattress-like response to changes in water mass, scientists had to process the GPS data to remove systematic errors called common mode error (CME). CME acts essentially as “noise” that masks the hydrologic signal. Using an independent component-analysis filter, the team was able to statistically separate the raw GPS data into CME and hydrologic signals. This allowed them to discard the signal that was noise and extract the subtle hydrologic signal they sought.

With the filtered GPS data, scientists were able to determine the daily magnitude and location of the surface depression and from this calculate the daily mass of water that caused it.

In other words, the study that published by the journal Science Advances demonstrates that it is possible to robustly quantify daily changes in water storage following extreme precipitation events like major hurricanes. It allows us to see how much water is temporarily stored on land after a major hurricane, where it is stored, and how long it takes for stored water to dissipate over time.