Tracking the Weight of Hurricane Harvey's Stormwaters Using GPS data

On August 26^th 2017 Hurricane Harvey struck the Gulf Coast as a category four cyclone depositing 95 km³ of water, making it the wettest cyclone in US history. Water left in Harvey's wake should cause elastic loading and subsidence of Earth's crust, and uplift as it drains into the ocean and evaporates. To track daily changes of transient water storage across the Gulf Coast, we use Global Positioning System measurements of vertical and horizontal surface motion with independent component analysis to remove systematic biases and extract the hydrologic signal. From the filtered timeseries we find a clear migration of land subsidence over a seven-day period from southwest Texas to western Louisiana of up to 21 mm, followed by five weeks of gradual uplift. From inverting the GPS positions, we estimate a third of Harvey's total water was initially captured on land (25.7 ±3.0 km³), suggesting the rest of the total stormwater deposited was rapidly lost via surface runoff and groundwater flow (58.0 ±6.0 km³, or 61%), within the first seven days, and the remaining stored water was gradually lost at a rate of 1 km³/day over the following five-week period, primarily by evapotranspiration. These results demonstrate that GPS networks can track the spatial extent and daily evolution of terrestrial water storage following transient, extreme precipitation events, with implications for improving short-term operational flood forecasts and understanding the response of drainage systems to large influxes of water.