Quantifying Changes in Water Loading in the U.S. Southwest via Comparison of GNSS, GRACE, and SWE Data Sets

We use geodetic data to show that hydrologically distinct sub-regions in the Southwest United States act independently of one another. The limited number of Global Navigation Satellite System (GNSS) stations and resolution of Gravity Recovery and Climate Experiment (GRACE) make hydrological partitioning difficult to unravel, especially in the Colorado River Basin which comprises a diversity of climates due to its highly variable topography. Here, we compare GNSS station vertical displacement data, GRACE surface mass change data, and snow water equivalent (SWE) data using elastic surface displacement modeling and signal localization techniques. We focus on a region composed of Arizona, New Mexico, Colorado, and Utah, allowing for the examination of variations in the Colorado River Basin, the primary source of water for the region's municipalities, agriculture, and ecosystems. We demonstrate that the accumulation and melt of snow have a first-order control on the timing of vertical displacement in this region. There exists a region-dependent seasonal partitioning between when GNSS and GRACE sense changes in the distribution of terrestrial water storage. In the Wasatch Range of central Utah, GNSS stations sense loading due to changes in the snowpack one to 2 months in advance of GRACE; in the Southern Rocky Mountains of Colorado, GNSS stations sense loading due to changes in the snowpack one to 3 months in advance of GRACE; and in the lower Colorado River Basin of Arizona, GRACE senses loading due to changes in river runoff three or more months in advance of GNSS stations.