GRACE and GRACE-FO Observations of Small-Scale Hydrological Signals: Case Study of Surface Water in the Lake Mead
NASA GRACE (2002-2017) and GRACE Follow-On (GRACE-FO; 2018-present) satellite missions consist of two identical satellites following each other at a distance of about 200 km in the same polar orbit at 500 km. The perturbation in satellite orbit is due to gravity field of the Earth, which is in turn caused by Earth’s mass distribution. As such, if we are able to measure temporal changes in gravity field of the Earth, we can model mass variation in the Earth system. The principal observation that enables GRACE and GRACE-FO satellites to measure time-variable gravity field is the inter-satellite tracking observation with an unprecedented accuracy of micro- to nano-meter. GRACE/GRACE-FO mass change observations over land provide accurate estimate of Terrestrial Water Storage (TWS) variation caused by e.g. dynamics of groundwater and snowpack. However, the biggest limitation of GRACE TWS change observations is their coarse spatial resolution of about 300 km. To examine the capability of GRACE and GRACE-FO for observing small-scale hydrological signals, we analyzed 22 years (2002-2024) of TWS change data over the Lake Mead area. To extract the surface water signal of Lake Mead, we removed the soil moisture and snow water equivalent (from the GLDAS Noah model) as well as groundwater storage (from the WGHM model) from the GRACE/GRACE-FO TWS change signal. We then compared the resulting GRACE-derived surface water with the in situ surface water data of Lake Mead. We will present our preliminary results for this analysis and discuss the similarity as well as discrepancy of the GRACE-observed and in situ surface water of the Lake Mead. Our results can shed light on using GRACE/GRACE-FO for observing small-scale hydrological signals at spatial scales of much smaller than 300 km such as the snowpack over Sierra Nevada and Groundwater storage changes in Central Valley, California.