Can we use the Issyk-Kul region as a test site for future satellite gravity missions?

The region around Lake Issyk-Kul in the Tian Shan Mountains in Kyrgyzstan is densely observed with in-situ monitoring stations and used to test and validate satellite observations. Although Lake Issyk-Kul lies at 1600 m elevation, it does not freeze over in the winter months. The hydrology of the region is dominated by the storage in several large to medium-sized endorheic lakes (e.g., lakes Issyk-Kul and Balkhash), artificial reservoirs (e.g., Kapshagay Reservoir), and the snow cover during the winter months. In addition, melting glaciers play a significant role in the region’s hydrology. In 2016, the GFZ Helmholtz Centre for Geosciences, Germany, and Central-Asian Institute for Applied Geosciences (CAIAG), Kyrgyzstan, installed two climate monitoring stations and several GNSS-controlled tide gauges for the monitoring of the environment and Issyk-Kul lake level variations. The in-situ observations and the ice-free winters make Lake Issyk-Kul an ideal test site for calibrating satellite altimetry. 

NASA and DLR plan to launch GRACE-C (Gravity Recovery and Climate Experiment – Continuation) in 2028 in the same orbit as GRACE. ESA plans to launch a Next Generation Gravity Mission (NGGM) in 2032, flying in an inclined and lower orbit. GRACE-C and NGGM will form the Mass-Change and Geosciences International Constellation (MAGIC), which aims to increase mass transport products' spatial and temporal resolution significantly. With this study, we investigate if and how we can use the region to evaluate the MAGIC future satellite gravity mission.

In order to assess the suitability of the Issyk-Kul region as a validation site for MAGIC, we investigate the behaviour of the different hydrological storage compartments. The recently published G3P data set (Global Gravity-based Groundwater Product) compiles a harmonised set of satellite observations of root-zone soil moisture (RZSM), snow water equivalent (SWE), glacier mass change, and terrestrial water storage (TWS). Information about surface water storage (SWS) can be derived from satellite altimetry. This data set allows us to understand the hydrological drivers of TWS variability. Variations of SWS explain the strong interannual variations beyond the linear trend well. However, the different lakes of the region show quite distinct interannual variations. With the current spatial resolution of GRACE and GRACE-FO, these variations cannot be separated. However, this separation would be a prerequisite for the region as a test site for future gravity missions.

By simulating realistic MAGIC observations of the region, we can assess their spatial resolution and, thus, if the region around Lake Issyk-Kul may serve as a test site for MAGIC. First results show that with the higher spatial resolution, we can discriminate between the SWS signals of the different lakes.