Expected performance of future MAGIC data-assimilated Terrestrial Water Storage&#160;(TWS) products
Authors: Nitschke Annika , Daras Ilias 
Published: September 2024

The planned MAGIC mission, a collaboration between ESA and NASA, is expected to deliver&#160;an extended record of the global mass transport time series with improved accuracy,&#160;temporal and spatial resolutions. ESA&#8217;s involvement in MAGIC is through its NextGeneration Gravity Mission (NGGM) and NASA contributes with its GRACE-C mission. One&#160;of the key deliverables is terrestrial water storage (TWS), which is vital for assessing&#160;changes in climate and managing water resources efficiently. As an essential climate&#160;variable, TWS plays an important role in providing information regarding extreme events.&#160;While the GRACE and its Follow-On mission distribute global TWS anomalies (TWSA), their&#160;coarse spatial resolution (around 150,000 km&#178;) constrains detailed analysis of smaller&#160;basins. Given that freshwater is often sourced from localized aquifer systems, enhancing&#160;spatial resolution is necessary for effective local water management. Furthermore,&#160;improvements in spatio-temporal resolution would allow advances in early flood warning&#160;applications. To overcome these limitations, data assimilation (DA) techniques have been&#160;developed to combine GRACE observations with land surface models (LSMs), making it&#160;possible to downscale and disaggregate TWSA information into its individual components.This study evaluates the performance of data assimilation utilising GRACE-type and MAGIC&#160;error information within the LSMs NOAH-MP and CLSM, with a focus on two regions in&#160;South America and Europe. The model runs cover the period from January 2003 to&#160;December 2006, using data generated during the ESA Science Support study for MAGIC&#160;Phase A. The data is based on closed-loop simulations with a 30 day repeat orbit, containing&#160;the hydrology, ice and solid Earth (HIS) signal along with atmosphere-ocean errors, ocean&#160;tide errors and instrument noise. In total 12 years of monthly data were produced spanning&#160;from January 1995 to December 2006 with spherical harmonic coefficients up to degree and&#160;order 90. A reference HIS signal, acquired from the ESA ESM over the same period is used&#160;to compute retrieval errors.&#160;The results demonstrate that MAGIC data assimilation offers advantages across climatically&#160;different regions independent of LSMs chosen. Furthermore, this study indicates that unlike&#160;previous data assimilation studies, it will be possible to assimilate MAGIC into smaller basins&#160;sizes, seen by the relative improvements of MAGIC DA over GRACE-type DA. Lastly, it is&#160;shown that in case of MAGIC DA post-processing can be considerably reduced, such as&#160;removing the need of filtering up to degree and order 60. Thus, leakage quantification issues&#160;due to the applied filter would be alleviated achieving more straightforward uncertainty&#160;quantification.Overall, MAGIC data assimilation offers substantial improvements in TWS estimation and&#160;trend correction compared to GRACE-type data assimilation, demonstrating its potential for&#160;improving hydrological applications