Joint assimilation of GRACE Total Water Storage Anomalies and In-Situ Streamflow Data into a Global Hydrological Model
Authors: Schulze Kerstin , Kusche Jürgen , Gerdener Helena , Engels Olga , Döll Petra , Müller Schmied Hannes , Ackermann Sebastian , Shadkam Somayeh 
Published: March 2022

&lt;p&gt;&lt;span&gt;Global hydrological models simulate water storages and fluxes of the water cycle, &lt;/span&gt;&lt;span&gt;motivated to assess water problems such as water scarcity, high flows and more generally the impact of anthropogenic change on the global water system. &lt;/span&gt;&lt;span&gt;However, the models include many uncertainties due to the model inputs (e.g. climate forcing data), model parameters, and model structure &lt;/span&gt;&lt;span&gt;which can lead&lt;/span&gt; &lt;span&gt;to&lt;/span&gt;&lt;span&gt; disagreements &lt;/span&gt;&lt;span&gt;when simulation results are compared to&lt;/span&gt;&lt;span&gt; observations. To reduce &lt;/span&gt;&lt;span&gt;and quantify&lt;/span&gt;&lt;span&gt; these uncertainties, &lt;/span&gt;&lt;span&gt;some of &lt;/span&gt;&lt;span&gt;the models are calibrated against in-situ &lt;/span&gt;&lt;span&gt;streamflow&lt;/span&gt;&lt;span&gt; observations or compared against &lt;/span&gt;&lt;span&gt;total water storage anomalies (TWSA) derived from the Gravity Recovery And Climate Experiment (GRACE) satellite mission. In recent years, TWSA data are integrated into some models via data assimilation &lt;/span&gt;&lt;span&gt;to directly improve the realism of the models&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In this study, we present our framework for jointly assimilating satellite and in-situ observations into the WaterGAP Global Hydrological Model (WGHM). &lt;/span&gt;&lt;span&gt;In addition to GRACE TWSA maps, for the first time here we experimentally jointly assimilate in-situ streamflow observations from gauge stations. &lt;/span&gt;&lt;span&gt;This&lt;/span&gt; &lt;span&gt;is in preparation for the&lt;/span&gt;&lt;span&gt; Surface Water and Ocean Topography (SWOT) satellite, which will be launched this year and is expected to allow the derivation of streamflow observations globally for rivers wider than 50-100m. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;GRACE assimilation strongly improves &lt;/span&gt;&lt;span&gt;the TWSA simulations in the Mississippi River Basin, e.g. the correlation increases to 91%, with which our results are consistent with previous studies. However, we find in this case that the streamflow simulation deteriorat&lt;/span&gt;&lt;span&gt;es&lt;/span&gt;&lt;span&gt;, f&lt;/span&gt;&lt;span&gt;or example, correlation reduces from 92% to 61% at the most downstream gauge station. In contrast, joint&lt;/span&gt;&lt;span&gt;ly assimilating GRACE data and streamflow observations from GRDC gauge stations improves the streamflow observations by up to 33% in terms of e.g. RMSE and correlation while maintaining the good TWSA simulati&lt;/span&gt;&lt;span&gt;ons. &lt;/span&gt;&lt;span&gt;In view of the upcoming SWOT mission, our data suggest that the &lt;/span&gt;&lt;span&gt;SWOT&lt;/span&gt;&lt;span&gt; data will help to further improve the structure and simulations of global hydrological models. &lt;/span&gt;&lt;/p&gt;