Coupling of downscaled GRACE data with SPHY derived hydrological components to generate snow water equivalent in the Himalayan catchment

Abstract

This study demonstrated the applicability of Gravity Recovery and Climate Experiment (GRACE) satellite data through the coupling of multisource datasets in cryospheric and hydrological assessments over the Himalayan basin. For this purpose, a coupled modeling framework was developed using the fully distributed Spatial Process in Hydrology (SPHY) model, forced by real-time satellite and hydro-observation datasets. To reduce uncertainty in the outcomes, a two-step calibration approach was performed: (i) utilizing the observed discharge at two gauges, namely Maneri and Rishikesh, and (ii) comparing the MODIS-derived snow cover area (SCA) with the SPHY-derived SCA. The observed versus modeled discharge was found to be comparable, with the corresponding coefficient of determination (R²) recorded as 0.64 and 0.66 for Maneri and Rishikesh, respectively. The seasonal snow cover computed by SPHY was validated against MODIS snow cover maps, yielding a coefficient of determination of 0.73 (2015). By analyzing the snow mass loading and unloading using GRACE Total Water Storage Anomalies (TWSA) and MODIS snow cover data, sixteen snow seasons (between 2002 and 2020) were identified in the snow-glacier-dominated sub-basins of the Upper Ganga River. The snow water equivalent (SWE) was computed after subtracting other hydrological/water-balance components in these sub-basins. A spatial downscaling of GRACE TWSA was performed utilizing the GLDAS variables, showing significant improvement in the GRACE TWSA at a 0.25° × 0.25° scale. The correlation analysis showed a good match between GRACE and GLDAS variables (R² up to ∼0.7). The downscaled GRACE-TWSA was compared with SPHY-TWSA and other cryospheric-hydrological components at different stations, with results found to be comparable. The results show that GRACE + SPHY SWE is comparable to GLDAS SWE in upstream regions, though some discrepancies exist downstream. For establishing the relationship between GRACE TWSA and hydrological components, cryosphere and hydrology variables such as snowmelt runoff, glacier melt runoff, baseflow, and rainfall-runoff were successfully separated at different stations (or sub-basins).