Reply on EC1

Abstract

Total Water Storage Change (TWSC) reflects the balance of all water fluxes in a hydrological system. The Gravity Recovery and Climate Experiment/Follow-On (GRACE/GRACE-FO) satellite missions are the only means of observing this state variable, distributed as coefficients of a spherical harmonic (SH) model. The well-known correlated noise in these observations requires filtering, scattering the actual mass changes from their true locations. This effect is known as leakage. This study explores the traditional basin and grid scaling approaches and develops a novel frequency-dependent scaling for leakage correction of GRACE TWSC in a unique, basin-specific assessment for the Indus basin. We harness the characteristics of significant heterogeneity in the Indus basin due to climate and human-induced changes to evaluate the physical nature of these scaling schemes. The most recent, WaterGAP Hydrology Model (WGHM v2.2d), with its two variants, standard (without glacier mass changes) and Integrated (with glacier mass changes), is used to derive scaling factors. For the first time, we explicitly show the effect of inclusion or exclusion of glacier mass changes in the model on the gridded scaling factors. Frequency dependant scaling factors, as a novelty, allow us to compare the differences of scaling seasonal and trend components separately. We employ time series analysis using the Lomb-Scargle periodogram to decompose the time series and visualize the effect of filtering on different time scales. We find that for the Indus basin, where mass changes of different frequencies are localized, frequency-dependent scaling factors for the entire basin perform similar to the gridded scaling factors, while both outperform basin scaling. The property of frequency-dependent scaling to keep the noise level unscaled can be extremely useful for applications requiring a high signal-to-noise ratio of TWSC observations. Apart from these novel developments and insights into the traditional scaling approach, our study encourages the regional scale users to conduct specific assessments for their basin of interest.