Insights into water storage response in a humid coastal monsoon region: joint inversion of GNSS and GRACE/GFO observations in Guangdong, China

Summary Global Navigation Satellite System (GNSS) and Gravity Recovery and Climate Experiment (GRACE)/GRACE Follow-On (GFO) observations provide complementary geodetic constraints on terrestrial water storage (TWS) changes, but either data source alone is often insufficient for interpreting water storage response in small humid coastal monsoon regions. We integrated vertical displacements from 49 GNSS stations with GRACE/GFO mascon solutions to estimate monthly TWS changes in Guangdong, China, from 2017 to 2024. An objective-space weighting strategy was used to balance local detail constrained mainly by GNSS with regional mass signals stabilized by GRACE/GFO. The joint solution retains GNSS-supported spatial detail not resolved by GRACE/GFO and reduces boundary artifacts relative to the GNSS-only inversion. It also shows the highest consistency with hydroclimatic water-balance estimates among the geodetic solutions, with a maximum correlation coefficient of 0.85 (95 per cent CI: 0.72–0.93). We further combined the joint solution with hydroclimatic data to interpret water storage response under different moisture conditions. At the seasonal scale, soil moisture storage (SMS) responds nearly synchronously to precipitation, whereas TWS changes peak about one month later. Both responses show weaker amplitudes and shorter lags along the coast, with the opposite pattern inland. For storage anomalies, TWS shows state-dependent memory, mainly 3–7 months under normal and wet conditions but 11–12 months during the prolonged 2021 drought, while SMS remains tied to a much shorter timescale. The joint TWS record also captures three hydrological droughts and two regional flood episodes from 2017 to 2024, including an extreme drought that persisted from December 2020 to January 2022. Together, these results show that joint inversion of GNSS and GRACE/GFO observations provides observation-based constraints on integrated TWS for interpreting storage adjustment, state-dependent storage memory and major hydrological extremes in humid coastal monsoon regions, beyond what can be inferred from meteorological indicators or model-based analyses alone.