Monitoring land subsidence induced by groundwater change using satellite gravimetry and radar interferometry measurements. Case study: Surabaya city, Indonesia

Land subsidence is considered a potential hazard often occurring in densely populated urban areas due to increasing freshwater demands from groundwater pumping. The Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry combined with Sentinel 1 interferometric satellite radar measurement has provided the possibility to monitor land subsidence induced by groundwater change. This study monitored land subsidence induced by groundwater change through satellite observations over Surabaya City, Indonesia, from 2014 to 2019. Persistent Scattered InSAR (PSInSAR) measurement was used to monitor land subsidence by using 114 SLC pairs. As for the groundwater perspective, Global Land Data Assimilation System (GLDAS v.2.2), which contains the Groundwater Storage Anomaly (GWS) derived from GRACE satellite observation, was used to understand groundwater’s spatial and temporal variation. The results show a satisfactory agreement of satellite radar measurement with ground measurement (R = 0.96, RMSE = 4.92cm), while satellite gravimetry measurement showed reasonably good agreement with radar measurement as well (R = 0.25). Regarding the magnitude and occurrence of land subsidence over Surabaya City, the result shows that, over the past 5 years, the southern part of the city had the highest subsidence ranging from -10 mm/year to -40 mm/year. Therefore, the results conclude the capability of both satellite gravimetry and radar measurements to monitor land subsidence over an urban area. Thus, this information could be considered as an important decision-making process for disaster management purposes.

Validation of GIA models in north-east Greenland using densified GNSS measurements and refined estimates of present-day ice-mass changes

<p>Models of glacial-isostatic adjustment (GIA) exhibit large differences in north-east Greenland, reflecting uncertainties about glacial history and solid Earth rheology. The GIA uncertainties feed back to uncertainties in present-day mass-balance estimates from satellite gravimetry. We present results from repeated and continuous GNSS measurements which provide direct observables of the bedrock displacement. The repeated measurements were conducted within five measurement campaigns between 2008 and 2017. They reveal uplift rates in north-east Greenland in the range of 2.8 to 8.9 mm yr<sup>-1</sup>. We used the observed uplift rates to validate different GIA models in conjunction with estimates of the elastic load deformation induced by present-day ice-mass changes and ocean mass redistribution. To determine present-day ice-mass changes for both the Greenland Ice Sheet and the peripheral glaciers, we combined CryoSat-2 satellite altimetry data with GRACE satellite gravimetry data. The different GIA models were consistently used in all processing steps. Our comparison between observed and predicted uplift rates clearly favours GIA models that show low rates (0.7 to 4.4 mm yr<sup>-1</sup> at the GNSS sites) over GIA models with higher rates of up to 8.3 mm yr<sup>-1</sup>. Applying the correction predicted by the GIA model favoured in north-east Greenland we estimate an ice-mass loss of 233 ± 43 Gt yr<sup>-1</sup> for entire Greenland (including peripheral glaciers) over the period July 2010 to June 2017.</p>

Continental Water Storage Changes Sensed by GRACE Satellite Gravimetry

Since its launch in March 2002, the Gravity Recovery And Climate Experiment (GRACE) mission has been mapping the time variations of the Earth’s gravity field with a precision of 2–3 cm in terms of geoid height at the surface resolution of 300–400 km. The unprecedented precision of this twin satellite system enables to detect tiny changes of gravity that are due to the water mass variations inside the fluid envelops of our planet. Once they are corrected from known gravitational contributions of the atmosphere and the oceans, the monthly and (bi)weekly GRACE solutions reveal the continental water storage redistributions, and mainly the dominant seasonal cycle in the largest drainage river basins such as Amazon, Congo, Mississippi. The potential differences measured between the twin GRACE satellites represent the sum of integrated surface waters (lakes and rivers), soil moisture, snow, ice and groundwater. Once they are inverted for estimating surface water mass densities, GRACE solutions are also used to establish the long-term mass balance of the ice sheets impacted by global warming, for quantifying the interannual variations of the major aquifers, as well as for surveying the hydrological signatures of intense meteorological events lasting a few days such as tropical hurricanes. This chapter describes GRACE gravity products and the different data processings used for mapping continental water storage variations, it also presents the most remarkable results concerning global continental hydrology and climate changes.

Interannual glacier and lake mass changes over Scandinavia from GRACE

SUMMARY The Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry observations have been widely used in the study of glaciers. However, there is still no detailed GRACE-based study of the glaciers over the Scandinavian Mountains (SCAMs), where the glaciers are debris-covered and the effects of glacial isostatic adjustment (GIA) are significant. In this paper, GRACE observations are combined with climate data to analyse interannual mass changes in glacier and lake areas over Scandinavia during the period from 2003 to 2016. An inversion algorithm, the constrained forward modelling method, is used to recover the signals of glaciers and lakes from GRACE observations. Our results show that the total glacier mass loss rate over Scandinavia is –1.0 ± 1.1 Gt yr–1 during our study period. We find that the glacier accumulation regime in different subregions of the SCAMs may be different. The glacier mass change in the central SCAMs tends to be mainly driven by precipitation. Two rapid transitions from dry/wet years to wet/dry years in the lake area in south Scandinavia are identified by multiple data. The transitions are likely caused by changes in atmospheric circulation, that is surface wind. The mass changes of Scandinavia can be primarily explained by the influence of winds. We find that the glacier area is controlled by both the northerly and southerly winds, while the lake area is mainly driven by the southerly winds. This discrepancy leads to the different mechanisms of mass change in glacier and lake areas. We also discuss the influence of GIA, and suggest that the GRACE-derived long-term hydrology trends over Scandinavia may be unreliable and need to be verified. Our study indicates that GRACE data have potential in detecting small-scale glacier changes.