Bridging the gap between GRACE and GRACE-FO using a hydrological model

Assessing groundwater drought in Iran using GRACE data and machine learning

Groundwater serves as a critical freshwater reservoir globally, essential for ecosystem conservation and human well-being. Drought conditions adversely impact groundwater systems by first reducing recharge, followed by declines in groundwater levels and withdrawal potential, which can result in agricultural setbacks and irreversible consequences such as land subsidence. The introduction of the Gravity Recovery and Climate Experiment (GRACE) project marked a significant advancement in monitoring terrestrial water storage anomalies (TWSA), encompassing both surface and subsurface water. Traditional methods for assessing groundwater storage anomalies (GWSA), such as piezometric wells, have proven to be costly and inefficient, often lacking sufficient spatial and temporal coverage. Although GRACE data offers valuable insights, its large-scale nature presents challenges for localized basin and aquifer studies, compounded by data gaps resulting from a 15-month interruption during the transition to the GRACE-FO project. This study investigates the status of groundwater across six major river basins in Iran utilizing data from GRACE and its complementary Global Land Data Assimilation System (GLDAS) over a 255-month period from 2002 to 2023. The Extreme Gradient Boosting (XGBoost) algorithm is employed for downscaling TWSA to a resolution of 0.25°, achieving a high Pearson correlation (R) of 0.99 and a root mean square error (RMSE) of 22 mm. The downscaled GWSA, derived from the balance equation, exhibits an average correlation (R) of 0.93 and RMSE of 39 mm with observational data. Following the application of the Seasonal Autoregressive Integrated Moving Average (SARIMA) model to fill GWSA time series gaps, this study models and forecasts GWSA trends through 2030 using historical data and SSP2 scenario projections of the canESM5 climate model. Results indicate an average groundwater depletion of 29 cm per year across Iran's aquifers from 2002 to 2023, with the Caspian Sea basin experiencing the most significant decline. The GRACE Groundwater Drought Index (GGDI) is calculated and compared with the Standardized Precipitation Index (SPI), revealing an 8-month lag in drought propagation from meteorological to groundwater sources in Iran. Furthermore, correlations between the GGDI and teleconnection indices highlight their substantial influence on drought conditions in basins adjacent to major water sources. The results of this study, by emphasizing the reliability of satellite data and machine learning models in groundwater drought monitoring, can assist policymakers in enhancing groundwater resource management, strategic planning, and identifying critical basins, particularly in regions with limited observational data.

Between flood and drought: How cities are facing water surplus and scarcity

Droughts and floods are weather-related hazards affecting cities in all climate zones and causing human deaths and material losses on all inhabited continents. The aim of this article is to review, analyse and discuss in detail the problems faced by urban ecosystems due to water surplus and scarcity, as well as the need of adaptation to climate change taking into account the legislation, current challenges and knowledge gaps. The literature review indicated that urban floods are much more recognised than urban droughts. Amongst floods, flash floods are currently the most challenging, which by their nature are difficult to monitor. Research and adaptation measures related to water-released hazards use cutting-edge technologies for risk assessment, decision support systems, or early warning systems, among others, but in all areas knowledge gaps for urban droughts are evident. Increasing urban retention and introducing Low Impact Development and Nature-based Solutions is a remedy for both droughts and floods in cities. There is the need to integrate flood and drought disaster risk reduction strategies and creating a holistic approach.

Autoregressive Reconstruction of Total Water Storage within GRACE and GRACE Follow-On Gap Period

For 15 years, the Gravity Recovery and Climate Experiment (GRACE) mission have monitored total water storage (TWS) changes. The GRACE mission ended in October 2017, and 11 months later, the GRACE Follow-On (GRACE-FO) mission was launched in May 2018. Bridging the gap between both missions is essential to obtain continuous mass changes. To fill the gap, we propose a new approach based on a remove–restore technique combined with an autoregressive (AR) prediction. We first make use of the Global Land Data Assimilation System (GLDAS) hydrological model to remove climatology from GRACE/GRACE-FO data. Since the GLDAS mis-models real TWS changes for many regions around the world, we further use least-squares estimation (LSE) to remove remaining residual trends and annual and semi-annual oscillations. The missing 11 months of TWS values are then predicted forward and backward with an AR model. For the forward approach, we use the GRACE TWS values before the gap; for the backward approach, we use the GRACE-FO TWS values after the gap. The efficiency of forward–backward AR prediction is examined for the artificial gap of 11 months that we create in the GRACE TWS changes for the July 2008 to May 2009 period. We obtain average differences between predicted and observed GRACE values of at maximum 5 cm for 80% of areas, with the extreme values observed for the Amazon, Alaska, and South and Northern Asia. We demonstrate that forward–backward AR prediction is better than the standalone GLDAS hydrological model for more than 75% of continental areas. For the natural gap (July 2017–May 2018), the misclosures in backward–forward prediction estimated between forward- and backward-predicted values are equal to 10 cm. This represents an amount of 10–20% of the total TWS signal for 60% of areas. The regional analysis shows that the presented method is able to capture the occurrence of droughts or floods, but does not reflect their magnitudes. Results indicate that the presented remove–restore technique combined with AR prediction can be utilized to reliably predict TWS changes for regional analysis, but the removed climatology must be properly matched to the selected region.

Adaptive DDK Filter for GRACE Time-Variable Gravity Field with a Novel Anisotropic Filtering Strength Metric

Filtering for GRACE temporal gravity fields is a necessary step before calculating surface mass anomalies. In this study, we propose a new denoising and decorrelation kernel (DDK) filtering scheme called adaptive DDK filter. The involved error covariance matrix (ECM) adopts nothing but the monthly time-variable released by several data centers. The signal covariance matrix (SCM) involved is monthly time-variable also. Specifically, it is parameterized into two parameters, namely the regularization coefficient and the power index of signal covariances, which are adaptively determined from the data themselves according to the generalized cross validation (GCV) criterion. The regularization coefficient controls the global constraint on the signal variances of all degrees, while the power index adjusts the attenuation of the signal variances from low to high degrees, namely local constraint. By tuning these two parameters for the monthly SCM, the adaptability to the data and the optimality of filtering strength can be expected. In addition, we also devise a half-weight polygon area (HWPA) of the filter kernel to measure the filtering strength of the anisotropic filter more reasonably. The proposed adaptive DDK filter and filtering strength metric are tested based on CSR GRACE temporal gravity solutions with their ECMs from January 2004 to December 2010. Results show that the selected optimal power indices range from 3.5 to 6.9, with the corresponding regularization parameters range from 1 × 1014 to 5 × 1019. The adaptive DDK filter can retain comparable/more signal amplitude and suppress more high-degree noise than the conventional DDK filters. Compared with the equivalent smoothing radius (ESR) of filtering strength, the HWPA has stronger a distinguishing ability, especially when the filtering strength is similar.