# Groundwater Storage Changes in China from Satellite Gravity: An Overview
## 1 Introduction  
Groundwater, as an important component of the global water cycle, is a vital resource of fresh water to sustain agricultural, industrial, and domestic development in many parts of the world, especially in arid and semi-arid regions [1, 2]. Globally, groundwater uses in irrigation account for \(\sim\)43% of the total consumptive irrigation water use [3]. Groundwater also supplies \(\sim\)50% of drinking water and \(\sim\)40% of industrial water globally [4]. More than 1.5 billion people worldwide rely on groundwater as the primary source of drinking water [1]. Due to extreme climate episodes, population growth, and extensive groundwater mining and over-exploitation, long-term groundwater depletion has occurred in many regions of the world, exacerbated by various adverse environment impacts, including groundwater quality deterioration, land subsidence, seawater intrusion, and soil salinization [5, 6]. Groundwater depletion has been considered as a serious threat to sustainability of water supplies. From a perspective of water balance, groundwater storage (GWS) change in an aquifer system is referred to as the budget between recharge (e.g., seepage from soil/surface water, and lateral inflow) and discharge (e.g., groundwater withdraws and lateral outflow) [7]. However, it is difficult to quantify groundwater recharge rates and discharge rates directly from in situ hydrological measurements [8]. Alternatively, water level observations from monitoring wells provide the principal source of information on groundwater systems. Groundwater level changes can be further converted to storage changes by multiplying by storage coefficients, e.g., specific yields for unconfined aquifers. However, groundwater levels are often poorly monitored with adequate spatial resolutions due to limited available well networks in many areas around the world. In addition to being monitored by in situ well observations, GWS changes can be simulated by hydrological models. At global scales, hydrological models can be divided into two categories: land surface models (LSMs) and hydrological water balance models (HMs). Most global LSMs developed by the atmospheric community or climate scientists can simulate water and energy fluxes between the land surface and the atmosphere in general circulation models, which generally can be used to predict soil moisture and water storage changes, but they mostly fail to simulate GWS changes [9; 10]. Global HMs, however, are developed for global water resources assessments; therefore, human water uses, e.g., irrigation water use, domestic and industrial water use, are considered in these HMs [10]. Thus, groundwater, as an important source of water use in many arid regions, is generally simulated in the HMs. However, accurate estimation of GWS changes and their trends from global HMs remains a challenge, due to the imperfect parameterization of HMs, the limited knowledge about groundwater recharge/abstractions, and the scarcity of independent ground observations to calibrate models [11]. At regional scales, numerical groundwater flow models can be used to simulate groundwater dynamics and GWS changes in a specific region. To construct a reliable regional groundwater model, however, detailed hydrogeologic data (e.g., hydraulic parameters, water level observations, geology, aquifer materials/channels), climate forcing data, and land use data, among others, need to be collected. It is often difficult to obtain a comprehensive dataset for regional groundwater modeling. Apart from ground well observations and global/regional models, independent remote sensing-based estimates of water storage changes provide the potential to monitor GWS changes from space. The Gravity Recovery And Climate Experiment (GRACE) twin-satellites provide a direct measure of monthly total water storage (TWS) changes globally at a spatial resolution longer than \(\sim\)300 km for over a decade and a half since 2002 [12; 13].