# Two Decades of Groundwater Variability in Peru Using Satellite Gravimetry Data
## 1 Introduction
### Hydrogeological Context of Peru  
Peru's diverse topography, encompassing coastal deserts, the Andes Mountains, and the Amazon Basin, creates a unique hydrogeological landscape heavily reliant on groundwater for agriculture, urban water supply, and ecosystems [12]. This heavy reliance on groundwater suggests that surface water storage in dams and reservoirs is either insufficient or not adequately distributed to meet agricultural and urban demand in various regions of the country. Coastal regions, such as Lima and Ica, face intense groundwater demand because of agricultural expansion, while Andean aquifers depend on snowmelt and precipitation, and the Amazon Basin benefits from high rainfall, but is vulnerable to deforestation [13]. This situation tends to decrease the GWS on the coast, keeps it conditioned in the Andes, and keeps it high or stable in the Amazon. These regional differences, combined with natural variability and anthropogenic pressures, underscore the need for comprehensive GWS assessments. Peru faces growing challenges in groundwater management because of climate change, rapid urbanization, and agricultural intensification [14, 15, 16]. Precipitation variability, exacerbated by El Nino and La Nina events, disrupts recharge patterns and complicates water resource planning [17]. While multinational studies have included groundwater assessments in Peru [18, 19, 20, 21, 22], recent research has focused on specific issues, such as aquifer characterization [13, 23], pollution and saline intrusion [19, 24], groundwater-induced landslides [25, 26], and sustainability in key regions like Ica and Piura [27, 28]. Despite these efforts, most national-scale assessments have prioritized surface water, leaving critical gaps in understanding groundwater dynamics [29]. While regional studies have characterized specific aquifers (e.g., Ica, Piura) [27, 28, 29], there is a lack of comprehensive analyses that quantify GWS trends across Peru's diverse coastal, Andean, and Amazonian regions over extended periods. Furthermore, few studies have integrated satellite gravimetry data with in situ measurements to validate GWS estimates or assessed the combined impacts of El Nino-Southern Oscillation (ENSO) events and anthropogenic extraction on groundwater depletion. This knowledge gap hinders the development of effective water management strategies to address increasing pressures from climate variability and overexploitation, particularly in vulnerable regions like Lima, Ica, Arequipa, and Tacna. Several studies have used GRACE and GRACE-FO satellite data to assess GWS changes at global and regional scales [30] reviewed GRACE's capacity to detect significant groundwater trends in highly stressed aquifers such as northwest India and California, emphasizing its usefulness while also noting spatial resolution limitations and the lack of validation with in situ data. More recently, [PERSON] et al. (2022) [31] applied GRACE-FO data combined with climate models to quantify GWS depletion in the Haihe River Basin in China, identifying rates between \(-1.86\) and \(-1.92\) cm/year. However, they also highlighted inconsistencies with field observations. Other authors have focused on improving spatial resolution. [PERSON] et al. (2024) [32] proposed a downscaling method integrating well observations to reduce GRACE errors in the Chinese plain, allowing for better detection of local trends. Similarly, [PERSON] et al. (2023) [33] analyzed terrestrial water storage (TWS) variability in large endrohei basins and found that precipitation (P) was the main driver of changes, rather than human extraction. Despite these advances, no comprehensive study has yet:  
Conducted a country-scale assessment of GWS variability in Peru over a 20-year period; integrated satellite gravimetry with both climate drivers and anthropogenic factors; and validated GWS estimations using local field data in the Andes-Amazon transitional region.