# Two Decades of Groundwater Variability in Peru Using Satellite Gravimetry Data
## 2 Materials and Methods  
### Study Area  
Peru, located in western South America, spans nearly 1.3 million km\({}^{2}\) and presents a strikingly diverse geography--ranging from the arid Pacific coast to the high Andes Mountains and the extensive Amazon basin (Figure 1). This variability in terrain and climate provides an ideal context for analyzing groundwater distribution through advanced monitoring technologies [13].  
The Andes, running north to south, shape the country's topography and hydrology, with snowmelt and rainfall contributing to aquifer recharge [23; 24]. To the east, the Amazon rainforest--one of the world's biodiversity hotspots--supports regional aquifer systems through abundant rainfall and river flow. In contrast, the hyper-arid coastal region features favorable geological conditions for coastal aquifer development [17].  
Key surface water bodies in the Andes, such as Lake Titicaca, play a vital role in regulating regional water balance [34]. Major river basins like the Maranon and Ucayali influence surface-groundwater interactions and recharge dynamics across Peru's hydrogeological zones [19].  
This study encompasses the entire Peruvian territory, analyzing GWS variability over two decades through satellite gravimetry and in situ observations. We categorized regions based on natural hydroclimatic boundaries to capture spatial variations in GWS behavior. Understanding recharge, extraction, and land use heterogeneity is essential for national-scale groundwater assessments.  
Peru covers approximately 1.28 million km2 along the western coast of South America and exhibits remarkable geographic and climatic heterogeneity, structured into three natural regions: the arid coastal plain, the Andean highlands, and the Amazon rainforest. The coastal region is extremely dry, with annual precipitation below 50 mm and high demographic pressure linked to intensive irrigation [35]. The Andes feature steep topographic gradients and a strong dependence on groundwater resources in rural areas. In contrast, the Amazon receives over 2000 mm of rainfall annually and hosts extensive aquifers, though groundwater monitoring remains limited [36].  
Hydrologically, Peru is divided into three main drainage basins (Pacific, Atlantic, and Titcaca). The Pacific slope, which hosts the majority of the population and economic activity, contains less than 3% of the country's renewable water resources and relies heavily on groundwater abstraction. The Amazon basin supplies the remaining 97% [36].  
Peru's climate is also strongly influenced by the El Nino-Southern Oscillation (ENSO). Between 1968 and 2006, pronounced interannual impacts were observed: El Nino events increased river discharge along the northern coast, while La Nina produced the opposite effect in various catchments. Intensive irrigation in coastal areas imposes significant  
Figure 1: Location of Peru in South America (yellow box, right). The map shows the country’s hydrographic regions, highlighting climatic and hydrogeological differences and their relationship to groundwater recharge and storage. Green: high precipitation (Amazon); Orange: Andes (rainfall recharge); Yellow: Pacific Coast (hyper-arid zone with coastal aquifers).  
pressure on aquifers, primarily due to inefficient systems that withdraw large volumes of groundwater under limited recharge conditions [35].  
This hydrological and climatic mosaic makes Peru an ideal setting for assessing decadal GWS variability through satellite gravimetry. The contrast between arid zones with limited recharge and rainfall-rich regions, coupled with ENSO influence and anthropogenic pressures, provides a unique opportunity to analyze both the spatial patterns and temporal dynamics of GWS using GRACE/GRACE-FO, supported by in situ observations.