Spatiotemporal groundwater recharge assessment under prolonged drought conditions in São Paulo State, Brazil

Following recent prolonged regional droughts, reliance on groundwater resources in São Paulo State, Brazil, has increased, emphasizing the need for robust evaluations of groundwater availability to support integrated water resources management. For this purpose, dependable methods for estimating groundwater recharge were applied. This paper focuses on one of the region’s most important aquifers, the Bauru Aquifer System (BAS), which spans roughly 42% of São Paulo State (≈ 117,000 km²). This paper assesses the spatial and temporal variability of recharge estimates in the BAS using four methods: RISE, RISE corrected, Total Master Recession Curve (MRC), and Annual MRC, based on data from 43 monitoring wells collected between 2009 and 2025. The estimated mean annual groundwater recharge was 147 mm (RISE), 205 mm (RISE corrected), 221 mm (TotalMRC), and 274 mm (Annual MRC), representing approximately 16% to 27% of annual precipitation. Methods that derive a master recession curve from longer time series tend to reduce bias by including a larger number of recession events, thereby decreasing the influence of locally sharp water level drops. Cross-correlation analyses between precipitation and groundwater levels showed that 75% of the wells exhibited the maximum positive correlation at a lag of 70–150 days (2.5–5 months), indicating a slow, delayed response of the aquifer to rainfall. These findings offer valuable insights into recharge quantification and groundwater response times to precipitation in the BAS.