Changes in Rainfall Regime: A Case Study in the Piracicaba River Basin, Brazil

Abstract

Changes in precipitation regimes associated with climate variability and climate change pose significant challenges to water resource management, agriculture, and land‐use planning, particularly in socioeconomically strategic regions such as the Piracicaba River Basin, Brazil. This study analyzed precipitation series from 1985 to 2022 using data from 159 regularly spaced grid points across the basin. Pettitt, Mann–Kendall, and Sen non‐parametric tests were applied to both annual totals and annual maximum 24 h rainfall. Significant trends were identified in 22 grid points for annual precipitation and in 19 points for annual maximum daily rainfall. Breakpoints were concentrated in the late 1990s and between 2009 and 2012. Annual totals exhibited consistent negative Sen slopes, indicating a cumulative reduction in precipitation. In contrast, extreme daily rainfall showed spatially heterogeneous and lower‐magnitude trends, with no evidence of widespread intensification. Declining trends were concentrated near the basin headwaters and the Cantareira Water Supply System, suggesting potential implications for regional water security. Notably, the detected changes preceded the 2013–2015 water crisis in São Paulo. These findings highlight the need for continuous monitoring and adaptive water management strategies under increasing climate variability.