LAUSR

Abstract The economic activities of the Sonora strongly depend on groundwater extraction, which is generally of recent meteoric origin. Understanding the hydrological functioning in this semi-arid region, located in northwestern Mexico, is crucial for integrated water management in a changing climate. This study presents a spatio-temporal analysis of the isotopic composition (δ18O, δ2H, d-excess) during the 2018 rainy season in the Sonora River Basin (SRB), a region influenced by the characteristic weather conditions of the North American Monsoon (NAM). The basin is located in the central region of Sonora and covers an area of approximately 27,000 km2. Precipitation was collected using six passive totalizers, distributed spatially from the coast of the Gulf of California up to 1,425 m asl inland. The rainfall isotopic composition is comparable with the Global Meteoric Water Line (GMWL) and with the records of the Global Isotope Network in Precipitation (GNIP) in Chihuahua, Mexico, between 1962 and 1988, and Tucson, Arizona (2018). The temporal variability of rainfall isotope ratios was Δ18O = 7.4‰ and Δ2H = 49.8‰, with a clear depletion pattern (low δ18O) as the monsoon rainy season progressed. The isotopic lapse rate was found to be −1.0‰ in δ18O per km of elevation as a result of the topography relief and rainout effects within the basin. The influence of tropical cyclones such as Hurricane Sergio (October 2018) provided the opportunity to isolate the effects of a tropical storm with respect to the deep convective precipitation, typical of the NAM systems over northwest Mexico.