Isotopic signature of precipitation in the semi-arid region of Ceará, Northeastern Brazil

Abstract

Abstract. The study of stable isotopes in the precipitation of Ceará was conducted on monthly rainfall samples between 2011 and 2019 and on daily rainfall samples between 2018 and 2019 in order to support the global knowledge of hydrological processes in semi-arid regions given the scarcity of such data in the Northeast of Brazil. Results showed that the frequency of rainfall sampling in semi-arid regions may induce an isotopic bias, due to the partial evaporation from the water collectors during monthly storage causing isotopic fractionation. In the absence of adequate data processing, this bias may lead to a wrong definition of the Local Meteoric Water Line (LMWL), which can have major consequences on the understanding of groundwater recharge processes. The processing of the monthly isotopic data (elimination of the evaporated data) and the analysis of daily data highlighted that the LMWL of Quixeramobim is characterized by a y-intercept value greater than +12 ‰, indicating that rainwater is composed of vapours from marine origin, but also from continental recycling. Analysis of δ18O monthly variations in relation to climatic factors suggested the importance of convective systems and relative humidity in the control of the isotopic composition of precipitation in a semi-arid tropical region.