Isotopic Characterization of Rainwater for the Development of a Local Meteoric Water Line in an Arid Climate: The Case of the Wadi Ziz Watershed (South-Eastern Morocco)

Anas El Ouali,Mohammed El Hafyani,Abderrahim Lahrach,Abdellah El Hmaidi,Mohamed Qurtobi,Abdennabi Alitane,Jacques Mudry,Anton Van Rompaey,Tibari El Ghali,Ali Essahlaoui,Allal Roubil

doi:10.3390/w14050779

Abstract

For any hydrological or hydrogeological system, the arrival of new rains is the input signal to the system. This isotopic signature of precipitation is of major interest in understanding the recharge processes of the aquifer system. On the scale of a given basin, staged stations at different altitudes and spread out in space allow this input signal to be well characterized and to draw the local meteoric water line. In south-eastern Morocco, specifically, in the Errachidia region, several chemical and isotopic studies of the waters of the various aquifers have been carried out. In the absence of a local meteoric water line, these studies were based on the use of the global meteoric water line (GMWL). Thus, the objective of this work is the isotopic characterization and the elaboration of the local meteoric water line of the rainwater of the Ziz watershed. This characterization of the input signal in the study area is based on 41 measurements of stable isotopes (δ18O and δ2H) relating to the precipitations collected during the period from December 2019 to November 2020 in four staged stations at different altitudes and spread over the space from upstream to downstream of the watershed. The linear relationship of δ2H as a function of δ18O describes the local meteoric water line (LMWL) by equation δ2H = 7.5 ± 0.3 δ18O + 4.6 ± 1.7; R2 = 0.93. This equation displays evaporation confirmed by the arrival of continental currents in an arid environment. The variation in precipitation δ18O as a function of the sampling altitudes for the rains highlighted the relationship δ18O = −0.0026 ∗ Z − 1.67, with R2 = 0.93, which means an altitudinal gradient of −0.26‰ per 100 m of altitude. In this regard, the development of the local meteoric water line and the determination of the altitudinal gradient for the first time in this arid to semi-arid region of the watershed will be of great use to researchers and water resource managers; for example, to help determine the groundwater recharge areas, determine the exchanges between surface water and groundwater, and analyze many other hydrological problems.

Highlights

For at least four decades in the Maghreb, climate change has resulted in a gradual rise in temperatures and a decrease in precipitous waters [1–6]
With the objective of determining a consistent local meteoric water line (LMWL) to be used for a variety of hydrological applications in this continental region, a precipitation weighted regression method [42,48] aiming to reduce the effect of rainfall amount of small precipitation samples on the calculation of the LMWL was used for the development of an updated LMWL with the equation δ2 H = 7.5 ± 0.3 ∗ δ18 O + 4.6 ± 1.7 and a correlation coefficient of R2 = 0.93
The slope of 7.5 ± 0.3 and intercept of 4.6 ± 1.7 of the LMWL are lower than those of the global meteoric water line (GMWL), indicating that precipitation are affected by non-equilibrium processes during formation of the atmospheric vapor masses and precipitation generating a variable difference between δ18 O and δ2 H because of the different kinetic effect during the fractionation of these two isotopes

Summary

Introduction

For at least four decades in the Maghreb, climate change has resulted in a gradual rise in temperatures and a decrease in precipitous waters [1–6]. In sub-arid climatic zones, establishing a precise hydrological balance capable of allowing the simulation of scenarios of active management of water resources is a delicate operation. The irregularity of the temporal distribution of precipitation, as well as that of the measured water heights, prevent the use of averages in forecasts. Isotope hydrology using chemical and isotopic data is a means of accessing the global balance and understanding of the hydrological behavior of aquifer systems [7]. These isotopic techniques have a wide field of application. They allow tracing the origin of the water; in particular, the determination of the average altitude of the recharge zone [8–10]

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