Elucidating the climate and topographic controls on stable isotope composition of meteoric waters in Morocco, using station-based and spatially-interpolated data

Abstract

Understanding the main controls on stable isotope variations in precipitation is fundamental for the interpretation of the hydrological cycle. However, spatio-temporal variations in δ18Op are poorly known in Morocco. Herein, we explore the relative influence of meteorological variables, spatial and orographic (altitudinal) effects, atmospheric circulation and moisture sources on precipitation stable isotopes in Morocco. Precipitation events and two-years-long monthly records from 17 rain-gauge stations in Morocco are investigated and compared in this study to global gridded records of monthly and annual stable isotopes in precipitation. We highlight that the main spatial controls on precipitation stable isotopes are the topography and the distance from marine source. The most depleted mean annual isotopes are located in the High Atlas Mountains (δ18Op=−9.56‰ and δ2Hp=−59.3‰), while the most enriched isotope ratios exist in southwestern Morocco (δ18Op=−2.35‰ and δ2Hp=−7.47‰). The well-constrained relationship between δ18Op and altitude describes a gradient of 0.11–0.18‰ per 100m. The seasonal variation is expressed by a general enrichment that reaches −4.8‰ during the dry season, related to the recycled vapor contained within the summer precipitation. Notwithstanding the scarcity of temperature and precipitation measurements, the amount effect is observed in multiple stations during several rain events and precipitation seems to have more influence on δ18Op than temperature. Backward moisture trajectories indicate a distinct depletion in δ18Op in extreme events originating from the Atlantic Ocean. The presence of a rain shadow effect is also revealed on the lee side of High Atlas Mountains, southeastern Morocco.