Quantifying the effect of moisture source and transport on the precipitation isotopic variations in northwest Ethiopian Highland

Abstract

Understanding the controlling factors for variations of precipitation isotopes is essential for accurate interpretation of isotope-based paleoclimate proxy data as well as better apprehension of the hydrological cycle. These are particularly important for Ethiopia, an arid region and important location for studying human evolution. Past studies have established the importance of moisture source on isotopic variation in precipitation, but few studies have identified exact moisture sources and quantified their contributions. In this study, we use a Lagrangian moisture diagnostic technique and daily precipitation isotope data collected at Debre Markos on the northwest Ethiopian Highland to quantify the contributions of different moisture sources to precipitation. We also quantitatively establish the relative importance of local climate conditions, climate conditions of moisture uptake locations, and different moisture sources on the isotopic variation in precipitation at the study site. Our results show that recycled continental moisture from adjacent areas is the most dominant moisture source throughout the year (67%), and its contribution is higher in the wet season (79%) than in the dry season (56%). Secondary moisture sources include the southern Indian Ocean (16%) for the wet season and the northern Indian Ocean (14%) for the dry season. Based on correlation analyses, we find that the most important control over the precipitation isotopic variation is the convective intensity at both the study site and the moisture uptake locations. Strong connective activities are linked to low isotopic values, and the seasonal movement of the deep convection region could explain the seasonal precipitation δ18O patterns with low values in the wet season and high values in the dry season. Other climate factors that affect the isotopic composition of precipitation include local temperature, relative humidity, rainfall amount as well as total rainout along the trajectories and relative humidity at moisture uptake locations. Our study could provide new perspectives for interpreting δ18O-based paleoclimate proxies in this region, as well as crucial information for understanding hydrocliamte processes in a region where sustainable water resources are of particular importance for socioeconomic development.