Assessing moisture sources of precipitation in the Western Pamir Mountains (Tajikistan, Central Asia) using deuterium excess

Tobias R Juhlke,Christiane Meier,Robert Van Geldern,Kim A Vanselow,Jakob Wernicke,Jamila Baidulloeva,Johannes A.C Barth,Stephan M Weise

doi:10.1080/16000889.2019.1601987

Abstract

Air moisture of oceanic origin can be subject to long-range transport and could contribute to precipitation at distant places. With ongoing climate change the relationship between water vapour sources and sinks is under constant evolution and plays an important role for water budget assessments. This study analysed monthly integrated and event-based precipitation samples from two field sites in the Western Pamir Mountains (Tajikistan) for stable isotope compositions of oxygen (δ18O) and hydrogen (δ2H) of water. The aim was to investigate water vapour sources and, in particular, to evaluate contributions of water vapour from the Mediterranean region. The latter has been often postulated as a potential moisture source region for Central Asia. Deuterium excess values (d), that serve as a fingerprint of moisture origin, were below 13‰ in monthly integrated samples, for most of the year. This indicates a smaller contribution of high Mediterranean moisture (∼20‰) to the Western Pamir Mountains than originally expected. A maximum d value of 19‰ was observed in March, which is not in agreement with the common observation of increased Mediterranean contribution during winter (DJF). A Lagrangian backward trajectory model (HYSPLIT) was applied to infer the general air mass origin for sampled precipitation events. A maximum contribution of ‘western’ moisture of 40% was also detected in March, while 40%–60% of moisture that contributed to precipitation events in winter was transported by trajectories that originated from the Northern Indian Ocean.

Highlights

Values for d18O range between –24.4& and –2.0&, whereby minimum values are reached in the cold season, while maximum values were observed during warm season
Deuterium excess values above 15& that are typical for cold season precipitation in the eastern Mediterranean and further east, as well as in the Persian Gulf and Kabul, were not as abundant in the Western Pamir Mountains during this time of the year
With respect to our hypothesis regarding the influence of Mediterranean moisture, our data evaluation of trajectories from the Western Mediterranean to the Western Pamir show a maximum moisture contribution during March and April, when it was not initially expected

Summary

Introduction

Stable isotopes of oxygen and hydrogen in water have become state of the art parameters in hydrological studies to delineate water sources (Yurtsever and Gat, 1981; Joussaume et al, 1984; Rozanski et al, 1993; Bowen and Revenaugh, 2003; Terzer et al, 2013; Stumpp et al, 2014; Galewsky et al, 2016) and investigate processes, such as evaporation (Craig and Gordon, 1965; Gibson et al., 2016) and transpiration (Dongmann et al, 1974; Helliker and Ehleringer, 2000). Choosing the oceanic reservoir as a starting point of the hydrologic cycle, evaporation of isotopically relatively uniform ocean water enriches the resulting water vapour in light isotopes. Condensation of this water vapour in clouds during formation of rain is assumed to be an isotopic equilibrium process, which only depends on temperature. The isotopic composition of precipitation is subject to several effects that mostly depend on the temperature during condensation. They include the altitude effect, the latitude effect, Tellus B: 2019.

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