Spatial characteristics of surface water and groundwater using water stable isotope in the Tarim River Basin, northwestern China Yucui Zhang, 1,2 Yanjun Shen, 1 * Yaning Chen 3 and Yun Wang 2,3 1 Key Laboratory of Agricultural Water Resources, Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China 2 Gaduate University of Chinese Academy of Sciences, Beijing 100049, China 3 State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang, 830011, China ABSTRACT Water resources management of the Tarim River Basin in China is hampered because it is poorly gauged, leading to inadequate understanding of the components of the hydrological cycle. This is an important problem to both ecology and society in this arid basin. In this paper, we attempt to investigate the isotopic characteristics of the surface water and groundwater in the Tarim River Basin and estimate the water evaporation loss from lakes. By measuring the water stable isotope composition and by using a mass balance model, we analyzed the spatial characteristics of the isotope composition, and isotopic water lines for surface water and groundwater were determined. Phreatic water evaporation was verified on the basis of the isotope composition results. The spatial variability of the Aksu River exhibited a ‘toward lighter–heavier’ change along the main stream for both δ 18 O and δD, and the distributions could be described by a cubic equation. The isotope composition of the evaporation vapour (δ E ) of the Aksu River decreased with distance. No regular trend was observed for the Tarim River between the isotope composition and distance, whereas δ E appeared to exhibit the same characteristics. This study is the first to use the isotopic method in the evaporation research for Bosten Lake. The water fraction lost [evaporation (E)/inflow (I)] in October was 0·25, which suggests that 0·68 × 10 8 m 3 input water was lost to evaporation. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS Tarim River; stable isotope composition; spatial variability; evaporation; Bosten Lake; Aksu River; Kaidu River Received 8 September 2011; Revised 28 June 2013; Accepted 30 June 2013 INTRODUCTION In arid regions of northwestern China, where the ecology and environment are vulnerable to disturbances from both human exploitations and climate change, water resources management encounters the dilemma of balancing water for ecological conservation and economic development (Chen et al., 2011). Water is the basic factor that determines the structure formation, development and stability of oasis ecosystems in this region (Chen et al., 2008). To understand the hydrological and ecological processes, it is necessary to study the characteristics of the hydrological cycle and water balance in detail (Shen and Chen, 2010). Evaporation loss of river and lake water plays an important role in the hydrological cycles in this region. Traditional observation is inconvenient for estimating evaporation loss in such ungauged catchments (Gibson et al., 1993), especially in the arid regions. Water stable isotope provides a possibility for understanding lake water balance, run-off generating and evaporation processes by using the fractionation mechanism (Tian et al., 2008), combined with regional climate and hydrology information. The isotope mass balance method has been applied successfully in river and lake research (Dincer, 1968; Zuber, 1983; Gonfiantini, 1986). Gonfiantini (1986) estimated the water fraction lost by evaporation from lakes, i.e. the ratio of evaporation to inflow (E/I), which is a key indicator for water balance. Simpson and Herczeg (1991) investigated the enrichment of δD in surface water evaporation throughout the Murray Basin in Australia. These researchers observed that a 1% loss of River Murray water under summer conditions by evaporation resulted in a mean increase in the value of δD of approximately 0·6–1·0% in the surface water. In Africa, Aly et al. (1993) observed that evaporation from the Aswan High Dam removed approximately 19% of the input water flow. In North America, Gat et al. (1994) estimated the evaporation from the Great Lakes on the basis *Correspondence to: Yanjun Shen, Key Laboratory of Agricultural Water Resources, Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China. E-mail: yjshen@sjziam.ac.cn ECOHYDROLOGY Ecohydrol. 6, 1031–1039 (2013) Published online 31 July 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1416 Copyright © 2013 John Wiley & Sons, Ltd.