## 1 Introduction  
Water is usually the single most limiting factor for provision of ecosystem services, and water scarcity is impacting human welfare worldwide, especially in arid and semiarid regions that are very sensitive to climate change and land use and land cover change [1; 2; 3; 4]. Agriculture in arid and semiarid regions relies heavily on irrigation of water diverted from rivers, and large-scale urban or agricultural development in these regions depends upon the ability to pump groundwater or convey freshwater over long distances from natural sources [5]. Besides, rapid socio-economic development drives land use change, which is altering the hydrologic system and increasing water needs for industrial, domestic and environmental uses, has potentially large impacts on water resources [6; 7]. However, coincident with rapid growth in water demand is the potential for substantial reduction in water supplies in arid regions [8]. Runoff of many rivers in arid regions showed a declining trend under the influence of the climatic and land use change during the past decades [9]. As a result, the traditional water utilization approach in these arid and semiarid regions is now facing a big challenge, which appeals to people to develop water-saving irrigation and enhance water use efficiency for sustainable water use [10]. Enhancing water use efficiency is a critical response to growing water scarcity [11], and it is necessary to carry out in-depth research on water use efficiency, which can provide valuable reference information for scientific water resource allocation to make more efficient use of limited water resources [12; 13; 14]. In particular, it is urgent to analyze water use efficiency in these arid and semiarid regions in order to improve the capacity of these regions to provide the key ecosystem services that are essential to human well-being [15; 16]. The water use efficiency or water productivity is the ratio of the net benefits from crop, forestry, fishery, livestock and mixed agricultural systems to the amount of water used to produce those benefits, and in the broadest sense it reflects the objectives of producing more ecological benefits at less social and environmental cost per unit of water consumed [11; 17]. Many of the benefits that humans gain from ecosystems are directly or indirectly related to freshwater, which are commonly referred to as water related ecosystem services, e.g., water purification and soil conservation [14; 18]. The ecosystems provide multiple ecosystem services, however, the current research on water use efficiency has generally focused only on crop water use efficiency, and there has been very limited research on water use efficiency for the provision of ecosystem services except crop production or primary production [11; 19; 20; 21; 22]. With a shift in resource management philosophy from production of food, fiber and forage to protection and restoration of ecosystems [23], it is surely necessary to analyze water use efficiency for the provision of other ecosystem services. There is an increasing consensus about the importance of incorporating ecosystem services into resource management decisions, but there are still some difficulties in quantifying these ecosystem services and the corresponding water use efficiency [24]. On the one hand, it is difficult to quantify the provision of some ecosystem services, and previous research has generally focused on ecosystem service valuation that assigns values to ecosystem services provided by different land use types andcan hardly provide the information on the quantification of ecosystem services [23, 25, 26]. More importantly, there is rarely available information on how to account for the impacts of the pattern of land uses or land-cover classes on the provision of ecosystem services [27].