## 1 Introduction
Fortunately, the Millennium Ecosystem Assessment (MA) has provided the conceptual framework to assess or quantify the provision of ecosystem services [21], and there have also been some models to quantify some essential ecosystem services, e.g., Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) [24] and the system for identifying and zoning ecosystem services (SIZES) [28]. On the other hand, there are still some difficulties in accurately estimating the water used for the provision of ecosystem services, which is generally represented with evapotranspiration (ET) [29]. There have been some reliable methods for quantifying ET at a point or field scale for specific sites, but it is fairly difficult to apply such methods for quantifying ET at large scales [5]. Fortunately, some models such as GLEAM model and MODIS-ET model have been proposed for estimating ET with remotely sensed data, which is able to capture land surface information from large geographic extents [5, 30, 31]. The Heihe River Basin is the second largest inland watershed in China, and the lower Heihe River Basin is a typical arid and semiarid region in Northwest China, where the water scarcity has been intensifying and posing a threat to the sustainability of agricultural production and provision of essential ecosystem services such as primary production and soil conservation [12, 13]. The precipitation is very limited in the lower Heihe River Basin, with the annual precipitation reaching approximately 37 mm and the maximum annual evapotranspiration exceeding 1000 mm. Since the precipitation is far less than the water requirement for crop and other vegetation, the water from Heihe River plays a key role in sustaining the agricultural ecosystem as well as other ecosystems [12]. However, runoff of Heihe River into the lower reach has declined significantly due to the impacts of human activities and climate change during the past decades [12, 13], which has led to a series of ecological problems such as desertification and grassland degradation, and consequently posed a great threat on the ecological safety of Northern China. For example, serious desertification has made the Badain Jaran Desert in the Ejina Banner become a main sand source of sandstorms influencing Northern China, especially the Beijing-Tianjin region, which has suffered from frequent sandstorms in the spring for years [14]. Besides, the lower Heihe River Basin is an important ecological barrier of China, where the soil conservation service of vegetation plays a key role in guaranteeing the ecological safety of Northern China [12, 13]. The Chinese central government started a program of water diversion in 2000 in order to tackle the water shortage issue and restore the ecological environment and control the desertification in the lower Heihe River Basin. Engineering measures have been applied since 2000, _i.e._, water diversion, lining diversion channels, irrigation efficiency improvement [15]--which have increased water supply to the lower Heihe River Basin and has allowed the ecological environment begin to recover. However, much remains unknown about the changes in ecosystem service provision and water use efficiency before and after the implementation of the program of water diversion [16], which can provide valuable information for improving the program of water diversion and the local ecosystem management. In particular, there has been very limited research on water use efficiency for provision of key ecosystem services such as the soil conservation service in the lower Heihe River Basin. The soil conservation service plays a key role in reducing the soil erosion that has great impacts on ecosystem productivity [32; 33; 34]. Besides, there are always strong links between measures for soil conservation and measures for water conservation, in particular there are also links between water use efficiency and biological soil conservation, and this applies equally in arid and semi-arid regions [35].