# Ecosystem Water Use Efficiency in the Three-North Region of China Based on Long-Term Satellite Data
## 1 Introduction  
Vegetation plays an important role in terrestrial ecosystems by absorbing carbon dioxide and consuming water during photosynthesis [1]. During the photosynthesis process, green vegetation could cause both a heating result by absorbing incident solar radiation and a cooling effect by transpiration [2, 3]. The net radiation results are usually cooled with spatial diversities [4]. Beyond these effects, vegetation can also produce organic matter through primary production and affect the carbon balance, regulating regional and global climates [5]. During primary production, vegetation may utilize different amounts of water depending on its species composition. This aspect has to be considered with great attention in plans of reforestation. It is obvious that, for ecological restoration, in areas of a limited water supply, we should use plants with limited water consumption and relatively sufficient organic matter production [1, 6, 7]. We think that for developing plans of restoration it would be very important to characterize the different areas of interest in terms of water use efficiency (WUE). Therefore, we propose in this paper the use of an index based on the ratio of gross primary production (GPP) to evapotranspiration (ET) that could be calculated by remote sensing data and could be useful to characterize the different areas of a region in terms of water use efficiency [8, 9, 10]. Realizing the importance of WUE, preliminary studies focusing on leaves and individual plants were first investigated with field measurements or gas exchange methods [11; 12] and were later extended to local scales, such as farmland [13; 14], grassland [9; 15], and forest [8; 16; 17; 18; 19]. However, these measurements are largely restricted to small scales. With the development of remote sensing techniques, global-scale productions of GPP and ET with sufficient accuracies have been produced, providing powerful tools to evaluate regional and even global ecosystem WUEs [20; 21; 22; 23; 24]. Both natural and anthropogenic factors could affect WUE. [PERSON] et al. [25] discovered that WUE shows a negative correlation with annual precipitation and temperature in China. [PERSON] et al. [26] found that the leaf area index, soil moisture, and vapor pressure may affect WUE. Other studies found that drought [27], aerosols [28], and CO\({}_{2}\)[29] are possible influencing factors. In addition to these natural factors, human activities may affect WUE through urban construction, land reclamation, and ecological restoration projects [30]. For example, [PERSON] et al. [31] found that the contribution of natural factors to WUE variation in northern China was approximately 84%, while the remaining contribution comes from afforestation and other human activities. In some ecologically fragile regions, human activities could contribute 80% to WUE variation [32]. Furthermore, the effects of natural and human activities may even counteract each other, making WUE variation more complicated [22]. Therefore, deeply investigating ecosystem WUE in regions is highly important and meaningful to evaluate the effects of these large scales of ecological restoration programs. The Three-North region (northeast, north, and northwest) of China is a typical ecologically fragile area experiencing serious environmental problems such as desertification and land degradation [33]. Acting as a connection between China and Europe, it has influenced the global economy since ancient times. With the implementation of the \"One Belt One Road Initiative\", the environmental problem of the Three-North region has attracted increasing attention [34; 35]. For example, overgrazing and grasslands reclamation make this area more vulnerable [36]. To overcome these problems, China has implemented a series of ecological projects since 1979 to restore vegetation cover in this region by planting trees on a large area [37].