# Remote Sensing of Ecosystem Water Use Efficiency in Different Ecozones of the North China Plain
## 1 Introduction  
In terrestrial ecosystems, vegetation cover plays an important role in energy and material balance through photosynthesis processes [1]. During photosynthesis, vegetation cover absorbs incident solar light and stores it as organic matter [2]. Furthermore, by producing oxygen and absorbing carbon dioxide, vegetation cover can adjust local, regional and global climate [3]. During these processes, the most important is primary production involving water consumption, carbon dioxide absorption, and organic matter production [4]. The primary productions of different vegetation types consume different amounts of water, which is one of the major restrictions on vegetation growth [5]. Therefore, it is important to determine the water consumption of different ecosystems, especially in different ecological function zones. WUE can be defined as the ratio between GPP and evapotranspiration (ET) [6; 7; 8] and is very helpful for understanding the carbon and water cycles of terrestrial ecosystems. Early studies on WUE mainly focused on the leaf level through in situ measurements and were time and labor intensive [9; 10]. The ecosystem water use efficiency based on in-situ eddy covariance measurements has been extensively investigated throughout the world [11; 12; 13]. As measurements have advanced, scales of WUE have been extended to farmland [14; 15], grassland [7; 16] and forest [6; 17; 18; 19; 20]. With the help of remote sensing technology, GPP and ET product data have provided abundant data sources for WUE research at the local [21; 22], regional [21; 23] and global scales [24; 25]. For example, WUE in most areas of the sandstorm source area in Beijing and Tianjin increased [26], and during the period from 1982 to 2015, the WUE of more than three-quarters of China increased [21]. The northern hemisphere WUE will be affected by phenological changes [27], and climate seasonal changes affect global WUE changes [28]. [PERSON], et al. [29] discovered that there were negative correlations between precipitation and WUE from 1979 to 2012. In addition to precipitation and temperature, leaf area index, soil moisture, water vapor and human activities are also potential influencing factors affecting WUE [30]. As discovered by [PERSON], et al. [31], climate factors contribute to 84% of WUE variations, while human activities contribute 16%. WUE, as defined by the ratio of GPP and ET, will be influenced first by GPP. Vegetation cover GPP is influenced by vegetation species, environmental conditions and human activities [32]. Therefore, it is necessary to analyze WUE considering vegetation species and environmental conditions. Ecological zones are defined as ecoregions where the majority of species and environmental conditions are similar. [PERSON], et al. [33] discovered that within the northern farming-pastoral region, vegetation dynamics show great spatiotemporal diversities. [PERSON], et al. [34] pointed out that vegetation dynamics and courses in different ecological zones were largely different. Therefore, it is necessary to deeply investigate the dynamics and potential factors for different ecological zones. The North China Plain is located in the hitherland of China, with a developed economy and convenient transportation. It is an important grain producing area in China and plays an important role in maintaining national security. The North China Plain has experienced environmental problems of land degradation and desertification [35]. In order to alleviate the above environmental and ecological problems, ecological restoration projects have been implemented to increase the vegetation coverage, etc., which have changed the carbon and water exchange between the atmosphere and the surface, thus affecting the spatiotemporal changes in WUE [26]. However, the shortage of water resources in the North China Plain and the large-scale decline of the groundwater level are important reasons for restricting the sustainable development of agriculture in the North China Plain.