# Spatio-Temporal Changes in Water Use Efficiency and Its Driving Factors in Central Asia (2001-2021)
## 4 Discussion  
### Spatial and Temporal Variation and Distribution of WUE in Central Asia  
It has been shown that the results of ecosystem WUE calculations based on the MODIS products GPP and ET are highly consistent with calculations based on flux tower data [20; 24]. Based on the MODIS products GPP and ET, [PERSON] et al. and [PERSON] et al. obtained a global mean annual WUE of 1.7 gCkg\({}^{-1}\)H\({}_{2}\)O [24; 26]. Using the same methodology, the mean annual value of WUE in this study is 3.011 gCkg\({}^{-1}\)H\({}_{2}\)O, which is similar to other ranges calculated in dryland areas, such as southern and northern Africa, Australia, and central South America, where the WUE of dryland ecosystems ranges from 2.5 gCkg\({}^{-1}\)H\({}_{2}\)O to 5.1 gCkg\({}^{-1}\)H\({}_{2}\)O [26]. In addition, the estimation of WUE based on the Google Earth platform significantly improved the efficiency of the study. The WUE of dryland ecosystems is higher than the global average due to the fact that vegetation in dryland ecosystems experiences long periods of drought and is more tolerant of dry conditions [46]. Vegetation in dryland ecosystems is more sensitive to climatic factors [32; 47]. Therefore, WUE monitoring and assessment of dryland ecosystems can contribute to a more comprehensive understanding of changes in the carbon-water cycle in dryland ecosystems and enhance the ability to predict future climate change [11]. We found weak inter-annual variability in WUE in Central Asia, with significant intra-annual variability and some seasonality (Figure 7). This is evidenced by an increase from April to a maximum in August each year, followed by a rapid decline, which is consistent with the conclusions obtained by [PERSON] et al. and [PERSON] et al. [27; 28]. High WUE is mainly concentrated in June, July, and August due to reduced transpiration and reduced water consumption due to the partial closure of natural vegetation stomata during the dry season [48]. In contrast, relatively adequate artificial irrigation means that crops are relatively less affected by drought stress. However, artificial irrigation causes significant evaporation of soil water, which, in turn, leads to a lower WUE for the crops [49]. It is noteworthy that the areas of Central Asia where inter-annual variation in WUE declined significantly were also concentrated in oases, possibly due to irrigation practices on arable land and irrational cropping structures during the monitored period. For example, [PERSON] et al. found that the irrigated areas of Uzbekistan, Turkmenistan, and the southern oases of Xinjiang, China were agricultural, and that the main crop during the monitoring period was cotton, which is a high water consumer [27]. Therefore, the conservation of dryland ecosystems requires a rational allocation of water resources and a trade-off between agricultural and ecological uses. It is not necessarily a good thing that crops in oases are over-irrigated, which reduces the efficiency of water use and also causes soil salinization [50; 51]; therefore, it is important to protect the ecosystem while ensuring food production security. In addition, this study found a significant increase in WUE in Central Asia in the Ustyurt Plateau, around the Aral Sea to the Lake Balkhash Basin, the Ili River Basin, and in central Tajikistan. It has been shown that the Ustyurt Plateau experiences long periods of drought, which, in turn, strengthens the drought resistance of the vegetation in the area [52], thereby increasing WUE [27]. [PERSON] et al. found that the increasing trend of actual net primary productivity (ANPP) in the Lake Balkhash Basin and the Ili River Basin was due to appropriate grazing and progressively warmer and wetter climate change in the basin. Not only has the climate in the basin become suitable for vegetation growth, but grazing intensity has also been reduced [53], which may lead to an increase in WUE in the region.