## 3 Results and Discussion
### Spatiotemporal Variation of the Water Use Efficiency for Soil Conservation  
The WUE-SC in the study area showed significant spatial heterogeneity and ranged between 0-98.69 t mm\({}^{-1}\) during 2000-2008, indicating that approximately 98.69 t soil loss had been reduced by using 1 mm ET in a 1 km grid cell at most. The WUE-SC was below 1.00 t mm\({}^{-1}\) in most part of the study area, and the regions with high WUE-SC mainly concentrated in the northeast part and southeast part of the study area, showing a spatial pattern similar to that of the soil conservation amount during 2000-2008. For example, the highest WUE-SC occurred in the low-coverage grassland and Gobi desert near the northeast broader region, low-coverage grassland, shrub forest and sandy land near the northeast broader region. The WUE-SC is also very high in the medium-coverage or low-coverage grassland in some part of Ejina Oasis. Besides, the average WUE-SC reached 1.10, 0.89, 1.68 t mm\({}^{-1}\) in 2000, 2005 and 2008, respectively, which was very low on the whole and first decreased slightly and then increased during the study period (Figure 5). In addition, the spatial pattern of WUE-SC kept consistent in most part of the study area, but it changed significantly in some regions in the southeast part and northeast part during 2005-2008. The WUE-SC decreased by 1-37 t mm\({}^{-1}\) in most regions in the northeast part of the study area, and it increased in only a few regions in the northeast part, with the increment showing a decreasing gradient from the broader to the inner part. In particular, the WUE-SC has decreased very obviously in the water body of East Juyanhai Lake, which has reappeared in 2003 due to the water diversion. By comparison, the WUE-SC increased significantly in the southeast part of the study area during 2005-2008, with an increment of 1-10 t mm\({}^{-1}\) in the regions around Gurinai Lake, and even 10-97 t mm\({}^{-1}\) in some parts near the southeast broader. Figure 4: Spatial pattern of the annual ET in 2000, 2005 and 2008. It has been reported that uneven changes in environmental factors can lead to spatially heterogeneous responses of WUE to environmental change [20], and this also applies to the WUE-SC (Figure 2). The average WUE-SC decreased by 19.09% during 2000-2005, and the average ET increased by 18.47%, while there was no significant change in the soil conservation amount (decreasing by 5.42%), indicating that the change in the WUE-SC was mainly due to the change in the ET during 2000-2005. The ET increased during 2000-2005 because more water had been allocated to the lower reach since the ecological water transfer project was implemented. In particular, East Juyanhai Lake near Ejina oasis reappeared in the year 2003, the water body of this lake greatly increasing the ET. By comparison, the average WUE-SC increased by 88.76% during 2005-2008, the ET only decreased by 3.89%, while the soil conservation amount increased by 82.15%, suggesting the change in the soil conservation amount made a great contribution to the increase of the average WUE-SC. Besides, there was no obvious change in the soil conservation rate during 2005-2008, which reached 57.36% and 55.98% in 2005 and 2008, respectively, indicating that the vegetation coverage change did not lead to significant change in the soil conservation amount. However, during 2005-2008 the potential soil loss increased remarkably by 88.76%, which led to the significant increase of the soil conservation amount and consequently an improvement in the average WUE-SC. In particular, the potential soil loss increased most obviously in the southeast part of the study area (Figure 2), which is close to Badain Jaran Desert, with an increment of 1000-5000/km\({}^{2}\) and an increment rate of 20%-50% in most parts of this region during 2005-2008.