## 3 Results and Discussion  
### Spatiotemporal Variation of the Soil Conservation Amount  
The results suggested that there was significant spatiotemporal variation of potential soil loss, soil conservation amount and soil conservation rate in the study area (Figure 2 and 3). The annual soil conservation amount in the study area ranged between 0-8822 t km\({}^{-2}\) a\({}^{-1}\), with an average value of 75.47, 71.38 and 137.18 t km\({}^{-2}\) a\({}^{-1}\) in 2000, 2005 and 2008, respectively. Besides, the annual total soil conservation amount of the study area first decreased from approximately 5.80 million ton in 2000 to 5.4 million ton in 2005, but then increased to 10.55 million ton in 2008, showing significant inter-annual variation. In addition to the inter-annual variation, the soil conservation amount also showed obvious time-series variation at the monthly scale (Figure 2). The climax of the soil conservation amount occurred during March and May in 2000 and 2008 and during April and June in 2005, while there was very limited soil conservation in other months, indicating the soil conservation mainly occurred in the spring (Figure 2). There are frequently strong winds in the spring in the lower Heihe River Basin, which lead to the enormous increase of potential soil loss; what is worse, the vegetation coverage rate is generally very low in the spring when most vegetation just begins to grow, making the study area extremely susceptible to the wind erosion. Although the soil conservation amount in the study area varied substantially across years, the overall spatial pattern of soil conservation was generally consistent during 2000-2008, with significant change only occurring in a few parts of the study area (Figure 3). Besides, the soil conservation  
Figure 2: The monthly soil conservation amount (SC) (Unit: thousand ton) and ET (Unit: mm) in 2000, 2005 and 2008.  
amount was very low in most parts of the study area, and a high soil conservation amount only occurred in a few regions such as the northeast border region and the area near Gurinai Lake in the southeast border region. The soil conservation amount in the northeast border region showed an obvious decreasing gradient from the northeast to the southwest during 2000-2008, and the soil conservation amount in the southeast border region also showed an obvious decreasing gradient from the border part to the inner part in 2008. It is not surprising that the spatial pattern of the soil conservation amount is similar to that of the potential soil loss since the latter is the maximum of the former, but the soil conservation amount is also influenced by other factors such as the vegetation coverage rate. What is more, the soil conservation amount varied greatly among different land cover types. For example, the high soil conservation amount in 2008 occurred in the Gobi desert in the northeast border region, the water body over East Juyanhai Lake, the low-coverage grassland and sandy land in the southeast border region, where the vegetation coverage rate was generally low. The lowest soil conservation amount occurred in the Gobi desert near the southwest border, where both the vegetation coverage rate and potential soil loss were very low. It is somewhat surprising that the high soil conservation amount mainly occurred in the regions with the low vegetation coverage rate rather than the high vegetation coverage rate, indicating that the potential soil loss may have exerted more significant impacts on the soil conservation amount. The soil conservation rate also showed significant spatial heterogeneity (Figure 3). The regions with the high soil conservation rate concentrated in the oases and irrigated areas along the main stream of Heihe River, where the vegetation coverage was in good conditions and the major land cover types included cultivated land, shrub forests, closed forest land and medium-coverage grassland.