Controls of vertical profiles of soil moisture content in the sites across the North-South Transect of the Loess Plateau

Background/Question/Methods

Soil moisture is one of the key limiting factors for plant growth in the water-limited areas, especially in the Loess Plateau, where the large-scale “Grain for Green Program” was implemented. Characterization of soil moisture content (SMC) profiles at the sites along North-South Transect of the Loess Plateau has profound implications for understanding hydrological processes of the terrestrial water cycle at the large spatial scale. We selected 45 sampling points at 11 sites along the North-South Transect of the Loess Plateau based on the precipitation gradient. Here, we asked how the SMC in different soil layers (0-1m, 1-2m, 2-3m) would vary along the transect and how the topographic variability would affected SMC in different soil layers along the transect. We also asked which factors dominated the SMC in different layers along the transect. The basic statistical features for each sampling points were analysed to characterize the variation of SMC along the transect. Paired-sample T test was used to assess the contribution of different topography factors to the overall variation of soil moisture. The mulit-linear regression analysis was applied to explore the relationships between the environmental factors and SMC in different layers.

Results/Conclusions

The vertical profiles of SMC at the sampling points along the transect revealed large variations, especially in the soil layer below 1m. The average SMC in the vertical profiles (0-3m) at the sampling points increased with the precipitation gradient increasing (R²=0.46). The SMC in the sampling points located in regions with annual precipitation (MAP) >500mm was higher than those in the sampling points located in regions with mean MAP <500mm. The vertical profiles of SMC under different land use types revealed different distribution patterns with the precipitation gradient changing. Paired T-sample test results showed that the topographic variability including slope gradient, slope aspect and slope position revealed no significant effects on the SMC in different soil layers along the transect. Multi-regression analysis showed that average SMC was mainly controlled by MAP and land use type in different soil layers (0-1m, 1-2m, 2-3m). It’s noted that our results were inconsistent with those results at the catchment scale that topographic factors would play significant roles on the SMC in each soil layer. The results indicated that the precipitation and land use type should be considered in the vegetation restoration and sustainable water management at regional scale in the Loess Plateau.