| Soil moisture is a fundamental property affecting plant growth, transport and transformation of soil nutrients, water and energy budgets in the soil-plant system. The spatial variation of soil moisture has important implications on all of these processes. The heterogeneity of soil moisture is affected by various environmental factors, such as soil texture, NDVI, total nitrogen, topographical factors like slope and aspect, etc. As the upper reach of the Heihe River Watershed is the main runoff producing area of the Heihe river, the distribution of soil moisture has great influence on runoff. Study of the temporal and spatial heterogeneity of soil moisture in this area is the key to understanding the regional water cycle process.This study chose the upper reach of the Heihe River Watershed as the study area. Based on the combination of soil-vegetation-DEM, both random soil sampling and points and in situ observations were designed for collecting soil moisture and related environmental data over the period of 2012-2014, in order to fill the data gap in the upper reach of the Heihe River Watershed. Subsequently, the relationship between spatial heterogeneity of soil moisture and environmental factors were examined, by using principle component analysis (PCA), regression kriging interpolation methods, and linear regression methods. Furthermore, the temporal stability and characteristic of soil moisture’s temporal and spatial heterogeneity were analyzed. The main findings are as following:1. Based on the collected soil moisture and environmental data at random sampling points, regression kriging interpolation methods were used to identify spatial pattern of soil moisture in the study area. The results indicated that soil moisture at both 0-10cm and 10-50cm depth had a relatively high water content in the eastern part of the study area and relative low water content in the western part of the study area. And there was strong variability in the western part of the study area.2. Correlation analyses were conducted between soil moisture and NDVI, soil texture, topographical factors like slope, elevation, aspect, total nitrogen and porosity. Results indicate that soil moisture at the two soil layer (0-10cm and 10-50cm) are significantly correlated to topographical factors. Thus, rotated principal component analysis (RPCA) was used to find the controlling factors of soil moisture, showing that vegetation and topography factors are the controlling factor of soil moisture in the upper reach of the Heihe River Watershed.3. At the watershed scale, soil moisture showed little temporal variations. The 0-10cm soil layer showed the highest temporal heterogeneity of the vertical distribution. And by using wavelet analysis, the soil moisture data (2013-7-19～2014-7-13) showed a cycle of 2.5 months and 4.6 days at annual, seasonal and weekly scales, respectively.4. The analysis of the 31 in-situ soil moisture data sets with respect to time stability resulted in the identification of representative sample locations to predict average soil moisture at different soil layer. And time-stability at the watershed scale proved to differ between the four vegetation forms.5. Regression analysis of soil moisture and environmental factors showed, R2=0.42, indicating 43% of the spatial variability of soil moisture can be explained by the environmental factors. Thus environmental factors alone cannot fully explain the variation of the soil moisture in the study area.6. Both direct and indirect methods were used to estimate average soil moisture at watershed scale, results indicating that indirect method were relatively more appropriate in the upper reach of the Heihe River Watershed. |
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