黄土丘陵区小流域土壤有效水空间变异及其季节性特征

基于213个样点土壤有效水数据,从流域、坡面和沟道三个尺度分析了黄土丘陵区典型小流域土壤有效水在春、夏、秋三个季节的空间变异特征。结果表明,土壤有效水均呈较强空间变异性,其中沟道土壤有效水均值和空间变异强度(标准差和变异系数)均显著高于坡面,坡面和沟道土壤有效水正态性明显好于流域尺度。不同尺度土壤有效水空间变异程度随均值发生变化,变异系数随均值增加呈指数递减趋势。流域和坡面尺度土壤有效水与坡向呈显著正相关关系且相关系数高于其与坡度和高程,而沟道尺度与各地形因子相关关系均较弱。土壤有效水空间变异呈现明显的季节性特征,秋季土壤有效水均值最高而空间变异性最低,夏季土壤有效水均值最低但变异系数最大。流域尺度夏季土壤有效水与高程相关系数显著高于春秋两季,而坡面尺度则相反。此外,土壤有效水抽样不确定性和估算误差随样点数量增加呈非线性递减变化趋势,当样点数超过20个时,增加样点数量的作用有限。

Spatial variability of available soil moisture and its seasonality in a small watershed in the hilly region of the Loess Plateau

Based on the available soil moisture (ASM) data collected from the 213 sampling sites over a small typical watershed in the hilly region of the Loess Plateau, spatial variability of soil available moisture at the scale of watershed, hillslope and gully and its seasonality (spring, summer and fall) was analyzed separately. Results show that the ASM, no matter at what scale, displays relatively strong spatial variability and ASM in gullies is higher in mean and spatial variability (Standard Deviation and Variation coefficient) than in hillslopes. The ASM also showed higher normality at gully and hillslope scales than at watershed scale. Spatial variability at all the three scales varies with the mean of ASM, and variation coefficient descends exponentially with the increasing mean. ASMs at the watershed and slope scales are closely and positively related to slope aspect, with correlation coefficient being higher than that its correlation with slope and elevation. However, the correlation coefficients of ASM with size of a gully and various landform factors are all low. The spatial variability of available soil moisture exhibits apparent seasonality. The ASM in fall, among the three seasons, is the highest in mean, but the lowest in variability. However, the ASM in summer is just the other way around. At the watershed scale, the ASM in summer is much higher than that in spring and fall in correlation coefficient with elevation, whereas it is in a reverse pattern at the slope scale. Besides, the uncertainty of the sampling of ASM and the estimation errors display a nonlinear descending trend with the increasing number of samples. However, when the number of sampling sites exceeds 20, the effect of increasing the number of samples becomes very limited. The findings presented here are expected to improve the understanding of seasonality of ASM spatial distribution of gully catchment in the Loess Plateau and could help design optimal sampling strategy of soil moisture.