黄土区小尺度坡面土壤含水率时空变异性研究

Temporal-Spatial Variability of Slope Soil Moisture at Small Scale on Loess Plateau

土壤含水率在水平和垂直方向上均具有高度的时空异质性,关于水平方向变异的研究取得了很大进展,而对垂直方向变异的研究较为缺乏。为掌握土壤含水率在剖面上的垂直变异特征,采用经典统计、地统计及分形分析相结合的方法,研究了黄土高原典型坡地剖面土壤含水率的时空变异性。结果表明:不同测定时间下的剖面土壤含水率均服从自然对数正态分布,在空间上均表现为中等变异性,沿土层深度方向的变化趋势均为增长型,且这种变化规律具有时间稳定性;剖面土壤含水率在整个研究区域尺度、微尺度上的半方差函数均可用指数模型进行很好地拟合,均表现出强烈的空间依赖性,并且这种规律几乎不随时间的变化而变化;不同测定时间下剖面土壤含水率在整个研究尺度上均表现出有限的自相似性,在小于2.00 m的间距尺度下具有稳定的自相似性,大于2.00 m间距尺度的自相似性比较微弱,分维数的大小与控制土壤含水率的主要过程有关;不同测定时间下剖面土壤含水率的空间自相关性基本一致,即当滞后距离小于3.20 m时为空间正自相关,大于3.20 m时为负自相关,等于3.20 m时为不相关,而在剖面土壤含水率之间的相关性达到了极显著水平。

Soil moisture has strong spatial heterogeneity in both vertical and horizontal directions. Considerable interests have been devoted to the assessment of horizontal direction whereas less of that are known in the vertical variation. In this study, we used classical statistics, geo-statistics and fractal methods to characterize the temporal-spatial variability of 6 m depth profile soil moisture in the vertical direction, on the typical slope land on the Loess Plateau. Profile soil moisture at different measurement times followed a log-normal distribution and showed a moderate variability, Soil moisture increased with the increasing of soil depth and behaved temporal stability during measurement periods. Profile soil moisture demonstrated a strongly spatial dependence as reflected by the nearly none nugget ratio, which did not change with measurement time, and the experimental variogram of soil moisture was fitted well with an exponential model. Profile soil moisture at different measure times showed a limited self-similarity at an entire study scale. It has a stable self-similarity at distances less than 2.00 m and weak self- similarity at distances larger than 2. 00 m. The value of fractal dimension was highly related to the main processes of control soil moisture. At each measurement time, the spatial auto-correlation of the profile soil moisture was the same, that is, it demostrated spatially positive correlation at distances less than 3.20 m whereas spatially negative correlation at distances larger than 3.20 m. The profile soil moisture at different measure times showed a significantly positive correlation.