Distribution of soil moisture over different depths in a small alpine basin

In this study we compare measurements of soil moisture at 0-6, 0-12 and 0-20 cm depth in order to examine how representative the surface measure is with regard to the deeper layers. Detailed soil moisture data were collected over three hillslopes in the 1.9 km2 wide Rio Vauz mountainous catchment, located in the central-eastern Italian Alps. The field work was carried out during summer 2005; water content values were collected at several points over the three hillslopes at 0-6 cm, 0-12 and 0-20 cm depth by means of two portable instruments being able to provide spot values: an impedance probe was used to collect surface soil moisture data while water content up to 20 cm depth was measured by a TDR probe. The comparison between soil moisture patterns at different depths is evaluated by examining: (i) summary statistical properties of the data set; (ii) scatter plots; (iii) spatial distributions via maps; (iv) evaluation of Pearson and Spearman correlation coefficients. Results show that the soil moisture generally decreases with depth. The degree of correlation between the data collected at the three depths is relatively high. The visual comparison of maps suggests a reasonable consistency of patterns; wet and dry areas on each hillslope are fairly the same at every sampled depth. Overall, spatial organization and spatial consistency are controlled by soil effects, reflecting high organization in the distribution of soils. This is consistent with the pedological characteristics of these soils, which exhibit a relatively homogeneous vertical structure.