Hillslope-scale variability of soil water potential over humid alpine forests: Unexpected high contribution of time-invariant component

Abstract

Soil water potential (SWP) gradients control water and energy fluxes in the Soil – Plant – Atmosphere Continuum. Exploring the spatial – temporal variability of SWP at the hillslope scale is an essential step towards developing ecohydrological models. In this study, SWP monitoring was carried out on two contrasting, alpine, humid forested hillslopes (i.e., coniferous and broadleaf forests) at three depths (10, 50, and 100 cm). The effects of static (soil properties and topography) and dynamic (precipitation dynamics and soil wetness conditions) factors on SWP dynamics and variability were investigated based on statistical methods, temporal stability, and variance decomposition analysis. The results showed that the SWP in the soil – root zone (10 and 50 cm depths) in the broadleaf forest exhibited higher variability and lower temporal stability than in the coniferous forest, likely because of the stronger evapotranspiration and more variable soil hydraulic conductivity. The relatively high SWP variability in the deeper soil layer (100 cm depth) of both forested hillslopes was due to the frequent lateral/preferential flow. Furthermore, topographic factors (i.e., distance of hillslope to stream and slope) and soil properties (clay and rock fragment contents) regulated the spatial patterns in SWP and influenced representative site selection in the coniferous and broadleaf forests, respectively. Decomposition of the spatial variance showed that the time-invariant component was the dominant contributor to the total spatial variance. An exponentially increasing relationship between the time-invariant component and mean SWP and a concave-up parabolic shape between the contribution of the temporal anomalies and mean SWP were observed. Our results provide an improved understanding of the temporal and spatial variability in terms of SWP in forested hillslopes.