Factors controlling the asymmetry of soil moisture and vegetation dynamics in a hilly Mediterranean catchment

Abstract

The interaction between hydrology, terrain, and vegetation, is an essential element in understanding complex landscapes, and in forecasting their evolution under climate change. In Mediterranean Europe, few studies have been performed on the interrelation between them. In this study, soil moisture, vegetation, and water table dynamics were measured on opposing hillslopes of a semi-arid, Mediterranean catchment in southern Spain, to quantify the aspect influence on ecohydrological dynamics of an oak-woodland savanna or “dehesa”. Observations of a network of 32 soil moisture sensors on two opposing hillslopes, a north-facing slope (NFS), and a south-facing one (SFS), detected a similar moisture dynamics, although the influence of the vegetation induces a lagged response where the NFS dries out earlier compared to the SFS. Analysis of successive soil water profiles after storm events suggests that the water moves downwards as an unsaturated preferential flow along the walls of the fractured granite material, with residence times between 16.3 and 36.7 days. Our results show that there is a direct coupling between the hillslope hydrology, the depth of the weathered bedrock, much higher on the NFS, and the vegetation dynamics. Vegetation is much denser on NFS, as LIDAR-derived biomass estimates were 29% higher than on the SFS. The evolution of the normalized difference vegetation index (NDVI), obtained from the Sentinel-2 time series, also shows completely opposing trends with a minimum value at the end of winter for the NFS, whereas the SFS reaches minimum values in summer. A significant correlation between NDVI and soil moisture was found for the SFS (Pearson’s R of 0.81). However, this correlation is very poor on the NFS, but a good correlation with water table was found (R=0.46). This shows how the deep weathering on the NFS allows for an additional water source, contained in the rock moisture and saturated zone, that represents an important source for the vegetation, whereas on the SFS only soil moisture supports the vegetation. These vegetation differences in turn directly impact surface soil moisture, leading to the observed, relatively similar trends on both hillslopes.