Hysteresis of soil moisture spatial heterogeneity and the "homogenizing" effect of vegetation

By partitioning mass and energy fluxes, soil moisture exerts a fundamental control on basin hydrological response. Using the design characteristics of the Biosphere 2 hillslope experiment, this study investigates aspects of soil moisture spatial and temporal variability in a zero-order catchment of a semiarid climate. The hydrological response of the domain exhibits a particular structure, which depends on whether topography-induced subsurface stormflow is triggered. The occurrence of the latter is conditioned by topography, soil depth, and pre-storm spatial distribution of moisture. As a result, a non-unique behavior of soil moisture spatial heterogeneity emerges, manifested through a hysteretic dependence of variability metrics on mean water content. Further, it is argued that vegetation dynamics impose a "homogenizing" effect on pre-storm moisture states, decreasing the likelihood that a rainfall event will result in topographic redistribution of soil water. Consequently, post-rainfall soil moisture dynamics associated with the effect of topography that could lead to the enhancement of spatial heterogeneity are suppressed; a potential "attractor" of catchment states emerges. The study thus proposes several hypotheses that will be testable within the framework of long-term hillslope experiments.