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. It is argued that 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-event 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-event states, decreasing the likelihood that a rainfall event will result in efficient topographic redistribution of soil water. Consequently, post-event soil moisture dynamics 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 are testable within the framework of the unique long-term Biosphere 2 experiment.