Variability of In Situ Moisture Measurements and Implications for Modeling Hillslope Processes

Abstract

This paper investigates changes in moisture conditions within regolith prone to shallow landslides. An intensive monitoring network was established to quantify the responses in soil water content, soil water potential, and positive pore-water pressures to rainfall events. The results show that the hydrological response of the soil is primarily a function of the storm characteristics, but this response can be modified by antecedent moisture conditions, topographic position, and heterogeneity of soil properties. The physical properties of the soil results in two modes of response in soil water conditions: a “damped” response resulting from matric flow, and a “spiked” response caused by preferential flow through macropores. Macropore flow is only significant during major storms, and when antecedent moisture conditions are high. Soil moisture conditions are highly variable in both time and space. Topography and soil properties contribute to this variability, but, since the soil properties are influenced by topography, separating the two effects is difficult. Likewise the piezometric response is affected by the variation in soil properties, which provide preferential flow paths through the regolith. Any response cannot be explained simply by rainfall and antecedent soil moisture. The high degree of temporal and spatial variability in measured soil moisture conditions, even at the detailed scale of this study, has important implications for field investigations and slope stability modeling, i.e. considerable variability has to be included in any modeling framework. Likewise the importance of preferential flowpaths, and temporal variation in flow dynamics, have a critical influence on runoff processes.