Stochastic soil moisture dynamics along a hillslope

Abstract

The spatial and temporal dynamics of soil water content along a hillslope is the result of a number of complex and mutually interacting processes. This paper deals with the role of subsurface, unsaturated, lateral water flow and its links to climate, soil, and hillslope characteristics. The analysis focuses on the soil moisture dynamics at the daily time scale, averaged over the plant rooting depth, and accounts for the stochastic nature of precipitation as well as for the non-linear dependence of transpiration and hydraulic conductivity on soil moisture. The lateral fluxes of soil moisture are described by means of the one-dimensional Richards equation, and the probabilistic soil moisture dynamics is numerically investigated considering different conditions of climate, pedology, vegetation, and hillslope geometry. From the analysis two different regimes emerge: a humid one, characterized by an unsaturated lateral flow with significant spatial gradients of soil moisture along the hillslope, and a dry one, in which the topography does not affect the spatial distribution of the soil moisture. In the humid regime, the long-term average spatial pattern of soil water content is studied at different points along the hillslope using the mean, rms, and pdfs of soil moisture, as well as the components of the long-term water balance. All these analyses show how the soil moisture dynamics is the result of complex and non-local interactions between climate, soil, vegetation, and hillslope shape.