Preferred states in spatial soil moisture patterns: Local and nonlocal controls

Abstract

Abstract. In this paper we develop a conceptual and observational case in which soil water patterns in temperate regions of Australia switch between two preferred states. The wet state is dominated by lateral water movement through both surface and subsurface paths, with catchment terrain leading to organization of wet areas along drainage lines. We denote this as nonlocal control. The dry state is dominated by vertical fluxes, with soil properties and only local terrain (areas of high convergence) influencing spatial patterns.We denote this as local control. The switch is described in terms of the dominance of lateral over vertical water fluxes and vice versa. When evapotranspiration exceeds rainfall, the soil dries to the point where hydraulic conductivity is low and any rainfall that occurs essentially wets up the soil uniformly and is evapotranspired before any significant lateral redistribution takes place. As evapotranspiration decreases and/or rainfall increases, areas of high local convergence become wet, and runoff that is generated moves downslope, rapidly wetting up the drainage lines. In the wet to dry transitional period a rapid increase in potential evapotranspiration (and possibly a decrease in rainfall) causes drying of the soil and "shutting down" of lateral flow. Vertical fluxes dominate and the "dry" pattern is established. Three data sets from two catchments are presented to support the notion of preferred states in soil moisture, and the results of a modeling exercise on catchments from a range of climatic conditions illustrate that the conclusions from the field studies may apply to other areas. The implications for hydrological modeling are discussed in relation to methods for establishing antecedent moisture conditions for event models, for distribution models, and for spatially distributing bulk estimates of catchment soil moisture using indices.

An approach to understanding hydrologic connectivity on the hillslope and the implications for nutrient transport

“…Also consistent with Grayson et al's [1997] results, we find a high correlation between the TOPMODEL topographic index and the spatial distribution of WTDs when the soils are wet (and much less so during dry conditions). We see that the mid-slope regions behave as a ridge-valley cutoff switch as far as shallow subsurface connectivity is concerned.…”

“…Hydrologic connectivity is the condition by which disparate regions on the hillslope are linked via subsurface water flow, and is a key determinant of the movement of nutrients down a hillslope [Hornberger et al, 1994;Creed and Band, 1998]. Grayson et al [1997] proposed that two distinct hydrologic states predominate: (1) a dry state, in which hillslope regions are hydrologically unconnected, the dominant flow is vertical, and spatial patterns of soil moisture are unorganized, and (2) a wet state, in which hillslope regions are connected via lateral subsurface flow and soil moisture patterns are organized spatially.…”

Observed spatial organization of soil moisture and its relation to terrain indices

“…When the catchment becomes extremely wet, extensive (near) saturated conditions exist, and the degree of organization decreases. Similar seasonal changes in behavior are likely to occur in a wide range of landscapes where there is significant seasonality [Grayson et al, 1997].…”

Combined effect of heterogeneity, anisotropy and saturation on steady state flow and transport: Structure recognition and numerical simulation

“…Because of the dynamics of water in soil and across the atmosphere-soil interface, the hydrological systems may swoop between different states [e.g., Grayson et al, 1997;Western et al, 2001]. The switching from one state to another depends on climate and soil storage and on their interaction [e.g., Western et al, 2002].…”