PREFERRED STATES IN SPATIAL SOIL-MOISTURE PATTERNS - LOCAL AND NONLOCAL CONTROLS

In this paper we develop a conceptual and observational case in which soil water patterns in temperate regions of Australia switch between t wo preferred states. The wet state is dominated by lateral water movem ent through both surface and subsurface paths, with catchment terrain leading to organization of wet areas along drainage lines. We denote t his as nonlocal control. The dry state is dominated by vertical fluxes , with soil properties and only local terrain (areas of high convergen ce) influencing spatial patterns. We denote this as local control. The switch is described in terms of the dominance of lateral over vertica l water fluxes and vice versa. When evapotranspiration exceeds rainfal l, the soil dries to the point where hydraulic conductivity is low and any rainfall that occurs essentially wets up the soil uniformly and i s evapotranspired before any significant lateral redistribution takes place. As evapotranspiration decreases and/or rainfall increases, area s of high local convergence become wet, and runoff that is generated m oves downslope, rapidly wetting up the drainage lines. In the wet to d ry transitional period a rapid increase in potential evapotranspiratio n (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 c limatic conditions illustrate that the conclusions from the field stud ies may apply to other areas. The implications for hydrological modeli ng are discussed in relation to methods for establishing antecedent mo isture conditions for event models, for distribution models, and for s patially distributing bulk estimates of catchment soil moisture using indices.