On the Origins of Different Types of Topographic Dependence and Temporal Instability in Catchment-Scale Soil Moisture Patterns.

Soil moisture is a key state variable in many hydrologic applications.  At the catchment scale, topography has been shown to be an important control on soil moisture patterns, but the nature of its influence often varies between catchments.  Some soil moisture patterns depend most strongly on the hillslope orientation with wetter areas occurring on hillslopes that are more protected from the sun.  Other soil moisture patterns depend on the valley configuration with wetter areas occurring in the valley bottoms.  Still other patterns are temporally unstable with the type of topographic dependence changing in time.  This research aims to identify the catchment characteristics that determine the type of topographic dependence that occurs and the strength of temporal instability in the soil moisture patterns.  A conceptual model is developed that estimates soil moisture patterns by assuming that the processes that affect soil moisture at any given point are in equilibrium.  This assumption allows the spatial pattern of soil moisture to be estimated as a function of the spatial-average soil moisture.  The model includes infiltration, variably-saturated lateral flow, drainage to deeper layers, and evapotranspiration.  These processes are described with functions that include various topographic attributes such as slope, specific contributing area, curvature, and the potential solar radiation index.  The model is tested through application to three catchments with extensive soil moisture data (Tarrawarra, Satellite Station, and Cache la Poudre), and it is shown to reproduce a substantial portion of the observed variation in each dataset.  An index is then developed to measure the type of topographic dependence (hillslope orientation or valley configuration), and another index is developed to measure the strength of temporal instability.  The dependence of these indices on the soil, vegetation, and climate characteristic are then explored.  These evaluations indicate that saturated horizontal hydraulic conductivity, pore disconnectedness, and a vegetation evapotranspiration efficiency parameter have strong effects on the organization and instability of the soil moisture patterns.  In contrast, annual potential evapotranspiration alone has less impact on the organization or its stability.