Evaluating Dominance and Temporal Evolution of Physical Controls of Soil Moisture: A Decadal Synthesis Study of SMEX02, SMEX03, SMEX05, CLASIC07 Field Campaigns

Soil moisture is a dynamic state variable at the interface of the surface, sub-surface and the atmosphere. It partitions the energy budget by affecting the thermal fluxes going into or out of the Earth system and influences the water budget by controlling runoff, infiltration and percolation. Owing to its strategic position in the hydrologic cycle, its temporal and spatial variability is of prime importance. Heterogeneous geophysical attributes at the surface namely soil, vegetation, topography and precipitation control its spatial and temporal distribution. The influence of each of these interdependent physical controls evolves with time and scale. This study investigates the effect of three physical controls i.e. topography, vegetation and soil over the Little Washita and Walnut Creek watersheds in Oklahoma and Iowa respectively. Point scale data collected from four soil moisture campaigns in the past decade (SMEX 02, SMEX 03, SMEX 05 and CLASIC 07) was used in this analysis. The spatial variability of soil moisture and the effect of different physical controls on soil moisture was assessed using Shannon entropy. This technique evaluates the effects of interdependent physical controls on soil moisture in a novel way. It was found that in Little Washita watershed, during wet conditions, topography is the dominant physical control whereas the dominance shifts to soil in dry conditions. In the Walnut Creek watershed, vegetation remained the dominant physical control with soil gaining dominance under certain specific conditions. We suggest that there exist specific moisture threshold conditions at which the dominant physical control changes from vegetation to soil or from topography to soil depending on the nature of heterogeneity present in a particular watershed.