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Soil Physics

Soil Hydraulic Conductivities and their Spatial and Temporal Variations in a Vertisol

Dep. of Biological and Agricultural Engineering, 2117 TAMU, Texas A&M Univ., College Station, TX 77843-2117. J. Maximilian Köhne now at Univ. of Rostock, Germany

^* Corresponding author (bmohanty{at}tamu.edu)

Knowledge of soil hydraulic parameters and their spatiotemporal variation is crucial for estimating the water and solute fluxes across the land–atmosphere boundary and within the vadose zone at different scales. The objective of this study was to determine soil hydraulic conductivities [saturated hydraulic conductivity, K_sat, and unsaturated hydraulic conductivity, K()] and their spatial and temporal variations in a clay-dominated biporous Vertisol near College Station, TX, using tension infiltrometers. The study was conducted within a 20- by 16-m plot across several seasons during a 21-mo period (May 2003–January 2005) to investigate the impact of varying disk sizes (measurement support) on K(), and the spatial and temporal variations of K() under natural environmental conditions due to pore space evolution. Infiltration occurred in a bimodal fashion consisting of preferential flow (occurring at soil water pressure heads [] = –0.05 to 0 m) and matrix flow (at = –0.2 to –0.1 m). Biological and structural macropores present in the soil resulted in gravity-dominated flow near saturation ( = –0.05 to 0 m) for all experiments. The Student's t-test of analysis of variance indicated that hydraulic conductivities were not affected by changes in the infiltration disk sizes. Although the K() values at four different locations within the plot did not show significant spatial variability, they demonstrated strong temporal variation during the 21-mo period based on the evolution of natural environmental conditions due to seasonal precipitation, root growth and decay, and structural pore space dynamics. Temporal trends of K() indicated that hydraulic conductivities close to saturation were positively correlated with antecedent moisture conditions reflecting liquid cohesion, water films bridging across cracking peds, and the activation of flow in biological and structural macroporosity in the biporous soil system.

Abbreviations: AMC, antecedent moisture content • REV, representative elementary volume