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SOIL PHYSICS

Determination and Characterization of Preferential Water Flow in Unsaturated Subsoil of Andisol

^a National Institute for Agro-Environ. Sciences, Tsukuba, Ibaraki 305-8604, Japan
^b Graduate School of Agriculture, Hokkaido Univ., Sapporo, Hokkaido 060-8589, Japan

^* Corresponding author (sadao{at}affrc.go.jp).

Experimental approaches for directly determining preferential water flow in field soils usually require artificial interruption of the soil matrix's water flow. This interruption may change the quantitative as well as the qualitative nature of the preferential water flow, particularly in soils such as Andisols where matrix water flow is the dominant process of drainage. To overcome this problem, we applied both the one-dimensional form of Darcy's equation and the soil water balance method, during and shortly after each rain event, for characterizing as well as determining the preferential water flow across a depth of 1 m in an Andisol. The volumetric water contents and pressure potentials at different soil depths were monitored by time domain reflectometry and tensiometry during a 7-yr period. Preferential water flow was detected only two to seven times per year; nevertheless, it accounted for 16 to 27% of the annual total drainage. These flows occurred under macroscopically homogeneous soil water conditions, with pressure potentials mostly below the air-entry pressure in the subsoil. The in situ unsaturated hydraulic conductivity at the depth of 1 m, determined to apparently explain the total drainage during a preferential water flow event, varied over more than one order of magnitude, with little change in water content, and the maximum value reached the saturated hydraulic conductivity. This indicates that preferential water flow occurred mainly through the largest macropores at this depth due to pore-scale physical nonequilibrium. Macropore-mediated preferential water flow and solute transport should be considered in unsaturated subsoil of Andisols.

Abbreviations: TDR, time domain reflectometry