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DIVISION S-1-SOIL PHYSICS

Prediction of Near-Saturated Hydraulic Conductivity in Three Podzolic Boreal Forest Soils

Dep. of Forest Ecology, Univ. of Helsinki, P.O. Box 24, FIN-00014, Helsinki, Finland

marja.mecke{at}helsinki.fi

Steady-state infiltration fluxes into the soil were measured with a tension infiltrometer at supply potentials of -0.35, -0.70, and -1.10 kPa, and the near-saturated hydraulic conductivities (K) were calculated using an exponential model. Measurements were conducted in four mineral soil horizons at three forest sites, representing contrasting textures. The analysis was concentrated on K at -0.35 kPa [K(-0.35)] since this potential corresponds to the 1-mm pore diam., which is often considered to be the limit between macropores and mesopores. The average K(-0.35) of the site varied in the parent soils of the three sites from 0.46 to 40.98 cm h^-1, while in the two uppermost horizons the variability was smaller: 0.30 to 0.69 cm h^-1. Three multiple linear regression models of log[K(-0.35)] were constructed by stepwise regression analysis. The retained water content at the seven potentials; textural fractions; dry bulk density; and Al, Fe, and C contents were suggested as predictor variables. In addition, simple functions of these variables were suggested. In Model 1, all horizons were included ; in Model 2, all horizons except the upper illuvial horizon were included ; and in Model 3, only the lowest horizon was included . Adding predictor variables increased r² in all models. The water content at -100 kPa, which depends on pore-size distribution and C content (which produce a strong retarding effect on water flow), were the most important predictors for K(-0.35). Similarly, by gradually excluding horizons where pedological and biological processes had changed the structure and pore-size distribution, r² increased from 0.86 (Model 1) to 0.99 (Model 3).

Abbreviations: , the constant in the exponential K() function • D_b, dry bulk density • K, hydraulic conductivity • K_s, saturated hydraulic conductivity • K_se, extrapolated saturated hydraulic conductivity • K(), hydraulic conductivity at water potential • _s, total porosity • (), water content at water potential • , water potential • TI, tension infiltrometer

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