TEMPORAL VARIATION IN THE HYDRAULIC CONDUCTIVITY OF A TILLED CLAY SOIL AS MEASURED BY TENSION INFILTROMETERS

Steady-state infiltration rates from tension infiltrometers were measured on ploughed and unploughed plots in a clay soil during the period June to October. Measurements were made both at the soil surface and at depths of 15 and 25 cm. Hydraulic conductivity in the water potential range zero to -11 cm was obtained using a piece-wise exponential K(psi) function and Wooding's solution for infiltration from a circular source. A two-line regression model showed excellent fits to paired (in K,psi) values on all measurement occasions. This may indicate the existence of a bimodal pore system, reflecting the contributions of macro- and mesopores to the measured K(psi) function. The break-point potential dividing the two pore systems varied between c. -4 and -6 cm. Significant variations in the K(psi) function between sampling occasions were found at the soil surface, but not at depths of 15 and 25 cm. Measured K(psi) values decreased during the growing season, particularly at potentials between -4 and -6 cm where reductions were up to one order of magnitude. This was attributed to soil structural breakdown by rain impact and surface capping or sealing. Hydraulic conductivity near the soil surface was significantly increased by disc harrowing in autumn. In contrast, no pronounced difference in the K(psi) function between ploughed and unploughed treatments could be discerned at 15 and 25 cm depths in the soil.