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

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