Hydraulic conductivity determination for simulation use in soil water equation

Unsaturated hydraulic conductivities of Kamphaeng Saen soil series were determined as functions of water content to a depth of 100 cm at 10 cm interval. The functions were obtained in a 3 step measurement. The first step was measurement for water saturated soil. Soil was sampled from the field undisturbed. Saturated hydraulic conductivities were measured by falling head permeater. The second step was for soil from less than saturation down to a matric potential of -0.8 x 10 to the fifth power Pa. The measurement was carried out in the field by the internal drainage method. Tensiometers were installed in the soil profile at various depths to monitor the change in matric potential. Water contents were calculated from the values of matric potential through the soil water characteristic functions previously obtained. By evaluating the hydraulic gradients and water contents as functions of time and depth, the values of unsaturated hydraulic conductivities were calculated from the continuity equation. The last step was for the drier soil with matric potential less than -0.8 x 10 to the fifth power Pa. Disturbed soil sample was packed in0992the soil column to determine the coefficients of diffusivity, and then converted the values into the hydraulic conductivity coefficients. The results of the study showed that the saturated hydraulic conductivity values of all depths were moderately slow, varying from 2.3 x 10 to the -6 to 6.3 x 10 to the -5 m s to the -1 with saturated volumetric water contents ranging from 40-60 percent. The unsaturated coefficients, K can be shown as a function of volumetric water content which takes the form K = a exp (b x function of water content) where both a and b were constants. The average overall function for all depths is K 9.3 x 10 to the -17 x exp [57.89 x function of water content] m s to the -1, indicating that the coefficients drop drastically as the soil water content decrea