Laboratory of Soil Physics, Department of Natural Resources, National Institute of Agro-environmental Sciences, Tsukuba, Ibaraki, 305, Japan.
1Present address: Department of Soil Science, Assam Agricultural University, Jorhat, Assam, 785013.
Soil water diffusivity, D(θ), evaluated by the time domain reflectrometry (TDR) technique based on depthwise measurement of soil water content variation from a drying soil column was compared with horizontal flow method involving both drying and wetting processes. Deviation of TDR measured water content data from experimental values was overcome by introducing slope and intercept of the lines to the original Topp's equation. The K-θ relation of soils involving sorption and desorption processes together could be resolved into straight line on a semi-logarithmic scale (R2>0.90), which indicates no hysteresis effect on K-θ function for computing water flux in soil. The D(θ) increased exponentially with increase in water content in all the soils (R2 > 0.85), regardless of methods of estimation. For light soils, D(θ) values obtained by wetting process were slightly lower than that of the drying process in the dry range, but the reverse was true in the wet range. For heavy soils, D(θ) obtained by wetting was slightly higher than that of the drying; however, the difference between the two was not large enough when the entire water content range was considered.
Time domain reflectrometry (TDR), K-θ relation, D(θ) values
