A STUDY OF THE WATER CHARACTERISTICS AND PORE SIZE DISTRIBUTION OF A TROPICAL ALFISOL
The water characteristic of the an alfisol predominantly sandy loam in texture was measured with undisturbed core samples in a pressure plate apparatus at pressures of 5, 10, 33, 60, 100, 1000, and 1500 kPa. The core samples were obtained at 20 cm depth intervals to 100 cm soil depth. The pore size distribution and the matric pressure head (hB) defining the boundary between the structural and matrix domains were determined using a derivative curve technique. The pore size distribution varied from dominantly unimodal to essentially bimodal distribution as the soil clay content increased with soil depth. The values of hB ranged from 120 to 250 cm. However, at 0.05 level of significance, there was no difference between the respective structural and matrix domain porosities determined using the derivative curve technique and the corresponding ones by an empirical method which assumed and an arbitrary hB–value of 100 cm. Structural domain porosity comprised more than half of the total porosity in the upper soil depths indicating a preponderance of macropores in that region. The measured water characteristic fitted well to both the closed-form bimodal model of Seki (2007) and the unimodal one of Kosugi (1996). However, the model of Seki had higher coefficients of determination and showed better fit over the entire range of measured data.