Mohawesh O., Fukumura K. and Ishida T. (2004) 
Abstract

   Soil water transport across the air-soil interface and in the upper layers of the earth’s surface is fundamental information in soil and water conservation practices. Accurate water transport models are increasingly needed to solve water - flow and contamination problems. To achieve this, better and simple means for characterizing the soil hydraulic properties are needed. Presently, our ability to create complex numerical models far exceeds our capacity to describe the physical system on which the models are based. The soil hydraulic properties play an integral role in any numerical solution to water flow and contaminant transport. Nevertheless, the problem of determining the soil hydraulic properties are confounded by the expense of experimentally measuring this properties and the large number of observations required to adequately characterize the spatial distribution due to commonly occurring field spatial heterogeneity of soil hydraulic properties. Yet, there is a growing need for more efficient methods to estimate the spatial heterogeneity of soil hydraulic parameters. Moreover, spatial variability in canopy structure is a second major source of variability of hydrologic fluxes

This study is being conducted at a commercial arable flat area in northeast part of Thailand. Dry bulk density, volumetric soil water content, Leaf area index (LAI) and plant height were measured on site at mesh points. Statistical and additional geostatistical analyses were applied for the measured properties. Spherical model best fitted all experimental semivariograms. The range of spatial dependence of dry bulk density and volumetric soil water content was 31 m and 24 m, respectively. The range of leaf area index and plant height was 35 and 32 m, respectively. Dry bulk density, volumetric water content, leaf area index and plant height exhibit a moderate spatial dependence. These semivariograms were used as a basis for further geostatistical assessment and to generate spatial fields of soil and canopy properties.

In order to estimate the heterogeneity of soil hydraulic properties with dry bulk density, soil hydraulic properties were measured in the laboratory for repacked soil samples compacted to pre-determined dry bulk densities. The experimental data was fitted to the van Genuchten’s and Mualem’s models. Effects of dry bulk density on soil hydraulic properties were deduced from variations in the van Genuchten’s and Mualem’s models parameters. The correlation coefficients between dry bulk density and the model’s parameters were between 0.84-0.97. The van Genuchten’s and Mualem’s models of soil water retention and hydraulic conductivity, respectively, were modified to be a function of original parameters and dry bulk density. Using these modified models and distribution of dry bulk density in the field, distribution of soil hydraulic properties can be predicted.

Proceedings of Participatory Strategy for Soil and Water Conservation Conference. P.105-113, ERECON, Tokyo- Japan.