Effective porosity of paddy soils as an estimation of its saturated hydraulic conductivity

Soil saturated hydraulic conductivity (Ks) is an important soil physical property. Some laboratory and field methods are expensive, time consuming and labour intensive. Indirect methods such as pedo-transfer functions (PTF) are available. Effective porosity or macroporosity (Øe) is approximately equals to porosity minus volumetric soil water content at the field capacity. According to Kozeny–Carman equation, Ks could be evaluated using Øe. Franzmeier estimated the Ks from Øe based on Ahuja et al. He found a strong relationship between Øe and Ks. This paper presents results of a study to characterize the effective porosity in lowland paddy fields and to show the possibility of using the effective porosity (Øe) in estimating the saturated hydraulic conductivity (Ks). Soil in lowland paddy fields forms its horizon as topsoil, hardpan and subsoil. Soil samples were collected randomly within a 2300-ha rice cultivation area where there are five dominant soil series. A total of 408 soil samples were taken from 136 sampling points and at three depths, namely, the topsoil, hardpan and subsoil. Ks values were measured in the laboratory using the falling head method. Soil bulk density and moisture content at −66 kPa were determined. The Øe was then calculated using the difference of the total porosity (Ø) minus the volumetric moisture content at −66 kPa. The Ks values ranged from 5.35×10−4 to 8.77×10−2 m day−1. The Db varied from 0.62 to 1.91 Mg m−3, and the values of the Dp ranged from 1.10 to 2.89 Mg m−3. The Ø ranged from 0.17 to 0.68 m3 m−3. The results of the Øe of the samples in this study were obtained by calculating the difference of the total porosity and volumetric moisture content at field capacity. For clayey soils, field capacity is taken at the suction of −66 kPa. The Øe varied from 0.05 to 0.55 m3 m−3, with the mean value of 0.24 m3 m−3. The regression equation of a power function shows a highly significant regression coefficient, r2 of 0.50 (n=400). This indicates that there is a strong relationship between Ks and Øe for the lowland paddy soils in the study area.