Point estimation of water retention in smectitic soils

Soil hydraulic properties are needed for the modelling of water and substance movement in unsaturated soils. Their direct measurements are difficult, time consuming and expensive. Therefore, these properties need to be estimated using indirect methods. The objective of this study is to evaluate the effect of microaggregates on the estimation of soil water retention. In this study, 74 disturbed and undisturbed soil samples were taken from Guilan province, northern Iran. Bulk density; water content at matric suctions of 5, 25, 33, 100 and 1500 kPa; and microaggregate-size distribution (MASD) were measured. The fractal parameters of the MASD were calculated and used to estimate water content. Estimation of water contents at different matric suctions was improved significantly by adding the fractal parameters of the MASD to the pedotransfer functions. The most improvement occurred for θ₁₅₀₀ prediction with relative improvements of 12·5% and 45·2% for training and testing, respectively. It can be due to the influence of soil microstructure on water retention at high matric suctions. Smectite clay with high specific surface area affects soil structure and microaggregates and, consequently, water retention. Using the MASD would be useful in the estimation of soil water content in smectitic soils.