Moisture dynamics as related to capillary and drainable pores in the Nile Delta soils

A b s t r a c t. The dynamic of soil moisture may be studied through water flux, storage, moisture conductivity and movement into the soil pore spaces. The volume of pore space in the soil and also the size, shape, type, continuity and distribution of the pores are important characteristics related to the storage, conductivity and movement of water and gases. The movement of water by gravitational forces in the natural soils occurs principally through the non-capillary pores (i.e. rapidly drained pores), while other movements occur in capillaries which may be classified as coarse and fine capillary pores. The unsaturated condition of soil water is a common state in nature after rainfall or as a result of irrigation, therefore, the purpose of this study was to develop equations to describe and estimate the unsaturated hydraulic conductivity K(θ) in relation to soil pore-size classes that contained the available water in the soil root zone. The equations were based on a water-retention curve, saturated hydraulic conductivity and pore-size function measured from undisturbed cores. The equations were applied to three soil profiles from the Nile Delta and compared with measured K(θ) data from two field experiments, which were conducted in clay in an unsaturated condition and in sandy soil areas using the internal drainage in situ method. The pore size function f(r) represents the fraction of the total pore whichvolume, which was contributed by pores with radii from 0 to ∞ at the prevailing degree of saturation. Data showed the applicability of the suggested equations for calculating unsaturated hydraulic conductivity in the soil pores even for soils with a high proportion of clay. K e y w o r d s: modelling unsaturated hydraulic conductivity, drainable and capillary pores, alluvial clay and sandy soils