Hydraulic conductivity and sorptivity at unsaturated and saturated conditions as related to water infiltration in soils

Abstract

Sorptivity (S) has been defined in terms of the horizontal infiltration equation. Atunsaturated conditions (at a very short time) S represents “maximum sorption capacity”,but in saturated conditions the sorption capacity decreases with the time. Over a long timeof infiltration, sorptivity was not studied as a soil water parameter that could bedetermined. The purpose of this study is to apply derived equations depending on theinfiltration functions to predict (1) soil water sorptivity (S) at infiltration capacity(unsaturated conditions) and at basic infiltration rate (Ib) (saturated conditions), (2) thehydraulic conductivity (Saturated Ks and unsaturated K(θ)) into capillary-matrix and noncapillary macro pores of soils. Five alluvial (saline and non-saline clay) and calcareous soilprofiles located in the Nile Delta were investigated for applying the assumed equations. Adecrease in S value was observed with an increase in soil water content. At steadyinfiltration rate (Ib), S decreased from $1.04 to 0.647cm.min^{-0.5}$ (i.e. S decreased by 37.79%)in average in calcareous soils and from $0.537 to 0.251cm.min^{-0.5}$ (53.25%) in alluvial claysoils. The steady Sw parameter was used in prediction of the hydraulic conductivities andthe basic infiltration rate $I_b$ , whereas, $S_w$ is a suggested term at steady infiltration rate. Thecalculated values of $I_b$ were corresponding to those obtained by infiltration experiment. Thisconfirmed the significance of steady $S_w$ as a new functional infiltration parameter. Amatching factor u was calculated as a ratio between predicted Ib and the measuredsaturated hydraulic conductivity, Ks. The mean values of u were 0.895, 0.685 and 0.360 forcalcareous, clay and saline clay soils respectively. Unsaturated K(θ) has been discriminatedinto saturated macro-pore $K(θ)_{RDP}$ and matrix unsaturated K(θ)h. The values of K(θ)RDP formacro pores remained higher than those for soil matrix pores $(K(θ)_h)$ in the studied soils.The highest value of K(θ) was obvious in calcareous soil profiles, while the lowest value wasexisted in saline clay soil. In conclusion, the predicted values of hydraulic conductivities ofsoil matrix (capillary) and macro (non-capillary) pores were reasonable and existed in thenormal ranges of the investigated soils, indicating that the proposed equations areapplicable and can be recommended to be used in coarse and fine textured soils with largescale of different properties