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. At unsaturated 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 time of infiltration, sorptivity was not studied as a soil water parameter that could be determined. The purpose of this study is to apply derived equations depending on the infiltration functions to predict (1) soil water sorptivity (S) at infiltration capacity (unsaturated conditions) and at basic infiltration rate (I_b) (saturated conditions), (2) the hydraulic conductivity (Saturated Ks and unsaturated K(θ)) into capillary-matrix and non-capillary macro pores of soils.  Five alluvial (saline and non-saline clay) and calcareous soil profiles located in the Nile Delta were investigated for applying the assumed equations. A decrease in S value was observed with an increase in soil water content. At steady infiltration rate (I_b), 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 clay soils. The steady S_w parameter was used in prediction of the hydraulic conductivities and the basic infiltration rate I_b , whereas, S_w is a suggested term at steady infiltration rate. The calculated values of I_b were corresponding to those obtained by infiltration experiment. This confirmed the significance of steady S_w as a new functional infiltration parameter. A matching factor u was calculated as a ratio between predicted I_b and the measured saturated hydraulic conductivity, Ks. The mean values of u were 0.895, 0.685 and 0.360 for calcareous, clay and saline clay soils respectively. Unsaturated K(θ) has been discriminated into saturated macro-pore K(θ)_RDP and matrix unsaturated K(θ)_h. The values of K(θ)_RDP for macro 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 was existed in saline clay soil. In conclusion, the predicted values of hydraulic conductivities of soil matrix (capillary) and macro (non-capillary) pores were reasonable and existed in the normal ranges of the investigated soils, indicating that the proposed equations are applicable and can be recommended to be used in coarse and fine textured soils with large scale of different properties.

Keywords

• Infiltration functions,soil pores,steady sorptivity,unsaturated hydraulic conductivity

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