Water flow in soil at small water contents: a simple approach to estimate the relative hydraulic conductivity in sandy soil

Abstract

Unsaturated soil hydraulic properties are key properties in the fields of soil science and civil engineering. Because of their strong dependence on water content, there are considerable experimental and numerical difficulties in their determination, specifically in the dry range. This situation is encountered regularly in arid and semi‐arid regions. The models commonly used for predicting the unsaturated hydraulic conductivity function rely on pore bundle concepts that account for capillary flow only and neglect film flow. Furthermore, the assumption of a local equilibrium between liquid water and its vapour is no longer valid at small water contents. Thus, with classical approaches, the experimental identification of hydraulic characteristics can fail at small water contents. To emphasize the weakness of capillary models, soil column experiments have been carried out with two sandy soils from Burkina Faso. Special care was taken to prevent any transport processes that are not directly related to liquid transport. Data from profiles of water content were used in an inverse numerical procedure to identify the coefficients of a new relative hydraulic conductivity function. Our results show that this simple approach is suitable for the analysis of flow processes at small water contents. It provides a simple, robust and inexpensive method to identify the properties of the unsaturated conductivity function that account for capillary and film flows. Highlights A new kind of soil column experiment is proposed to focus on the dry range. A new relative hydraulic conductivity function accounting for film flow is introduced. The weaknesses of classical relative hydraulic conductivity functions are emphasized. It provides a simple, cheap and robust method to identify the relative hydraulic conductivity.

Analysis of Non-Equilibrium Phase Change in Transfers at Low Water Content by Considering the Film Flow

“…As water content decreases, hygroscopic effects become predominant and adsorptive forces prevail over capillary ones. Liquid water is present in the form of adsorbed layers around soil particles and its thermodynamic state can differ markedly from bulk water (Kébré et al, 2017). Therefore, the modelling of transport phenomena at low water contents requires a specific description (Kébré et al, 2017).…”

“…Liquid water is present in the form of adsorbed layers around soil particles and its thermodynamic state can differ markedly from bulk water (Kébré et al, 2017). Therefore, the modelling of transport phenomena at low water contents requires a specific description (Kébré et al, 2017). In particular, it has been emphasized that vapour diffusion can contribute considerably to water transport, which leads to reconsidering the classical assumption of a local equilibrium (Smits et al, 2011;Ouedraogo et al, 2013;Massman, 2015).…”

“…Moreover, the physical processes that govern liquid water flow in the low water content range are still in question. In general, numerical modelling based on the Richards equation requires the complete determination of its hydraulic properties (i.e., the hydraulic conductivity and water retention functions) (Kébré et al, 2017). Numerous attempts have been made to derive theoretically the hydraulic conductivity function directly from the water retention curve based mainly on capillary bundle models (Burdine, 1953;Mualem, 1976).…”

“…Although fairly accurate at large water contents where water is held mainly by capillary forces, these descriptions generally underestimate the permeability at low water contents (Khaleel et al, 1995). The most used van Genuchten-Mualem permeability function is appropriate for intermediate water contents (w>0.04 kg.kg −1 ), which highlights the shortcomings of capillary bundle models in predicting the permeability function in this dry range of the soil (Kébré et al, 2017).…”

A Universal Model of Unsaturated Hydraulic Conductivity With Complementary Adsorptive and Diffusive Process Components

“…Fredlund and Xing (1994) proposed a model, without dividing contribution of capillary and film flow, that improved the estimation of unsaturated HC at the dry end. However, their model does not lead to a closed-form equation, and a numerical solution is required to determine the relative HCF (Kebre et al, 2017). By applying even more pressure and at very low saturation, water pathways become longer and sparse, which in turn limit the connectivity of liquid, and as a result, water movement is only viable through vapor flow (Kelleners et al, 2016;Lehmann et al, 2008;Shokri & Or, 2010).…”