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 Furthermore, the assumption of a local equilibrium between liquid water and its vapour is no longer valid at small water contents.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.HighlightsA 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.