• Sciences de l'ingénieur
  
• Sciences de l'ingénieur/Génie civil
  
• Sciences de l'ingénieur/Mécanique/Mécanique des matériaux
  
• Physique/Mécanique/Mécanique des matériaux
  
• Planète et Univers/Sciences de la Terre/Hydrologie
  
• Sciences de l'environnement/Milieux et Changements globaux
  
• Sciences de l'ingénieur/Mécanique/Mécanique des fluides
  
• Physique/Mécanique/Mécanique des fluides
  

The aim of this communication is to analyse the influence of water activity and total pressure on water evaporation. The system is composed of liquid and gas phases, separated by a plane surface, contained in a cylinder whose volume is regulated by a piston. Water activity is regulated by saturated salt solutions and pressure by the piston. The experimental device and procedures were defined to limit the temperature variation at the interface. A transient method is used. From a steady state, a volume increment is imposed; the resulting non-equilibrium leads to an increase in the partial pressure of water vapour to the equilibrium pressure imposed by the solution. Numerical calculation shows little variation in temperature in the gas-liquid interface under the experimental conditions. An evaporation model is adopted taking into account chemical potential discontinuity at the interface. The surface flux evaporation and chemical potential jump at the interface are deducted from the total pressure recording. In the neighbourhood of equilibrium, the surface flux of phase change is shown to be proportional to the chemical potential jump. The surface coefficient of evaporation increases with the total pressure of the gas phase and water activity.

The 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 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 Furthermore, the assumption of a local equilibrium between liquid water and its vapour is no longer valid at small water contents.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 introduced into 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 unsaturated conductivity function that account for capillary and film flows.

In this paper, we evaluate the performance of the most widely used semi-empirical models (van Genuchten (1980), Fayer and Simmons (1995) and Fredlund and Xing (1994)) and, an empirical model, the Modified Kovacs (MK) model for the determination of soil-water characteristic curve at the low water contents of two horizons of a soil from Burkina Faso. Combining terms from capillary state and adsorbed state of soil water gives a physical basis for the Modified Kovacs model. Our study confirms that the use of semi-empirical models requires somewhat large dataset covering the entire range of water content while the empirical model MK requires only basic geotechnical properties of soil and a few experimental points to adjust the parameters m and ac of the model. It appears that the MK model, by its simplicity and lower cost for the acquisition of experimental data, is the most appropriate.

The unsaturated soil hydraulic properties (the soil water characteristic and relative permeability curves) are key hydrodynamic parameters in the fields of soil science and civil engineering. Because of the strong dependency of these properties on water content, their determination is subject to considerable experimental and numerical problems. Difficulties increase when the soil approaches oven-dry conditions, a situation often encountered in arid and semi-arid areas. Moreover, computational costs of estimating the hydrodynamic properties can be relatively high also. After a short review of alternative modeling approaches for the hydraulic functions from saturation to oven-dryness, we present a theoreticalnumerical approach, along with a relatively simple, robust and inexpensive experimental method for inverse determination of van Genuchten/Mualem soil hydraulic parameters. For gravimetric water contents greater than 0.04, numeral results agreed well with experimental data, while some discrepancies were observed at very low water contents.

The dynamics of water content in the superfi cial layers of soils is critical in the modelling of land-surface processes. In arid regions, vapour flux contributes signifi cantly to the global water mass balance. To account for it in theoretical descriptions, most of the models proposed in the literature rely on the local equilibrium assumption that constrains the vapour pressure to remain at its equilibrium value. It implicitly amounts to consider an instantaneous phase change. Recent works underlined a retardation time and a decrease in phase change rate as the water content gets lower. Therefore, the objective is to revisit water transport modelling by rejecting the local equilibrium assumption. This requires developing a non-equilibrium model by taking into account the phase change kinetics. To assess the interest of this approach, a natural soil of Burkina-Faso has been experimentally characterized from independent tests and soil column experiments have been carried out. The comparison of experimental drying kinetics and water content profi les with computational predictions confi rms the reliability of this description. Liquid/gas non-equilibrium is signi ficant in a limited subsurface zone which defi nes explicitly the transition from liquid transport in lower layers to vapour transport in upper layers, i.e., the evaporation front. The overall moisture dynamics is governed by the coupling between water transport mechanisms (liquid fi ltration, vapour di ffusion, phase change) that mainly occurs in this transition zone.