Transfert d'eau dans un milieu poreux non isotherme Water Movement in Non Isothermal Porous Medium

Résumé

Les transferts d'eau dans le sol sont généralement, pour des raisons de facilité, supposés se dérouler dans des conditions isothermes. Les modèles proposés couplant les transferts hydriques et thermiques se heurtent aux difficultés inhérentes à la détermination des coefficients de transfert. Disposant de l'évolution de la température, de la succion et de la teneur en eau dans les profils du sol de lysimètre, une comparaison portant sur l'importance des gradients hydriques et thermiques dans le transfert d'eau en phases liquide et vapeur a été menée. Il se dégage clairement, que dans le domaine de teneurs en eau qui intéressent l'agronome (teneur en eau supérieure à celle au point de flétrissement), l'essentiel des transferts se fait via la phase liquide. En effet, les flux dus aux gradients de teneur en eau dépassent d'au moins deux ordres de grandeur les flux induits par les gradients de température. Ce qui confirme bien la validité des équations de Darcy-Richards (DARCY, 1856; RICHARDS, 1931) où l'effet de température est négligé.

Mots-clés:

• Diffusivités,
• phases liquide et vapeur,
• transfert de l'eau dans le sol

Abstract

In most studies of liquid water flow, isothermal conditions are assumed. Since the beginning of the last century, several models and studies concerning mass and heat transfer have been conducted, but results didn't agree on the effect of temperature and hydraulic gradients on water movement. Also, the relative importance of these different gradients is not well known. Models used to quantify the simultaneous transfer of water due to temperature and water content gradients require a predetermination of thermal and iso-thermal coefficients of water diffusivity. Moreover soil characteristics must be known, mainly the relationships between hydraulic conductivity (K), pressure head (h), and water content (Ɵ). Measurements of water content, pressure head and temperature were done in a loamy sandy clay soil within a weighable lysimeter. The relationships between hydraulic conductivity, pressure head and water content were obtained from field and laboratory data.

The water content gradient is generally less than those of temperature. Even with the maximum temperature gradient, the flux due to the water content gradient is more important than those due to the temperature gradient; the difference can reach 200 %. Various field data (temperature, pressure head and water content) for different field conditions concerning the mass transfer in both liquid and vapor phases resulting from hydraulic and temperature gradients were analyzed. Results show that when soil moisture is between the wilting point and field capacity, mass transfer occurs mostly in the liquid phase. This result confirms the validity of Darcy-Richards (DARCY, 1856; RICHARDS, 1931) equation where the effect of temperature is neglected. Also, the isothermal diffusivity of liquid water is very important when compared to all other diffusivities.

Keywords:

• Diffusivity,
• liquid and vapor water phases,
• water transfer in soil