Temperature dependence of the water retention curve for dry soils

Water retention curves (WRCs) are equivalent to water adsorption isotherms that display the soil water content as a function of water activity in the pore space. The use of water activity implies that pure (unbound) water at the given temperature is considered to be a reference state. In this study we measured the temperature dependence of WRCs for nine European soils under dry conditions (i.e., water activity < 90% relative humidity (RH), matrix tension < -1.5 MPa). The results show a significant temperature dependence of the WRCs. The absolute value of the adsorption enthalpy of water, Delta H-ads, which reflects this temperature dependence, increased with decreasing water content and thus deviated from the condensation enthalpy of a pure (unbound) water phase, Delta H-cond. These results are explained by the following facts : under increasingly drier conditions the interactions between water molecules and the mineral surfaces become more and more dominant because the sorbed water film becomes very thin. These interactions between water and minerals are stronger than those between pure water molecules. The observed temperature dependence of WRCs varied only a little between the studied soils. Therefore, the average equation, Delta H-ads (kJ/mol) 4: 9 lnRH - 66: 5, derived from our experimental data may serve as a good approximation of Delta H-ads for soils in general and thus allow the temperature extrapolation of WRCs (in the dry region down to 30% RH) between 5 degrees C and 40 degrees C without the need for additional experimental information.