Equilibrium, non-equilibrium and residual water: Consequences for soil water retention

Factors that affect water retention in soil are discussed in terms of thermodynamic and hydraulic equilibria. It is assumed that the Groenevelt and Grant (2004) water retention equation describes soil water that is at or close to thermodynamic equilibrium. It is shown that immiscible displacement, as occurs in a pressure cell apparatus in which air displaces water, can leave residual soil water that is not in thermodynamic equilibrium. We describe this residual water as being in hydraulic equilibrium. As hydraulic equilibrium is approached, the outflow of water becomes at first very slow and then essentially ceases. An empirical equation for water retention in bi-modal soils is used for soils that exhibit residual water content. It is found that residual water is that which remains after the connected (drainable) textural pore space has emptied by Darcian convective flow. The point at which Darcian flow ceases is called hydraulic cut-off. The water remaining after hydraulic cut-off is the residual water that moves much more slowly mainly through vapour-phase diffusion. The suction of residual pore water can be significantly smaller than the air pressure that has been applied in a pressure cell apparatus. Models for predicting the amount and pressure of residual water are presented and tested using experimental data for 14 soils from France and Poland. Some consequences for soil water retention studies and for the status of soil water in the field are discussed.