Quantification of transient specific yield considering unsaturated-saturated flow

Abstract

Specific yield is one of the most important hydrogeological parameters, and is a key factor connecting flow processes in the unsaturated and saturated zones. In this study, an innovative expression for the dynamic (or time-dependent) specific yield is proposed considering the coupled unsaturated-saturated flow process. The new specific yield equation includes parameters such as saturated water content, residual water content, pore characteristic parameter, initial depth of water table, time-dependent depth of water table, initial pressure head, and time. The involving parameters in this new specific yield equation reflect the impacts of lithology, initial water table depth and other factors. This new equation approaches an asymptotic (steady-state) value which is the same as reported previously for a shallow water table condition. Both advective and diffusive unsaturated flow processes are taken into consideration, which is in contrast to a previous study that ignored the diffusive unsaturated flow process. The model established in this study reveals the complete dynamic process of variation of water content and water head in the unsaturated zone. The newly developed specific yield equation can be incorporated into groundwater flow theory considering a dynamic water table reflective of a physically based unsaturated-saturated flow process.