Weighting the differential water capacity to account for declining hydraulic conductivity in a drying coarse-textured soil

Abstract

Water availability to plants growing in coarse-textured soils during a drying cycle relies on the declining abilities of the soil to release water (differential water capacity) and to deliver it to the plant (unsaturated hydraulic conductivity) under varying evaporative demand. In this context, the availability of water can be quantified using the concept of the integral water capacity, IWC, in which the differential water capacity is weighted by means of a restrictive hydraulic function before integrating. We argue here that the diffusivity is an appropriate component of the restrictive hydraulic function, which leads to the employment of the so-called 'matric flux potential' (which we propose to re-name as the 'matric flux transform'). As the starting point to apply the diffusivity function, we choose the inflection point of the water-retention curve drawn on semi-log paper, which, for the Groenevelt-Grant equation, occurs at a matric head, h, of precisely k0 metres. An illustrative example of the procedures is provided for a coarse-textured soil, which reveals that the restrictive function may not be sufficiently restrictive for all cases. We therefore apply an additional weighting coefficient to account for varying sensitivity of different plants to hydraulic restrictions.