An analysis of the effect of water table depth on plant-available soil water and its application to a yellow-brown sand

Abstract

Abstract The effects of a high water table on soil water storage and capillary rise into the root zone are analysed using the basic equations describing soil water statics and dynamics. When applied to a Pukepuke black sand, the analysis predicts that the equivalent depth of plant-available water increases substantially from 26 to 183 mm as the water table rises from 1.2 to 0.4 m. The effect of lateral flow from higher to adjacent lower areas of yellow -brown sand is shown to be important over distances of 400 m, and can provide further plant-available water. Traditional ways of estimating plant-available water storage are shown to be quite inappropriate where the water table is less than c. 1 m below the root zone depth. It is suggested that in sandy soils drains should be either shallow (e.g., 0.4 m deep) or very widely spaced (e.g., 400 m apart) if they are not to make the soil more droughty. The analysis presented is applicable to any soil and groundwater situation for which the relevant soil water retentivity and conductivity data are available.