A study of the specific yield in land-drainage situations

Abstract

From the definition of the specific yield in an unconfined aquifer, it is shown that the true specific yield is a function of the horizontal position and time and may be obtained from the measurements of localised fluxes and localised water-table movements. Less precise definitions of the concept - the average, bulk and virtual specific yields - are defined. With the assumption of vertical flow above the water table, the specific yield is derived as the sum of the air content at the surface and the ratio of the rate of change of the volume of water held in the moisture profile above the water table to the rate of rise or fall of the water table. Since the latter ratio can be small in certain circumstances, it is argued that the air content at the surface by itself is often a fair approximation to the specific yield. Results of measurements of all the specific yields, obtained in non-steady state drainage experiments using a completely permeable drain installation and a plastic drain installation in a hydraulic model sand tank, are presented. The variations of the specific yields with time are compared and discussed in relation to the varying moisture profile. With the use of a virtual specific yield given by the air content at the surface in the midplane position between drain lines, calculated assuming steady state conditions with a stationary water table, and also with the use of the experimentally obtained relationship between water-table height and drain discharge for a given drain installation, rise and decay curves for the water table are calculated and compared with experiment, and fair agreement found.