Perspective on ‘Delayed yield’

The phenomenon of ‘delayed yield’ has received wide attention in the literature during recent years. Considerable work has been devoted to the development of physically based mathematical models capable of successfully reproducing observed aquifer behavior. As a result of this effort, there appears to be at present a consensus of opinion among the main contributors to the theory of delayed yield as to an acceptable physical and mathematical interpretation of this phenomenon. In spite of this apparent consensus of opinion, there is still a considerable amount of controversy in the literature about some fundamental aspects of unconfined well hydraulics. The outstanding issues concern the physical assumptions underlying various delayed yield models, changes in the flow regime of an unconfined aquifer as a function of radial distance from the pumping well, the relationship between Boulton's α and other physical characteristics of the aquifer, the validity of conditions required in order to represent unconfined flow by integrodifferential equations, and the behavior of unconfined aquifers at large values of time. The present paper offers a perspective from the author's viewpoint on these and other related issues.