Theory of flow in unconfined aquifers considering delayed response of the water table

A new analytical model is proposed for the delayed response process characterizing flow to a well in an unconfined aquifer. The present approach differs from that of Boulton [1954b, 1963, 1970] and Boulton and Pontin [1971] in that it is based only on well‐defined physical parameters of the aquifer system. Therefore it provides a possible physical explanation for the mechanism of delayed water table response and eliminates the conceptual difficulties encountered with Boulton's theory of ‘delayed yield from storage above the water table.’ Contrary to prevailing belief the process of delayed response in a homogeneous anisotropic phreatic aquifer can be simulated by using constant values of specific storage and specific yield without recourse to unsaturated flow theory. The results suggest that, in the absence of significant infiltration at the ground surface, compressibility may often be a much more important factor than unsaturated flow above the water table. Current methods of analyzing field data from unconfined aquifers do not usually consider compressibility. The present theory shows that such methods are limited in their application to relatively large values of time.