Unsaturated Stream-Aquifer Connection When the Riverbed Itself Desaturates

In a previous series of 3 articles in this Journal we have looked at the situation when the aquifer desaturates while the riverbed (clogging layer) remained saturated. However it is quite possible that the riverbed itself will desaturate, particularly if the river stage is low and especially when the river goes dry. In this article we look at the situation when indeed the riverbed desaturates as the capillary pressure at the interface between the bottom of the riverbed and the top of the unsaturated zone in the aquifer exceeds the riverbed entry pressure, while the river itself does not go dry. Utilizing different soil textures for a riverbed we found that the seepage rate, following a slight increase in value, tends rapidly to an asymptotic limit which is not much higher than the seepage value at incipient desaturation of the riverbed. Thus the assumption that the riverbed does not desaturate may lead to grossly exaggerated values of the seepage rate.