The nonsteady state of the water table in drained land

The interesting papers by Maasland [1959] and Isherwood [1959] on the nonsteady state of the water table probably develop the discussion of this topic as far as is useful at present. The purpose of this note is to draw attention to an assumption implicit in all work of this kind which needs further examination before progress becomes possible. The assumption is that there is such a quantity as a constant specific yield f, namely the volume of soil water released per unit surface area per unit fall of the water table. Nonsteady-state moisture profiles. Moisture profiles above a water table which is falling after rainfall cessation may be described, in crude semi quantitative terms only, in some such manner as proposed in Figure 1 [see Luthin, 1957, p. 33]. One may suppose an initial stage indicated by the full line and subsequent stages of which the broken line indicates an example. If one supposes that all such profiles are similar, being merely displaced vertically, then the yield of water, which is transported across the water table, is given by the shaded area between the profiles, and this, in turn, by a simple geometrical argument, is equal to the rectangular area f dh where dh is the fall of the water table and f is the difference between saturation and field capacity, to use a somewhat elusive concept. As so defined the specific yield / is evidently a soil moisture constant. This naive treatment simply ignores and avoids the difficult problem of the water movement and moisture profile development in the unsaturated soil above the water table, a problem in the field of nonlinear diffusion [Luihin, 1957, chap. I]. It is easy to demonstrate some common cases where such a treatment is greatly in error.