In determining the spacing required between parallel agricultural drains, an important consideration is the length of time required to lower the water table at its point of maximum height to the extent that it will be below the root zone of the crop under cultivation. This ti me is a function of, among other factors, the available depth of permeable material through which flow to the drains can take place. When this depth is considered as variable, the resulting partial differential equation, even using the simplifying Dupuit-Forchheimer assumption, is nonlinear, hence difficult to solve analytically.