By means of the Green-Ampt analysis, it is shown that the drainage from an initially saturated column of porous material after the water table is suddenly lowered to a given level from the surface on which infiltration is maintained at a rate q during the drainage, may be described to a good approximation by the equation: 1- Q′ Q ′ ∞ = exp [-(F 0-q)t Q ′ ∞ ] where Q′ and Q′ ∞ are the quantities of water drained at time t and at infinite time, respectively, and F 0 is the initial flow rate from the column. Experimental confirmation of this equation was obtained with measurements on the drainage from a sand column. Experiments also showed that the equation described the drainage from a column when a steady infiltration rate on the surface was changed to a lower one, although the Green-Ampt analysis could not be applied to this case. The Green-Ampt approach was also used, with supporting experimental results from a sand column, to estimate the drainage from a column of porous material with a moving water table. This analysis was applied to investigate the delayed yield effect during the water-table drawdown over a field-drainage installation.