Model Calculations for the Extraction of Soil Water by Ceramic Cups and Plates

A numerical method is developed with which the flow of moisture towards suction units in soil can be evaluated. For a number of selected problems the unsaturated moisture flow equation is solved twice: (i) for radial flow towards a suction plate or suction cup; (ii) for vertical downwards flow under the same conditions, but without a suction unit in the soil. The calculations are performed for a flat soil without a plant cover. It is found that even for little vacuum, large differences may exist between the seepage rate as determined from a suction unit as compared to the seepage rate of the unextracted soil. It is also shown that the radius of influence of a suction unit, which extracts water continuously, can be several feet wide. It is concluded that in order to be reliable, data collected with suction units should be analyzed with care.