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In order to study effects of water table depth on specific yield in the soil profile, an experiment was conducted on
a sandy loam soil using one and two dimensional drainage and the estimating method of this parameter was evaluated in
the laboratory. For one dimensional drainage, a galvanized cylinder 45 cm in diameter and 100 cm length compacted with
a sandy loam soil was used at the bottom of which a small drain 5 mm in diameter hal been installed. A lab model
compacted with the same soil was used for the two dimensional drainage. At the bottom of the model, two drains each 5
mm in diameter and 100 cm spacing were installed.
In general estimated and measured results showed that the specific yield of this soil increases with increaing water cable depth. In both one and two dimensinal flow, specific yield in the surface layers were approximately close and for deeper layers, the value of this parameter was more in one than two dimensional drainge, The reason for this was attributed to more energy loss in tow dimensional drainge. Also, measured values in bothcases were about 35% in one and 57% in tow dimensional flow lower than those of estimated because of air entrapment due to incomplete saturation and drain resistance.
The reason for increasing specific yield with water table depth in the soil profile, was drainage due to remaining water in upper layers causing creation of higher suction as the water table drops. In other words. small pores are able to drain in the upper layers with increasing water table depths. Results of estimates in two dimensional drainage on another sandy loam soil showed that the specific yield is approximately the same in this soil profile. Thus, The assuption of constant specific yield seems valid for this soil.