Evaluation of Subsurface Drainage System Performance under Preventing Water Flow from upper Side Condition

The downward flow to the drain pipe، has a greater influence on the movement of soil particles toward drain envelope as compared with the horizontaland radialflow. In this study، by installing of a singular subsurface drainage systemconsisting of threedrain pipeswith drain spacing of 20 m and drain depth of 1. 5 m، in one hectare field of Sari Agricultural Sciences and Natural ResourcesUniversity، the effects of the elimination of downward flow to the drain pipe was investigated on the water table level and drainage flow. Prevention of direct entry of the vertical flow into drain pipe was performed by placing a layer ofplastic coveron the sand envelope of themiddle drain pipe. Water table level fluctuations were measured in holes which were dug in each drain trench and at 0. 5، 1. 5، 5 and 10 m spacing apart from each drain at intervals of 5، 15، 25، 35، 45، and 55 meters from collector ditch. Water table depth and drain discharge were measured from April 21 to December 21، 2011. The average drainage discharge fromno plastic drain (drain A) was more than corresponding value for plastic covered drain pipe (drain B) about 12 % and there was a significant difference (p=0. 05) between drainage water volume of drains A and B. The average depth of water table levels within the trench of drain A was 9. 1 cm more than the corresponding water table depth of drain B. Also، the average depths ofwater table in the 0. 5، 1. 5، and 5 m spacing apart drain A were approximately 5. 2، 2. 9، and 0. 05 m higher than of thosevalues of drain B. Sediment load of drain A was 74% more than that of drain B،indicating the considerable role of the inflow to drain from its upper part on the consequent sediment transport into drain pipes.