PERFORMANCE EVALUATION OFA SINGULAR SUBSURFACE TILE DRAIANGE SYSTEM OF SWABI SCARP

A well maintained subsurface tile drainage system plays an important role in the reclamation of waterlogged and saline area as well as improves crop productivity. A field study was conducted to evaluate the performance of subsurface tile drainage system during the years 1998 and 2011 in terms of depth to water table from the ground surface, drainage coefficient hydraulic conductivity and yield of major crops. The system was installed under Swabi Salinity Control and Reclamation Project (SCARP) in July, 1997 in the North-West of Pakistan. The study area is being drained by nine subsurface tile drains spaced 100 to 150 m and length ranged from 500 to 700 m at design depth of 1.5 m from the ground surface. Depth to water table and discharge of tile drains were measured on a weekly basis, while yields of major crops were recorded at harvest stage. Average depth to water table from ground surface ranged from 1.0 to 1.6 m and 0.8 to 1.2 m during the study periods 1998 and 2011 respectively. The water table was slightly (8%) higher in 2011 as compared with 1998 due to blockage of half of the laterals. Sugarcane and rice crops were not affected by the re-emerging water logging during 2011 due to blockage of tile drains while on the other hand wheat and maize were found sensitive to water logging as there yields were 42 and 9% lower under shallower water table condition. The overall mean drainage coefficient of 1998 was 1.65 mm day^-1, while in 2011 it decreased to 0.7 and 1.3 mm day^-1 in partially blocked and unblocked tile drains command area due to blockage of some of the tile drains. An average hydraulic conductivity before the project (1995) was 0.79 m day^-1 and it increased to 1.31 m day^-1 in 1998 and showed a further increase to 1.71 m day^-1 in 2011. It can be concluded that the major goals of the installation of singular subsurface pipe drainage system have been achieved during the early years after completion of the project as the project was successful in lowering the water table and increasing crop yield but the long term performance was adversely affected by improper maintenance of the main drain, subsurface tile drains and plantation of poplar (Poplus nigra) trees across the laterals.