Evaluation of Subsurface Drainage System in Unsteady Flow Conditions (Case Study: Research Farm of Shahid Chamran University of Ahvaz, Iran)

Karoon River is one of the most important water resources for irrigation of farming lands in Khuzestan, Iran, which its quality has decreased in early years. Due to increases in water tables and salts in soils using Karoon River for irrigation, a subsurface drainage system was installed at an agricultural research farm at Shahid Chamran University of Ahvaz to overcome these problems. The research farm contained seven laterals (diameter 125 mm, length 200 m, spacing 30 m, and depth 1.5 m) and one collector (diameter 300 mm). This study was conducted on this farm to evaluate above mentioned drainage system. To this end, (including outlet discharge from laterals and collector, outlet salinity from laterals and collector, salinity changes before (B) and after (A) irrigation), relative groundwater depth (RGWD), salt export indicator (SEI), and relative electric conductivity (REC) during three irrigation periods were determined. The result showed irrigation increased the drainage rate greatly. But it is notable this trend decreased after 52 hours. The drainage water salinity fluctuated during this study while high salinity was found in the first irrigation. The results also showed that drainage water salinity of the collector was nearly constant, so the maximum salinity, determined during second irrigation, was only 1.04 and 1.03 times more than maximum drainage salinity during first and third irrigations, respectively. Soil salinity decreased after irrigation but it was increased after several days due to capillary rise. RGWD was in the range of 0.9-1.1 and showed that the drainage system had sufficient efficiency for the control water table. In addition, SEI, for each irrigation period was in the standard value but maximum SEI was obtained during third irrigation and it was 1.27 and 1.42 times more than first and second irrigations, respectively. Maximum REC was determined for a distance approximately 150 m from collector before irrigation which this index was 1.28 and 1.26 times less than distance 100 and 50 meters, respectively. According to the results, the installation of the drainage network had a good performance to control the water table and soil salinity.