Control of Salinity and Nitrate Losses in Drainage Water under Water Table Management

Controlled drainage is one of the strategies to reduce negative environmental effects in conventional drainage systems. This study examines the effect of controlled drainage on quality and quantity of drainage water, specifically amounts of salt and nitrate in drainage water, and nitrate uptake by maize (SC 704) with sandy loam soil texture in Karaj. The experiment was performed as a randomized complete design including three treatments with three replications: free drainage (FD) and controlled drainage with 40 cm (CD40) and 60 cm (CD60) controlled water tables. Irrigation intervals were based on MAD = 0.65 in FD treatment. For all treatments, irrigation depths were determined based on deficiency of soil moisture from field capacity with 70 percent application efficiency. According to the research hypothesis, irrigation losses cause rising water table during the season, but the results showed that 30 percent irrigation losses during implementation in CD treatments did not raise the water table to the expected control levels due to increased consumption of plant. So between two water table control treatments, there was no significant difference. In FD treatment, which drainage water discharge was facilitated, both plant water consumption and yield were reduced. Effect of water table management was significant (p< 0.01) on nitrate and salt balance. Volume of drainage water in FD treatment was 34.8 percent of total gross irrigation whereas in CD treatments no drainage water was produced. In FD treatment, due to appropriate leaching conditions, salt and nitrate levels in drainage water was 63 and 20.8 percent of amount of input salt and nitrate, respectively. While in CD treatments due to controlled conditions, salt and nitrate leaching was not observed. At the end of cultivation period, measured values showed that nitrate accumulation in soil profile were greater in FD treatment than CD treatments. This observation shows that under CD treatments, as compared with FD treatment, plant nitrogen absorption is increased which is reflected by higher crop production. At the end of the growing season, however due to lack of leaching in CD treatments, a higher salt accumulation is observed in soil profile but this can be managed with leaching of salts during the off- season periods.