Effect of water table fluctuation on canola leaf area index

Improving soil water situation in the paddies by subsurface drainage systems provides suitable condition for the cultivation of winter canola. In order to evaluate the effect of subsurface drainage on the winter canola growth in paddy fields, an experiment was conducted as split plot in a randomized complete block design with three replications. The main factor involved farm drainage system consists of three conventional drainage systems with 0.90 m and 0.65m depths at 15 and 30m distances (D0.90L30, D0.65L30 and D0.65L15: D and L indicate drain depth and spacing, respectively) and a subsurface drainage system with four drain lines spaced 15 m apart with a depth of 0.65 m and 0.90 m as alternate (Bilevel). The sub factors were also the days after planting. During the growing period, to determine leaf area index (LAI), 1326 canola leaves were sampled and were analyzed by direct measurement and Digimizer software. The DataFit9 software was applied to examine the mathematical equations governing the canola LAI under different drainage systems. Water table rising was affected by precipitation and its falling was related to the performance of different subsurface drainage systems. In this regard, Bilevel system with mean depth to the water table of 64.7 cm had higherLAI. Critical LAI was found in 100 to 114 day after planting and its maximum (4.71) was observed in Bilevel. Even mathematical functions with the orders of 6 and 8 showed more accurate estimation other than equations for LAI in different drainage systems. Base on the results, subsurface drainage with proper depth and spacing could improve canola LAI by controlling the water table below the root zone.