The effect of trenched subsurface drainage on the drain water quality and solute transport in Soil in mid and end season drainage of rice

Proper design and management of irrigation and drainage systems improves the quality of drainage and conserve soil and water resources. In paddy fields, due to the high clay and impenetrable layer, the type of drainage and its envelope have a great effect on the movement of water in the soil and, consequently, solute transport. So, recognition of solute transport in this condition can lead to effective design and management of drainage to achieve proper drain water quality in paddy fields. So a box with dimensions 300×60×100 cm (in length, width and height) was considered. A Polyethylene drain pipe with 10 cm in diameter was laid in the sand and gravel trench with 20 cm width and 50 cm height. The rice transplant was transferred to the box and applied phosphate and urea fertilizer according to paddy fields custom. Flood irrigation method with 5 cm above the soil was performed and mid and end drainage was applied at tillering and harvesting stages, respectively. During experiment, the soil solution and drain water was sampled and chemical parameters including electro conductivity (EC), sodium adsorption ratio (SAR) and nitrate and ammonium concentration was measured. Results showed that mean EC of mid season drainage (567 ds/m) was 51% more than Mean EC of end season drainage. Nitrate and ammonium concentration of drain water of end season drainage reduced 12 and 39% than mid season drainage, respectively. The results indicated that hardpan layer has an effect on the accumulation of solute, so that at the end of the experiment, the accumulation of solute and nitrate on the top of this layer comparing with drain depth were 11 and 47 percent, respectively.