Subsurface Drainage to Enable the Cultivation of Winter Crops in Consolidated Paddy Fields in Northern Iran

Subsurface drainage is a prerequisite to grow winter crops in the consolidated paddy fields in Northern Iran. A four-year study (2011–2015) was conducted to quantify the effects of subsurface drainage on the saturated hydraulic conductivity, water table, drain discharge and winter crop yields. Subsurface drainage systems with two drain depths of 0.65 and 0.90 m and two drain spacings of 15 and 30 m were installed at the consolidated paddy fields of Sari Agricultural Sciences and Natural Resources University, Iran. During four successive winter seasons, the water table depth and drain discharge were measured daily. Soil saturated hydraulic conductivity was measured twice; before drainage system installation and four years following the installation. Canola grain yields were determined at harvest of each cultivation season. During the study period, the soil saturated hydraulic conductivity increased with the highest increase in the top 0–30 cm. The deeper drains were more effective in controlling the water table compared to the shallow, and the daily drain discharge of the deeper drains in the fourth year were higher than those of shallow drains. The canola grain yield of all drainage systems increased significantly by the seasons, and the largest difference in canola grain yield between first and fourth seasons was 2191 kg·ha −1 (318% increase) in the fields with 0.90 m drain depth and 30 m drain spacing. Totally, it became clear that installation of subsurface drainage systems with 0.90 m depth and 30 m spacing in the paddy fields of Northern Iran can be recommended to achieve high yield of winter crop, soil condition improvement, and multi-purpose land use.