Investigating long-term effects of subsurface drainage on soil structure in paddy fields

Subsurface drainage systems provide suitable conditions for annual cropping and crop diversification in northern Iran's paddy fields. Such structural and managerial changes affect soil structure through influencing saturated hydraulic conductivity and effective porosity of the soil. The aim of this research was to investigate the long-term effects of subsurface drainage systems (D0.90L30: drainage system with 0.90 m depth and 30 m spacing, D0.65L30: drainage system with 0.65 m depth and 30 m spacing and D0.65L15: drainage system with 0.65 m depth and 15 m spacing) on the soil saturated hydraulic conductivity and drainable porosity. Water table and drain discharge were measured daily during different growing seasons (2011-2015) following installation of the subsurface drainage systems in the paddy fields of Sari Agricultural Sciences and Natural Resources University. The saturated hydraulic conductivity was calculated using Hooghoudt and Glover-Dumm equations and the drainable porosity was calculated with Taylor's method. Computed hydraulic conductivities by the Hooghoudt and Glover-Dumm equations were in the range of 0.002 to 1.95 m day−1 and 0.003 to 2.5 m day−1, respectively, that highest value was related to D0.65L15 and lowest value was related to D0.90L30. So, the shallow drainage systems were more effective in improving the soil hydraulic condition compared with the deep ones. The drainable porosity in the D0.90L30, D0.65L30 and D0.65L15 systems ranged from 0.002 to 0.314 m3 m−3, 0.003 to 0.13 m3 m−3 and 0.004 to 0.126 m3 m−3, respectively. In conclusion, the installation of subsurface drainage especially at the shallower depth will improve paddy soil structure during time.