Comparison of steady- and unsteady-state drainage equations for determination of subsurface drain spacing in paddy fields: a case study in Northern Iran

In Northern Iran paddy fields, waterlogging is a serious problem. Provision of subsurface drainage seems to be a prerequisite for optimal crop production. Considering specific characteristics of the paddies, application of a suitable equation for planning of subsurface drainage systems is of great importance. In this study, five steady-state equations including Hooghoudt, Kirkham, Dagan, Ernst, and Ernst–Hooghoudt; and three unsteady-state equations including Glover–Dumm, Van-Shilfgard, and De-zeeuw–Hellinga were implemented to compute drain spacing. Required field data were obtained from 52 ha of paddy fields of “Haraz Human-Agriculture Resources Development Center,” located in Mazandaran province of Iran. The computed drain spacings were evaluated using dynamic equilibrium concept and DRAINMOD model. The minimum and maximum drain spacings were found to be 26 and 63 m corresponding to Glover–Dumm and Ernst equations, respectively. Based on the dynamic equilibrium concept, the averages of the computed water table depths for the drain spacings obtained by the unsteady-state equations were 0.5–0.63 m as compared with those obtained by the steady-state methods which were in the range of 0–0.43 m. According to the DRAINMOD model simulations, the maximum relative yield was 80.2 % which was obtained when the drain spacing was 26 m and it reduced to 73.5 % when the drain spacing was increased to 63 m. The computed drain spacings suggested by the unsteady-state equations were found to be more suitable than those obtained from the steady-state equations for the study area.