Assessment of Unsteady- State Drainage Equations to Determine the Spacing of Subsurface Drains in Paddy Fields

Subsurface drainage is required to eliminate drainage problems and to provide year-round cropping conditions in large areas of the Northern Iran,spaddy fields.With regard to the specific characteristics of these fields such as presence of hard layer of clay at shallow depth and high rainfall in the wet seasons, using an appropriate equation is important for the design of subsurface drainage systems.In this research, the applicability of four unsteady- state drainage equations including Glover–Dumm, Van-Shilfgard, Dezeeuw– Hellinga, and Kraijenhoff– Maaslandwas evaluated to determine subsurface drain spacing in paddy fields. Measurement of water table depth was made daily during a canola growing season, from November 2011 to April 2012, in different treatments of subsurface drainage pilotof Sari Agricultural Sciences and Natural Resources University. Performance of each equation was evaluated using the statistics including efficiency (EF), percent error (PE), coefficient of determination (R2), root mean square error (RMSE) and mean deviation (AD).Among different equations, the Dezeeuw– Hellinga was the best equationfor estimating water table depth during the study period.