The suitability of controlled drainage and subirrigation in paddy fields.

: DRAINMOD, an agricultural water management simulation model, was used to investigate the suitability of controlled drainage/subirrigation scenarios in 52 ha of northern Iran paddy fields which planted in April to September and the rest of the time being waterlogged and pounded and could not be cultivated because of high water table, low permeable top soils and unsuitable drainage systems. The model as run for 30 year climate data. Various drain depth and spacing combinations and different weir setting were simulated. Simulation results showed that the xcess soil moisture was the main factor for yield reduction and the best conditions of controlled drainage and subirrigation were obtained when the depth of weir setting was the same as drain depth and these systems acted as conventional drains. Based on results, controlled drainage and subirrigation were not suitable because of high rainfall in the region. But the conventional controlled drainage system can be used for these fields, so that in wet season the system acts like a conventional or free drainage system and in paddy rice growth periods, it operates as controlled drainage by installing a weir setting outlet drain.