Evaluation of Deep Percolation Rates at a Transect of Paddy Fields During Crop Season

The common method of irrigating rice in paddy fields of Iran, like most countries, is flooded irrigation. The water required in this method is too much. However, because of water shortage in recent years, and malfunctioning of irrigation systems, it is needed to use water in a reasonable way and increase water use efficiency. Therefore, it is necessary to know water loss amounts at the paddy fields. The deep percolation (DP) was measured by closed- and open-bottom rings in 4 locations, and 7 sites at each location, of paddy fields in Somae-Sara city, Guilan province. These locations were selected on the base of different physiographic units. The average DP of these locations was also monitored during plant growth season. The measurements were performed twice a week. Results showed that the rate of DP varied during the season, and could take a positive or negative value. The most important factors of these variations were the lateral seepage (from surrounding rice fields) and the high perched groundwater table in paddy fields.