Studies on Water Saving Irrigation as a Means of Preventing Cold Water Damage to Paddy Rice (4)

Abstract

It was previously reported by the authors that water saving irrrigation is very effective in preventing cold-water damage to paddy rice. The irrigation control, however, does not always seem to be available when the air temperature is lower than the water temperature of an irrigation channel and when a long spell of drought continues. Such climatic conditions will be detailed with a view to finding out when and where water saving irrigation works most effectively.From the results of the statistical analysis on the climatic data of the Tohoku District during the past twenty to forty years, the following were obtained: The mean minimum temperature during five-day period, 12.3°C, and the rainfall during ten-day period 40mm and less, are considered as the climatic limit on water saving irrigation.Judging from the temperature limit, the application areas of water saving irrigation program are presumed to cover all the cold-watered paddy fields in the southern parts of the Tohoku District and the cold-watered paddy fields in the plains of the northern Tohoku District.The effective application term on water saving irrigation generally comes after June 15 everywhere. Prior to that time the irrigation is feared to affect the growth of rice plant on account of high frequency of low temperature.According to our observation during the past ten years, the mean daily amount of the evapotranspiration in the paddy field was 3.1mm in June and 3.5mm in July. As the effective rainfall is considered to be 70 to 80 percent of each rainfall, the rainfall of 40mm and less during ten-day period will cause the paddy soil to be gradually dried up.The frequency of an effective no-rain-day (included a day with rainfall of 1.4mm and less) also acted as other indicator of drought. By checking the areas where the rainafell is less than 40mm in ten-day period and the no-rain-day frequency shows more than 70 percent, it was found out that drought visits the northeastern parts of the prefecture in mid June.Thus, the rainfall of ten-day period, 40mm, and the frequency of an effective no-rain-day, 70 percent, may be considered as the drying index on water saving irrigation.It may be concluded that the reading of the normals of the climatic data will tell us when and where water saving irrigation program will succeeded.