Water balance of low and flat paddy field was studied in the Muda Irrigation Project area in which plot-to-plot irrigation is practiced under low canal density of 10 m per ha.
　During the off season 1979, from 1st March to 20th August, a total water supply of 2,092 mm was observed. Water consumption during presaturation period before transplanting of 61 days was extremely high amounting to 973.8 mm, of which 765.7 mm or 79% was consumed as water loss that included evaporation of 353.7 mm and drained water of 412.0 mm. During the period from transplanting to maturing of 112 days, a total of 1118.2 mm was supplied by irrigation and precipitation. Water consumption was 614.6 mm for evapo-transpiration and 604.5 mm for drained water. Consequently, the existing water was decreased by 100.9 mm. An increase in irrigation canal density will be effective to reduce presaturation requirement. It is considered that major reason for the great loss amounting to 79% is the prolonged presaturation period due to difficulties of implementing separated irrigation between nursery beds and main fields and to intricate microtopography which obstructs water delivery. On the other hand daily water consumption of 10.0 mm during the growing period of rice plants in the present study is a reasonable value for ill-drained paddy fields.