Abstract
The survey on the water management has been carried out within a small area (7. 6 ha) which is irrigated by a small reservoir called “Osawa Shin-ike”. This area is included in the “Toban irrigation project” area, where three main dams and a considerable length of waterways are now being constructied in order to supply irrigation water.
The characteristics of water management in the area derived from the survey are as follows.
(1) Average release of the reservoir is 4.5mm deep each time throughout the irrigation period. Water is released every day except on rainy days from the start of irrigation to the end of August, and in September intermittent releases are carried out.
(2) Water level in the paddy fields are variable. The highest level is 7 cm, which is far under the crest of the drainage ditch. This fact shows there is no waste of water due to a spillover.
(3) Paddy fields are supplied with water usually by a rotation interval of two days, but the interval is changeable after “nakabosi” drainage because of difference in the demand of each field.
(4) A waterman supplies water to each field within two hours at a time.
(5) The supplied water quantity varies dependent on the differing water requirements, but the submerged duration in each field is kept constant.
(6) Upper and middle portion of the area is supplied with water of the “Shin-ike” release at 75% of total supply, and lower portion gets 20%. The weigh mean ratio of reused water is 45. 3% within the area.
(7) Average utilization rate of rainfall (utilized rainfall/supplied water) is about 8%.
