A dissertation submitted in partial fulfilment of the requirements for the Degree of Doctor of Engineering, School of Engineering and Technology

Thesis (Ph.D.) - Asian Institute of Technology, 1986

There have been proposed 3 farm water management alternatives for supplemental irrigated lowland rice fields with a field-to-field water distribution to maximize effective rainfalls, which consequently minimize amounts of irrigation water and field runoff. All the alternatives involve a rotational supply with 7-days intervals and an adjustment of the irrigation water amount according to the amount of rainfalls of the past week. The allowable depth of stored water in the fields is 10 cm for the alternative 1, but is increased to 20 cm for the alternatives 2 and 3 which also need a modified water supply distribution over the fields. The water distribution is manually operated in the alternative 2, but is automatically operated with the help of a newly developed water level difference control device (cheaply and simply constructed) in the alternative 3. Results from simplified field experiments in the wet season of 1984 were encouraging, leading to real field experiments in the wet season of 1985. It can be concluded that with the proposed management alternative 1 the effective rainfalls will be in the range of 60-80% but they can be brought upto 100% for the other alternatives whereas the effective rainfalls under the present actual field water management are zero. Moreover, results from a simulation model indicate, under the conditions of the proposed management alternative 3, reductions of the amounts of irrigation water by 2/3 and field runoff by 4/5 of the expected present actual amounts. These results reveal a high potential for the farm water management improvements. In addition, the operation of the irrigation system in accordance with the proposed management alternatives is also presented.