A Case Study of the Water Management of Paddy Field and Irrigation Pond after Farmland Consolidation in the Terraced Paddy Field Area

Abstract

Farmland consolidation was carried out in the terrace paddy field areas in Ishikawa prefecture, and the production basis was greatly improved. Water management investigation was carried out in this region, and the situation after the improvement was examined. The results obtained are as follows: 1) Almost three years were needed after the farmland consolidation for all beneficiaries to have a confirmation that the consolidation had improved the conditions of water utilization. In 1999, the total water supply for the paddy field during the irrigation season was 910mm, 46% of which was from irrigation ponds and 54% from rainfall. The ratio of available rainfall was 77%. 2) Irrigation pond managers carried out supply management following the standard measurement of supply discharge indicated by the levels of the storage set for each Irrigation pond. They had to watch the timing of optimal supply discharges for 24 hours, which was as hard a labor as had been required before the consolidation. 3) The storage rate of the irrigation pond kept decreasing from the start of irrigation. A critical phase would be caused if the storage rate went down to 50% or less by the middle of June (the beginning of intermediate drainage from the paddy field). In order for irrigation ponds to be filled and to maintain storage discharge after the intermediate drainage, the simulation result indicated that about 700mm of rainfall, which corresponds to a period of four return years, would be needed from the beginning of irrigation until the end of the rainy season.