Abstract
This study is conducted to verify how the water quantity and quality of irrigation water for paddy fields be affected by the runoff EC flux where the watershed is small and the paddy fields are within the watershed. In order to verify this facts, a watershed which encompasses an area of 205 ha was selected and is categorized as small. The irrigation water quantity and quality at intake point of the paddy field are measured. The data regarding rainfall, runoff, and EC values of the outlet of the watershed are also recorded. Further, a phisycal model (WATER QUALITY TANK MODEL) of water quality is developed to verify how irrigation water quantity and its water quality affect runoff quantity and runoff pollutant load. The model is applicable for different land use and is tried to simulate the fluctuation of water quality in the present study. The results indicate the importance of paddy fields within that small watershed in reducing the runoff EC flux. Further, the model simulation results accord well with those observed at the outlet of the watershed. And the authors entered the data, which were taken by Suzuki et al., into the model and tried to estimate the discharge and water quality of irrigation water for a term of 1985. As the results, the estimated water irrigation quantity was just as same as today and EC value was slightly greater than that of today. Other important water quality parameters such as nitrogen and phosphorus are underway and will be presented next.
