Phosphorus Runoff Loads from a Group of Paddy Fields and a Forested Watershed in the Sengari Reservoir Basin

抄録

  In the Sengari reservoir which supplies domestic water to Kobe City, Japan, total phosphorus (TP) concentration remains higher than the prescribed environmental standard value. Since 88% of the reservoir basin is forested and 10% occupied by paddy fields, the study sites were set in a group of paddy fields with 7 plots and a forested area of 111 ha. Field observation was conducted to estimate TP loads emitted from the paddy fields and the forested watershed, measuring TP concentration and discharge in an agricultural drainage canal as well as in a mountain stream. Water samples were collected once a week during the irrigation season (from May to mid-September) and intensively collected during floods both in the irrigation and non-irrigation seasons. Data loggers were operated to record water depths with an interval of 10 minutes, to determine the discharges with relevant hydraulic methods. As a result of hydrological analysis, we found that less frequent heavy rainfall events cause significant intensification of TP loads both from the paddy fields and from the forested watershed. Based on our observation, half of the annual TP loads both from the paddy fields and the forested area were estimated to be produced in a few percentage points of time, which correspond to around 10 days. In addition, the amount of TP load from the paddy fields during the non-irrigation season was found to be almost the same as that during the irrigation season.