Abstract
There are widely distributed paddy fields along Lake Kasumigaura. In many of these areas, irrigation drainage systems have been used to discharge water to the lake at night and to irrigate the fields with lake water in the daytime. In 2009, cyclic irrigation was introduced in which discharged water was stored in a bank-based water canal at night and was irrigated to the paddy fields again in the day. In 2010, cyclic irrigation was carried out after sediment was dredged in the bank-based water canal. The load reduction effect of this cyclic irrigation was examined by comparing it with the early method of irrigation used in 2008. Results of the comparison showed that on days in which the weather was good, the load to the lake could be managed by cyclic irrigation. However, during periods of rainfall, the load to the lake increased. It was observed that an accumulation of load in the bank-based water canal was discharged to the lake. Therefore, when sediment was dredged in the canal, the load to the lake decreased. In particular, the total phosphorus (T-P) load decreased greatly. The net effluent load of T-P was - 0.3 g·ha- 1·d- 1 before dredging, but it was - 6.4 g·ha- 1·d- 1 after dredging.
