Influence of saltwater wedges on irrigation water near a river estuary

Paddy fields irrigated using water from the Shinkawa River in Niigata, Japan, are subject to adverse effects associated with the occurrence of saltwater wedges in the river. The extent of saltwater intrusion in the river varies, depending on river discharge and tidal amplitude. In this study, field observations and numerical simulation of the Shinkawa River estuary were applied to ascertain the influence of saltwater wedges on paddy cultivation during the irrigation period. The saltwater wedge was surveyed using an echo-sounding profiling system (SC-3); results indicated that the longitudinal profiles of the saltwater wedge were 4.7 and 2.2 km from the river mouth sluice gate at the high water mark of spring tide and at the low water mark of neap tide, respectively. According to the vertical profiling using electrical conductivity (EC) meters at bridges crossing the river, there was a rigid interface between freshwater and saltwater with a thin halocline zone of approximately 0.25 m. These profiles were well reproduced by the one-dimensional, two-layer, unsteady flow model. The results of the model simulation and EC observation indicate that when the vertical position of the halocline rises to 1.2 m below the inlet of the water intake pumping station, the EC value rises sharply to 1,500 μs/cm, which is an unsuitable level for irrigation water. It is estimated that for about 23 % of the total irrigation period, pump operation is halted as a result of this mechanism.