Sadao Nagasaka

Characteristics of effluent load from a watershed including irrigation ponds

This paper describes an attempt to estimate the mass budget of irrigation ponds within a watershed and the possibilities to control the effluent load of nutrients from the watershed located in the Kyotanabe area. The paddy field lots were irrigated by the ponds and several mountain streams. In the study watershed, there were two ponds located on the upper and lower sides of a paddy area. Water could be pumped up from the lower pond to the upper pond as necessary. At the ponds, the total amount of nitrogen and phosphorus in the inflow loads including the sediment release was larger than those in the outflow loads. In SS, the effect of load reduction in the ponds was high. It is assumed that organic load reduction was not expected in the ponds. The study result indicates that it is possible to remove the nutrients in the ponds when a larger amount of water is pumped up than when the ponds are only in irrigation use. Moreover, the removal capacities of the nutrients could probably be increased in both ponds by controlling the amount of sediment releases.

Improvement of Leaching Methods for Acid Sulfate Soil

The area of acid sulfate soil (ASS) is distributed on a coastal plain in Southeast Asia. If improved, the area is useful for agriculture. One of major improvement methods of ASS is leaching with a large amount of fresh water. But in some case, it may be difficult to secure enough water. In this study, column experiments and a solute transport model, Hydrus-1D, were used for suggesting the most suitable reclamation method using minimum water. In column experiments, efficiencies of leaching, percolation and inundation, were compared. In addition, soil water quality in the field was measured at Banna Experimental Station in Thailand for estimating the difference between the result of the laboratory and field experiments. In this survey, soil water was collected from the area treated by an amendment and from the area without amendment. As a result, the sulfate leaching efficiency by percolation was better than that by inundation. From model calculation, sulfate transport was also estimated to be leached more efficienctly by percolation. A survey of soil water quality in the field showed similar results from the column experiment of using the percolation method.