2008 Volume 24 Issue 1 Pages 65-76
Total outflows of suspended solids (SS) and nutritive salts such as phosphorus from farmland as well as residential and industrial sectors are important parameters that have recently received more consideration in monitoring and managing the water quality of Japan’s rural watersheds. However, the pollution source is generally hard to specify because the source is spread widely across farming areas. To evaluate the environmental impact of agricultural activities on the water quality and resolve any problems, it is necessary to quantitatively measure the SS and nutritive salt concentrations of river water in conjunction with the dynamics of land use in the basin. This preliminary study used satellite images to estimate the acreage of major crops, and related the agricultural land use to the SS and total phosphorus (TP) concentrations and the river flow to develop a simple model for estimating the SS and TP loads per unit paddy area. We periodically measured the SS and TP concentrations and flow rates (m3/s) in four medium or small rivers located in Aichi Prefecture in central Japan. The measurements were carried out in May when puddling and rice transplanting are actively under way in the study area. Measurements were only taken when no heavy rain was recorded on the sampling day and previous day. The total area of the river basins in this study was approximately 3600 ha. We estimated the cultivated areas of paddy rice and winter wheat in each river basin during the study period using SPOT and RADARSAT images. The estimated average SS load per unit paddy area of the whole study area was 462 kg/ha in May 2005. The TP and SS loads were highly correlated (r=0.97, p<0.01). Due to the large variation in estimated SS loads (304 kg/ha – 710 kg/ha) between river basins adjacent to each other, further investigation is required to examine the relationship between the SS load and changes in land use in the area, such as recent widespread early-season cultivation and direct seeding of rice.
