Abstract
Nitrogen concentration of stream waters are investigated in 13 small watersheds. Table 1 shows the land use and pig density of each watersheds. NO3-N concentrations are quite different in watersheds, maximum value is 19.7 mg/Z, and minimum value is 0.7 mg/Z (Table 2). It depends on land use and especially on pig density (Fig. 3). NO3-N concentrations measured in ricegrowing season are lower than the values measured in non-ricegrowing season. During rice-growing season the water flowed out of upland area is almost irrigated and flooded within paddy fields in lowland area. Then nitrate caused from chemical fertilizer and pig slurry in upland is removed by both denitrification and aqua-plant uptake in the paddy. The nitrogen outflow model with nitrogen removal function of paddy fields were proposed in previous paper. Measured values are compared with the values calculated by this model. For non-ricegrowing season (Fig. 5), and for rice-growing season (Fig. 7), the calculated values are nearly equal to measured values. Large difference is appeared in watershed L, where lots of pigs were fed in the past. It means that large amount of nitrogen flowed out of the pig farms in the past remains in the subsoil and groundwater.
