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This study was conducted to evaluate the impact of land use system and N loadings to the environment estimated from N budgets on quality of stream water in Hokkaido, Japan. A case study was carried out in three towns of southern Hokkaido, which are Shiraoi, Yakumo, and Shizunai, characterized by intensive poultry farming (IPF), dairy cattle farming (DCF), and race horse farming (RHF), respectively. The estimation of N budgets using an N flow model indicated that the highest disposal N (880 Mg N yr−1) was generated in the IPF town and it resulted in 250 kg ha−1yr−1 surplus N in croplands. The disposal N was much lower in the DCF and the RHF town (102 and 71 Mg N yr−1, respectively) than that of the IPF town. Cropland surplus N in DCF town was 31 kg N ha−1yr−1, whereas RHF town had negative N balance. The linear regression analysis indicated that NO3-N concentration in stream water was significantly correlated with the proportion of upland field in drainage basins. The regression slopes varied among the towns, and it was the highest for IPF (0.040), intermediate for DCF (0.023) and the lowest for RHF town (0.006). The multiple regression analysis showed that regression slopes were significantly correlated (R2= 0.77 at 5% level) with livestock disposal N and cropland surplus N. Therefore, we assumed that these regression lines were the baselines for non-point source pollution, and the regression slopes were determined to act as impact factors of stream water quality. However, two sampling sites in the IPF area were scattered above the baseline. This fact strongly suggests that the area was affected by point source pollution.

River water quality was evaluated with respect to eutrophication and land use during spring snowmelt and summer base flow periods in Abashiri (mixed cropland-livestock farming) and Okoppe (grassland-based dairy cattle farming), eastern Hokkaido, Japan. Water from rivers and tributaries was sampled during snowmelt and summer base flow periods in 2005, and river flow was measured. Total N (TN), NO 3 -N, and Si concentrations were determined using standard methods. Total catchment and upland areas for each sampling site were determined with ArcGIS hydrology modeling software and 1:25,000-scale digital topographic maps. Specific discharge was significantly higher during snowmelt than during base flow. In both areas, TN concentrations increased, whereas Si concentrations decreased, with increased specific discharge, and were significantly higher during snowmelt. The Si:TN mole ratio decreased to below or close to the threshold value for eutrophication (2.7) in one-third of sites during snowmelt. River NO 3 -N concentrations during base flow were significantly and positively correlated with the proportion of upland fields in the catchment in both the Abashiri ( r =0.88, P <0.001) and Okoppe ( r =0.43, P <0.01) areas. However, the regression slope, defined as the impact factor (IF) of water quality, was much higher in Abashiri (0.025) than in Okoppe (0.0094). The correlations were also significantly positive during snowmelt in both areas, but IF was four to eight times higher during snowmelt than during base flow. Higher discharge of N from upland fields and grasslands during snowmelt and the resulting eutrophication in estuaries suggest that nutrient discharge during snowmelt should be taken into account when assessing and monitoring the annual loss of nutrients from agricultural fields.