Abstract
To contribute to the prediction of rainfall-related disasters, specific concentration factors that indicate the suspended solid (SS), total nitrogen (TN), and total phosphorus (TP) load intensities to river water for each land-use type were calculated using a simple watershed land-use model across Japan by applying the following multiple regression equation, according to the land-use ratios and published SS, TN and TP data.
 
 C=i=14aixi 
 
 C: SS, TN and TP concentrations (mg L-1); ai: SS, TN and TP specific concentration factor for land use i; xi: ratio of land use i; land use: 1 paddy fields, 2 upland fields, 3 forests, 4 urban areas.
 
 The land-use ratios for watersheds, whose lower ends were observation points of river water quality, were determined by the GIS technique using a published database of DEM and LULC mesh data. The SS specific concentration factor was 15.4 (from a 95% lower limit value of 12.0 to a 95% upper limit value of 18.8), 11.5 (7.4 to 15.6), 3.9 (2.6 to 5.1), and 11.2 (9.2 to 13.2) for paddy fields, upland fields, forests and urban areas, respectively (n=5103). The TN specific concentration factor was 1.67 (from a 95% lower limit value of 1.34 to a 95% upper limit value of 2.01), 4.08 (3.64 to 4.51), 0.76 (0.67 to 0.90), and 3.57 (3.38 to 3.76) for paddy fields, upland fields, forests and urban areas, respectively (n=3256). The TP specific concentration factor was 0.146 (from a 95% lower limit value of 0.119 to a 95% upper limit value of 0.172), 0.172 (0.138 to 0.206), 0.044 (0.033 to 0.055), and 0.267 (0.253 to 0.282) for paddy fields, upland fields, forests and urban areas, respectively (n=3256). These specific concentration factors had regional tendencies, such as suburban or rural, intensive or extensive agriculture, and so on.
Highlights
Increased nitrate in drinking water, and the eutrophication process by which a body of water becomes enriched in dissolved nutrients that stimulate the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen are all major environmental issues (Cooper 1993; Kumazawa 2002; Nagumo et al 2004)
We previously developed a simple model to estimate nutrient concentrations of river water based on the land-use ratios of watersheds and evaluated the impacts of land-use type on the total nitrogen (TN) and total phosphorus (TP) in river water, and the NO3-N concentration in shallow groundwater in Kagawa Prefecture. (Mochizuki et al.. 2013; Yoshikawa et al 2015)
The result for the TN specific concentration factors for forests tended to be large in high population density areas compared with low population density areas (Fig. 7c), which agrees with the findings of Itoh (2004), who interpreted the reason to be the deposition of nitrogen compounds from urban areas
Summary
Increased nitrate in drinking water, and the eutrophication process by which a body of water becomes enriched in dissolved nutrients that stimulate the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen are all major environmental issues (Cooper 1993; Kumazawa 2002; Nagumo et al 2004). Hojito et al (2003) estimated the nitrogen loading and nitrogen concentration in discharged water for each prefecture by assuming that surplus nitrogen in cropland soil will readily dissolve in water percolating through the soil They considered fertilizer and manure nitrogen and absorbed nitrogen by crops and nitrogen in rainfall, nitrogen fixation, nitrogen deposition and denitrification.
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