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Prior to the hydrological calculation, the initial conditions of water distribution in the watershed were estimated by steady-state calculation under a constant condition. First, all of the grid points except the atmosphere and surface were assumed to be filled with water. Then, calculations for 10,000 days under the assumption of uniform precipitation and evapotranspiration (3.26 mm/day in precipitation and 2.18 mm/day in evapotranspiration) were used considering the annual average precipitation, temperature, day length in the area, and Thornthwaite's equation. Running the model for 10,000 days resulted in a steady state that reflected the proper starting conditions for the predictive simulation. In this initialization, the water budget in paddy fields was calculated by the SCS-CN method. The initial parameters shown in Tables 1 and 2 were used in this initialization.

Table 1

The Manning's roughness coefficients used in the model

Manning's roughness coefficient [–]
Land useUsed for the initializationUsed for the calibration and validationLiterature valuesa
River and lake 0.035 0.035 0.035 
Waste land 0.1 0.1 0.1 
Paddy 0.6 – 0.6 
Other crop field 0.2 0.2 0.2 
Forest 0.4 0.4 0.4 
Golf course 0.4 0.4 – 
Transport 0.05 0.05 0.05 
Other 0.1 0.1 0.1 
Urban development 0.05 0.05 0.05 
Manning's roughness coefficient [–]
Land useUsed for the initializationUsed for the calibration and validationLiterature valuesa
River and lake 0.035 0.035 0.035 
Waste land 0.1 0.1 0.1 
Paddy 0.6 – 0.6 
Other crop field 0.2 0.2 0.2 
Forest 0.4 0.4 0.4 
Golf course 0.4 0.4 – 
Transport 0.05 0.05 0.05 
Other 0.1 0.1 0.1 
Urban development 0.05 0.05 0.05 

aNILIM (2006).

Table 2

Values of permeability and porosity

Permeability (mDarcy)
Porosity (m3/m3)
LayerInitialCalibratedLiterature valuesaInitialCalibratedLiterature values
Topsoil (II) 10,000 5,000 10,000b/5,000c,i 0.4 0.4 0.4b 
Alluvium (IV-1) 1,000 10,000 100–1,000/10,000b–90,000d 0.2 0.17 0.2b 
Loam (IV-2) 1,000 1,000 100–1,000b,j 0.2 0.2 0.2b,j 
Clay (IV-3) 100 0.287 1–10b,k/0.01–0.1e,k/–100f,l 0.2 0.2 0.2b,k 
Layer A (V-1) 10,000 25,000 10–10,000b,m/100–100–100,000f,h 0.2 0.2 0.1–0.2b,m 
Layer B (V-1) 100 200 100b,o/20–10,000g,o 0.2 0.05 0.2b,n/0.15h,o/0.05–0.10h,p 
Layer C (VII) 0.1–1b,q 0.5 0.05 0.05b,q 
Permeability (mDarcy)
Porosity (m3/m3)
LayerInitialCalibratedLiterature valuesaInitialCalibratedLiterature values
Topsoil (II) 10,000 5,000 10,000b/5,000c,i 0.4 0.4 0.4b 
Alluvium (IV-1) 1,000 10,000 100–1,000/10,000b–90,000d 0.2 0.17 0.2b 
Loam (IV-2) 1,000 1,000 100–1,000b,j 0.2 0.2 0.2b,j 
Clay (IV-3) 100 0.287 1–10b,k/0.01–0.1e,k/–100f,l 0.2 0.2 0.2b,k 
Layer A (V-1) 10,000 25,000 10–10,000b,m/100–100–100,000f,h 0.2 0.2 0.1–0.2b,m 
Layer B (V-1) 100 200 100b,o/20–10,000g,o 0.2 0.05 0.2b,n/0.15h,o/0.05–0.10h,p 
Layer C (VII) 0.1–1b,q 0.5 0.05 0.05b,q 

aLiterature values are described in the dimensions of [L · T–1], and converted to [mDarcy] under the assumption of 1 Darcy = 0.01 cm/s.

bGeosphere Environmental Technology (2006).

cTsutsumi et al. (2005).

dOchiai (1968).

eYasuhara et al. (1991).

fHiguchi et al. (2004).

gJAEA (2008).

hNILIM (2006).

iValues of sandy topsoil.

jValues of the Kanto Loam Layer.

kValues of the Jysoso Clay Layer.

lExtrapolated range of value for clay by a regression formula based on various sizes of soils.

mValues of the Shimousa Layer.

nExtrapolated range of value for sand, fine sand, very fine sand and silt by a regression formula based on various sizes of soils.

oValues of the Kazusa Layer.

pValues of mud layer and clay layer.

qValues of the Bedlock.

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