Niigataryo district is adjacent to Tokyo and consists of 4, 233 ha of low swampy land. It is located in the lower reaches of Nakagawa River in Saitama Prefecture. The district is surrounded by rivers, and the river beds are comparatively high. Therefore, a fair quantity of seepage water under levees is observed in non-irrigation period, and the quantity increases in irrigation period owing to artificial heightening of the water level of surrounding rivers. The water balance is shown by the following formula :
P= (
O-I) +
E+ (
G2-
G1) +Δ
S where P : precipitation,
O : runoff,
I : inflow of surface water,
E : evaporation from the district,
G2 : outflow of ground water (
G2 is negligible because of the low level in the district),
G1 : inflow of groundwater, ΔS : amount of change in water volume in the district. The results of the investigation are as follows.
(1) Non-irrigation period in 1952 and.1953.
i)
I=0,
Qb =
G1-Δ
S,
E=
P-O+
Qb, where
Qb : base flow separated from runoff curve.
E is the only unknown factor, as
Qb is known. Therefore,
G1 can be calculated.
ii) The mean value of
E obtained by the above formula is 1.7mm/day, and the mean value of
G1 (mean depth of 4, 233 ha) is 0.3-0.4mm/day.
(2) Irrigation period in 1954, 1955, 1959 and 1960.
i) The values of
E and
G1 are unknown, but E can be approximately presumed from the value of
Ep as fo llows :
E=
Ep× α where
Ep : evaporation from evaporimeter, α : a constant (0.9-1.2) characterizing each 10 days of irrigation period.
The value of
E obtained for the district is 3.6-4.0 mm/day.
ii) The caluculated value of
G1 is 2.0-2.5 mm/day, and 40-50
l/sec per kilometer of levee length.
(3) The value of G
1 correspond to the gradient of ground water level under levees.
(4) The results of caluculation in the district proved to be applicable in other districts of similar conditions.
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