The amount of seepage water under the levee was obtained by means of water balance and hydraulic calculation methods, and the two results were compared. As an example, investigation was carried out at the site of Project II, newly reclaimed area located in the southern part of the Hachirogata Lagoon, Tohoku District, Japan. It is 187 ha in area and consists of sandy soil. The reclaimed land is usually surrounded by outside water level which is higher than inside water level.
Water balance in such case of swampy lowland is generally expressed by the following equation :
P= (D₂-D₁) +E-G₁+ΔS
Where, P : precipitation, D₁; inflow of surface water, D₂ : amount of water pumped out, E : evapotranspiration, G₁ : amount of seepage water under the levee, ΔS : amount of change in water valume.
During the non-irrigation period, the inflow of surface water (D₁) equals zero. P, E and D₂ can be obtained by measurement and pump operation records. ΔS can be obtained from the change of ground-water level. The amount of seepage water under the levee (G₁), therefore, can be obtained indirectly by using the above equation. During the irrigation period, G₁ is presumed by taking in account the difference between the inside and outside water levels of the levee, basing on G₁ in non-irrigation period. G₁ in this period can be checked from the above equation by substituting D₁ obtained by measuring the water duty per unit area.
On the other hand, G₁ is obtained by hydraulic calculation. The value of G₁ in this method was confirmed to accord with that in the water balance method. In the case of hydraulic calculation, however, the permeability coefficient is obtained by using G₁ in the water balance method.
The estimation of the amount of seepage water under the levee is very difficult. In the planning of drainage for swampy lowlands and reclaimed lands, however, it is required to estimate the amount of seepage water under the levee. From the above result, it is proved that the method in which the water balance and hydraulic calculation methods are combined can be usefully employed in that case.

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