It is very important to clear up relations between variation of groundwater and soil mass deformation in landslide disaster, so far as the mechanism is elucidated. As an existing condition of groundwater in landslide area is very complex, there are some fairly difficult problems to make clear the relation between the existing condition of groundwater and the disaster.
Judging from conditions of eruption or loss circulation of water during bore hole drilling and from geological condition and fissure condition in bore holes in landslide area, it seems that some layers with groundwater exist in landslide area. And it is conjectured that each groundwater shows different behavior and gives different effects to the deformation of soil mass.
So we carried out investigations emphasizing on the behavior of each groundwater in one landslide area in the Rokko mountainous district. The landslide area has hydrogeologically some specific characters in the geological composition.
As the results, the followings are shown.
There are roughly four types groundwater, that is,1. shallow residental groundwater in colluvial soil layer (A); 2. shallow flowing groundwater in colluvial soil layer (A'); 3. middle groundwater in the Kobe group (B); and 4. deep groundwater in granitic rocks (C).
The existing conditions of these groundwaters are as follows: A is free groundwater; A' is free groundwater and artesian water under some conditions; B is free groundwater and partially artesian water under some conditions; and C is artesian water.
Supply sources of each groundwaters are as follows: A is infiltrating water of rainfall; A' is infiltrating water of rainfall and deep groundwater; B is infiltrating water of rainfall and deep groundwater; and C is infiltrating water of rainfall coming from caprock of granite.

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The groundwater in Hohu plain is recharged by the seepage from Saba river, creeks and paddy fields. The inflow to and the outflow from the groundwater are balanced in a short period, but for a long period, the level is decreasing by several centimeters a year during the last fifteen years. It brings about gradually the sea-water intrusion to the fresh groundwater.
Flow route of the groundwater can be detected by the head and the temperature distributions, and by 1 m depth ground temperature survey.
The main large wells in the area penetrate the lower aquifer which is separated by an aquiclude from the surface zone, so the vertical flow must be considered to be dominant for the whole groundwater flow system. The analysis of the flow and the balance of the groundwater, using quasi-three dimensional model for underground structure, results that the total recharge is 67.5× 10⁶ m³/year,60% of which,39.9 × 10⁶m³/year, is discharged from the wells, and the rest flows to the sea.

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