(1) Lake Tairo (Tairo-Ike) is a crater lake locating in the southern part of Miyake Island (Miyake Jima) which situates about 200 km south of Tokyo. As the intake amount for waterworks from the lake and its surrounding groundwater increased in recent years, the subsidence of lake water level brought many serious problems. Thus, the decision of the limit of intake amount was needed.<br> The lake was created by volcanic eruption about 2, 000 years before, which was estimated by ¹⁴C dating of carbonate woods obtained under scoria or tuff breccia around the crater. The lake is completely occluded, i.e., it lacks in and out-flow rivers. It is assumed that exchange of water masses may be made through groundwater flow.<br> Bathymetric map of the lake and surface geology near it is shown expressed in Figs. 2 and 3. Water level of the lake in normal condition is about 3 meters above sea level. Therefore, the deep layer of the lake lies under the sea level. The lake surface receives a slight influence of tidal motion up to 2cm, but seawater intrusion is not observed. Some of the results of limnological observations are shown in Fig. 4. High value in electric conductivity in summer is the effect of inflow of abnormal groundwater.<br> (2) Hydrological observations were made at many stations in the island, including permeability of soil, amount and water quality of springs, soil moisture, occasional river runoff during and after the rainfall and lake level. Locations of the observations are shown in Fig. 1.<br> Time variation of spring discharge is considerably small. Small variation in lake level means that the lake can tolerate large amount of intake and the discharge is compensated by groundwater inflow.<br> Local difference in rainfall is extremely large. For example, annual totals for Stations 1, 5 and 6 were about 3, 200mm/y, but it was more than 4, 200mm/y at St. 7 in 1972.<br> Results of the hydrological observations are summerized in Fig. 5 in five days' mean or total. Change in water level roughly corresponds to that of rainfall, but it is largely affected by the amount of intake.<br> (3) It was assumed from the above observations that the lake water would be a part of main groundwater system of the island flowing towards the sea. Time variations of spring discharge and main groundwater flow are considerably small as mentioned above, as the result of averaging mechanism through the groundwater compartment.<br> Climatological water balance for Miyake Island is illustrated in Table 1. Since annual mean precipitation is about 3, 000 mm/y and evapotranspiration is about 900mm/y, the excess of water is about 2, 100mm/y. The lack of persistent river flow in the island means that the outflow of excess water takes in the form of coastal and under-sea springs.<br> (4) Water balance relations for the lake are expressed by Eqs. 1, 7, 8 and 9, in which V means volume of water in the lake basin, Q_in is the inflow, Q_out is the outflow, W is the intake amount for waterworks, E is evaporation from lake surface, S₀ is the surface area of Tairo-Ike, S is total catchment area of the lake and P means precipitation amount. Evaporation (E_w) and evapotranspiration (E_S) were estimated by Penman's method descibed by Eq. 4. The coefficient f was decided as 0.7.<br> Two main factors in these equations (Q_in and Q_out) cannot be measured directly in this case. To obtain the amount of groundwater inflow (Q_in), the result of annual mean water balance (Eq. 7) and the discharge from spring (Miike Spring, St. 2 in Fig. 1) were used. The amount of discharge was convereted into runoff height by use of Eq. 3, where SM means catchment area for Miike Spring.