climatological conditions were investigated by the year climatic consideration of water balance in Yamagata prefecture, Northeast Japan. The water balance was evaluated by “bookkeeping procedures” based on the Thornthwaite's potential evapotranspiration. The data used here were monthly precipitation amount and monthly mean temperature recorded at 31 meteorological or AMeDAS stations in Yamagata and the surrounding prefectures for sixteen years 1967-1982 (Fig. 1).<br>The amount of the potential evapotranspiration was 700-720mm in the Shonai plain, 680mm-700mm in the Yamagata and Yonezawa basins and about 650mm in the Shinjo basin (Fig. 2). Along the border between the Shinjo basin and the other two regions, the potential evapotranspiration becomes 660mm-680mm.<br>According to the warmth index by Kira (1945), the Shinjo basin was classified into the cool temperate and the other two into the warm temperate zone. These character of temperature regions were also supported by sporadic distribution of vegitaton such as laurel forest (Machilus thunbergii, Quercus salicina) in the Shonai plain and Yamagata basin and the deciduous broadleaved forest (Fagus crenata) in the Shinjo basin. Moreover, it was revealed by the important relatioship between the potential evapotranspiration and paddy rice production that the 670mm of potential evapotranspiration is significant for the paddy rice cultivation (Fig. 6). From the energy-budget equation, the about 50W/m² of the net radiation can be calculated by using the 670mm of potential evapotranspiration and the zero of Bowen ratio.<br>The number of dry months, which the amount of the potential evapotranspiration exeeds that of the monthly precipitation, is about 1.5 in the Shonai plain and Shinjo basin and reaches 2.5-3.0 in the Yamagata and Yonezawa basins (Fig. 3). The distribution of the maximum values of water deficit for the sixteen years (1967-1982) shows almost the same pattern with that of the number of dry months; it is about 50mm in the Shonai plain and Shinjo basin and reaches above 100mm-200mm in the Yamagata and Yonezawa basins (Fig. 4).<br>At Yamagata the dry months occurred in April to October and most frequently in June and August (Table 3). On the occasion of water deficiency, however, the occurrence frequency of the dry month increases on the whole and specially in July (Table 4). This suggests the significance of the precipitation in the “Baiu Season”, one of the rainy seasons in Japan, for the occurrence of water deficit. Water deficiency occurred thirty six times for ninety-two years (1891-1982) and tends to continue about two years. Taking this tendency into account, it can be said that a drought with water deficiency comes on the average every three to four years. The frequency and the amount of water deficiency tend to increase in the period when the sunspot numbers change from the maximum to the minimum, especially in one to two years after the maximum of sunspot number (Table 2).<br>It can be also considered that the condition of water balance as described above influenced on the distribution of Quercus-forest. The Quercus serrata community is typical of the secondary forest in the low land. This forest shifts into the Quercus-mongolica var. grosseserrata community or Fagus crenata-Quercus mongolica var. grosseserrata community, which is typical of the montane secondary forest, as increasing an altitude. In general, the altitude of this boundary decreases gradually toward north. In Yamagata prefecture, this boundary runs 400m-600m of altitude in the Yamagata and Yonezawa basins, 300m or so in the Shinj