In the present study, the distribution maps of climatic water balance components were provided by climatic year method. The water balance was evaluated by “bookkeeping procedures” for each year based on the Thornthwaite's system (1948) as shown in Table 1. The data used here were monthly precipitation amount and monthly mean temperature recorded at 321 stations over the northeastern part of Japan during the period 1967-1982.
The distribution of the mean annual total of potential evapotranspiration (PET) is shown in Figure 1. The local differences of PET can be seen among basins and plains. The amount of PET is greater than 720 mm in the Kanto Plain and the Echigo Plain, Central Japan, and less than 540 mm in the southeastern part of Hokkaido.
Figure 2, Figure 3 and Figure 4 show the distribution of mean annual water surplus, mean annual water deficit and mean annual number of dry months, respectively. The distribution pattern of water surplus (Fig. 2) shows orographic effects of mountain ranges facing the Japan Sea ; the large amounts are observed along the mountains and the small amounts are observed over basins and plains on the Pacific side of the mountains. As shown in Figure 3 and Figure 4, the distribution pattern of water deficit and dry month is almost the same. The dry month is defined here as the month in which the monthly potential evapotranspiration exceeds the monthly precipitation. The features of these distribution are more complicated than that of water surplus. Large amounts of water deficit are appeared in some basins and plains blocked by mountains to the south.
We would like to study the significance of water balance components as environmental elements in the future. In this study, an interpretation was given for the geographical distribution of beech trees (Fagus crenata) in Hokkaido. AOYAMA (1986b) showed that the PET indicates a similar thermal indicator for vegetation to Kira's warmth index (1945). The border between cool-temperate deciduous forest and subarctic coniferous forests runs along the western foot of the central mountains in Hokkaido, where annual amount of PET is about 560 mm. The northern limit of beech trees is located further south, in the Kuromatunai lowland, where the PET is about 600 mm. KIRA (1971 a, 1976) interpreted that the difference of location of these borders was caused by the advance of beech trees from south during th period since the last retreat of the ice. In addition to this interpretation, we can indicate climatic reasons to set the northern limit of the beech trees. At the root of the Oshima Penninsula, north of the Kuromatunai lowland, region with the amount of PET less than 560 mm spreads and reaches to the Pacific coast. Moreover, a large amount of water deficit is recognized along the coast of the Japan Sea. In general, the beech trees dominate in the cool-temperate deciduous forest under the condition of wetter maritime climate. Consequently, it can de considered that the thermal condition as indicated by PET on the coast of the Pacific side and the moisture condition as indicated by water deficit on the Japan Sea coast make a barrier for the advance of beech trees toward the north.
