ON THE ANNUAL RUNOFF RATIO OF JAPANESE RIVERS

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The annual runoff ratio of Japanese rivers is discussed from the standpoint of water balance, Using available data on precipitation and runoff, the authors calculated yearly ann-ual water balance for eight drainage basins which seem to have relatively favorable condition among Japanese rivers for this kind of calculation. Locations of these basins are shown in Figure 3 by dotted line. The results are plotted in Figure 1 together with the results of three experimental basins, i. e. the Manna River, Takashima and Kamabuchi Forest Experime-ntal Watersheds. In Figure 1, the annual runoff ratio is expressed by broken lines and P, R, and E represent annual precipitation, annual runoff, and annual evapotranspiration respectively. In constructing the figure, we expected that if both accuracy and representativeness of the data are good enough, the plotted points would distribute within a narrow band nearly parallel to 45°-lines which indicate annual evapotranspiration. If not so, the result may not be considered to be reliable. By this crosschecking method we concluded that the results of six drainage basins except the Gono River are erroneous, because points are distributing beyond the range of climatologically acceptable amount of evapotranspiration. The distribution of annual actual evapotranspiration in Japan calculated by the Thornthwaite's method based on climatological stations mostly located on the flat land is shown in Figure 3. As Japan is mountainous in relief, it is reasonable to think that the actual evapotranspiration from the mountainous region is much lower than the values of Figure 3. One example is shown in Figure 4 for a mou-ntainous area in northeastern Honshu. Values in Figure 4 are based on the estimated temp-erature at each mesh point and location of the figure is indicated by a rectangle in Figure 3. Relation between the annual precipitation and the annual evapotranspiration for four drainage basins whose data are found to be reliable is shown in Figure 2. As a general trend, the annual evapotranspiration decreases either with increasing or decreasing annual precipitation. Thus, under the Japanese climate, an optimal value of annual precipitation for maximizing the annual evapotranspiration seems to exist within the range from 1, 500mm to 2, 000mm. It is interesting that the annual precipitation of Japan is found within this optimal range. Water balance data of experimental basins are also collected (Table 2 and Figure . 5) and the evapotranspiration losses from these basins are compared with the value of Figure 3. As the result, it is found that the former is larger than the latter for drainage basins in flat or hilly regions and smaller for those in mountainous regions. The annual actual evapotranspir-ation in Japan seems to change from 400mm to 1, 000mm depending on climatological conditions. According to the results described above, high value of annual runoff ratio for Japanese rivers reported so far is not acceptable and re-investigation is recommended especially for rivers whose annual runoff ratio is reported to be more than 90%. Obtained results may be important for future water resources development and utilization.
The Association of Japanese Geographersの論文

ON THE ANNUAL RUNOFF RATIO OF JAPANESE RIVERS

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概要

The Association of Japanese Geographers | 論文

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