The annual water balance in the Shinjo Basin was calculated by a climatological method. The Shinjo Basin, which is 205km2 in area, is situated in the northern part of Yamagata Prefecture (Fig. 1) and receives abundant snowfall in winter. The drainage from this basin flows into the Sake River which is a tributary of the M ogami River.
It is said that the data of water balance components (precipitation, runoff, evaporation) of a drainage basin may contain considerable errors and that the accuracy will get worse in the order of precipitation, evaporation and runoff. So the author wanted to estimate runoff from precipitation and evaporation. Empirically, 60_??_70% of a small pan evaporation value are regarded as the actual evapotranspiration of the area. The author adopted 65% of this value in this calculation.
On the other hand, the author calculated evaporanspiration from the drainage basin by using the Penman's method for potential evaporation (Table 5). In the case of the Shinjo Basin, the actual evapotranspiration was obtained by multipling the Penman's value by the factor of 0.80 (Fig. 5). The result of water balance obtained from mean value for nine years are as follows; precipitation: 2220mm/year, evapotranspiration: 568mm/year, runoff: 1652mm/year, runoff ratio: 0.74.
Compared with the observed values in the Kamabuchi Experimental Watershed which lies about 15km to the north of this basin, the calculated ones for the Shinjo Basin shows close agreement. Consequently, the calculated runoff from precipitation and evaporation can be considered as appropriate for the Shinjo Basin.
