Table 3 shows the results. The ratios of runoff and penetration were about 70–80% and 20–30%, respectively, in the grids analyzed here, indicating that the surface runoff is two to three times larger than the penetration within the grids of 70,000–80,000 in the area located at Mt. Tsukuba under the wet conditions assumed in this study. On the other hand, there are about 10 grids from the top of Mt. Tsukuba to the river (Figure 16). It is a very simple calculation under the assumption that only a straight line flow from the top of the mountain to the river occurred, but 3% (i.e. 0.7¹⁰) of water precipitated at the top of Mt. Tsukuba is estimated to reach the river directly by surface runoff. Conversely, 97% of water precipitated at the top of Mt. Tsukuba penetrates into the subsurface zone at least once.