The author estimated the soil moisture at Owada, from the potential evapotranspiration, the function of the air temperature. and the amount of precipitation, by Thornthwaite's method.
Fig. 1 shows an example of soil moisture, observed by drying method and estimated from climatological data. Observed data show the observed soil moisture of the bared land, and estimated data show the estimated soil moisture of the land with vegetation. In winter, estimated soil moisture are nearly 95-100%, but observed soil moisture change remarkably. In summer, as a general rule, both soil moisture change similarly way. Only in midsummer (July and August), the estimated data are smaller than the observed data. Because in this season, the transpiration by vegetation is very large, and the evaporation from the bared land is prevented by the mulch of dried soil.
Fig. 2 shows the seasonal change of the soil moisture, Fig. 3 shows the variability of the soil moisture, estimated from potential evapotranspiration. The variability or the range of soil moisture variation, is in maximum, 100%, and the mean value is very small in July, August and September. Because in this season drought occur very often, and sometimes, soil is saturated with water, as it rains heavily. In the last decade of June, as it rains heavily, so the mean of soil moisture is large and the range of variation is small. In winter, mean value is very large and range of variation is very small, because the soil moisture is always nearly saturated with water, as the transpiration by vegetation is very small in this season.