The author computed the potential evapotranspirations (PE) and drew the distribution map of water duties of field irrigation in Japan by applying the results on his papers, Studies on the Potential Evapotranspiration I through IV.
Our country lies between Latitude 45 (Wakkanai, Hokkaido Island) and 31 (Kagoshima, Kyushu Island) degrees north. The author's experimental formula of PE on the pasture e=C₂t. can be applied in the area south of Latitude 40 degrees north, so we computed PE in the area of Latitude 40 to 45 degrees north by Thornthwaite' s method. The computed PE is shown in Table 1. The meteorological data used in the computations are the mean values of the years between 1921 and 1950.
By applying the idea that the yield and its quality of crop is the best when the soil moisture in the root zone is kept at the field capacity, we can obtain the net water requirement in the irrigation project by computations.. Drought occurs in Japan usually in the period between the later part of July and August. We computed the lack water content in each August of the thirty years in the statistical years mentioned above and set the third largest year to be the standard year of the irrigation project. In this case, we assumed that the effective rainfall is a half of the actual rainfall.
.We set the effective soil moisture in the root zone between the wilting point and the field capacity into three stages of 100, 80 and 60 millimeters, and irrigate to the full extent of the field capacity when the available moisture decreased to a half of the effective moisture. We computed the minimum frequency of irrigation between July 1 and August 31 in the standard year as shown in Table 5.
.The required net water requirements are obtained by dividing applied water content per once by the minimum frequency of irrigation. The distribution map of the net water requirement of pasture is shown in Fig. 3. The net water requirements of the other crops are obtained by multiplying the figure in Fig. 3 to the evapotranspiration coefficients of the crops.

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The numerous variances of percolation in paddy fields are reported in this paper, and studies on these were made to approach an effective method that would reduce the percolation through paddy fields of volcanic ash soil.
These studies have distinctly revealed an extream inequality of percolation intensity within the paddy field of volcanic ash soil. This indicates that the functions of the reclamation methods commonly practiced are only limited to transformation of surface, totally leaving the treatment of sub-soil intact. Therefore, this means only a work of outward reformation.
On the other hand, dressing of bentonite can not be expected to effectively reduce the percolation unless the work was done as a step in a reclamation that includes a treatment of sub-soil.
In reclaiming paddy fields of volcanic ash soil emphasis must be made on the treatment of subsoil that has an excessive rate of percolation. A lack of due consideration to this point. would make bentonite dressing ineffective as a means of reducing percolation.
Following are the major points made clear in this report :
1) The results of mixing bentonite (less than 0.7 t/10a, -200 mesh) with the surface soil after paddy field reclamation showed that the percolation in the initial year was far over the criterion value of 3.0-4.0 cm/day. The percolation rate after the bentonite mixing was 9.8-50.4 cm/day in Oideno field on the foot of Mt. Iwate.
2) The bentonite treatment is effective to some degree in reducing percolation in paddy fields. But the high permeability of sub-soil exceeds the bentonite effect. Therefore, the priority must be given to elimination of the effect of the high permeability.
3) The value o percolation intensity within one paddy division are quite inuniform with a wide range. They form L-type frequency curves. In other words, in one paddy division a few portions with excessive rate of percolation raises the mean value of the division. Therefore, the amount of irrigation water may be saved considerably if the percolation intensity were equalized at the mode intensity. Based on actual measurements in new paddy fields, it was calculated that M₀/M≅0.05-0.30, S/M≅1.0-3.0. Here M means the mean value, M₀ means the mode value and S means the standard deviation of percolation intensities. From these it can be said that a uniformity of percolation should be achieved in reclaiming paddy fields.
4) In paddy fields of volcanic ash soil a considerable inequalty remains even in 60-year-old field. In this case the paddy had been reclaimed by a common method that lacked a treatment on sub-soil. Therefore, it can be said that farming only is inadequate to erase this inequality. In the old field surveyed, M₀/M≅0.4-1.0, and S/M≅0.25-1.0.
5) The characteristics of percolation inequality in one division was as follows : The high percolation rate was seen in the cut sub-soil part, and less percolation was seen in banked sub-soil. Also, the percolation rate in the foot area of ridge was bigger than that in the central part. However, percolation in the ridge foot is affected by sub-sA too, and the percolation rate in the ridge mounted on cut sub-soil was bigger than that in the banking part.
6) Although the inequality of percolation was seen bath in cut and banking parts, the percolation was more uniform in the banking part than in cut part.
The results stated above suggest that an effective way to unify and reduce the percolation within one division is to spread the banked and rolled horizon of sub-soil to the entire area of the division. This point is a guide to our future researches, and we plan to do minute studies in the issues following this.

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