The sieve analysis can be regarded as a statistical operation for obtaining a unique grading in each soil, on the assumption of the continuity of grain diameter. From this point of view, after synthetic researches into the descriptive limits of soil grains, several points were stated on the choice of sieves for classifying sand grains:
1. Descriptive limits of soil grains should be regarded as indicators of the soil properties bearing a close interrelation with the grading and used to classify soil grains and soils descriptively, on the basis of grain diameter.
2. Grade limits of sand grains are limits for classifying sand grains statistically and should be chosen so as to gain a unique grading.
3. Considering the diversity of grading, logarithmic transformation of grain diameter in the grain-size accumulation curve, the fact that 2 mm alone is common to all limits for describing soil grains, and the allowable error of sieve opening of JIS standard sieves, the geometrical series approving of 2^1/8as the common ratio (2^1/8 scale) should be adopted as a fundamental scale to grade limits (grade scale).
4. Grade limits for sand grains are based on 2^1/8scale, and it is necessary to choose suitable grade limits in due consideration of the actural condition of the grading and the required measures on this grading. In this case, it is desirable to preserve the geometrical series.
5. If the statistical measure is to be acquired from the sieving data, colog 2^1/8diameter in mm (2^1/8colog. scale) will be employed.

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It is often repored that soil dressing is effective in the control of leakage. However, only a few papers have appeared regarding the aspect and mechanism of leakage control by soil dressing. In the present paper, the author aimed at the explanation of the mechanism of leakage control of soil dressing in common paddy fields and examined the quantity and quality of pores in mixed particle systems.
The quantitative and qualitative changes of pores after the mixing of particle systems of different grain size was examined for glass particles and sand grains. The changes appear more conspicuously when the grain size ratio is smaller, that is, when the grain size of the added soil is smaller as compared with that of the original soil. The smallest porosity is obtained between 50 and 70 %(wt, %) of added soil for the original soil, but the minimun porosity does not correspond to the minimum percolation. The coefficient of permeability is smaller for even larger percentage of fine particles (as compared with the large grain size of the original soil). In addition, a slight change in porosity considerably influences the coefficient.
The theoretical minimum porosity, or the closest packing, is the case that the pores between large grains are gradually filled with small particles and the total volume does not change at all. On the actual conditions, some volume change takes place after mixing, and the porosity becomes minimum when soil is added above the theoretical mixing ratio. Therefore, the observed value of minimum porosity was above its theoretical value.
Generally speaking, the quantitative and qualitative changes of pores appear more conspicuously for mixtures of particles of uniform size. The changes were not so conspicuous for mixtures of particles with a certain range in grain size distribution.
The present paper refess to the results of experiments of soil dressing upon most simplified and abstract conditions. The author is looking forward for extending these studies in a more practical direction.

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This paper presents the full analyses of runoff plot experiments observed by automatic recording instruments on rainfall, surface runoff, soil moisture variation, water movement in soil, etc., to make clear the infiltration characteristics of the sloping clayey soil surface with three.different types of surface covering. The outline of these plots was previously published.
These analyses are summarized, as follows:
(1) The initial, rain differs remarkably in each other plot according to the structural characteristics of surface soil and covering, and its average value for bare soil surface, dense turf covering and forest covering equals to 1.4 mm, 6.1mm and 18.0 mm respectively. But of all plots, the same correlation exists among the initial soil moisture, the initial mass rainfall and the rainfall intensity in the initial stage of surface runoff; the more the initial soil moisture is, the less is the initial mass rainfall and the lower is the rainfall intensity above mentioned; the latter decreases linearly.
(2) Of all plots, the functional relation between infiltration intensity and rainfall intensity is approximated in the extreme accuracy by the expression of linear function as in the analyses of rain simulator infiltrometer experiments previously published, as follows:
i=αγ+ (1-γ) γ₀
i: infiltration intensity;γ: rainfall intensity;
γo: minimum rainfall intensity for surface runoff occuring and average infiltration intensity through the parts of ground surface covered with surface sheet flow;
αa: the areal ratio of the portion of ground surface not covered with surface sheet flow to the whole ground surface.
(3) According to the relation above mentioned, effective mass rainfall for surface runoff (Δ_e) is exactly the residual from mass rainfall, that is mass rainfall (ΔR) on the portion (1-α) of ground surface covered with surface sheet flow minus mass infiltration (γ₀·ΔT) through the portion.
(4) Surface runoff coefficient varies conspicuously on the ground surface where both (1-α) and γ₀ are comparatively large.

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The infiltration characteristics, as to the rainfall intensity and the mass rainfall, of mountainous slopes (bare soil surface, dense turf covering and forest covering with a medium stand of miscellaneous trees) composed almost homogeneously about the surface soil and covering were elucidated previously with observed data of rain simulator infiltrometer experiments and runoff plot experiments.
Based on these analytical results of infiltration characteristics above mentioned, the authors have analyzed the hydrological data of a small, experimental, forest watershed (4.4 ha) of a relatively simple composition and have made clear its characteristics, as follows:
R_e= (1-α_b) (R-γ₀b·T_R)...(1)
R_e: effective mass rainfall or rainfall excess for the surface runoff;
R: mass rainfall; T_R: duration of rainfall;αb, γ₀b: constants.
The equation (1) is the most suitable one for the intensity-relation between rainfall and surface runoff during the early period of rainfall (total mass rainfall <80 mm). The same abrupt change of the intensity relation given above at the limit of mass rainfall occurs as in the analytical results of the forest covering experimental plot previously reported (Plot 4).

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Veihmeyer and Hendrickson long contended that transpiration does not decrease materially until soil moisture falls almost to the permanent-wilting percentage.
On the other hand, Davis and Hynes reported that decreased moisture content resulted in decreased evapotranspiration rate.
Tamai reported that the evapotranspiration rate remained constant in the upper half of available moisture.
The present study, using floating lysimeters at Hiratsuka, Kanagawa Pref., indicates that the evapotranspiration rate of upland rice remains constant until soil moisture tension reaches nearly pF 3.0 in the upper half of root zone and pF 2.7 in the lower half.
Such a vertical distribution of soil moisture gives an index of water supply.
Thus in our country the measuring method of soil moisture extraction pattern should be improved on the basis of the above facts.
The relation of soil water content to evapotranspiration deserves further studies.

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On the experiments in the laboratory and the paddy fields, the authors made clear the mechanism and the effect of the percolation control of water by using bentonite at the time of “Shirokaki” methods (puddling preparation of paddy field for transplantation, that is one of the most important works for rice-plant culture). From the above experimental results an effective Shirokaki, method was shown from the standpoint of saving water.
From the experimental results both in the laboratory and in the field, the.effect of percolation control was in the order of kneading, complete and gross shirokaki.
The most significant point of this paper is that we could expect the same effect for percolation control of water as using bentonite by only kneading or complete shirokaki in the soil of such a composition as was used in this experiment (clay content: 23 to 28%).
However from the angles of the physical tests of soils, from the growing conditions of rice plants or from the management of paddy field, complete shirokaki does not seem to be a desirable method.
Therefore, both from the standpoint of saving water and of the yield, it was made clear that the kneading shirokaki was the most suitable method.

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As one of the series of studies on percolation through the dike with free surface, this paper especially refers to the free surface and seepage surface in percolation.
Water-tanks glazed in one side were used in the experiments (see Fig. 1). Special features of these devices are:
(i) Tensiometers are put in the capillary zone;
(ii) A mercury thermal regulator, a relay, and a heater are connected in order to keep constant water temperature in percolation.
Special features of the experimental procedures are:
(i) A probe potentiometer was used in order to persuit the free surface;
(ii) Free surfaces were seeked at the three stages ‘not cut off’, ‘half cut off’ and ‘cut off’ after banking several centimeters up to the upper limit of the capillary zone (see Table 1).
The author compared the results with the equation (2) by Dupuit and Casagrande and the equation (4) when h_s is obtained from the experimental results.
He found that the equation (2) did not come true when a capillary zone existed and the equation (4) coincided nearly with the experimental results in the downstream 1/4 of the dike when h_s was the experimental value, and the equation (2) came true in the upstream 3/4 part when ‘cut off’.
Future investigation are necessary to obtain the free surface and seepage surface for different materials and forms of the dike.

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In the previous paper (I), the author described on the free surface and seepage surface in percolation with free surface.
This paper deals with the results of the streamline, stream velocity, discharge, and capillary potential in percolation.
A solution of potassium permanganate was injected into the dike by a special injector, the flow of the colouring matter, was traced, and gained the streamline and velocity (see Figs. 1-19).
In this case, as shown in Table 1 in the previous paper (I), experiments were made at three steps “not cut off”, “half cut off” and “cut off”, and their difference was made clear.
In the case “cut off”, the experimental and theoretical values of the streamlines and the velocity distribution, when the seepage surface “h_s” was taken into consideration, were shown in Figs. 3, 6, 13, and 16.
As for the percolation discharge, Fig. 21 shows the author's experimental values plotted on the diagram of T.G.Chapman.
As for the capillary potential, a tensiometer of porous unglazed pottery connected to the manometer was put in the capillary zone under percolation, and the capillary potential (pF values) was measured, as shown in Fig. 22.

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