This paper is the second of three reports dealing with the nature cracking paddy field after drainage of residual water.
The first paper presented the effect of developing cracks on drying. The test was done with a drying box in a near steady state conditioned room. In order to examine these properties in field conditions, several tests were made with a lysimeter (2×2×2 m, two kinds of clayey soils were used respectively), which represented field conditions after the lapse of several years.
Soil moisture was measured with a tensiometer which was set at depths of 5, 10, 20, 30, 40, and 50 cm in the lysimeter to know drying characteristics of clayey soil. A glass block electrode was used to measure a loss of soil moisture from the cracked surface by an electric resistance method.
And at the same time, the width of cracks which developed in the soil was measured at the points where the cracks intersected lines set at intervals of 25 cm both longitudinally and transversely in a 1×1 m square selected in the lysimeter.
To evaluate variations of soil hardening with drying, a penetrometer analysis was conducted on alternate days in the drying period.
The tests were done without cropping, and the results of the tests were as follows;
1) Drying characteristics of clayey soil in the lysimeter are described with dividing into two layers, an upper layer (0-20 cm) and a lower layer. Drying of the upper layer is divided into two or three stages: the stage in which the soil dries quickly to pF 2 in soil moisture and the following stage in which it dries slowly. And on the lower layer, the soil does not dry till the soil on the upper layer dries to pF 2 in soil moisture. In such clayey soil, most of moisture is lost by evaporation from soil surface.
2) A crack, when its width becomes 1.0 cm or so, accelerates drying of soil markedly, especially in the aforementioned second stage.
Cracks developed in the soil in the lysimeter cause the evaporation surface to increase more than twice.
3) There seems to be some relation between the total water loss from soil and the maximum width of cracks.
The drying of clayay soil is accelerated by the existance of cracks at the begining of drying period.
4) According to the results of the penetrometer analyses, there seems to be some functional relation between the pF value of soil moisture and the corn index with the initial bulk density used as a parameter.
5) The corn index of the lower layer increases gradually with the repetition of cyclic drying.
All these data show that the drying of clayey paddy field ought to be recognized an associated behavior of soil-water system, such as drying-shrinking-cracking-accelerating of drying= increasing of soil hardness.

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This paper is the third of a series of three dealing with the characteristics of the drying and the cracking in clayey paddy field after drainage of residual water.
In the first and the second paper the authors clarified the drying characteristics of the crack developing field, and in this paper the nature of these cracks and the drying characteristics are reported.
Tests were made on the development of cracks and on the evapolating properties of soil moisture in the cracks of the lysimeter (2m×2m×2m) which was reported in the second report.
And then, in order to examine these properties under the field conditions, the artificially made up model cracks were used in the near steady state conditioned room.
A Soil Atmometer was used for indirect examining of the evaporation properties in the cracks, and a Mariot water supplying device, for the direct measurement of evaporated soil moisture.
The thermister thermometer was used for thermal measurement.
The experimental results are as followes.
1) Nature of the crack;
(i) In the semi-homogenized clayey paddy field soil where puddling was carrid out, the cracks occur under much moistend condition as pF 1.3-pF 1.5 of soil moisture. But these cracks do not extend so much. As the soil moisture decrease to pF 1.8-pF 2.0, another new cracks occur in the lysimeter clay, and they develop rapidly.
(ii) As the development of the length of cracks is so rapid, they reach final dimensions within two days after drainage of residual water, so it may be thought as constant in length to study cracks in conly nection with the drying characteristics of clayey soil.
As for the width of cracks, though it has a parabolic relation with elapsed time, it extends rather slowly than its length.
The depth of crack increase with its width in a linear relation. And then, the dimensions of cracks may be represented by its width.
(iii) An amount of cracks on the clayey soils are representable by the width of the largest crack there.
2) Evaporation in the crack.
(i) Evaporation in the crack remarkably decreases with depth. And there is linear relation between evaporation and the saturation deficit of the air in the crack.
(ii) Under the same environmental condition, there is a parabolic relation between the saturation deficit and the ratio of the width to the depth of the crack.
(iii) Dueing to the differences of the saturation deficit in the crack, water vapor diffusion occures and causes the drying of deep place. Thus, these cracks are useful for the drying of clayey paddy field.

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Following the preceding report on an ill-drained paddy field, we give an account of the features of underdrainage in well-drained paddy fields. It is demonstrated that, in the reclaimed land from the sea in Kojima Bay, paddy fields with the combined mole-pipe drainage or the pipe drainage are dealt with in connection with mechanisms on the methods of underdrainage.
We explained in full the conditions of soil and successively investigated the amount of discharge from underdrainage, level of groundwater table, soil moisture and bearing capacity.
So we knew that the amount of discharge from pipe drains was about 1. 5 1/sec/ha when water moved downward through the subsoil to a depth of 5 cm below the surface, and on the contrary the amount of discharge from combined mole-pipe drains was 39 1/sec/ha and its downward speed reached to 40 cm for 4 hours. These effects are distinguished from the effects of those results in pipe draining. There are some differences in the condition of cracks between both experimental fields, resulting in a great difference in the coefficient of permeability. It is indicated that we have to investigate the mechanisms of the combined mole-pipe drainage system because soil cracks due to the different systems of underdrainage show themselves in subsoil in different ways.

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There are some reclaimed lands from the sea in Kojima Bay from which we selected, as experimental fields, one paddy field with combined mole-pipe drainage and two paddy fields with pipe drainage. We investigated the relation among cracks, percentage of shrinkage due to drying and permeability at the three experimental fields from the results of our study on such problems as circumstances of the fields, irrigation, laying out of drainage and meteorological conditions since reclamation.
Consequently, the coefficient of permeability was found to be K=10^-3-10^-2 cm/sec, 10^-3 cm/sec and 10^-4 cm/sec in the field with combined mole-pipe drainage system and the two fields with pipe drainage system, respectively.
The amount of cracks was found to be largest in the combined mole-pipe drainage system, and there was a difference in the amount of cracks between the two fields with pipe drainage system.
There was little difference in the maximum length of cracks (about 80 cm) among the three experimental fields. The cracks are presumed to have been caused by a drought in 1951.
Therefore, the magnitude of cracks and the coefficient of permeability depend on conditions in past-day circumstances, i. e. laying out of drainage system.
It has been explained that the combined mole-pipe drainage system has a great effect on the acceleration of cracks because water moves downward through the subsoil rapidly.

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As a result of investigation on the relation among the amount of discharge, ground-water level and water pressures in pipe drain, in mole drain and in cinder filter, the method of combined molepipe drainage has been found to be more effective than other methods of underdrainage as summarized below:
1) Drain discharge (Q)-watertable height (H) curve makes an S and the curve may be divided intofour parts i. e. higher part, transition part, straightly falling part and lower part.
2) Diagrammatic higher part illustrating discharged water flowing fully through both pipe drain and mole, lower part illustrating drain-discharge only. On the otherhand, the straight line between higher and lower parts represents discharge in process of flow passing through positions of moledrains vertically so that flows in mole are vanishing.
Assuming one approximate discharge equation with respect to combined mole-pipe drainage, it has been found that numerical values of the coefficient of permeability coincide with the numbers obtained by reckoning backward the equation by substituting in it the differential pressures between (H) and water pressure in mole drain or water pressure in pipe drain.
We can explain Q-H curve satisfactorily as a result of the above. It is also pointed out that the amount of dischange will be increased by eliminating the cause for full flow.

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This study is a actural proof of reclamation of the tillage road type on sloped field, combination of tortoise shell pattern arrangement of farm road which was devised by authors with investigation of sharp of the field.
The theory and experiment of arrangement of tortoise pattern: Both sides of the turning section of zigzag parts (>) and (<) of sloped farm road are arranged on a contour line, and connects them with two contour farm road, then the shape of these roads become tortoise shell pattern.
The size of the arrangement of field plot is, rectangular form with 200 m of longside situated on the contour line, 61 m width with original slope of 8°, 31 m width with slope of 30° and it is necessary to settle the farm road with 6 m intervals along contour line of each field.
The cross angle of small farm road and tillage road becomes symmetrically at the upper and lower parts at the upper and lower parts on the each farm section and the more degree of slope increases the more inclination becomes distinct. The characteristics of arrangement of tortoise shell pattern is such one that, making the use of this declination and circling movement with the maximum cross angle, smooth handling of machinery will be practicable. Generally, the circling movement of machinery is accompanied by the running loss, even-numbered farm roads must be settled.
From the result of the running experiment of cross angle, it was proofed that, if the cross angle of the direction of movement is more than 140°, it will be all right. The authors clarified the minimum circling radius of many types of vehicles which is necessary for the turning point of circling alignment of the tortoise shell pattern. Actural proof of reclamation: The experimental farm is complicated steep slope of 0.8 ha, 40-80 mwidth with original slope of 15°-20°.
Total of the reclamation works repuired 8.8 hr/10 a with remove of surface soil of 5.9 hr/10 a with deep cultivation of 2.9 hr/10 a. This is pure operation efficiency and practicable works.
On the conservation of farm land, result ing from the application of sistematic counterplan, except the management of leakage of water of under ground ground water, a satisfied result was obtained.
On the steep sloped field with complicated gradient, it is difficult to apply the normal type of tortoise pattern shell, but it is possible to apply the modification of this pattern.
A pursuit investigation of the reclamation technic of this pattern was carried out, from the aim of popularization. It was found that it is necessary to simpliby the arrangement of tillage farm road on the complicated gradient. Reclamation cost of this experiment was 97, 000 yen/10 a. Compared with the general horizontal terraced field, this method required 1.2 times of cost, but it is lower than the expected cost and is within the range of possible works. From the result of the actural proof experiment, the authors could find the possibility of excution, on modanization of a basis and application of farm road arrangement.

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It is very important to find a trajectory of a flying water drop as a foundamental object of theoretical study of sprinkler sprays. In this study the author computed factors of tne most foundamental motion of a water drop under such a real condition as accompanied by no wind, by air drag force and by evaporation during a flight, using electronic computer.
The summary is as follows:
1) The air drag which follows the motion of a water drop of sprinkler sprays is a inertia resistance yielding to Newton's law, and the applicable minimum critical speed is 0.03 m/sec. at a water drop diameter of 0.5 mmφ and 0.008 m/sec. at 4.0 mmφ.
2) It is able to be assumed that a water drop holds spherical shape on an average during its flying, and the equation of motion is as follows:
x direction
_??_(1)'
y direction
_??_(2)'
Among a ordinary flying time, there is no variation of diameter influences on the trajectory.
3) The author searched for a approximate equation of Drag coefficient _CD as a function of Reynold number Re, and introduced it in the initial term of the computation using a electronic computer. Moreover, the author used known values under a most real condition as various coefficients used for calculation.
4) From the result of computation, continued trajectory of flying water drops with various diameter was obtained in various cases in which the nozzle angle is constant and the pressure varies.(Fig. 3)
5) Trajectory of flying water drops in various nozzle angle was obtained.(Fig. 5) Further, the speed, direction and the energy of water drops at the point of dropped point were able to be computed.

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The influence of wind on sprays emitted by a sprinkler is remarkable, so the author carried out a theoretical study of the behavior of the water drop.
1) On the assumption that the wind velocity is the same as the average wind velocity, the author considered a case of uniform velocity distribution in which the velocity is uniform vertically (in height), besides the velocity profile following to logarism law on each degree of roughness (earth resistance).
2) When wind velocity is parallel to X axis in x, y, z coordinatse, the following equations are realized in x, y, z directions:
x direction
_??_...(3)'
y direction
_??_...(4)'
z direction
_??_(5)'
Further, when the wind velocity yields to logarism law, next equation is used.
_??_(2)
3) Computed results were obtained in five directions in which the water spray direction is parallel, has angle of 45° and 90° to the wind respectively, and directions in one side of wind was selected as the object of computation, under the consideration of symetry.
4) The trajectory of a water drop emitted by a nozzle parallel to the wind, under conditions of venous pressure and wind velocity.
5) Projection of trajectory to the x, y plane of a water drop emitted in the direction inclined to wind was obtained, then the phenomenon of drift by wind became clear.
6) The variations of the behavior of a water drop caused by the vertical difference of the distribution of wind velocity, and by the variation of roughness on the earth (earth resistance) were found.

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The author measured the deformation and the size of a water drop in sprinkler sprays by direct photographing, Confirmed the adaptability of theoretical equation to the experimental sprinkling water distribution pattern by the upper limit function, and assumed the sprinkling water distribution on slope field as a application of the results of these studies, then obtained satisfying results respectively.
1) It was able to introduce theoretically the deformation of a flying water drop in sprinkler spraying, especially the critical velocity for causing division.
2) The author succeeded to take a photograph of water drops just befor they reach the earth with ordinary camera, and he could measure the water drop size, and moreover could to confirm a theoretical equation of motion of flying water drop.
3) Instead Of the former can test arranged in lattice type, the can test arranged in vadial type is able to (a) remove the influence of concentration of drop water during to astriction of projected water at the time of beginning and finish of a test, (b) make easy the statistical treatment of the depth of sprayed water on con centric circles.
4) By using can test arranged in radial type, it was able to obtain a regular and proper water sprinkling distribution pattern of the main nozzle, excepting indefinite repulsion sprinkling water and sprashed sprinking water distribution by breaker, and effective experimental results for the theoretical study of sprinkler sprays were obtained.
5) It was confirmed by the upper limit function that the theoretical equation of the motion of a flying water drop and its size distribution closely resemble to the values of sprinkling water distribution pattern which is experimentally obtained, then the accuracy of the equation was confirmed.
6) The author was able to assume a sprinkling water distribution pattern on slope fields, from the theoretical trajectory of flying of water drop and water drop size distribution pattern by the use of plan distribution pattern under no wind condition where the sprinkler is thought to show its proper value.

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In this paper, the authors report at first the influence of the variation of the depth of the least permeable layer in the whole soil layer on the percolation of layered soil, when the ground water level exists at infinite depth (under the open condition).
Secondly, the authors describe the depth in which the coefficient of permeability appears in the least permeable layer (it is called “the coefficient of proper permeability”).
-Method of experiments-
The measurement of the discharge of percolation flow and the distribution of pressure head was made, by the use of two kinds of soils. The lower layer soil of 102.0cm in thickness of the cement test standard sand was used as a coarse grade soil, and the upper layer soils of clayey loam having seven steps from 0.3 to 10.0cm in thickness were used as fine grade soils.
-Results of experiments-
In the case of the clayey loam, the coefficient of permeability of which is small valve the value of the coefficient of proper permeability does not appear when the soil layer is thin.
In this case, the friction resistance loss of percolation water is small, a lowering of pressure is little, and the control toward the velocity of percolation becomes small.
In the case of thin soil layer which does not indicate the coefficient of proper permeability, calculated values in which the values of the differences of pressure head in upper and lower ends in the least permeable layer was used, near the measured values.
But according to the increasing of the depth, the calculated values approach to the value of measurement in which the pressure head of lower end makes zero.
In the case in which soil layer is thin and does not indicate the coefficient of proper permeability, its permeability is called “the degree of permeability”.
In this paper, the coefficient of proper permeability appears at the thickness of 10.0cm, but the thickness of soil layer differs according to the change of soil texture, soil particles and porosity etc.

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