Recently, a master plan for sewerage construction, for complete coverage of city, town and village areas has been devised in Japan. This plan has be able to make progress regarding the construction of small sewerage systems in rural areas.
One of the purposes of this plan is to make sewerage disposal as economical as possible. The economical estimation of sewerage construction is made on the assumption that population of each district will increase. Therefore, the economical influence due to the decrease in the population is not considerd exactly. However, in rural areas, a decrease in population happens in general.
Economical estimation changes for the worse, as the population decreases. The effect of a decreasing population on the economical estimation is different between each sewerage system. So that a suitable sewerage system for a district changes as the population decreases. Then, we must consider the features of a population district, when we plan sewerage construction from the economical point of view.
In this paper, authors discussed three points.
1. Analysis on the character of population changes in rural areas.
2. Analysis on the economical estimation under the condition of a decreasing population.
3. Suggestions on how to make a master plan in population decreasing area, from the economical point of view.

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It is difficult to arrange a sewer piping network systematically in rural areas lacking an effective landuse plan in Japan. And now, we are compelled to arrange sewer piping network to fit the settlement as it is, by the construction of a small sewerage system in rural areas.
Considering the cost of sewerage construction and the increase in population, we must come up with a solution to the following two problems.
1. To what extent, we need to include into the sewerage project area.
2. How to treat sewage from houses, which will be constructed near the projected area after the construction of a sewerage system.
Economically efficiency must also be examined at first in a small sewerage project. However, we cannot settle the area only from the economical point of view. This is because we seldom meet with such a case in which settlements are opposed to each other and we are not able to arrange the pipe in a suitable way.
To settle a suitable area, many experiments are necessary in projected areas. So in order to solve these two problems, as mentioned above author has investigated and compared many cases, and has come up with a planning method on the basis of many cases in this paper.

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This series of studies is aimed at clarifying the mechanism of evapotranspiration from rice-planted paddy fields. Paddy fields are unique among crop fields because they are ponded throughout the rice growing season. In this report, the authers present the micrometeorology and water budget observation results regarding paddy fields, and discuss the energy distribution of the rice plant canopy. The results show that when rice plants sufficiently covered the water surface, the canopy was able to be maintained at relatively low temperatures and high humidity compared with that of the ambient atmosphere. This condition caused the sensible heat flux from both the upper atmosphere and the water surface to go to the canopy. As a result, the evapotranspiration exceeded the net radiation which was supplied to the canopy.

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In the present study, determination was made of the basic evapotranspiration mechanism by analysis of flux and water movement in a rice plant canopy and the development of a “two layer model”. First, a comparison between potential evapotranspiration as calculated by the Penman method and actual evapotranspiration measured by the water balance method was carried out. Actual evapotranspiration was found to exceed the potential one by a factor of about 1. 0 with growth of rice plants.
Secondly the reason for this is considered to lie in differences between heat and water transfer mechanisms.
Thus, the “two layer model” was proposed and distribution of heat components was analyzed using observed data, such as evapotranspiration, temperature, vapor pressure and net radiation. In addition, certain parameters such as K_P, K_W and K_T, which represent the transfer coefficients defined in this paper, were calculated and the conductivity of water movement in a plant body itself was evaluated.
According to the results obtained, the occurence of extensive evapotranspiration can be reasonably explained from the fact that heat transfer under a canopy is relatively difficult but movement of water within a plant body is relatively easy.

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The effects on micrometeorology of dense vegetation sprinkling at high temperature in summer are discussed on the basis of field experimental results.
The effects were expressed in terms of differences or rates based on values of a control field.
Generally the values of the sprinkling field differed from those of the control field before the first sprinkling in a day, because of the remaining water in the soil from the day before.
The effects were compared with those in the case of sparse vegetation described in the previous paper.
The main results obtained are as follows:
1) The effects on air temperature at heights of 1.5, 1, 0.3m were-0.1, -0. 1, -0.9°C before sprinkling, -2.7, -3.4, -3. 6°C during sprinkling and-0.4, -0.6, -1.8°C after sprinkling respectively. In comparison-with the effects in the case of sparse vegetation, the effects are much the same at a height of 1 m above the ground, slight at a height of 0.1m above the canopy top and small inside the canopy.
2) The effect on canopy surface temperature was noted only during sprinkling and was-2.3°C. This is nearly equal to that in the case of sparse vegetation.
3) The effects on relative humidity at height of 1.5, 1, 0.3m were+1.6, +2.3, +5. 8% before sprinkling, +13.0, +17.2, +20.4% during sprinkling and+2.4, +3.6, +12.8% after sprinkling respectively. In comparison with the effects in the case of sparse vegetation, the effects are much the same at a height of 1m above the ground, slight at a height of 0.1m above the canopy top and small inside the canopy.
4) The effects on upward longwave radiation were?0. 6% before sprinkling, -3.5% during sprinkling and-0.4% after sprinkling. These are smaller than the effects in the case of sparse vegetation.
5) The effects on net radiation were-1.8% before sprinkling, +8.9% during sprinkling and +2.7% after sprinkling. The effects during sprinkling were greater than those in the case of sparse vegetation.
6) The effects on latent heat flux were +1% before sprinkling, +48% during sprinkling and +11% after sprinkling. The effects during sprinkling were somewhat larger than those in the case of sparse vegetation.
7) On regard to dense and sparse vegetations, the ratio of biomass derived from variation in the canopy surface temperature, was equal to the ratio of vegetation indices.
8) Canopy surface temperature excess induced theoretically was equal to the value observed. Thus the constant difference between theory and observation in sparse vegetation is due to the bare ground between canopies.

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One of the many causes for drought is due to low precipitation, an amount below the mean value. In case it continues for several months, the occurrence of drought phenomena can be seen many times. A new method for depth-duration analysis (D-D analysis) concerning droughts, based on the characteristics and mechanisms with a time lapse of successive low precipitation, is discussed.
The definition of “duration preciptitation” is given first as precipitation accumulated for any duration of months beginning from any particular month. On the basis of monthly precipitation for the last several years, the occurrence probabilities of every duration precipitation for every month is derived stochastically.
Many interpretations and applications of the D-D analysis of low precipitation, according to the duration precipitation, are possible and shown:
1) Restoration or maintenance duration of low precipitation regarding any month is capable of being evaluated,
2) Maintenance and judgement of the design precipitation in any duration can easily be estimated with the occurrence probability regarding the residual duration precipitation after calculating the accumulated precipitation with a time lapse within its duration.
The same argument and interpretation on the occurrence probability of duration precipitation is applied to high precipitation, amount up the mean value, also.
The depth-duration analysis shown in this paper is an adequate method that could be applied to predicting and provisions for drought phenomena, and management and control planning of water resources.

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This paper deals with the flood runoff characterstics in the experimental basin comprised of a woodlands, a golflinks and a playground, as part of the study of the effects of land-use condition on runoff characteristics.
The flood runolf characteristics in each basin were examined comparatively from the view points of the time of concentration, the coefficient of peak runoff, the equivalent roughness of the kinematic runoff model and the parameter of the storage function model.
The following results were obtained:
(1) By converting a woodlands to a golflinks, a baseball ground and a playground, the concentration time is reduced by 3/4-1/2, and the coefficient of peak runoff increases several times.
(2) Rain water flow over a slope can be simulated by using the kinematic runoff model of the Manning type, regardless of how land is used.
(3) The watershed model for applying the kinematic model can be extremely simplified.
(4) The proper values of the equivalent roughness (m^-1/3⋅s) are as follows: 1.0 for a woodlande, 0.2-0.3 for a golflinks and 0.03-0.05 for a playground.

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The coefficients of permeability of Takizawa volcanic ash soil and Tozawa clayey soil were observed. The molding water content of the soils was varied in seven or eight steps. The compacted soils were percolated with boiled pure water during a long period of time up to 579 days.
The permeabilities of soils varied with time in the following four stages.
1. The permeabilities decreased a little in the first stage. This stage appeared in only three cases.
2. The permeabilities of Takizawa soil remained constant in the second stage which continued for a long period of time in some cases. The second stage was not found in thecases of Tozawa soil molded in a low water content.
3. In the third stage, the permeabilities abruptly increased up to seven times and six times of the initial values in the cases of Takizawa and Tozawa, respectively.
4. In the fourth stage, the permeability continued to decrease gradually in the cases of Takizawa whereas it decreased rapidly in the cases of Tozawa.
Some characteristic features of the variety of permeability were considered from the standpoint of the structure of the compacted soil.

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In this paper, quantitative relationships between the aggregate and some physical and engineering properties of soil are researched using the artificial aggregate forming in sandy soil followed by their measurement.
Five varieties of test fields, divided with a Latin square, controlled by the clay content, CaCO₃ content and oil cake content, were prepared for the research. Measurements of the aggregate, dry density, pF-moisture relationships, consistency and soil resistance were carried out from 1983 until 1985.
The following relations were found.
(1) Dry density of soil (Y₁) decreased proportionally to degree of aggregation (X). Relationship between Y₁ and X can be expressed as follows:
Y₁ =-0.0110 X + 1.31 (γ=-0.469***)
(2) Soil moisture corresponding to pF 2 (Y₂) and pF 3 (Y₃) increased proportionally to the degree of aggregation (X). They are expressed as follows:
Y₂ = 0.501 X + 34.2 (γ=0.459***)
Y₃ = 0.537 X + 14.4 (γ=0.606***)
(3) Relationship between the plasticity index (Y₄) and degree of aggregation (X) was proportional and expressed as follows:
Y₄ = 0.613 X + 0.990 (γ = 0.698***)
(4) Engineering properties of soil may be evaluated partially both aggregate and consistency of soil.
Based on the above, it is possible that the relationship between the aggregate and some physical and engineering properties of sandy soil is expressed quantitatively, and that soil structure is classified according to every 10% degree of aggregation.

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Analysis of water-stable aggregates for 17 nonvolcanic ash types of soil and 17 volcanic ash types of soil was carried out to evaluate the structural development of clayey types of soil. In addition, the significance of the size of water-stable aggregates from the viewpoint of the physical properties of undisturbed soil samples was investigated, and a few problems concerning the analysis of water-stable aggregates were discussed.
The results obtained are summarized as follows.
(1) The extent of the difference between the aggregates distribution of fresh samples and that of air-dried samples were dependent on the soil types (nonvolcanic ash soil or volcanic ash soil) and soil layers (surface soil or subsoil).
(2) The aggregate percentages of separate aggregate size obtained for the fresh samples showed a significant correlation with the physical properties of undisturbed soil samples (saturated hydraulic conductivity and water retentivity), but that for the air-dried samples did not.
(3) The large-size aggregates (8-2mm) of the fresh samples were negatively significant and the small-size aggregates (1-0.1mm) were positively. Accordingly, it was considered that the soil types which are high in small-size aggregates have a well-developed soil structure.
(4) Generally speakig, volcanic ash types of soil had a tendency to have high small-size aggregates, in comparison with nonvolcanic ash types of soil.

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