Macropores in soils of agricultural fields make the soils nonuniform and also govern the water movement phenomena occuring there. We examined the pore size which divided macropores from micropores and the sample size when sampling from soils with macropores. The former, we statistically analyzed relationships between saturated hydraulic conductivity and pore characteristics of undisturbed cylindrical soil samples. The latter, we simulated the sample size which represent the crack length in a unit area when sampling soils with cracks. The results of the statistical analysis showed that saturated hydraulic conductivity was correlated best with the porosity for the pores whose equivalent diameter was greater than 0.1〜0.03mm. And, the result of the simulation showed that the representative sample size for the crack length in a unit area had to contain 20〜50 units of mesh. On the basis of such results, we discussed an approach to make clear water movement phenomena in soils with macropores. And, following 3 subjects were proposed to be investigated.
①understanding of macropore system
②water flow in each macropore
③size of block and governing law when averaging water movement phenomena in a field soil.
In this paper, radiographs of soil macropores were obtained by using soft X-ray radiography and several quantitative analysis were carried out by image processings. The soil samples tested were andosols which have a lot of macropores and duralumin samplers were used due to their strong X-ray transmittance property. The methodologies and experimental results are as follows :
1) By injecting liquid contrast agent (CH2I2) into the soil samples, the fluidities were recorded using a X-ray TV video camera. It can be seen by the video recording that the fluid flowed with pulsatory motions in the macropores connected with each other.
2) The spatial structures of macropores (in 3D graphics) were drawn from the stereoradiographs using a personal computer resulting to the visual observation on the spatial continuities of soil pore structures and the calculation of their actual lengths.
3) The coefficient of the permeabilities were calculated by using the fluid velocities and the diameters of the main macropores, respectively, and compared with the values of the permeability tests. Differences were found among the permeability values of the main macropores and of the soils.
The results of this experiment show the importance of a more reliable information on soil macropore structures through the improvement of the experimental procedure.
Differences in soil structure according to land-use and soil properties were examined visually using soft X-ray. Consequently, the following results were obtained.
(1) Differences in paddy field Soil structures in plow layers were different from the subsoil. In the plow layer, unique soil structures were formed in a year after spring puddling. After puddling, pores outside the aggregates were predominant to other structures, but in the lower plow layer, vertical tubular-pores were formed gradually according to rice root penetrating. In the subsoil, tubular-pores were predominant to other structurs and no structural variation was observed in the course of the year.
(2) Differences in soil type In Andosol, the pore diameter was larger than in Gray lowland soils and Brown lowland soils because of its aggregation. In sandy loam, tubular-pore structure was rarely formed because of its single-grained structure.
(3) Soil structure in upland field compared with paddy field The plow layer in the upland field was rich in pore structures but poor in tubular-pore structures. In the subsoil, however, tubular-pore structures were predominant as well as in the paddy field, but tubular-pores had no principal direction, which is different from the paddy field. In the upland field converted from paddy field, vertical tubular-pore structure which could be seen in the paddy field decreased in number with time as affected by drainage or drying.
By the using the Warrick’s analytical solution with one dimensional water extraction, iso-matric potential ditributions were estimated under the steady state conditions of two dimensional moisture flow. The results estimated were compared with the experimented ones measured at the fields cultivated with sugercanes and cucumbers in the south-west Islands of Japan which showed the climate characteristics of frequent rainfall. As a result, in order to estimate the outlines of the wetted areas as the dimensions designed for the irrigation schedules and systems of the drip methods, it is recommended that the analytical solutions are applied to the limited field conditions, of which the crop cultivation are carried out in the periods of a few rainfall.
Soil moisture flow under a drip irrigation on the sugarcane field is simulated by a finite element method and the outline of the method is described. The calculated results of moisture distribution compare favorably with the observed results. Soil moisture movements are simulated under the dry condition and wet condition of initial soil moisture. In the case of dry condition, irrigated water does not transfer to the main root zone below the crop. It is anticipated that the role of water uptake by the root distributed below the drip line is important. In the case of wet condition, irrigated water transfers to the main root zone but deep percolation loss is large. The locational relation of main root zone and dripline are important factor in determining the interval of dripline. Arbitrary distribution pattern of root is considered in the finite element method, therefore the method is useful tool in designing of drip irrigation.
We have investigated the effect of puddling on percolation rate, water pressure distribution and quality of percolating water in a flooded rice field on diluvial plateau covered by Kanto loam of volcanic ash soil.'The field is well-drained type. Irrigation period is from May to August. In May, field is flooded by irrigation water and rice is transplanted. In September rice is harvested. After the rice, there is no crop in the field. Soil is usually puddled before transplantation. However in 1988, we did not puddle in order to compare the results measured in 1987 with puddling. We measured :
1) Rainfall, the change of the level of surface water, soil temperature by automatic recorder.
2) Evapotranspiration, daily water requirement in depth, and percolation rate.
3) Water pressures were measured by tensiometers and piezometers in the depth of 10, 20, 30, 40, 60, 80cm.
4) Percolating waters in the depth of 10, 20, 30, 40, 60, 80cm were sampled and the concentration of NO3-N, NH4
-N and EC, pH, ORP were measured.
5) SoiFs component phases and hydraulic conductivities of each layer were measured at each stage. Average percolation rate measured inside in 1987 was 0.6cm/d, but in 1988 the value of inside increased to 4.5 cm/d due to nonpuddling as shown in Fig.1. Pressure distribution of percolating water also changed due to puddling (Fig. 4).
The concentration of NH4 -N in percolating water of 20cm depth increased to 4 mg/1 at the period of basal apply of chemical fertilizer in May, and after that it gradually decreased. In August it became zero. In the subsoil, the value of NH4-N concentration in 1987 was also zero. However in 1988, the concentration increased to about 3 mg/1. It is considered that NH4 flow down due to large percolation rate.
Actual ridging, furrowing and ditching practices were observed on steep farmland of deep Kuroboku soil, rhe farmland had been reclaimed at original slope in 1972 and has ununiformed sloping fields with small valleys aside or inside. Airphotograph taken 3 years after reclamation showed that many gullies had occurred along these concave valley bottoms. Analysing the directions and the geomorphic features of furrowing practices shows that ridge and furrow tended to be formed in the steeper side direction of a field lot. In these fields, many ditches were observed to be dug across ridges and furrows. The troublesome ditching practices were suggested to be done in the sense of safety drainage; the steeper or the more gentle these ditches became, the shorter they were because a steep ditch would be eroded to cause a gully and a long gentle ditch would cause downside erosion by an overflow. The actual furrowing and ditching practices are supposed to be affected by the field lot arrangement and the replant failure of a monocultural cropping system but also show that the both soil conservation and water drainage practices are necessary together. These are implicated with the safe overland flow control and the effective subsoiling not to cause overland flow at a concave valley bottom.