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Measurement of soil water flux in andisols at a depthbelow a root zone of about 1 MeterShuichi Hasegawa c , Seiko Osozawa a c & Hideto Ueno b ca Shikoku National Agricultural Experiment Station , Zentsuji , 765 , Japanb Chugoku National Agricultural Experiment Station , Fukuyama , 721 , Japanc National Institute of Agro-Environmental Sciences , Tsukuba , 305 , JapanPublished online: 04 Jan 2012.
To cite this article: Shuichi Hasegawa , Seiko Osozawa & Hideto Ueno (1994) Measurement of soil water fluxin andisols at a depth below a root zone of about 1 Meter, Soil Science and Plant Nutrition, 40:1, 137-147, DOI:10.1080/00380768.1994.10414286
To link to this article: http://dx.doi.org/10.1080/00380768.1994.10414286
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Soil ScL Plant Nutr., 40 (I), 137-147, 1994 137
Measurement of Soil Water Flux in Andisols at a Depth below a Root Zone of about 1 Meter
Shuichi Hasegawa, Seiko Osozawal, and Hideto Ueno2
Nation'al Institute of Agro-Environmental Sciences, Tsukuba, 305 Japan
Received April 21, 1993; accepted in revised form July 30, 1993
A method which uses the pressure head to predict unsaturated hydraulic conductivity is presented to calculate the soil water flux in a field. Hydraulic conductivities in the primary drying and wetting processes were measured with core samples in the laboratory and the hysteresis between the hydraulic conductivity and pressure head was taken into account. Hydraulic gradients were measured every hour with tensiometers installed in the field. This method was applied to analyze the water movement at 94-cm depth in Hydric Hapludands. Downward or upward flow of water by summing soil water fluxes was examined using the water balance method. Amounts of downward flow determined by our method after heavy rain in a wet soil were slightly larger in the soybean plot but smaller in the bare plot than those obtained by the water balance method due to non-uniform infiltration. Water balance equation which used values of upward flow across a 94-cm depth estimated reasonably well the evapotranspiration from the soybean plot and the evaporation from the bare plot during a dry period. Effect of initial soil water conditions on downward flux by rainfall was well monitored by our method.
Key Words: Andisol, hysteresis, soil water flux, water balance.
Measurement of soil water flux into and out of the root zone of a crop is indispensable for understanding its water use especially during dry spells and for evaluating the amounts of water and nitrate nitrogen which were not taken up by the crop but must reach ground water. Magnitude and duration of the soil water flux are strongly influenced by the water transmission properties of the soil. Andisols, which are major cultivated soils for upland crops in Japan, display high water transmission properties even though the soil texture ranges from Lie to He. Saturated hydraulic conductivities of Andisols are often of the order of 10-2 cm S-1 (Tabuchi 1963) and hydraulic conductivities at - 3 kPa are commonly higher than the order of 10-5 cm S-1 (Iwata 1971; Hasegawa and Maeda 1977). Due to such higher water transmission properties of Andisols, it is expected that most of the water supplied by rainfall infiltrates into soils uniformly without surface ponding.
When the soil water moves uniformly and no preferential flow occurs, the soil water flux is expressed by the product of the hydraulic conductivity and hydraulic gradient. Under field conditions, the hydraulic conductivity which can be measured based on the drying process
Present addresses: 1 Shikoku National Agricultural Experiment Station, Zentsuji, 765 Japan; 2 Chugoku National Agricultural Experiment Station, Fukuyama, 721 Japan.
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138 S. HASEGAWA, S. OSOZA WA, and H. VENO
of the soil (e.g., Ogata and Richards 1957; Rose et al. 1965) is expressed as a function of the soil water content as the hysteresis between them is negligible. Soil water flux is then calculated using the hydraulic gradient measured with a tensiometer at two depths and the hydraulic ·conductivity corresponding to the average soil water content between the two depths determined with a neutron probe or by direct sampling (Reicosky et al. 1977). This method is useful when the soil water content changes gradually like the upward soil water flux toward the root zone. Unlike the measurement of the soil water pressure, frequent and precise measurements of the soil water content during infiltration by rainfall are rather difficult.
Use of the pressure head to predict the hydraulic conductivity, however, is associated with another problem, because the hydraulic conductivity as a function of the pressure head shows a large hysteresis between the wetting and the drying processes. If the hysteresis is taken into account, only the measurements of the pressure head in the field may enable to calculate the soil water flux. Unsaturated hydraulic conductivities in both the drying and the wetting processes are measured accurately by the constant head method according to Richards and Moore (1952) in the laboratory. Major drawback of the laboratory method is that the size of a soil core is too small in comparison with the field size.
In this paper we measured the unsaturated hydraulic conductivities in both the primary drying and wetting processes as a function of the pressure head in the laboratory and made a simple assumption to take account of the hysteresis of the secondary processes. Values of the hydraulic conductivities obtained by this method and hydraulic gradients measured with tensiometers installed in the field were applied for calculating the soil water flux at 94-cm depth in Andisols.
EXPERIMENTAL PROCEDURES
Experimental field. Field experiments were conducted in a field of the National Institute of Agro-Environmental Sciences (latitude 36"01'N and longitude 140"07'E). Average annual rainfall recorded from 1981 to 1990 was 1,229 mm. The soil belonged to Hydric Hapludands (Soil Survey Staff 1992) which extended to a 200-cm depth, below which non-volcanic clay soils were present. The soil profile was divided into six layers, ApI (0 to 20 cm), Ap2 (20 to 37), 2Bl (37 to 79), 3B21 (79 to 108), 4B22 (108 to 155), and 5B23 (155 to 200). The top four layers were used in this study. The soils of the ApI and Ap21ayers had been disturbed by ordinary and deep plowings in the past unlike the soils of the 2BI and 3B21 layers which contained a large number of fine tubular pores « I mm) formed by decayed roots of dead plants. No visual cracks or fissures were observed throughout the profiles. Two soil surface conditions, bare and vegetation were prepared. Soybean (cv. Tachinagaha) was sown on May 8, 1991 in an area covering 15 X 10 m2 with a row width of 65 cm. A bare plot was prepared beside the soybean plot. Rooting depth of the soybean plants was determined on August 30.
Pressure head. Tensiometer cups 18 mm in diameter and 60 mm in length were installed at depths of 6, 22, 32, 41, 55, 67, 83, and 105 cm in the soybean plot and 6, 25, 35, 45, 57, 67, 85, and 103 cm in the bare plot below the surface. Pressure heads measured with pressure sensors were recorded every hour from July to August except during the period July 13 to 17. Pressure sensors with an accuracy of ± I cm water head were calibrated once a month to compensate for the drift caused by long term use.
Soil water content. Soil cores I-m long were taken on July 4, II, 31, and August 8.
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Soil Water Flux at I-m Depth in Andisols 139
A thin steel tube, 27.5 mm in inside diameter and 1,100 mm in length, was driven into the soil to I-m depth and was retrieved. The soil cores were sectioned every 5 or 10 cm to measure the soil water content profiles using the oven-dry method. Two sets of soil cores were taken from each plot.
Meteorological data. Data of rainfall and pan evaporation (USDA class A pan) were obtained from a weather station located less than 1 km from the experimental field.
Unsaturated hydraulic conductivity and curve of soil water characteristics. Unsaturated hydraulic conductivities of all the layers were measured in the laboratory according to the method of Richards and Moore (1952). Three undisturbed core samples 10 cm in diameter and 4 cm long were taken from each layer. The soil core saturated with water by capillarity was sandwiched between a pair of filter membranes and was placed in a pressure chamber. Water was supplied with a constant head and the hydraulic gradient was measured with two fine ceramic cups, 2 cm apart, inserted from the lateral wall of the core. Flow rate was measured after a steady state condition was attained. Hydraulic conductivities were measured at several pressure heads by applying air pressure to the chamber in the primary drying process up to 150 cm and in the wetting process from this head to near saturation. Secondary processes of the hysteresis cycle were not examined. Curves of soil water characteristics in the drying process were obtained using the pressure plate method with separate undisturbed soil samples taken from each layer.
CALCULATION OF SOIL WATER FLUX AND WATER BALANCE
Soil water flux was determined by the following equation:
(1)
where q is the soil water flux, PI and pz are the pressure heads at two depths with a distance of I, and k is the hydraulic conductivity at the mean pressure head. Pressure head, which is negative in unsaturated soil, was represented by the water head in centimeter. Pressure heads measured with two tensiometers placed at 83- and 104-cm depths in the soybean plot, and at 85- and 103-cm depths in the bare plot in the 3B21 layer were used in the calculation. The mean depth was 94 cm in both plots. Since we obtained only two primary curves of hydraulic conductivities against pressure heads, and appropriate methods to predict hydraulic conductivities in the secondary drying or wetting processes have not been developed, we made a simple assumption to take into account the hysteresis of the secondary process as illustrated in Fig. 1. When the pressure head decreased at A on the primary wetting curve, hydraulic conductivities in the secondary drying process fell on point B of the primary drying curve whose horizontal distance from A was twice the distance between the two primary curves. However, we ignored the tertiary hysteresis of the rewetting process which occurred between A and B. Hydraulic conductivities in this wetting process fell on the line AB. The same method was also applied when the pressure head increased.
As the accuracy of the pressure head was ± I cm in the field, an error of about 10% was inevitable in the hydraulic gradient. The values of the soil water fluxes due to this error were 1 and 0.1 mm d- I when the hydraulic conductivities were 1 X 10-5 and 1 X 10-6 cm S-I,
respecti vely. Summation of the changes of the soil water fluxes at a given depth with time during a
given period is equal to the difference between deep percolation DP and upward flow UP
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140 S. HASEGAWA, S. OSOZA WA, and H. UENO
Pressure head (negative)
Fig. 1. Method to evaluate hysteresisaffected hydraulic conductivities in relation to two primary curves.
across the depth.
Wetting
1xlO-7'---'-_'---'-_L---L-L---L_L--L_L---I a -100 -200
Pressure head (em)
Fig. 2. Hydraulic conductivities of 3B2l layer.
DP-UP=~q. (2) The difference, DP- UP is also obtained by the water balance equation. When a given depth is deeper than the root zone, DP- UP is expressed as:
DP-UP=S1-S2+P-E, (3) where S1 and S2 are the amount of water stored within the depth on the first and the last days of the period, P and E are the amount of rainfall and evaporation or evapotranspiration during the period.
RESULTS AND DISCUSSION
Physical properties of the soil Physical properties of the soil were characterized by low bulk densities and high
hydraulic conductivities at high pressure heads (- 3 to - 4 cm) in all the layers as shown in Table 1. Higher bulk density in the Ap2 layer was due to field operations using agricultural machines, but the hydraulic conductivity near saturation remained high. Hydraulic conductivities in the drying process of individual layers decreased to about one tenth of the initial values at - 30 cm and were of the order of 10-6 to 10-7 cm S-1 at -100 cm. Figure 2 shows the unsaturated hydraulic conductivities of the 3B211ayer. Open and closed symbols indicate the drying and the wetting processes, respectively. Hydraulic conductivities at pressure heads below -150 cm were estimated by extrapolating the drying curve by visual examination. The ratios between the maximum and the minimum hydraulic conductivities at given pressure heads in three samples were less than 1.8 in both processes except near saturation.
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Particle Layer
Depth density
(cm) (g cm-3 )
Api 0-20 2.57 Ap2 20-37 2.58 2BI 37-79 2.75 3B21 79-108 2.75
June --t--- July
Soil Water Flux at I-m Depth in Andisols 141
Table 1. Physical properties of the soil.
Water content at pressure head Hydraulic conductivity
Bulk at pressure (drying process) density (cm3 cm- 3) (cm S-I)
(g cm-') -10 -60 -100
-1,000 -3--4 -30
-100 em em
0.75 0.64 0.44 0.42 0.36 2.2 X 10-3 2.2 X 10-4 1.2 X 10-' 0.80 0.64 0.53 0.49 0.40 1.6 1.3 2.1 0.55 0.75 0.70 0.67 0.62 0.5 0.7 0.7 0.57 0.75 0.68 0.66 0.56 1.1 1.4 2.3
, I I ~ .,
1 10 20 31
-\- August -\ Fig. 3. Distribution of rainfall in July and August, 1991.
Hydraulic conductivity in the wetting process was approximately one third of that of the drying process at - 30 cm and this difference was the largest in the hysteresis cycle.
Rainfall Figure 3 shows the record of daily rainfall in July and August in 1991. A relatively dry
period until August 19 followed frequent rainfall events from June to July 20. Eighty millimeters of water was added to the wet soils from July 4 to 6 and more than 100 mm of water was supplied from August 20 to 21 after the dry period.
Downward soil water flux associated with rainfall in a wet soil Soil water flux from July 4 to 11 was calculated. Figure 4 shows the pressure head
profiles measured at noon (12:00) on July 4, 6, and 11. The soil was initially wet and there was no appreciable difference in the pressure head between the soybean and the bare plots. Pressure head increased due to heavy rainfall on July 6 but did not become positive at any soil depths and then decreased due to deep percolation with time. Pressure head profiles in both plots on July 11 were not different from each other except at 6-cm depth. Soil water fluxes at 94-cm depth for both plots together with the distribution of rainfall are shown in Fig. 5. Soil water flux increased with the increase of the pressure head but the values were different between the two plots especially immediately after heavy rainfall on July 6. Maximum soil water flux of 5.0 mm h-1 in the soybean plot and 2.0 mm h- 1 in the bare plot appeared when the pressure head at 94-cm depth decreased to -21 and - 31 cm, respectively, and the value of the hydraulic gradient was nearly one. Hydraulic conductivity at - 21 cm was about three times larger than that at - 31 cm as shown in Fig. 2, suggesting that the maximum pressure head influenced significantly the soil water flux. There were no appreciable differences in the flux between the two plots after July 9.
Validity of our method to calculate the soil water flux by Eq. 1 can be verified by
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142 S. HASEGAWA, S. OSOZA WA, and H. UENO
Pressure head (cm) -400~ ______ ~-3,0~0 ________ -_2,0_0 ________ -~lrOO~ ______ ~00
-----------------------:~~---~=;~~ July ""~~
4 6 11 " \ Soybean '~~
Bare '\ I
July
20
40 ----
! ~
600. " Q
80
100
Fig. 4. Pressure head profiles at 120 noon (12:00) on July 4, 6, and II.
7 4 9 10 12 ~ 0~---k,_--~r__+----+_--_4----4_--~----~
~ T I I I 6 7 11
~1O ~ p
.C;; ~ 20
\ 1 ..• ·-::;=--
vP
-- Soybean
----- Bare
Fig. 5. Downward soil water fiux associated with rainfall in a wet soil.
Table 2. Water balance from July 4 to II (unit: mm).
Stored water on July 4 Stored water on July 11 Rainfall Pan evaporation Deep percolation by water balance method Deep percolation by fiux method
Sl S, p
E" DP DP
Soybean Bare soil
552.2 (557.1, ;;47.3) 557.3 (549.4, 565.2) 538.9 (543.6, 534.2) 543.9 (543.8, 544.0)
76.8 83.7
80.0 16.5
76.9 54.6
comparing Eqs. 2 and 3. As the soil water flux between July 4 and 11 always moved downward, the left hand side of Eqs. 2 and 3 represented deep percolation. The amounts of deep percolation at 94-cm depth determined by Eq. 2 were 83.7 mm in the soybean plot and 54.6 mm in the bare plot, respectively. To calculate the amount of deep percolation using Eq. 3, data of stored water, rainfall, and evaporation or evapotranspiration are needed. The amount of water stored at the 0- to 94-cm depth was obtained from the soil water content profiles measured on July 4 and 11 (Table 2). The amounts of water within the replications were not the same on both days and in both plots. Considering that 1% difference in volumetric soil water content in 1 m soil column is equivalent to 10 mm water and that the
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Soil Water Flux at l-m Depth in Andisols 143
soil profile was not homogeneous but stratified, these differences may be due to the spatial variability of the soil. The average values of stored water suggested that the bare plot contained 5 mm more water than the soybean plot but the differences in the amount of stored water in the respective plots between two days, Sl - S2' were 13.3 and 13.4 mm. The above results are in agreement with the pressure head profiles in which there was no appreciable difference between the plots and a slight difference between July 4 and 11 (Fig. 4). Total amount of rainfall during this period was 80 mm. Evaporation Ea and evapotranspiration Et
were not measured in the experiment. Since the soils were wet during this period, we assumed that Ea and Et corresponded to the pan evaporation Ep as a first approximation. The amounts of deep percolation calculated from Eq. 3 for both plots were 76.8 and 76.9 mm, respectively, as shown in Table 2. The amounts of deep percolation determined by the flux method were about 10% higher in the soybean plot and about 30% lower in the bare plot than those obtained by the water balance method. Judging from the pressure head profiles and the amount of stored soil water, the amount of deep percolation calculated from the flux method in the bare plot was lower than expected. If we did not take into account the hysteresis between the hydraulic conductivity and pressure head and considered the values of the primary drying curve, the amount of deep percolation determined by Eq. 2 would become 127.2 mm for the soybean plot and 106.2 mm for the bare plot. These values were much larger than the amount of rainfall and were unrealistic.
The discrepancy in the amount of deep percolation between the flux method and the water balance method may be ascribed to the following factors. A major reason may be the existence of non-uniform infiltration. Saffigna et al. (1976) pointed out that stream flow along the stem of potato plants, leaf drop from the outer foliage, and runoff from the sides of a hill resulted in non-uniform infiltration. Ghodrati and Jury (1990) supplied 100 mm of water to a loamy sand soil without visual cracks and channels, and they observed that vertical fingering of dye traces extended more than twice as deep as the mean displacement of the dye. These studies suggest that non-uniform infiltration must have occurred in our field due to heavy rainfall, because non-uniform infiltration leads to a higher or lower pressure head in a certain part of the cross-section than the average, and to the difference in downward soil water flux from one area to another. With the progress of drainage, the pressure heads at a given depth became identical due to the redistribution of water as indicated in the pressure head profiles of both plots on July 11 (Fig. 4).
Upward soil water flux during a dry period Changes in the values of soil water fluxes and pressure heads in the 3B21 layer of both
plots from July 21 to August 20 are illustrated in Fig. 6. Daily soil water flux was obtained using pressure heads at midnight (24:00). The soil water fluxes in both plots moved upward by July 26, and 6.5, 23, and 16 mm of water supplied by rainfall in August were absorbed in shallower depths and did not result in downward flow through the 94-cm depth. The upward flux in the bare plot increased to 0.5 mm d- 1 on August 1 for about one week then decreased due to rainfall. On the other hand, the upward flux in the soybean plot reached a value of 1.0 mm d- 1 on August 2 and the flux values ranged between 0.9 and 1.1 mm until August 20. Root length distribution measured on August 30 showed that the rooting depth was shallower than 80 cm, indicating that direct absorption of water by the roots was not included in the upward flux. Water uptake by roots in depths shallower than 80 cm generated larger hydraulic gradients at the 94-cm depth as shown in Fig. 6 and transmitted more water through this depth in comparison with the bare plot.
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144 S. HAS EGA W A, S. OSOZA W A, and H. UENO
-300
-100
., "0
lOf 20 (mm)
21 July
Daily rainfall
-.51 5
1 August
Upward
I I I 16 79
23 Depth 83 em
104
21
i ~Hr~~~~~----------------------~-+-=~=~~~----~ ,,~-~
B-1 " ~ -2 " Downward
'0
'" Fig. 6. Pressure head and soil water flux during a dry period.
Table 3. Water balance from July 31 to August 8 (unit: mm).
Soybean Bare soil
Stored water July 31 Sl 506.6 511.5 Stored water August 8 S2 524.9 529.4 Rainfall P 29.5 Upward flow UP 8.7 4.0 Pan evaporation E. 20.9 Evapotranspiration E, 19.9 Evaporation E. 15.6
The total amount of upward flux calculated by Eq. 2 during this period in the soybean plot was 23.3 mm. The upward flow was clearly important as a source of water for crop growth even though this amount was much smaller than the evapotranspiration during this period. The total amount of upward flux in the bare plot was 7.7 mm which was approximately one-third of that in the soybean plot.
If the upward flow in the water balance equation is neglected, evaporation and evapotranspiration should be underestimated compared with the actual values. The evaporation from the bare plot and the evapotranspiration from the soybean plot were predicted by Eq. 3 using the amount of stored water in the soil profiles measured on July 31 and August 8, the amount of upward flow obtained from Eq. 2 and rainfall during this period. Table 3 shows the results of the calculation. As the rainfall amounted to 29.5 mm during this period, the amounts of stored soil water on August 8 were larger than those on July 31. The difference in the amount of stored soil water, S2 - Sl was almost the same in both plots and only the amounts of upward flow were different between the two plots. As indicated in the table, the pan evaporation was the largest and the evapotranspiration was close to it, whereas
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August 21l 21
o rtJ
20
....... 0 'i .rl
Soil Water Flux at I-m Depth in Andisols
22 23 24 25 26 27 28
- Soybean
---- Bare
~ ~------- --- -~
.':1\'_ ...
>< S 0.5 p S I;::l~
~ 1.0
~ <= ~
, , ,
.~
~ V .,-
., -'
~ 1.5 c:l
29
1"
'. - -
Fig. 7. Downward soil water flux associated with rainfall in a dry soil.
145
30
the evaporation was the lowest. It is reasonable to assume that the evaporation from the bare plot was lower than the pan evaporation. As the 35 cm surface soil was too dry to measure the pressure heads with the tensiometers except after the rainfall on August 7, the evaporation from the soil surface should be lower than the potential evaporation. The values of both the evaporation and the evapotranspiration were about half of that of the pan evaporation during this period when the upward flow was not taken into consideration. The values of upward flow and deep percolation are essential to calculate the evaporation or the evapotranspiration based on the water balance as stressed by van Bavel et al. (1968). In our experiments, an error incidental to the water balance method was caused by the estimation of stored soil water by direct soil sampling.
As the drying process at 94-cm depth induced by evaporation as well as evapotranspiration was so slow in comparison with infiltration, the pressure head at the cross-section of 94-cm depth in each plot was considered to be in equilibrium. Therefore, the tensiometer readings during the drying period were more representative than those during infiltration.
Downward soil water flux associated with rainfall in a dry soil After the relatively dry period, 108.5 mm of water was supplied by rainfall from August
20 to 21. Before the rainfall, soils drier than a value of - 500 cm pressure head were found at 67 and 35 cm depths in the soybean and the bare plots, respectively. Figure 7 shows the soil water flux and the rainfall distribution. The total amount of deep percolation associated with this rainfall event until August 30 in the soybean plot was 62.3 mm and that in the bare plot was 47.9 mm. The maximum pressure head which generated the peak downward flux in the soybean plot was - 30 cm and that in the bare plot was - 40 cm. As we did not measure the soil water content profiles during this period, it is difficult to evaluate the amount of deep percolation by the water balance method. However, the cause of the discrepancy in the values between the two plots must be the same as that observed under the wet conditions.
Changes in downward fluxes with time through the dry soil indicated in Fig. 7 and those through the wet soil indicated in Fig. 4 show the effect of soil water conditions on the water
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146 S. HASEGAWA, S. OSOZA WA, and H. UENO
movement. Maximum soil water flux in each plot was observed several hours after rainfall on July 6, whereas more than 15 h were needed before the maximum val ue of the soil water flux was attained after the rainfall event on August 20. Such a rapid infiltration into the wet soil was also observed for the second peak of rainfall on August 21. These phenomena could be described by the piston flow model as well as the Richards' potential equation of water movement. According to the piston flow model, water infiltrating into a wet soil expels a large amount of water in the soil ahead, which results in the rapid progression of the wetting front in comparison with a dry soil. The above phenomena were well monitored in the field by our method.
CONCLUSION
Soil water flux at a depth below the soybean root zone was estimated in an Andisol field. Downward soil water flux at 94-cm depth associated with heavy rainfall occurring twice and amounting to 65.0 and 85.5 mm for 24 h took place under unsaturated conditions due to the high unsaturated hydraulic conductivity of the Andisol. However downward soil water flux at a specific site at a given depth during infiltration is likely to be more or less affected by non-uniform infiltration and may not be representative of the mean value at that depth in the field. Considering the fact that the highest soil water pressure affected the magnitude of the soil water flux significantly, the evaluation of the highest soil water pressure by infiltration which fluctuates horizontally at the same depth is likely to improve the reliability of our method. At the tailing time of drainage and during the period when water flowed upward, the magnitude of the soil water flux was less than a few millimeters per day so that the soil water was redistributed to equilibrate the pressure head. Our method enabled to estimate adequately the upward flow during this period. It is important to consider that an Andisol can supply water at the rate of 1 mm d- 1 toward the root zone during dry spells. We need an improved method to represent the hysteresis between hydraulic conductivity and pressure head.
Acknowledgments. We thank Dr. H. Seino for providing the meteorological data, Mr. H. Katou for his helpful discussions, and Prof. S. Iwata for his valuable comments on this study. This study was partly supported by a research grant on "Improvement of Function of Water Conservation and Management in Agriculture and Forestry" from MAFF, Japan.
REFERENCES
Ghodrati, M. and Jury, W.A. 1990: A field study using dyes to characterize preferential flow of water. Soil Sci. Soc. Am. J., 54, 1558-1563
Hasegawa, S. and Maeda, T. 1977: Structure change in soils affecting water retention and unsaturated flow. Trans. Jpn. Soc. Irrig. Drain. Reclam. Eng., 72, 22-29 (in Japanese with English abstract)
Iwata, S. 1971: Measurement of unsaturated permeability. J. Sci. Soil Manure, Jpn., 42, 441-442 (in Japanese)
Ogata, G. and Richards, L.A. 1957: Water content changes following irrigation of bare-field soil that is protected from evaporation. Soil Sci. Soc. Am. Proc., 21, 355-356
Reicosky, D.C., Doty, C.W., and Campbell, R.B. 1977: Evapotranspiration and soil water movement beneath the root zone of irrigated and nonirrigated millet (Panicum miliaceum). Soil Sci., 124, 95-101
Richards, L.A. and Moore, D.C. 1952: Influence of capillary conductivity and depth of wetting on moisture retention in soil. Trans. Am. Geophys. Union, 33, 531-539
Rose, C.W., Stern, W.R., and Drummond, J.E. 1965: Determination of hydraulic conductivity as a function
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Soil Water Flux at l-m Depth in Andisols 147
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