沙漠研究 22-1, 143 -146 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Desalinization of a Salt-affected Field
Using a Rice Husk Underdrainage SystemKoji INOSAKO*1), Keigo YASUNAGA2), Naoyuki TAKESHITA3), Tadaomi SAITO1) and Mitsuhiro INOUE4)
Abstract: Excess salt and water must be drained from fields to avoid salt accumulation in dry regions. An underdrainage systemeffectively maintains adequate soil salinity and moisture. However, the installation of modern underdrainage systems is too
expensive for developing countries. Therefore a more affordable underdrainage system is required. A rice husk underdrainagesystem (RHUS) is one alternative. In this study, model experiments clarified the process and the limit of desalinization in RHUS. A
leaching experiment using a soil tank was performed in a laboratory, and the results were analyzed by a numerical model. The
HYDRUS 2D/3D code was used for the numerical experiments. The results clarified the following: 1) when leaching water is
supplied to the surface soil, the salt and water in the plow layer immediately move to the rice husk zone and are discharged out of thesoil system; 2) the water and salt movements in the no-tilled zone (NTZ) were very small, and it is very difficult for RHUS to desalt
the NTZ and avoid the re-accumulation of salt; 3) making a deep plow zone enhanced the desalinization ability of RHUS; 4) theconnection between plow and rice husk zones is very important to increase the desalinization performance of RHUS.
Key Words: Arid region, Leaching, Numerical analysis, Salinity, Solute transport

1. Introduction
Salt accumulation is a great problem for agriculture in arid
and semi-arid regions. Before planting, farmers often irrigate
their fields with a large amount of water to reduce the soilsalinity. This temporary moves the excess salts to the lower
soil layers, but after the crop is harvested, the salt easily returns
to the surface layers in impermeable soils (Yamamoto, 2009).
For sustainable agriculture, excess salts must be removed
from fields. Although an open-channel drain is an ordinary
facility for the salt and water management of a field, it is noteffective in removing salts from fields (Kitamura et al., 2006).
Therefore, an underdrainage system is necessary for effective
desalinization of salt-affected fields.
High permeable filter materials and drain pipes are used inmodern underdrainage systems. A construction guide of sucha system has been written by the Japanese institute of irrigationand drainage (1993) and its performance is very good.
However, the cost is unreasonable for farmers. Especially, it
is financially difficult for developing countries to introduce thesystem into all salt-affected farmlands. Therefore, thedevelopment of a more affordable underdrainage system isneeded for these countries. The rice husk underdrainagesystem (RHUS) is one alternative. Inosako and Ohara (2004)
applied it to a salt-affected paddy field in Tanzania andevaluated its performance. The amount of drainage withRHUS was less than a modern underdrainage system with acorrugated pipe. However, the salinity concentration of thedrained water from RHUS was three times that of the modernsystem, and the salt load removed by these systems was about
the same. Therefore, they concluded that RHUS effectively
removed the accumulated salts from salt-affected paddy fields.
However, the process and the desalinization limit in RHUS
were unknown and the construction method was notestablished. In this study, we did leaching and numericalexperiments using a soil tank to clarify the process and limit ofthis system.

2. Materials and Method
2 .1. Leaching experiments in a laboratory
2.1.1. Soil tank
Leaching experiments were performed in a laboratory witha 50-cm long, 100-cm high and 15-cm wide soil tank (Fig. 1)that has a ceramic filter at the bottom to simulate soil condition.
A 4-cm diameter outlet was made in its lower left corner.
2.1.2. Outline of experiments
Light clay soil was packed into the tank by a hydraulicfilling method to a depth of 60 cm with a bulk density was 1.20g/cm3. An 8000 cm3 solution of NaCl with a concentration of
0.03 mmol/cm3 was poured into the soil, which was dried bylamps until salt accumulation was confirmed on its surface.
Next, a 10-cm long, 60-cm deep ditch was made on the* Corresponding Author: inosako@muses.tottori-u.ac.jp
4-101, Koyama-cho Minami, Tottori, 680-8553, Japan
1) Faculty of Agriculture, Tottori University3) Kurashiki City Office2) MEC INC.

4)Arid Land Research Center, Tottori University

(cm/s), t is time (s), ρb is the bulk density (g/cm3), Kd is the
distribution coefficient (cm3/g), D is the hydrodynamic
dispersion tensor (cm2/s), and q is the Darcy-Backingham fluxvector (cm/s).
The plow changes the surface layer’s soil structure, andparticularly increases the volume of macropores. Therefore,it changes the soil hydraulic properties. The Durner model(Durner, 1994) was adopted as a soil hydraulic model that canconsider the effect of artificially made macropores:Se θ  θrθs  θ rn w1 1  1h 1Fig. 1. Schematic diagram of the soil tank, (a) filled soil zone (FZ),(b) rice husk zone (RHZ), (c) plow zone (PZ), (d) no-tilledzone (NTZ).

tank's left. Rice husk was placed in the ditch up to 40 cm
deep with a bulk density of 0.12 g/cm3 and called the rice husk
zone (RHZ). The remaining part of ditch was filled withsurface soil and called the filled zone (FZ). Except for the FZ,
a 10-cm deep area of the surface zone was plowed with a handhoe; and called the “plow zone (PZ)”. The bulk density of theplowed layer was 1.09 g/cm3. The tilled zone (TZ) consistedof both of the FZ and PZ, whereas the no tilled soil zone (NTZ)
was the remaining area.
Eight thousand cm3 of tap water at a concentration of
0.00085 mmol/cm3 was added to the soil tank for leaching andsettled for 24 hours. Then, the outlet was opened and theamount of discharged water and concentration of solute weremeasured. The bottom of the filter gave a pressure head of-10 cm during the experiment.
2.2. Numerical analysis
2.2.1. Numerical model
HYDRUS 2D/3D software ver.1.11 (Šimůnek et al., 2006)
was used for numerical analysis. In this model, water flow
and solute transport were calculated using following equations; K h  1t(1)b K d   C   DC  qC t33

 m1

n2  m22(3)2where θr and θs are the residual and saturated volumetric watercontent, αi, ni, and mi are the experimental constants, wi is theweighting factor for the subcurves , and w1+w2=1.
2.2.2. Simulation of leaching experiment
We conducted a simulation of a leaching experiment using
the soil tank to analyze the water and solute movements in
RHUS. A simulation domain was the rectangular column ofsoil in the tank. The simulations were divided into four steps.
In the first step, salt accumulation occurred by evaporation.
The soil tank was uniformly filled with light clay soil. The
initial pressure head and concentration were given as -1.0 cm
and 0.03 mmol/cm3. We assumed that evaporation of 6mm/d continued for two days. The boundary condition at thesoil bottom (the lower boundary condition) was given at aconstant pressure head of -10 cm.
The results of the first step were given as the initialconditions for the second step. The TZ and RHZ were newly
set up in the simulation domain. The hydraulic and solutetransport parameters of these materials are shown in Table 1and Figure 2. The variable and constant pressure heads wereused as the upper and lower boundary conditions of thesimulation domain, respectively. The variable pressure heads
were -10 cm in the first 300 seconds, -1 cm from 300 s to 3600s, and -0.1 cm from 3600 to 32,200 s. The condition wasarbitrarily determined as the amount of supplied water became
8000 cm3.
Table 1. Hydraulic and solute transport property of materials. Itwas assumed that the FZ parameters were the same as the tilled
light clay soil. λL and λT were the longitudinal and transverse
dispersivity.
Material(2)where θ is the volumetric water content (cm /cm ), h is thepressure head (cm), K is the hydraulic conductivity tensor

  w 1   h 

Rice huskKsρbλLλT

Kd(cm/s)(g/cm3)

(cm)

(cm)

(cm3/g)

0.33

0.12

0.48

0.048

0-5

Light clay soil

2.7x10

1.20

3.10

0.31

1.12

Tilled light clay soil

0.01

1.09

3.10

0.31

1.12

30

0.2
0010

ECwFlow

010

121010Pressure head (cm)

31004500

1000

1.0

Time (s)Fig. 3. Change of drainage rate from soil tank.

Fig. 2. Soil water retention curves of materials.

0.5

FZ

depth

lengthdepth

lengthCase 1002010

Case 2

45000

Case 354000Solute flux (mmol/s)

Table 2. Size of the FZ and PZ in the numerical experiment 2.
PZ

1.210EC w(dS/m)

0.4

1.4

2030.6

Flow (cm /s)

33

Water content (cm /cm )Tilled light clay soil
Light clay soil
Rice husk

0.8

0.4

Observation
Estimation

0.3
0.2
0.1
00100(cm)200300Time (s)Fig. 4. Comparison of solute flux between observation and estimation.

3. Results and Discussion
3.1 Features of desalinization in RHUSFigure 3 shows the changes of the discharge rate and ECw
in drainage from the outlet. The maximum rate reached 30
cm/s at the beginning and exponentially decreased. It became
1/10 of the initial rate after 180 s. The electric conductivity ofthe effluent was stable around 1.2 dS/m. Although the ECwof the leaching water was 0.00085 dS/m, the water dissolved
the salts in the surface layer and moved to the rice husk zone ina day. Desalinization of the soil surface occurred.

NTZ
Degree of satulation (%)

In the third step, the model simulated the condition wherethe soil tank was left standing on the floor for 24 hours using
the results of the second step as the initial conditions. Theupper and lower boundary conditions were no flux andconstant pressure head of -10 cm.
In the fourth step, the seepage condition was given at theoutlet point as a boundary condition. The results of third step
were used as the initial condition. Moreover the boundary
conditions were identical of them in the third step.
2.2.3. Numerical experiments
Numerical experiments were conducted to clarify theeffective desalinization factors in RHUS. The focal points
were the effect of the plow zone thickness and the tilled zone
structure on desalinization. The PZ thickness was changed a0, 5, 10, 15, and 20 cm in numerical experiment 1.
Numerical experiment 2 was conducted under the conditions inTable 2. In cases 2 and 3, the PZs were not connected to therice husk zone. Moreover, in case 3, the PZ was in contact
with the right side of the calculation domain. Therefore,
20-cm deep and 10-cm long part on the RHZ was NTZ.

100TZ80

RHZ

6001000

2000
Time (s)3000

Fig. 5. Change of saturated ratio of each zone.

Figure 4 compares the solute flux between the observationand estimation in the leaching experiment. The estimation
was in good agreement with the observation. The numericalmodel reproduced the leaching experiment well.
Figure 5 shows the change of degree of saturation of eachzone from the beginning of the drainage up to 3600 s. Since
the degree of saturation of the NTZ was very high and stable,
drainage in this zone did not proceed. The salt, whichpercolated into this zone, couldn’t be drained by RHUS. Thereduction of the degree of saturation of TZ was also small,because most of the water drained from this zone had already
moved to the RHZ during the leaching (simulation of thesecond and third steps). Only the water stored in the RHZ
was discharged from the outlet. The salts and water in theNTZ were not removed and only the TZ was desalted byRHUS.
3.2. Effective factors on desalinization in RHUS
3.2.1. Thickness of plow zone
Numerical experiments were performed to clarify therelationships between the solute flux and the PZ thickness. Inall simulations, the initial conditions of the pressure head andthe solute concentration were the same as the soil tankCum. solute flux (mmol)

706050403005101520Thickness of plow zone (cm)Cum. solute flux (mmol)

Fig. 6. Relationships between cumulative solute flux and PZ thickness.

leaching experiments using a soil tank in laboratory andnumerical experiments. This study clarified the following: 1)
when leaching water was supplied to the surface soil, the salt
and water in the plow zone immediately moved to the ricehusk zone and were discharged out of the soil system; 2) thewater and salt movements in the no-tilled zone were very small,and it was very difficult for RHUS to desalt the no-tilled zone
and to avoid the risk of the re-accumulation of salt; 3) making a
deep plow zone enhanced the ability of desalinization ofRHUS; 4) the connection between plow and rice husk zones is
crucial to increase RHUS desalinization.

34

Acknowledgement
This study was financially supported by “the collaboration
study of Arid Land Research Center, Tottori University”,
Global-COE project of JST, and the Grants-in-Aid for
Scientific Research of JSPS.

32302826
Case 1Case 2

Case 3Structure of tilled zone

ReferencesFig. 7. Effect of structure of tilled zone on cumulative solute flux.
3simulations. The amount of the leaching water of 8000 cm
was also the same.
As shown in Figure 6, the solute flux was proportional tothe PZ thickness despite the same amount of leaching water.
Therefore, a deep PZ increases the soil’s permeability and theamount of salt transported to RHZ, suggesting that PZ plays acrucial role for increasing the desalinization efficiency inRHUS.
3.2.2. Effect of TZ structure on desalinization in rice husk
underdrainage system
Figure 7 shows the effect of the TZ structure on thecumulative solute flux. Although the areas of all cases were
the same, their cumulative solute fluxes were clearly different.
In particular, the presence of FZ greatly influenced
desalinization in RHUS, because the cumulative solute flux incase 1 was the largest among them. The results suggest thatthe connection between PZ and RHZ is crucial to effectively
desalinize fields using RHUS.

4. Conclusions
A rice husk underdrainage system (RHUS) is a reasonable
method for desalinization. In this study, the process and thelimit of desalinization in RHUS were investigated throughDurner W. (1994): Hydraulic conductivity estimation for soils
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