Infiltration Characteristics of Tropical Soil Based on Water Retention Data

215原著論文

原著論文

水文・水資源学会誌第21巻第3号（2008）

Infiltration Characteristics of Tropical Soil Based on Water Retention Data

Ⅰ．INTRODUCTION

Infiltration is the physical process of water

entering the soil from its surface. The amount of

water that infiltrates into the soil and its variation

with time depend upon slope, soil structure,

surface roughness, soil texture, surface cover,

hydraulic conductivity and surface water content(Leonard and Andrieux, 1998). It plays important

role for agricultural planning, environmental

research and policy analysis such as development

of plant irrigation, fertilizer and soil nutrition

movement, surface and subsurface water pollution,

and groundwater recharge (Netto et al., 1999;

Dingman, 2002).

In the tropical region such as Indonesia, soils

often receive high precipitation and subject to

Muhamad ASKARI 1) Tadashi TANAKA 2)

Budi Indra SETIAWAN 3) Satyanto Krido SAPTOMO3)

1)Ph.D. student of Graduate School of Life and Environmental Sciences, University of Tsukuba（Ibaraki 305-8572, Japan）

2)Graduate School of Life and Environmental Sciences, University of Tsukuba（Ibaraki 305-8572, Japan）

3)Department of Agricultural Engineering, Bogor Agricultural University,

Kampus IPB Darmaga（Bogor 16680, Indonesia）

水文・水資源学会誌J. Japan Soc. Hydrol. and Water Resour.Vol. 21, No.3, May 2008 pp. 215 - 227

Water retention and hydraulic properties of undisturbed tropical soils collected from many regions of

Indonesia were analyzed to estimate infiltration characteristics of the soils. Soil texture was classified based on

International Society of Soil Science (ISSS) classification. The van-Genuchten model was used to estimate the

relationship between water content and matrix potential at pF=1, pF=2, pF=2.54, pF=4.2. The 165 soil water

retention data were used to optimize parameters of the model and to find the air entry value. Green-Ampt and Philip's

infiltration models were applied to characterize soil infiltrability of each textural type. The Nash and Sutcliffe's

efficiency was used to evaluate numerical simulation of cumulative infiltration of Green-Ampt's infiltration model

compared to the results of laboratory experiments. The 165 soil samples were classified and were optimized into 10 ISSS

textural types: heavy clay, sandy clay, sandy clay loam, sandy loam, sand, light clay, clay loam, loam, silty clay, and silty

clay loam. The results of performance evaluation of Green-Ampt's infiltration model showed that Green-Ampt's

infiltration model can describe infiltration characteristics by using soil water retention and hydraulic properties

data. The tropical soils based on soil texture exhibit contrasting infiltration characteristics as indicated by

infiltration rate, length of wetting front and sorptivity. The characteristics of soil infiltrability are mainly influenced by

hydraulic conductivity, initial water content, and matrix potential at the wetting front.

Key words: soil water retention, tropical soil, ISSS texture classification, Green-Ampt, wetting front, sorptivity

216 原著論文

loose their top soils due to run-off and soil erosion

especially when their surfaces openly exposed to

the atmosphere. In this situation, soil infiltrability

reduces with times because of the crust formation,

or the exposure of subsoil whose relatively dense

on the soil surface after its top soil is being

removed by soil erosion process. Soil infiltrability

which is variant of soil textures also changes with

their bulk density and initial water content. These

conditions become big constraints to measure

infiltrability of soils in the fields. Therefore,

numerical simulation models of soil infiltrability

will be very important in understanding this

process.

Many water flow problems near the soil surface

can only be solved numerically due to soil

heterogeneity, non-linearity of soil physical

properties, non-uniform root water uptake and

rapid changing boundary conditions. Water flow in

the vadose zone in term of infiltration process is

predominantly vertical, and commonly can be

simulated as one-dimensional flow in many

applications (Romano et al., 1998). By running the

one-dimensional model at various locations,

horizontal variability of meteorological conditions,

crop characteristics, soil properties and drainage

conditions is accommodated and regional water

can be determined (Bresler and Dagan, 1983;

Hopmans and Stricker, 1989).

Soil water retention and hydraulic data which

are collected at a great number of soil physical

laboratories (Rawls and Pachepsky, 2002; Hodnett

and Tomasella, 2002) enhance the applicability of

the some equations related to infiltrability of the

soil such as Richards-Darcy's, Philip's and Green-

Ampt's infiltration model (Wang et al., 1997;

Romano et al., 1998; van Dam and Feddes, 2000;

Braud et al., 2005; Regalado et al., 2005; Kozak

and Ahuja, 2005). Most of studies concerning the

application of soil water retention and hydraulic

properties on simulation of the soil infiltrability

have been widely conducted at subtropical region.

On the contrary, the characteristics of soil

infiltrability at tropical region using those data

have not been extensively studied. In Indonesia,

the application of soil water retention and

hydraulic data on the estimation of soil

infiltrability has been recently conducted on sand

and silty clay textures (Saleh, 2000) and sandy

clay texture (Hermantoro, 2003) using Richards-

Darcy's infiltration model.

The objective of present study, therefore, was to

optimize soil hydraulic function using van

Genuchten's equation and to estimate the

infiltration characteristics of various tropical soil

textures using the optimized soil hydraulic

function data.

Ⅱ．INFILTRATION MODEL

The one-dimensional, downward-infiltration

flow system is depicted schematically in Fig. 1a.At fixed time t 0 after the instantaneous ponding

of water depth hc at t = 0 on the top surface of the

soil (Z = 0), position coordinate Z being taken as

positive downward. Volumetric water content =(Z, t) for independent Z and t in general, Z0 is

the depth above which = 0 at time t (for zero

J. Japan Soc. Hydrol. and Water Resour.Vol. 21, No.3, 2008

Fig. 1 (a) Schematic diagram of the ponded-water, onedimensional downward water-infiltration flow problem;and (b) two exemplary graphs of water-content profilescorresponding to the uniform soil column in (a)(Swartzendruber, 2000).

217原著論文

air entry value), and 0 Z Z0 is the region of

constant water content 0. At t = 0; the uniform

soil had been at constant initial water content n

throughout its semiinfinite extent Z 0.

Corresponding to the flow column in Fig. 1a,

schematic graphs (profiles) of water content

against depth Z at fixed time t are shown in Fig.1b. The profile for a general soil is indicated by

the solid heavy curve that is constant at 0 for 0

Z Z0 (water content changes little as tension

increase up to a point of inflection). This more or

less distinct point represents to the tension at

which significant volume of air begins to appear in

the soil pores and is called air entry tension. As

tension increases beyond its air entry value, the

water content begins to decrease rapidly and then

more gradually (from 0 at Z0 to n at Zn). At

very high tensions, the curve again becomes nearly

vertical reflecting a residual water content ( n

after Zn).

Misra et al. (2003) catagorized Green-Ampt and

Philip's infiltration model as the mathematical

solutions to physically based theories of

infiltration. Green-Ampt assumed a piston-type

water content profile (Fig. 1b) with a well-defined

wetting front. The piston-type profile assumes the

soil is saturated at a volumetric water content of

0 (except for entrapped air) down to the wetting

front. At the wetting front, the water content drops

abruptly to an antecedent value of n, which is the

initial water content. The soil-water pressure head

at the wetting front is assumed to be hf (negative).

Soil-water pressure head at the surface, h0, is

assumed to be equal to the depth of the ponded

water. Corresponding to Fig. 1b, the Green-Ampt

profile (heavy broken line) remains at 0 for 0

Z L but drops in abrupt, stepwise manner to n at

Z = L and remains at n for Z L.

Using the assumption described above, the

Green-Ampt's infiltration equation takes the form(Hillel, 1980):

(1)

where i (t) is the infiltration rate at time t, I (t) is

the cumulative infiltration at time t, and is equal to

Z・( 0－ n), KS is the hydraulic conductivity

corresponding to the surface water content (the

saturated hydraulic conductivity), and Z is the

length of wetting front.

The mathematical and physical analysis of the

Philip's infiltration model (Philip, 1957)

separated the infiltration process into two

components - that was caused by a sorptivity

factor and was influenced by gravity. Sorptivity is

the rate at which water will be drawn into a soil in

the absence of gravity; it comprises the combined

effects of adsorption at surfaces of soil particles

and capillarity in soil pores. The gravity factor is

due to the impact of pores on the flow of water

through soil under the influence of gravity. The

Philip's model takes the form of a power series but

in practice an adequate description is given by the

two-parameter equation:

(2)

where i is infiltration rate, Sp is sorptivity, t is time

and Kp is a gravity factor related to hydraulic

conductivity. Sorptivity indicates the capacity of a

soil to absorb water and is the dominant parameter

governing the early stages of ilfiltration. As the

time increases, the parameter Kp becomes

important in governing the infiltration rate.

Ⅲ．METHODS

1. Classification of soil texture

Soil texture was classified based on

International Society of Soil Science (ISSS)

classification using distribution of sand, silt, and

clay fractions. The classification was conducted by

using the triangle textural references as shown in

Fig. 2.

2. Laboratory experiment

The infiltration experiment was conducted on

standard sand and loam (2 mm-sieved) soil types(Setiawan, 1992), and silty clay soil type (2 mm-

sieved) (Askari et al., 2006).

( )p

p

Kt

S

ti += − 21

2

+−==

Z

Zhh

K

dt

dI

i

f

S

0


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The apparatus used in infiltration experiment of

standard sand and loam had 20 cm of diameter and

50 cm of soil column length consisted of 10 pieces

of 5 cm soil rings. The length of soil column was

lengthened because there was 40 cm of non-

continues macropore. During the infiltration,

pressure head profile was measured by using a

pressure transducer. Output terminal of the

pressure transducer were connected to data logger.

A marriote tube was applied to supply water into

the soil surface indicating cumulative volume of

infiltration and a weighting balance was used in

order to measure draining water from the bottom

of the soil matrix. The weighing balance was

connected to a second personal computer.

The apparatus used in infiltration experiment of

silty clay adopted those in infiltration experiment

of standard sand and loam although was more

simplified and was manually operated. The

apparatus as shown in Fig. 3 had 5 cm of diameter

and 25 cm of length consisting of 5 pieces of 5-cm

soil rings. A marriote tube was applied to supply

water into the soil surface indicating cumulative

volume of infiltration. A weighting balance was

used in order to measure the change of soil column

weight during the water was infiltrated.

Inspite of the apparatus being used were

dissimilar, the measuring procedures were

commonly not different. The soil sample would be

manually compacted as uniformly as possible into

soil ring started from lower part of the soil

column. Equation (3) would be used to calculate

weight of the soil needed to be compacted into the

fixed volume of the soil ring in order to get the

expected dry bulk density. The compaction should

be done gently enough in order not to destroy soil

aggregate.

(3)

where Ws is soil weight, W is mass wetness, b is

dry bulk density, and V is volume of the container.

The physical characteristics and soil water

retention data for simulation refered to the data

presented by Askari et al. (2006), Saleh (2000)

and Setiawan (1992).

2. Numerical analysis

The 165 data of soil physical and hydraulic

properties consisted of percentage (%) of sand,

silt, and clay fractions; bulk density; organic

matter (carbon organic); saturated water content,

water content at pF 1, pF 2, pF 2.54, pF 4.2; and

saturated hydraulic conductivity which are

collected from many regions of Indonesia such as

( ) VρWWbs

1+=


Fig. 2 The soil texture references for ISSS classification. X％,Y％, Z％ represent percentage (％) of sand (0.02-2.0mm), clay (< 0.002 mm), and silt (0.002-0.02 mm)respectively (Verheye and Ameryckx, 1984 in Teh andRashid, 2003).

Fig. 3 Schematic diagram of infiltration apparatus.

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Flores, Kotawaringin Barat, Samarinda, Kutai, dan

Gorontalo (Hikmatullah and Sulaeman, 2006) were

used to optimize the parameters of soil hydraulic

function of van Genuchten (van Genuchten, 1980):

(4)

where ( ) is effective soil water content, s and

r are the saturated and residual water content,

is matrix potential, and , n and m are empirical

parameters. Modification of in Eq. (4) to be

1/ (Setiawan, 1992) will give a parameter that is

called air-entry value ( ae). Empirical parameters

of van Genuchten's soil hydraulic function are

computed from measured retention data points by

employing non-linear regression techniques, with

constrains 0, n 1, and 0 m 1 (Pereira and

Allen, 1999). Solver Add-In on Microsoft Excel

were used to optimized the parameters.

The infiltration characteristics using the Green-

Ampt's infiltration model were divided into

infiltration rate and length of wetting front. The

infiltration rate was calculated by Eq.(1).

Meanwhile, the length of wetting front was

calculated by the Eq. (5) below (as the result of

integration of Eq. (1)):

(5)

The Eq. (5) has 7 variables which are KS, 0, n,

t, Z, h0, and hf as we had previously explained

concerning infiltration model. KS and 0 were

obtained from saturated hydraulic conductivity and

saturated soil water content data respectively. n

was assumed to be equal to residual soil water

content ( r) because there were no data of soil

water content when soil sample was taken. Xie et

al. (2004) stated that if no initial water content is

obtained, it is assumed to have initial water

content equal to the residual water content. h0 was

0 cm H2O with the assumption that the soil surface

was in saturated condition without ponded water,

and hf was obtained from air entry value resulted

from the optimization of soil hydraulic function.

Another variable, Z, was determined by employing

Newton-Raphson method (Burden and Faires,

1993) on the Eq. (5).

Another infiltration characteristic that is

sorptivity, Sp, was calculated using the following

equation (Philip, 1969 in Angelaki et al., 2004):

(6)

4. Performance evaluation

Several statistical measures are available for

evaluating the performance of a model. These

include correlation coefficient, relative error,

standard error, volume error, coefficient of

efficiency (Hsu et al., 1995 in Mishra et al., 2003),

among others. The Nash and Sutcliffe efficiency(Nash and Sutcliffe, 1970 in Mishra et al., 2003)

was one of the most frequently used criteria. This

criterion is analogous to the coefficient of

determination and is expressed in percentage form

as:

(7)

where D1 is the sum of the squares of deviations

between computed and observed data:

(8)

and D0 is the initial variance which is the sum of

the squares of deviations of the observed data

about the observed mean, expressed as:

(9)

where Y0 is the observed data, Y and Y－ stand for

computed data and mean of the observed data,

respectively.

The efficiency varies on a scale of 0 to 100. It

can also assume a negative value if D1 D0,

implying that the variance in the observed and

computed infiltration values is greater than the

model variance. In such a case, the mean of the

observed data fits better than the model. The

efficiency of 100 implies that the computed values

are in perfect agreement with the observed data.

( )200 ∑ −= YYD

2

01 ∑

−=∧YYD

1001Efficiency

0

1 ×

−=D

D

( )( )fnSp

hhθθKS −−=00

2

2

( ) ( ) ( )

−+−−=

−f

f

n

S

hh

Z

LnhhZt

θθ

K

0

0

0

1

( )( )( )mn

rS

r

ψ

θθ

θψθ

α+

−+=1


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Ⅳ．RESULTS AND DISCUSSION

1. Classification of soil texture

According to the classification of International

Society of Soil Science (ISSS), the 165 soil

samples were classified into 10 textural types (Fig.4). As seeing in Fig. 4, light clay and heavy clay

are predominant (51 %). The rest (49 %) are

divided into sandy clay, sandy clay loam, sandy

loam, sand, clay loam, loam, silty clay, and silty

clay loam. Based on the existing data, the other

two textural types which are loamy sand and silty

loam, are not available.

2. Optimization of van Genuchten's soil

hydraulic function

The parameters of soil hydraulic function of

each textural type was computed from measured

retention data points by employing non-linear

regression techniques (least square error) with

constrains the saturated water content equals to

water content at total pores, the residual water

content equals to water content at pF 4.2, 0, n

1, and m =1-1/n. Besides the least square error

criteria, we should also consider the soil bulk

density data as another selection criteria of the

optimized data. This is useful in order to reduce the

effect of variability of soil structure in the field.

van Genuchten's soil hydraulic function was

able to produce best-fitting for all soil textural

types with coefficients of determination (R2) in the

range of 0.907 to 0.995 (Table 1). Soil textures

dominated by sand fraction 50 % which are clay

loam, sandy clay, sandy clay loam, sandy loam,

loam and sand have saturated hydraulic

conductivity which is equal to the addition of sand

fraction value except for loam which has high

saturated hydraulic conductivity due to the highest

silt fraction among others.

There is interesting phenomena that saturated

hydraulic conductivity of clayey-soil which are

heavy clay and light clay are higher than

remaining soils except for sandy loam, loam and

sand. These phenomena are caused not only by

pore size and its distribution but also by the

highest soil organic matter of clayey-soil. In

addition, it is might be strongly influenced by

montmorillonitic mineral content of clayey soil.

Scanning electron microscrope observation

indicated that the montmorillonitic soil had thicker

crust comprising either small particle with a very

developed washed-in zone underneath or large

ones with fine material between them (Wakindiki

and Ben-Hur, 2002).

Sandy clay has the lowest residual water content

among others. In contrast, silty clay has the

highest residual water content among others. It is

clearly indicated by silt and clay (fine mineral)


Fig. 4 Distribution of soil textures from the 165 soil samples data.

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fraction content although low soil organic matter

of silty clay. Silty clay loam has the highest

saturated soil water content among others due to

particle size distribution of this soil is dominated

by silt and clay and also due to high soil organic

matter content. In contrast, clay loam has the

lowest saturated water content among others.

Mitchell (1993) stated that the smaller soil

particles performed, the larger contact area

increased among its particles. As a result, the

higher micropores with steady structure occurred.

Thus, this soil has water content relatively higher

than one which is composed by larger particle(Saxton and Rawls, 2006). The occurrence of

higher soil organic matters not only will strengthen

the soil aggregate but also enhance soil capacity in

holding and storing water. This is because soil

organic matter minimizes soil compaction,

provides pores, and is able to store a quantity of

water which corresponds to a multiple of the

organic matter's weight (Emerson, 1995).

3. Performance evaluation of Green-Ampt's

infiltration model

Figure 5 shows the comparison between

observed and computed cumulative infiltration of

silty clay, standard sand, and loam soil textures as

the result of laboratory experiment. Generally,


Table 1 Saturated hydraulic conductivity and van Genuchten's soil hydraulic function parameters for 10 ISSS textural types oftropical soil.

Fig. 5 The comparison between observed and computedcumulative infiltration of silty clay, standard sand,and loam soil textures.

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sand has the highest cumulative infiltration

followed by loam and silty clay respectively. The

value of efficiency of each soil textures indicated

that numerical simulation of Green-Ampt's

infiltration model agreed well with measured data.

These good agreements between experimental and

numerical results confirmed that Green-Ampt's

infiltration model can describe infiltration

characteristics using soil water retention and

hydraulic properties.

4. Infiltration characteristics of tropical soils

Optimized soil hydraulic function (as shown in

Table 1) is the input data for the estimation of

infiltration characteristic of each soil texture by

using Green-Ampt's and Philip's infiltration model.

The infiltration characteristic derived from both

models using the optimized soil hydraulic function

are infiltration rate, length of the wetting front, and

sorptivity. The three infiltration characteristics

were using the same data as shown in Table 2.Figure 6 shows time dependence of infiltration

rate in 10 soil textures of tropical soil. Sand has

the highest initial infiltration rate among others

followed by loam, sandy loam, heavy clay, light

clay, sandy clay, sandy clay loam, silty clay, silty

clay loam, and clay loam respectively. The same

pattern in final infiltration rate is also showed by

the same order.

In one side, these phenomena are equal to the

decreasing of saturated hydraulic conductivity

especially for sand, loam, sandy loam, silty clay,

silty clay loam and clay loam respectively. In

another side, infiltration rate pattern of heavy clay,

light clay, sandy clay and sandy clay loam are also

influenced by a significant difference between

matrix potential in the entry surface and matrix

potential in the wetting front (Table 2).

Subramanya (1984) stated that the distribution and

pore size of soil will directly influence its

infiltration rate. A loose, permeable, sandy soil

will have a larger infiltration capacity than a tight,

clayey soil. The existing of soil organic matter

when water flows under the soil, the soil pores will

not be covered by clay particles or damaged soil

aggregate.

Figure 7 shows results of numerical simulation

indicating advances of length of the wetting front

in the soil matrix. Since the water infiltrates

through the soil surface only, the length of the

wetting front vertically downward as the time

increases. With the same time elapsed, sand has

the biggest Z value and is followed by sandy loam,

loam, heavy clay, light clay, silty clay, sandy clay

loam, sandy clay, clay loam, and silty clay loam

respectively. The addition pattern of Z value is

different with infiltration rate caused not only by

the influence of wetness increment between

saturated water content and the initial soil water

content but also the difference between matrix

potential in the entry surface and matrix potential

in the wetting front (Table 2).


Table 2 Parameters of Green-Ampt's infiltration model for 10 ISSS textural types of tropical soil.

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Fig. 6 Time dependence of infiltration rate in 10 ISSS textural types of tropical soil using Green-Ampt's infiltration model.

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Fig. 7 The length of wetting front during 1.5 hours infiltration in 10 ISSS textural types of tropical soil.

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Figure 8 shows sorptivity characteristics as a

result of antecedent soil water content changes.

Generally, the sorptivity value is decreasing along

with the increasing of antecedent soil water

content. On the other hand, the effects of

adsorption at surfaces of soil particles and

capillarity in soil pores are decreasing along with

the increasing of antecedent soil water content.

As the water content approaches saturation,

sorptivity tends to zero and the infiltration rate

becomes equal to the field saturated hydraulic

conductivity. As the time increases, gravity factor

becomes important in governing the infiltration

rate. This implies that the steady infiltration rate

reached after a long time should be largely

independent of the antecedent water content(Philip, 1957).

Although the sorptivity is decreasing along with

the increasing of the antecedent soil water content,

the response for each soil texture is different. It is

seen clearly that the texture of sand, loam, sandy

loam, heavy clay and light clay give drastically

decreasing sorptivity value among others. The

drastic changes are equal to saturated hydraulic

conductivity, wetness increment between saturated

water content and the initial soil water content, and

difference between matrix potential in the entry

surface and matrix potential in the wetting front.

Furthermore, the value of soil sorptivity can be

used to indicate the origin and the environmental

condition of soil in the field. Materechera et al.(1993) stated clearly that soil from planted

treatments had higher sorptivities than soil which

had not been planted due to biopores left by the

roots.

Ⅴ．CONCLUSIONS

Parameters of van Genuchten's soil hydraulic

function of tropical soil were optimized for 10

ISSS soil textures which are heavy clay, sandy

clay, sandy clay loam, sandy loam, sand, light

clay, clay loam, loam, silty clay, and silty clay

loam. Most of them are dominated by sand mineral

fraction. The results of performance evaluation of

Green-Ampt's infiltration model using standard

sand, loam, and silty clay soil textures showed that

Green-Ampt's infiltration model can describe

infiltration characteristics using soil water

retention and hydraulic properties data. The

tropical soils based on soil texture exhibit

contrasting infiltration characteristics as indicated

by infiltration rate, length of wetting front and

sorptivity, in which the characteristics of soil


Fig. 8 The profile of sorptivity characteristics as a result of antecedent soil water content changes. Initial water content equals to-10, -50, -100, -250, -346.7, -500, -750 and -1000 cm H2O of matrix potential.

226 原著論文

infiltrability are mainly influenced by hydraulic

conductivity, initial water content, and matrix

potential at the wetting front.

This study can be used to estimate soil

infiltrability in a field scale with previously known

its soil properties. However it still needs to

consider inhomogeneous of initial water content in

the soil profiles.

ACKNOWLEDGEMENTSThe authors would like to thank Mr. Trisnadi

and Mr. Rudiyanto of Department of Agricultural

Engineering, Bogor Agricultural University for

their assistance to achieve laboratory experiment

and numerical simulation of infiltration. The data

of soil physical and hydraulics properties were

provided by Mr. Yiyi Sulaeman of Indonesian

Center for Agricultural Land Resources Research

and Development. We are also grateful to

anonymous reviewer for their help to improve the

quality of this paper. This study has been

supported by Directorate General of Higher

Education of Ministry of Education of Indonesia

through The Postgraduate Program Scholarship(BPPS).

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227原著論文

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Xie M, Esaki T, Cai M. 2004. A time-space based approach formapping rainfall-induced shallow landslide hazard.Environmental Geology 46 : 840-850.

（Received：Sep.13,2007, Accepted：Feb.8,2008）


土壌水分保持データに基づく熱帯土壌の浸透特性

熱帯土壌の浸透特性を予測するため，インドネシアの多くの地域から採取された未撹乱土壌の水分保持特性と水理特性を分析した．採取した土壌の土性は国際土壌科学学会の分類法に基づいて分類した．また，pF=1，pF=2，pF=2.54，pF=4.2で体積含水率と土壌水分張力との関係を予測するためにvan-Genuchtenモデルを適用した．このモデルのパラメータを最適化するために，また空気浸入値を予測するために合計165個の土壌水分保持特性データを使用した．それぞれの土性の浸透能特性を明らかにするために，Green-AmptとPhilipの浸透モデルを適用した．さらに，室内実験結果との比較において，Green-Amptの浸透モデルによる累積浸透の数値シミュレーション結果を検証するために，Nash and Sutcliffeの効率係数を使用した．本研究の結果，合計165個の土壌試料が，国際土壌科学学会分類法に基づいて分類され，重粘土，砂質粘土，砂質粘ローム土，砂壌土，砂土，軽粘土，埴壌土，ローム土，シルト質粘土，シルト質粘ローム土の10タイプに分けられた．Green-Amptの浸透モデルの性能評価試験結果から，Green-Amptの浸透モデルは土壌の水分保持特性と水理特性のデータを使用することによって，各土壌の浸透特性を評価できることが明らかとなった．また，熱帯土壌の土性の違いは，浸透速度，浸潤前線深度，およびsorptivityに関して，著しい対照を示した．さらに，熱帯土壌の浸透特性は主に，透水係数，初期水分量，および浸潤前線先端における土壌水分張力の大きさによって影響されていることが明らかとなった．

キーワード：熱帯土壌, 国際土壌科学学会分類法, 土壌水分保持特性, Green-Amptモデル, 浸潤前線, sorptivity

Muhamad ASKARI 1) 田中　正 2)

Budi Indra SETIAWAN 3) Satyanto Krido SAPTOMO 3)

1)筑波大学大学院生命環境科学研究科博士後期課程（〒305-8572 茨城県つくば市天王台1-1-1）

2)筑波大学大学院生命環境科学研究科（〒305-8572 茨城県つくば市天王台1-1-1）

3)インドネシア・ボゴール農科大学農業工学研究室（ボゴール農科大学Darmaga Campus, Bogor 16680, Indonesia）

Infiltration Characteristics of Tropical Soil Based on Water Retention Data

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