ΐῒῑῌῐ
J. Jpn. Soc. Soil Phys.
No. +*3, p. ./῍/* ῎,**2῏Analysis of Surface Runo# in a Sloping Sandy Soil in Northeast Thailand Using Soil Water Storage Capacity
Hiromasa HAMADA*, Hiroki WATABE**, Toshitsugu MOROIZUMI***and Somsak SUKCHAN***** Japan International Research Center for Agricultural Sciences,Owashi +ῌ+, Tsukuba, Ibaraki, -*/ῌ2020, Japan** Chugoku Shikoku Agricultural Regional Administration O$ce, Azashosakai,ῌ+, Kawabata, Itano, Tano-gun, Tokushima, 113ῌ*+*,, Japan*** Okayama University, Tsushima-naka ,ῌ+ῌ+, Okayama, 1**ῌ2/-*, Japan**** Land Development Department, ,**-/0+ PhaholyothinRd., Lardyao, Chatuchak, Bangkok, +*3**, Thailand
Abstract
In Northeast Thailand, permeable sandy soils are widely distributed. Most rainfall may inﬁltrate the ground and there may be little surface runo#. However, soil erosion caused by surface
runo# has been reported. To investigate the mechanism by which surface runo# occurs, weselected a sloping area where the top soil consisted of a permeable sandy layer overlying a very
low permeability clay layer. We deﬁned the soil water storage capacity of the sandy layer at aparticular time as the di#erence between the maximum observed water content and the observed
water content at that time. We assumed that when the amount of rainfall exceeded the soil water
storage capacity, the excess rainfall became surface runo#. As a result of monitoring of soil watercontent in the sandy soil layer over a period of one year, the total amount of runo# was estimatedto be about -*ῌ of the total rainfall and the periods for which our analysis showed that soil water
storage capacity was exceeded were followed by rises in the water level of a pond that surface
runo# ﬂowed into.
Key words : Northeast Thailand, Sandy soil, Soil water storage capacity, Sloping layered soil,
Surface runo#+.

Introduction

The Northeast Thailand region accounts for
one-third of the total area of Thailand. The
topography of the region is undulating hills,
and sandy soils occupy about 2*ῌ of the area.
Annual rainfall is +,***ῌ,,*** mm, and falls
mainly during the rainy season, May to October (Kohyama and Subhasaram, +33-). Agriculture in the region is dependent on rainfall, but
the highly variable monthly rainfall results inunstable crop yields. To use the available rainwater e#ectively, it is essential to identify themovement of rainwater after it reaches theground surface. The permeability of the widelydistributed sandy soils in Northeast Thailand
is very high. Consequently, most rainfall may
inﬁltrate and there may be little surface runo#.
However, damaging soil erosion caused by surface runo# has been reported (Mitsuchi et al.,+320 ; Sombatpanit et al., +33/). It was di$cult
to explain the mechanism of the occurrence of
surface runo# from the viewpoint of the permeability of surface soil. Watabe et al. (,**.)
hypothesized that surface runo# would occur
after the sandy soil overlying a clay soil with



46

+*3

,**2

Fig. + Experimental site
(N : +0῍3῍ῌ E : +*,῍.2῍)very low permeability was ﬁlled with water.
However, their research has not been veriﬁed
experimentally. In this study, we measured thewater content of the sandy soil overlying avery low permeability clay soil on a hill slope
to verify the mechanism by which surface
runo# occurs.
,.

Materials and Methods,. + Study site
We selected a sloping ﬁeld covered by sandy
soil in the Nong Saeng village, about -/ km
south of Khon Kaen city. In the rainy season,
farmers in this area cultivate rice in the lowland, and sugarcane in the upland (Fig. +). The
surface slope of the sugarcane ﬁeld in ourstudy area was about - . There were two
ponds in the lower part of the sugarcane ﬁeld.
The water level in these ponds was not a#ectedby groundwater and subsurface ﬂow (Hamadaand Sukchan, ,**2), indicating that when surface runo# occurred, the water level would
increase markedly.
The soil from the surface to + m depth was
Loamy Sand (LS), 1ῌ clay, ,ῌ silt and 3+ῌ
sand. The soil below + m depth was Sandy Clay(SC), ,1ῌ clay .ῌ silt and 03ῌ sand. The

saturated hydraulic conductivity was on theorder of +*. cm/s in the LS layer and on theorder of +*0 cm/s in the SC layer (Hamada etal., ,**0). From the permeability values, weinferred that rainwater inﬁltrated the groundand is retained in the LS layer above the SClayer.
,. , Field investigation and analytical method
We measured soil water content (at two sites,Site + and Site ,), rainfall, and water level in apond (P-+ : area -3* m,) (Fig. +).
Soil water content in the LS layer was measured using Proﬁle Probe (Delta-T Device, PR +/0) at depths of +*, ,*, -*, .*, 0* and +** cm atintervals of about one week between September ,*, ,**- and September ,*, ,**.. Using
these data, we estimated the soil water storage
capacity of the LS layer. The soil water storage capacity on a particular day was deﬁned as
the di#erence between the maximum recorded
volumetric water content of the LS layer (June+0, ,**.) and the measured value on that day.
We assumed that when the amount of rainfall
was less than the soil water storage capacity,
all rainfall would inﬁltrate, and when theamount of rainfall was greater than the soil
water storage capacity, the excess would become : Analysis of Surface Runo# in a Sloping Sandy Soil in Northeast Thailand Using Soil Water Storage Capacity

47

Fig. , Estimation of soil water storage capacity (at site + on April 1, ,**.)

surface runo#. The process for estimation of thesoil water storage capacity is shown in Fig. ,.
We measured water levels in the pond P-+ byusing survey instruments, GPS positioning(Sokkia, RADIANTM IS) in order to check theoccurrence of surface runo#. First, we measured the elevation of the bottom of the pond.
We then calculate the depth of water in thepond as the di#erence in the elevation between
the water surface and the bottom of the pond(error : *.*, m). The water-level measurements were recorded at intervals of one to two
weeks from March +, ,**. to September +1,,**..
Rainfall data was collected at intervals of +*minutes with a rain gauge (SEC, WeatherBucket).
-.

Results and Discussions

Figure - shows daily rainfall and calculated
soil water storage capacity in the LS layer atsite + and , (squares and triangles). The totalrainfall was +,00 mm during the period of ourinvestigation. The soil water storage capacity
was calculated at intervals of about one week.
It was therefore necessary to determine thedaily soil water storage capacity in order toestimate the daily amount of the surface
runo#. We calculated the daily soil water storage capacity by using the following equation :WnWn+PnEn

ῌwhere on day n, Wn is soil water storage capacity (Wn * indicates surface runo#), Pn is daily
rainfall, and En is daily evapotranspiration(September ,+, ,**- was day one). We assumed
that deep percolation beyond a depth of + m
was negligibly small. Daily values of E used inEquation (+) were chosen by trial and error. We
used + mm/day from September ,+, ,**- to January +2, ,**. and as - mm/day from January+3, ,**. to September ,*, ,**., chosen so that,Wn values (Solid line in Fig. -) would follow

48



+*3

,**2

Fig. - Daily rainfall and estimated weekly and daily soil water storage capacity

Fig. . Estimated surface runo# (Total : -1. mm)

the trend of calculated capacities using weekly
observed soil water content. Using Wn obtainedwith Equation (+), we estimated the daily surface runo# as the di#erence between the amountof rainfall and soil water storage capacity.
Our analysis results showed that the surface
runo# occurred between September ,* to October ,*, ,**-, in the middle of June, ,**., between July ,* and August ,*, and on Septem-ber ,*, ,**. (Fig..). The total amount of runo#
during the period of our study was estimatedto be -1. mm, which was about -*ῌ of the total
rainfall (+,,00 mm). The temporal relationshipbetween daily surface runo# and variations ofwater level in the pond (Fig. /) shows that theperiods when the water level rose in the pondcoincided with periods of surface runo#, thus
validating our hypothesis that runo# occurs

 : Analysis of Surface Runo# in a Sloping Sandy Soil in Northeast Thailand Using Soil Water Storage Capacity

49

Fig. / Relationship between estimated surface runo# and depth of the pond (P-+).

when the amount of rainfall exceeds soil water
storage capacity.
..

Conclusion

This study identiﬁed the mechanism bywhich surface runo# occurs in a sloping area inNortheast Thailand, where the soil from thesurface to + m depth is LS layer and overlies avery low permeability SC layer. We deﬁned
soil water storage capacity as the di#erence
between the maximum water content we observed in the LS layer and observed values.
We assumed that when the amount of rainfall
exceeded soil water storage capacity, the excesswater became surface runo#. The periods
during which we estimated that surface runo#
occurred coincided with rises in the water levelof the pond that surface runo# ﬂowed into,
thus validating our hypothesis that runo# occurs when the amount of rainfall exceeds soil
water storage capacity.
Acknowledgement
We would like to acknowledge the usefulsuggestions for this research from Dr Shuichi
Hasegawa of Hokkaido University. We would
also like to thank Mr Chikara Ogura, from theNational Agriculture and Food Research Or-ganization, for his assistance in our ﬁeld investigations.
Reference
Hamada, H. and Sukchan, S. (,**2) : Monitoring ofradon-,,, concentration in surface waters ofthe Ban Phai subwatershed northeast Thailand using a method to concentrate radon by
air-circulation, JARQ, .,, 0-ῌ02.
Hamada, H., Moroizumi, T. Watabe, H., Srisuk K.
and Hasegawa, S. (,**0) : Soil water and groundwater in Nong Saeng, JIRCAS Working
Report, .1, pp.,1ῌ-+, Tsukuba.
Kohyama, K. and Subhasaram, T. (+33-) : Salta#ected soils in northeast Thailand, theirsalinization and amelioration. ADRC technicalpapers, No. ++, pp. +ῌ/., Khon Kaen.
Mitsuchi, M., Wichadit, W. and Jeungnijiirund, S.
(+320) : Outline of soils in the northeast plateau, Thailand, ADSC Technical papers, No.+,
pp. +ῌ2*, Khon Kaen.
Sombatpanit, S., Rose, C.W., Ciesiolka, C.A. andCoughlan, K. J. (+33/) : Soil and nutrient loss
under rozelle (Hibiscus subdari#a L. var.altissima) at Khon Kaen Thailand, Soil technology,
2 : ,-/ῌ,.+.
Watabe, H., Hamada, H., Moroizumi, T. and Hasegawa, S. (,**.) : A mechanism of surface runo#at a sloping sandy ﬁeld in Northeast Thailand,
Proceeding of Symposium of Japan Society ofSoil Physics, +2ῌ+3, Okayama, in Japanese.

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