ΐῒῑῌῐJ. Jpn. Soc. Soil Phys.
ῐ῎῍ῑῒ῏
No. ++*, p. 13῍21 ῎,**2῏Relationship between Soil Water Content of a Surface
Sandy Soil and Groundwater Level in a Study Site
in Northeast Thailand
Hiromasa HAMADA* and Somsak SUKCHAN*** Japan International Research Center for Agricultural Sciences,
Owashi +ῌ+, Tsukuba, Ibaraki, -*/ῌ2020, Japan** Land Development Department, ,**-/0+ Phaholyothin Rd.,
Lardyao, Chatuchak, Bangkok, +*3**, Thailand
Abstract
In some areas of northeast Thailand, groundwater is conﬁned by a clay layer, and the waterlevel in wells drilled into the aquifer is above the ground surface, suggesting that water could be
supplied from the aquifer to the surface soil and making it possible to cultivate crops withminimum irrigation. We measured the groundwater level in wells and the water content of thesurface sandy soil during the dry season. In recharge areas, the soil water content was usually lessthan the depletion of moisture content for optimum growth in the dry season, and less than thepermanent wilting point in the late dry season (from January), indicating that in these areas,
irrigation would be required for the cultivation of crops. In the discharge areas, where the waterlevel in wells drilled into the conﬁned aquifer was higher than the ground surface, the soil watercontent was more than the depletion of moisture content for optimum growth throughout the dryseason, indicating that in these areas it would be possible to cultivate crops with minimum
irrigation during the dry season using natural upward ﬂow.
Key words : Northeast Thailand, Conﬁned groundwater, Soil water content of sandy soil, Minimum irrigation, Natural upward ﬂow+.

Introduction

The main industry of northeast Thailand isagriculture, which depends on rainfall. Only+*ῌ of the area used for agriculture is irrigated(Prapertchob and Humnath, ,**.). The topography is undulating hills, and farmers growrice in the lowlands during the rainy season,and sugarcane and cassava in the uplands
throughout the year. Though annual rainfall is+,*** to ,,*** mm, little rain falls during themonths of the dry season (November to April).
Therefore, it is essential to develop water resources to enable cultivation during the dryseason. Sandy soils cover about 2*ῌ of thisregion (Kohyama and Subhasaram, +33-). In
some areas, a clay layer at depths of + to . m, of
which permeability is on the order of +*ῌ0 cm/s, is overlaid by the sandy soil. Beneath theclay layer is a weathered sandstone aquifer. In
the lowlands, groundwater in the aquifer is
conﬁned by the clay layers and sometimes thewater level in observation wells drilled into theaquifer is higher than the ground surface(Hamada et al., ,**0). In this case, water could
be supplied to the surface sandy soil throughthe clay layer. It might be possible, therefore,
to cultivate some crops with minimum irrigation using natural upward ﬂow during the dryseason. In this study, we measured water re-80

Table +++*

,**2

Elevation, depth, screen, period of measurement and land use

No.

Elevation(m)

Depth(m)Screen(m)Period of measurement

Land use+ῌ++ῌ,,,*1,*1,*,

1-*-*-ῌ1,0ῌ-*,0ῌ-*

April 1, ,**. to April ., ,**/
April 1, ,**. to April ., ,**/
September -*, ,**/ to May ,., ,**1-+3*-*,0ῌ-*

April 1, ,**. to April ., ,**/.

+13-*,0ῌ-*

April 1, ,**. to April ., ,**/

Fallow
Fallow
Forestry
Paddy (rainy season)
Fallow (dry season)
Paddy (rainy season)
Fallow (dry season)

tention characteristics of surface sandy soil
and monitored the water content in ﬁelds andthe groundwater level in observation wells
during the dry season to examine the inﬂuenceof the groundwater level in the conﬁned aquiferon the surface sandy soil water content and toconsider the possibility of minimizing irrigation. Studies of soil water movements in Thailand have been reported, the relationship between shallow groundwater above the clay
layer and moisture damage to cassava (Taniyama and Miura, +33+), the relationship between variation of soil moisture and surface
runo# (Watabe et al., ,**/ ; Funakawa et al.,,**1), variation of soil water content in thesandy soil (Watanabe et al., ,**., Oda and Ogura,,**2 ; Moroizumi et al., ,**2), and regional
groundwater ﬂow (Srisuk et al., ,***). However, no reports on the inﬂuence of the groundwater level in a conﬁned aquifer on soil moisture have been published.
,.

MethodWe selected Nong Saneg village, -/ km southof Khon Kaen city as the study site. The soil
from the surface to + m depth is Loamy Sand
(LS), of which permeability is on the order of+*. cm/s, and from + to . m depth is Sandy
Clay (SC), of which permeability is on the orderof +*0 cm/s. A weathered sandstone aquifer
underlies the SC layer (Hamada et al., ,**0).
We selected two sites in the recharge area andtwo sites in the discharge area for the measurements of groundwater level and soil water content considering the topography (Hamada,,**/). Fig. + is a map of the observation sites
and information about the observation wells
and the measurement periods is listed in Table+. Locations + and , are in the uplands, andlocations - (at the foot of a hill) and . (a ﬂatland) are in the lowland where farmers cultivate rice in a rainy season. We measured thegroundwater level in the observation wells atintervals of one to two weeks during the period
from ,**. to ,**1.
In the soil investigation, we measured thewater retention characteristics of the sandy soil
and monitored the soil water content, to examine the possibility of the cultivation of plants
with minimum irrigation. For the measurement of the water retention characteristics soil
samples were collected as +**-ml cores at thepoint between location , and location - in September ,**. (Fig. +). We collected nine samples,
three samples at each depth, /ῌ+* cm, ,2ῌ-- cm
and 3/ῌ+** cm. The water retention characteristics were measured by the pressure plate
method. For monitoring of soil water content,
we collected disturbed soil samples at every +*cm depth by using an auger and measured thewater content (by weight : w). The measured
values indicate the average water content of
the soil layers +* cm thick. We conducted ourﬁrst soil investigation in December ,**/. After
that, we measured the soil water content at
locations +ῌ- between October ,**0 and April,**1 at intervals of one to three months. The
soil water content at location . was measured
on January +,, ,**1 to conﬁrm that the soil
water content where the groundwater level inΐῒῑῌῐ : Relationship between Soil Water Content of a Surface Sandy Soil andGroundwater Level in a Study Site in Northeast Thailand

81

Fig. + Observation sites (No. + : Two wells)Longitude : ,0-1**ῌ,03+**, Latitude : +1202**ῌ+123/** (UTM coordinates).

the observation well was above the groundsurface would be high. The monthly rainfalls
during the investigation at location . were+.,.. mm in October, ,**0, no data in November
and December (due to equipment problems), +..
mm in January, -., mm in February, /., mm in
March and ,3.2 mm in April ,**1(until April ,1).
-.

Result and discussion-. + Variation in groundwater level
The variation in the groundwater level isshown in Fig. ,. The water level in well +ῌ+ (1m deep) was higher than that in well +ῌ, (-* mdeep), indicating that these wells are in therecharge area. The water level in well , wasmore than +* m below the ground surface, indicating that the soil water content of the surface sandy soil was not a#ected very much by
the groundwater. The water level in well - washigher than the ground surface and that in well. also rose above the ground surface after May,**.. In these areas, groundwater levels inholes which were drilled through the clay layer
were higher than the surface, inferring thatwater from the conﬁned aquifer could be
supplied to the surface sandy soil through theclay layer and could keep the soil wet evenduring the dry season.
-. , Water retention characteristics
Figure - shows the water retention characteristics of the sandy soil. According to theengineering manual for irrigation and drainage
by Japanese Institute of Irrigation and Drainage (+33*), the lower limit of available soil moisture for crops is regarded as the point at whichcrops begin to be damaged. The soil watercontent is called the depletion of moisture content for optimum growth (qog). In this study, weused the depletion of moisture content for optimum growth as the lowest point of soil moisture at which irrigation is not required, andadopted the value of +** kPa (Suction). The
bulk density of the samples (rb) was +...῎*.*2(Mg m῍-). Water content (w) was calculatedusing volumetric water content and bulk density (w῏q/rb). The qog of the sandy soil is *.*12m- m῍- (w : *.*/+ῌ*.*/1 kg kg῍+), and the permanent wilting point (qwp) is *.*+3 m- m῍- (w : *.*+ῌ*.*+. kg kg῍+). When the soil water content ismore than *.*/1 kg kg῍+, plants could growwithout irrigation. When the soil water content is less than *.*+- kg kg῍+, the plants might

82ΐῐ῎῔῕ῑ ῒ ++* ῏ ῌ,**2῍Fig. , Groundwater level (upper : No. ,, Lower : No. +, No. - and No. .).

Fig. - Water retention characteristics.

die.
-. - Variation of soil water content
Figure . shows the soil water content measured on December ,* and ,+, ,**/. The soil
water content to a depth of + m at location ,,where the groundwater level was +,.,- m deep,
was less than qwp. At location -, where thegroundwater level was *./ m above the groundsurface, the soil water content was more thanqog, indicating that plants could grow there
without irrigation. The water content at location +, where groundwater level was ..,, mdeep (well +ῌ+) was similar to that at location ,.
Figures /ῌ2 show the soil water content nearῐ῏῎ῌ῍ : Relationship between Soil Water Content of a Surface Sandy Soil andGroundwater Level in a Study Site in Northeast Thailand

Fig. .

Soil water content (December ,*ῌ,+, ,**/).

Fig. /

Water content (No. +).

83

84

++*,**2

Fig. 0 Water content (No. ,).

the observation wells during the dry season(October ,**0 to April ,**1). At location +, thesoil water content tended to increase withdepth. From December +, ,**0 to April ,1 ,**1,
the values, except for some from more than 1*
cm depth, were less than qog, and most values atdepths above -* cm, were less than qwp (Fig. /).
Most values of the soil water content at location , from November +- were lower than qog
and after December +, ,**0, became less thanqwp (Fig. 0). In these areas, irrigation is required
for the cultivation of crops in the dry season.
The soil water content at locations - and .
was more than qog throughout the period of theinvestigation. Moroizumi et al. (,**2) reported
that the actual evapotranspiration during thedry season in the lowland of northeast Thailand where soil is same as our study sites was+./ mm day+. Using this value, the decrease ofsoil water content in the sandy soil to a depthof + m during the period of between December+,, ,**0 and April ,1, ,**1 was calculated to beabout ,** mm. However, our observation showedthat the decrease of soil water content in thesandy soil was about 0* mm, less than the estimated value. Moreover, the soil water content in location . in a ﬂat land, where subsurface ﬂow from the upland didn’t a#ect thesoil moisture, was more than qog, suggestingthat some water was supplied to the surface
sandy soil through the clay layer in locations and .. Therefore, it should be possible to cultivate crops there with minimum irrigation using
natural upward ﬂow.
..

Conclusion

In this study, we examined the inﬂuence ofthe groundwater level in the conﬁned aquifer
on water content of the sandy soil to a depth of+ m in northeast Thailand. The soil water content in the area where the groundwater level inobservation wells was higher than the groundsurface was more than the depletion of moisture content for optimum growth, even during
the dry season, indicating that it should bepossible to cultivate crops there with minimum
irrigation using natural upward ﬂow.

ῐ῏῎ῌ῍ : Relationship between Soil Water Content of a Surface Sandy Soil andGroundwater Level in a Study Site in Northeast Thailand

Fig. 1Water content (No. -).
This land was used for rice ﬁelds until the end of November.

Fig. 2 Water content (No..) : January +,, ,**1.

85ΐῐ῎῔῕ῑ ῒ ++* ῏ ῌ,**2῍86

Acknowledgements
We would like to acknowledge the usefulsuggestions for this research from Dr Toshitsugu Moroizumi of Okayama University.
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