IJERD – International Journal of Environmental and Rural Development (2020) 11-2

Research article
erd

Effect of Excavated Small Drainage Channels on
Desalinization in Northeastern Thailand
NAHO NOHARA
Hokkaido University, Sapporo, JapanTADAO YAMAMOTO*
Hokkaido University, Sapporo, Japan
Email: tady@env.agr.hokudai.ac.jp

TAKASHI KUME
Ehime University, Matsuyama, JapanKATSUYUKI SHIMIZU
Tottori University, Tottori, JapanCHULEEMAS BOONTHAI IWAI
Khon Kaen University, Khon Kaen, Thailand
Received 27 January 2020

Accepted 29 October 2020(*Corresponding Author)

Abstract Salt-affected soil is a serious problem in Thailand, and most of this soil is found in
the northeastern part of Thailand. The total area of saline land in Northeast Thailand is 2.85
million ha, which accounts for about 17% of this area (Land Development Department,2011).
The objective of this study is to evaluate the effect of desalinization by digging drainage
channels in salt-affected fields during rainfall. The study was conducted on a high salinity
field in Khon Kaen, Thailand. In April 2019, the main drainage channel of a depth of 1 m,
was excavated across the center of the field in the south-to-north direction. Additionally, side
ditches were installed along the circumference of the west part of the field. We constructeddrainage channels to promote field drainage and effectively induce salt leaching with rainfall.
Field observations were conducted eight times, especially in the rainy season, from August
2018 to September 2019. Soil apparent electricity conductivity (ECa) was measured using anelectromagnetic induction meter in three distinct depth ranges, i.e., 0.375 m, 0.75 m, and 1.5m, from the soil surface. Soil salinity was represented as contour maps based on spatiallyinterpolated data and spatial distribution and temporal changes were analyzed. The soil ECa
in the field gradually increased from upstream to downstream. Moreover, it was higher near
the surface and lower deep underground. By comparing the soil salinity observed in the laterpart of the rainy season in both the years, ECa was observed to have decreased by 20% from
August 2018 to September 2019. The ECa values at depths of 0.375 m and 0.75 m in the west
part of the field, which had side ditches, were significantly lower than those in the east partof the field two months after the drainage channels were constructed. However, a contrary
result was obtained after a heavy flood. These results indicate that drainage channels
contribute to salt leaching with rainfall. Our results suggested that excavating drainage
channels could effectively reduce soil salinity during the rainy season under good drainage
conditions.
Keywords saline soil, desalinization, leaching, drainage, electric conductivity

INTRODUCTION
Soil salinization is a major global problem limiting crop productivity. Urgency exists to prevent andimprove soil salinization for sustainable agriculture. Salt-affected soils are found in more than 100Ⓒ ISERD

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countries and typically occur in arid and semi-arid regions (Rengasamy, 2006). The Food andAgriculture Organization (FAO) estimated that globally the total area of salt-affected soils is 831
million ha. Salt-affected soils have also been developing in monsoon zones in South and Southeast
Asia. Thailand is one of the countries where improving saline soil is an urgent need.
Salt-affected soil is a serious problem in Thailand, and most of this soil is found in thenortheastern part of Thailand. The total area of saline land in Northeast Thailand is 2.85 million ha,which accounts for about 17% of this area (Land Development Department,2011). The potential
source of salt in this area is halite in the Maha Sarakham Formation (Wongsomsak, 1986). Apartfrom its geographical environment, human activities such as deforestation associated with
agricultural development and traditional salt making have contributed to accelerating soil salinization(Arunin et al., 2015; Löffler et al., 1988). People in this area are impoverished because of low
agricultural productivity resulting from infertile land and salt-affected soil. To combat this problem,the mechanism of reducing soil salinization was elucidated, and studies using various approaches tomitigate soil salinity have been attempted from more than 30 years (Miura et al., 1991;
Patcharapreecha et al., 1990). However, the people in this area are still suffering from salt-affectedsoil. In other words, these desalinization approaches do not have widespread application. Some
farmers abandoned their farmlands and gave up farming because with salt-affected soil being formed
opportunities to expand farmlands did not exist. To enhance agricultural development and encourage
the agricultural community, ameliorating salt-affected soil and develop them for farmlands are necessary.
One of the common strategies for combating salt-affected soil is leaching salts from upper tolower soil depths (Qadir et al., 2000). To effectively facilitate the leaching process, an open or
subsurface drainage system is commonly adopted (Ritzema, 2016). This approach is generally
considered to decrease the saline groundwater table and removes leaching water from the soil profile.
However, installing a drainage system to control salinity is a costly proposition (Datta et al., 2000)
and raises concerns about high salinity drainage water. In this study, a conceptual model of the
Cascading Salt Using System (CSUS) (Kume et al., 2019a, 2019b) has been considered. This model
focuses on water and salt flow and proposes to reconsider salt as a useful resource. In this system,
different types of crop cultivation are practiced considering feasible salinity levels. For instance, ricepaddies can be cultivated upstream and halophilic crops can be cultivated downstream where the salt
concentration is higher. Moreover, table salt can be made from high salinity discharge water that isobserved at the lowest downstream level. Production of halophilic crops and salt are expected to raise
farmers’ income.
This study is the first step of the CSUS project to decrease soil salinity in a large salt-affected
field by excavating small drainage channels. Open drainage channels that are currently installed in
the field considered in this study are not aimed towards completely removing salts from the field andmitigating soil salinity for low salt-affected levels. These drainage channels are not large; therefore,
they are inexpensive and easy to install for farmers. The objective of this study is to evaluate theeffect of desalinization by digging small drainage channels.
METHODOLOGY
Study Area
The study was conducted in a high saline soil field in Ban Phai district (16°03′ , 102°69′ E), Khon
Kaen (Fig. 1). This area experiences a tropical savanna climate (Köppen climate classification Aw)
with marked alternation of the rainy season (from May to October) and dry season (from ovember
to April). The range of annual precipitation is from 1,100 mm to 1,500 mm, and the mean annual
temperature is 27°C. The overview of the field under consideration in this study is shown in Fig. 2.
This field with an area of approximately 0.8 ha has no vegetation because of high soil salinity;
however, the upper area of the field is used for rice cultivation in the rainy season. For improving thesaline soil in the field, a strategy to install small drainage channels was adopted by the Land
Development Department. The drainage channels are expected to promote field drainage and induce
salt leaching with rainfall. In April 2019, a main drainage channel with a 1-m depth was excavatedⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-2

across the center of the field in the south-to-north direction. Additionally, side ditches with a depthof 1 m were installed along the circumference of the west part of the field. The gradient of thedrainage channels was approximately 2.5 / 1000. The water flow is in the direction of the arrow in
Fig. 2. The drainage ultimately runs downstream through the hume pipe under the road lying on thenorth side of the field. Daily precipitation data (Fig.3) in the urban area in Khon Kaen was obtained
from the Japan Meteorological Agency website.
Field Survey
Soil salinity was measured using an electromagnetic induction meter, EM38-MK2 (Geonics Limited),
which measured apparent soil electrical conductivity (ECa) in units of millisiemens per meter (mS/m).
This instrument has two receiver coils, each of which, are separated by 1 m and 0.5 m from thetransmitter providing data from effective depth ranges of 1.5 m and 0.75 m, respectively, when
positioned in the vertical dipole orientation, and 0.75 m and 0.375 m respectively, when positioned
in the horizontal dipole orientation. We measured soil salinity in vertical and horizontal dipole
orientation in 1-m intercoil spacing as well as in horizontal dipole orientation in 0.5-m intercoil
spacing, thereby obtaining three distinct depth ranges of data, i.e., 0.375 m, 0.75 m and 1.5 m, fromthe soil surface. The measurement interval was about 10 m. Field observations were conducted eighttimes, especially in the rainy season, from August 2018 to September 2019 and three times in the dry
season, i.e., February, March, and December 2019.

Fig. 1 Location of the study area

Fig. 2 Overview of the study area

Fig. 3 Daily precipitation in Khon KaenⒸ ISERD

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Data Analysis
The soil ECa data obtained from EM38-MK2 in the field were used as a database for mapping the
distribution of the field soil salinity. They were interpolated using the Inverse Distance Weighted
method with 1-m intervals and represented as contour maps to analyze spatial and temporal
distributions. Because ECa measured using EM38-MK2 affects soil moisture, data under similar
groundwater level conditions were used for comparing soil salinity changes. The observation datesof the data used for this analysis are shown in Fig. 3 in blue (in the rainy season) and red (in the dry
season) dots. Moreover, averages of field ECa were calculated based on the interpolated data.
RESULTS AND DISCUSSION
Spatial Distribution and Temporal Changes of Soil Salinity in the Field
Soil salinity in the field was represented as contour maps. Fig. 4 shows field soil salinity at a depth
range of 0.75 m in August 2018 as well as September 2019 before and after excavating the drainage
channels. The soil ECa gradually increased from upstream to downstream in the field associated withwater flow. Identical results were obtained at the other measurement dates as well as the other two
depths, i.e., 0.375 m and 1.5 m. Comparison of both the figures showed that the soil ECa in September
2019 was lower than that in August 2018 in the entire field. Table 1 shows the average EC a in thefield at three different depth under similar groundwater level conditions in the rainy season. The soil
ECa has a tendency to be higher near the soil surface and lower deep underground. In the rainy season
in 2019, the soil ECa gradually decreased from the early part of the rainy season in May to the endof the rainy season September. The specific decrease was from 1398 mS/m to1290 mS/m, 1367 mS/m
to 1205 mS/m, and from 1106 mS/m to 944 mS/m at the depths of 0.375 m, 0.75 m, and 1.5 m,
respectively. The total rainfall in this period was 655.4 mm. Leaching is considered to haveprogressed during rainfall.

Fig. 4 Distribution of soil salinity in the field at a depth of 0.75 m before the installation ofdrainage (left) and after the installation of drainage (right) in the rainy season
Moreover, the soil ECa decreased after excavated drainage channels. The soil ECa in September
2019 was 20% lower than that in August 2018, even though both these periods were the later part of
the rainy season. This result indicates that installing drainage channels promotes salt leaching by
rainfall in the rainy season.
Additionally, soil salinity was compared before and after excavating drainage channels in thedry season. Fig.5 shows field soil salinity at a depth range of 0.75 m in March 2019 as well as
December 2019 before and after excavating the drainage channels. The averages of the soil ECa in
the dry season are also shown in Table 1. After installing drainage, the soil ECa in December 2019
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was lower than that in March 2019, before installing drainage. Additionally, Fig. 5 clearly shows thatthe soil ECa decreases after excavating the drainage channels. Miura, K., and Subhasaram, T. (1991)
indicated that accumulation of salt on the soil surface in the dry season was associated with theelevation of the groundwater table to a critical level at the end of the rainy season and continuity ofcapillary pores above the groundwater level. In this study, comparing the soil ECa under similar
groundwater level conditions, this continuity of capillary pores should be prevented due to thegroundwater level after excavating the drainage channels was relatively lower than before that. This
result agrees with the earlier result in the rainy season and supports the assumption that drainage
channels effectively function as a desalinization method.
Table 1 Averages of field ECa (mS/m) at three depths
Depth (m)Rainy season
May 2019 June 2019
1398
1277

September 20191290

Dry season
March 2019
December 2019
1133
944

0.375

August 2018
-

0.75

1470

1367

1286

1205

1109

954

1.5

1155

1106

1077

944

947

840

Fig. 5 Distribution of soil salinity in the field at a depth of 0.75 m before installing drainage(left) and after installing drainage (right) in the dry season
Comparison of Salinity Changes under Different Drainage Intensity
In the field under consideration in this study, the intention of drainage channels is different between
the west and east parts of the field. The west part of the field is surrounded by a side ditch; therefore,drainage and leaching efficiency in the west part are expected to be better than those in the east partof the field. To compare the desalinization effect on both the sides, we compared the soil ECa in thewest part to that in the east part of the field. Fig. 6 shows the average soil ECa at a depth of 0.75 m
in the west and east parts of the field. The error bars show the standard deviation. The average of thesoil ECa before the introduction of drainage was higher in the west part of the field than that in theeast part of the field, i.e., 1498 mS/m and 1442 mS/m, respectively. However, the soil ECa was lower
in the west part than that in the east part of the field two months after the drainage installation, i.e.,
June 2019. The average field ECa was 1205 mS/m and 1340 mS/m in the west and the east part of
the field, respectively. Moreover, significant difference (Mann-Whitney U test; p <0.01) with thesame result was obtained at a depth of 0.375 m, while no significant difference between the west andthe east parts of the field was observed at a depth of 1.5 m. This result indicates that intensive drainage
enhanced salt leaching in the west part of the field. However, in September 2019, the soil ECa in thewest part was higher than in the east part of the field. Before measurement in September 2019, thisarea experienced heavy flooding, and some parts of the drainage system were broken; therefore,Ⓒ ISERD

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excess water in the field could not be immediately removed. This may have caused higher salinity inthe west part of the field in September 2019. In December 2019 after the flood, the same tendency
was observed as that before the flood even though it was the dry season.

Fig. 6 Averages of the soil ECa in the west andthe east part of the field at a depth of
0.75 mFig. 7 Divided five areas depend on the
distance from drainage channels

Fig. 8 Changes of the average ECa in five areas in the rainy season
Subsequently, we compared the soil ECa near the drainage channels and far from them toconduct a detailed examination of the field salinity changes. The field was divided into five areas(Fig. 7) based on the distance from the drainage channels, i.e., the area 0-10 m from the main drainage
channel in the west part of the field (area A), the area 0-10 m from the side ditch in the west part of
the field (area B), the middle area of the west part of the field (area C), the area 0-10 m from the main
drainage channel in the east part of the filed (area D), and the area 10-25 m from the main drainage
channel in the east part of the field (area E). We compared the values of the soil ECa in the five areasat three depths during the rainy season under a similar groundwater level condition. Fig. 8 shows theaverage soil ECa in each of the five areas at depths of 0.375 m and 1.5 m as an example. The soil ECa
generally decreased in all the areas and at all the depths from August 2018 to June 2019. However,in September 2019, after the flood, the soil ECa suddenly increased in areas A and B at a depth of
0.375 m, while it slightly decreased in area C, which was surrounded by A and B,. This result
indicates that salt was supplied from the soil surface layer in C to the A and B areas. Alternatively,
this may suggest that salt reaccumulated from the drainage water after the flood. Conversely, at a
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depth of 1.5 m, the soil ECa in areas A, B, and D near the drainage channel, considerably decreasedin September 2019 in contrast with small changes before that. The drainage and leaching effect wasconsidered to be less at a depth of 1.5 m because the groundwater level was over 1.5 m in theordinarily rainy season, but the depth of the excavated drainage channels was 1.0 m. However, thegroundwater level in the field was low in July and August 2019 because low rainfall was experienceduring this period. Therefore, a large amount of rainfall caused by the flood infiltrated deep
underground in an instant and this made the salt leach away, especially from near the drainage
channel and deep underground. These results indicate that drainage channels contribute to saltleaching during rainfall and drainage systems operating in a satisfactory manner are important fordesalinization.
CONCLUSION
We evaluated the effect of small drainage channels during rainfall on desalinization in the highsalinity field in Khon Kaen, Thailand by measuring field soil ECa using an electromagnetic induction
meter. After analyzing the spatial distribution and temporal changes of the field soil salinity, we
found the following results. The soil ECa gradually increased in the field from upstream todownstream. The soil ECa was higher near the surface and lower deep underground. In 2019, the soil
ECa gradually decreased from the early part rainy season to the end of the rainy season. By comparing
the soil salinity before and after excavating the drainage channels in the same season, the soil ECa
was observed to have decreased after drainage installation. Moreover, the soil ECa in the west part of
the field, where drainage was more intensive, was significantly lower than that in the east part of the field
two months after the drainage channels were constructed. However, later in the rainy season in 2019,the value of the soil ECa in the west part of the field was higher than that in the east part of the field.
The drainage channels were considered to have not worked well after the heavy flood. The same
holds true of the result obtained by comparing the ECa changes based on the distance of the drainage
channels. Our results suggest that the excavated drainage channels can effectively reduce soil salinity
during the rainy season. Overall, the desalinization effect could be observed in the field soil ECa
throughout the year. Additionally, desalinization is assumed to continue to progress by repairing thedrainage channels.
It was estimated that heavy flood in September 2019 would be equivalent to once in 50 years
scale from local interviews. Due to such heavy flood, several collapses of the channel slope,
sedimentation in the channel, and damage to the Hume concrete pipe at the most downstream point
occurred. In order to restore the drainage function and leaching effect, it is necessary to repair themurgently.
ACKNOWLEDGEMENT
This work was supported by Grant-in-Aid for Scientific Research (KAKENHI; No.17H04630) (B)from the Japan Society for the Promotion of Science (JSPS) and The Mitsui & Co. Environment
Fund (Grant No. R17-0039).
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