IJERD – International Journal of Environmental and Rural Development (2020) 11-1

Research article
erd

Evaluation of Water Shortages in Agricultural Water Use in
the Sangker River Basin, Cambodia
CHEY VANNA
Graduate School of Agriculture, Hokkaido University, Sapporo, JapanTADAO YAMAMOTO*
Research Faculty of Agriculture, Hokkaido University, Sapporo, Japan
Email: tady@env.agr.hokudai.ac.jp

TAKASHI INOUE
Research Faculty of Agriculture, Hokkaido University, Sapporo, Japan
Global Station for Food, Land, and Water Resources, GI-CoRE, Hokkaido University, Sapporo,
Japan
Received 27 January 2020

Accepted 25 March 2020(*Corresponding Author)

Abstract The shortage of agricultural water in the Sangker River basin of Cambodia is
becoming a severe problem, and the river flow seems to have decreased. The present study
assesses the excess and deficiency of irrigation water resources in this river basin for
agricultural production. The Soil and Water Assessment Tool (SWAT) is used to analyze thewater balance at the main headwork in this river from 2014 to 2018. SWAT was used toestimate the available volume of stream water at the ungauged point of the Kang Hot
headwork, and the model performance was evaluated at a gauged point downstream by the
Nash–Sutcliffe efficiency (NSE). The model was calibrated manually and automatically byusing data from 2007 to 2013. Furthermore, the flow rates in the two main irrigation canals
were measured from June 2018 to October 2019 to obtain the actual irrigation water supply,
and the evaluation was made by checking the difference between supplied water volume andirrigation water demand. The model output showed good agreement between observed andsimulated monthly streamflow during the validation period (NSE = 0.69, RSR = 0.55). From
the results of water balance analysis, water shortage rates exceeding 20% of the monthly
water requirement occurred mostly between April and June from 2014 to 2017. Furthermore,
there was excess water supply in 2018 because of abundant river flow that year. The actual
irrigation water supply during the study period showed rates of water shortage of 52% and
41% in the left and right main canals, respectively, at the headwork. This study provides new
insights for field technicians to consider irrigation planning for present and future water
resources management and development for sustainable irrigation agriculture.
Keywords SWAT, water balance, irrigation, rice paddy, water supply and demand

INTRODUCTION
Many river basins worldwide are facing perceived water shortages because of increasing demands
for water from all sectors. In Cambodia, increasing water demands and competitiveness among
sectors and between upstream and downstream of the river system are also becoming crucial, and theconcern is that the country will face highly restricted water resources (Technical Service Center &JICA, 2014). Accordingly, the shortage of agricultural water is becoming a serious problem in theriver basins, and the streamflows appear to be decreasing. Contributing 22% of gross domestic
product collectively, agriculture, forestry, and fishing are essential to the growth of Cambodia’s
economy, and of the 80% of Cambodian people who live in rural areas, 65% rely on those threesectors (USAID, 2019). The Royal Government of Cambodia continues to enhance the managementof water resources, develop irrigation systems, and undertake repair and maintenance of the irrigation
infrastructure where required. Furthermore, headwork and multipurpose dams have been constructedⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

in the river basins to meet the increased demand for water and flood control. The Sangker River basin
in northwest Cambodia is one of the largest agricultural production areas in the nation and faces
similar water-resource issues. An irrigation project has been implemented in this area, but improved
management is required to solve the water-deficit problems.
The state-of-the-art method for estimating water deficiency is to analyze the balance between
the water supply and the demand for irrigation water (IW). Masona et al. (2018) revealed a shortfall
in IW use by applying the relative water supply (RWS), the ratio of water supply to water demand;
an RWS of 1.6 showed that the targeted scheme received much more water than its IW demands. To
assess water deficits in the Heibe River basin of China, Ji et al. (2006) established a model of thebalance between water supply and demand based on meteorological, hydrological, land use, andsocio-economic data. They found that the water supply was insufficient to meet the water demandsof the various irrigation districts, and indeed the Pingchuan irrigation district has already experienced
a water supply crisis. By comparing the actual water supply with crop water requirements, Shakir etal. (2010) revealed an annual water shortage of more than 40% in the Upper Chenab Canal of
Pakistan. However, with regard to the spatial distribution of operational parameters, calculating the
supply-and-demand balance of water resources does not resolve the regional problem of waterresource utilization on a relatively large scale in an inland basin (Bormann et al., 1999). Therefore,
the present study considers the water balance of a regional scheme by incorporating a physically
based model.
In the present study, the Soil and Water Assessment Tool (SWAT) hydrological model is usedbecause of its widespread use in predicting runoff and sediment yield (Arnold et al., 1998). Herein,
SWAT is used to estimate the streamflow at the headwork of the main canals. These streamflow data
are then used to evaluate the deficit of water resources in the irrigation project area. The present
results could help to identify the root of the water shortages that occur frequently not only in theirrigation project area but also across the river basin.
OBJECTIVE
This study aimed to define the available stream water to the irrigation project area and identify theextent to which the water supply of the system is in deficit and/or excess. The specific objectives are(i) to assess the surplus (excess) and deficiency of IW from the Sangker River at the Kang Hot
headwork point and (ii) to evaluate the actual IW uses and the demand in the Kang Hot Irrigation
Project area.
METHODOLOGY
Study Site
The present study was conducted in the Sangker River basin situated in the Battambang province ofCambodia and with a total drainage area of 6,053 km2 (Department of Hydrology and River Works,n.d.). The elevation ranges from 13 to 1,400 m above sea level, and the annual rainfall ranges from
695 to 1,787 mm. The focus was on a 3,062-km2 area to the southwest of the city of Battambang thatcovers the middle and upper basin of this river system (Fig. 1). The Kang Hot Irrigation Project isonly one of the huge numbers of water uses in the study area, with a total command area of 72,000
ha. However, the water is taken from the Sangker River to the project area at the Kang Hot headwork
and transported to the paddy fields through the left and right main canals, where the irrigated areas
are only 11,200 ha and 42,000 ha (Kodoma, 2018), respectively (Fig. 2).
To assess the surplus and deficiency of IW, the balance between the available water supply fromthe river and the IW demand was analyzed at the Kang Hot headwork from 2014 to 2018. The actual
IW use in the Kang Hot Project area was also evaluated by comparing the difference between theactual volume of water intake from the river and the IW requirement from June 2018 to October
2019.

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

Fig. 1 Location map of study area

Fig. 2 Kang Hot Irrigation Project area

Availability of Stream Water
In the study area, the supply of agricultural water depends on irrigation from the river source. For
the latter, the available volume of stream water was estimated using the SWAT hydrological model.
SWAT Modeling
SWAT is a river-basin-scale model developed to predict the impact of land management practices
on water, sediment, and nutrient yield, including agricultural activity, in large, complex watershedswith varying soils, land use, and management conditions over a long period of time (Winchell et al.,2013). In the present study, SWAT was set up with the basin outlet at the Samdach Hun Sen Bridge
as a gauging station located in Battambang City (Fig. 1). The watershed and sub-basins weredelineated within the basin using 30-m-resolution DEM data (ASTER-GDEM 2) in the
ArcSWAT2012 interface. The 3,062 km2 of the catchment area accounts for 51% of the entire
Sangker River basin (Fig. 2). A sub-basin was defined as a drainage area with a threshold of 5,000 ha,
resulting in 26 sub-basins that were subdivided further into 123 hydrological response units based on
slope, soil, and land use, the data for which were retrieved from the FAO-UNESCO Soil Map andthe Mekong River Commission (MRC-2010), respectively. The available meteorological data ofdaily rainfall and temperature data at four stations obtained from the Ministry of Water Resources
and Meteorology (MOWRAM) for 14 years (2007-2018) had to be preprocessed by the model. The
observed streamflow data collected from MOWRAM at the Samdach Hun Sen Bridge between 2007
and 2018 were used for model calibration (2007-2013) and validation (2013-2018). The SWAT
model was calibrated manually by editing the most sensitive parameters based on recommendations
by Abbaspour et al. (2015). In addition, the SUFI-2 algorithm packaged in SWAT-CUP was used forautomatic model calibration and validation and sensitivity and uncertainty analyses (Abbaspour etal., 2007). The model performance was evaluated using the quantitative statistics of the Nash–
Sutcliffe efficiency (NSE) and the ratio of the root mean square error to the standard deviation of
measured data (RSR) suggested by Moriasi et al. (2007). After fixing the SWAT for the Sangker
River at the Samdach Hun Sen Bridge gauging station, the flow rate at the Kang Hot headwork was
extracted from the database of model results.
Irrigation Water (IW) Demands
The IW demands were calculated mainly for rice-based farming in the Kang Hot Project area. Basedon its cropping calendar, the unit IW requirement was estimated with a 5-d time step using
IR = (ETo × Kc + PR +Lp – ER) / IE,(1)Ⓒ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

where (i) IR is the unit IW requirement, (ii) ETo is the crop reference evapotranspiration (calculatedusing the Penman–Monteith equation with different time steps using meteorological data), (iii) Kc is
the crop coefficient following the FAO’s guideline for the case of rice paddy fields (Allen et al.,1998), (iv) PR is the percolation rate (because of the absence of field measurement records, this wascollected from the Master Plan Study Team of JICA in 2007, which applied a dial gauge and openended-cylinder method in the study site), (v) Lp is the land preparation requirement [(needed tosaturate the root zone and depends on soil type and rooting depth, which was estimated by assumption(i.e., sandy loam = 250 mm, clay loam = 200 mm, clay = 170 mm)], (vi) ER is the effective rainfall
calculated by following empirical relationships developed in Japan (Dastane, 1974), and (vii) IE is
the irrigation efficiency, which refers to the conveyance efficiency of the distribution system and wascollected from MOWRAM and the Master Plan Study Team of JICA in 2009. The IW demand comesfrom the unit IW requirement multiplied by the area to be irrigated, which gives a volume of IW
needed per unit time. The IW demand is computed as
ID = IR × A,(2)

where ID is the IW demand and A is the area.
Estimation of Actual IW Supply
The actual continuous flow rate for each of the main canals was estimated by three steps. (1) Waterlevel loggers were installed in the left and right main canals near the Kang Hot headwork during thestudy period from June 2018 to October 2019. (2) Each canal flow rate was calculated directly fromthe mean canal-flow velocity (using a current meter over ten time differences) and the flow canalcross-sectional area. The water level in each canal at the moment of measurement was also recorded.
These measurements were then integrated into a rating curve (H-Q curve) of the water level and thecanal flow rate for each canal (Buchanan and Somers, 1976). (3) By using this H-Q curve, the recordsof the daily water level were converted into the daily canal flow rate and aggregated at a monthly
volume.
RESULTS AND DISCUSSIONSWAT Calibration and Validation
Figure 3 shows the results of the observed and simulated streamflow at the Samdach Hun Sen Bridge
gauging station in Battambang City (Fig. 1) with a monthly time step in the periods of calibration(2007-2013) and validation (2014-2018). The graphs show that the SWAT model tracked theobserved data accurately both calibration, and validation periods. This was confirmed by NSE valuesof 0.58 and 0.69 in the calibration and validation periods, respectively (Moriasi et al., 2007). From
the SWAT results at the gauging station, the available water resources at the Kang Hot headwork
could be estimated for 2014-2018 from the database of the simulation results.
Analysis of Balance Between Available Stream Water and IW Demand
To evaluate the excess and deficiency of IW from the Sangker River at the Kang Hot headwork point,
Fig. 4 shows the monthly averages of available stream water and IW demand at the headwork, andFig. 5 shows the observed deficit rate for the 5-year study period (2014-2018). The results show thatthe availability of stream water varies over a year and from 1 year to the next. With regard to boththe irrigation area and cropping patterns in the Kang Hot Project area during this study period, they
were supposed that the IW demands did not change much. If the water shortage is less than 10-20%of the monthly requirement, then the production losses are not very serious (Brouwer et al., 1992).

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

RainfallObserved

Simulated
0
100

600

200
500

Validation period
NS: 0.69/RSR:0.55
Good

Calibration period
NS: 0.58/RSR:0.64
Satisfactory

400

300
400
500

300

600

200

700
800

100

900

0

1000

Jan-07 Jan-08 Jan-09 Jan-10 Jan-11 Jan-12 Jan-13 Jan-14 Jan-15 Jan-16 Jan-17 Jan-18

Fig. 3 Monthly simulated and observed flow rate for calibration and validation period
in Samdach Hun Bridge gauging station of Sangker River
River Discharge

Irrigation Water Demand

600

0
100

500

200
400

300

300

400
500

200

600

100

Rainfall (mm.month-1)

Discharge volume ( 106 m3.month-1)

Rainfall700

0Jan-14800

Jan-15Jan-16

Jan-17Jan-18Fig. 4 Balance of available stream water and irrigationwater demand at Kang Hot headwork
Deficit rate (%)
0

90

100

80

200

70
60

300

50

400

40

500

30

600

20

700

10
0
Jan-14800
Jan-15Jan-16

Jan-17Jan-18Fig.5 Monthly water deficit rate at Kang Hot headworkⒸ ISERD36

Rainfall (mm.mo-1)

water deficit rate (%)

Rainfall100

Rainfall (mm.month-1)

Streamflow ( 106 m3.month-1)

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

The water balance calculation in Fig. 4 indicates that water deficit always occurred during theApril–May dry season every year from 2014 to 2017, with shortage rates of 50-99% of monthly
requirements. In the dry year of 2015, the area faced 5 months of subsequent water scarcity (withdeficiency rates of 30-99%). Each year, the stream water was surplus in the July-November wet
season. In 2018, heavy rainfall increased the streamflow volume, which means that the stream water
could meet the IW demand in a given area for the whole year.

( 106 m3.month-1)

Water supply volume

Irrigation water demand

60
50
40
30
20
10
0
Jun-18

Sep-18Dec-18Mar-19Jun-19Sep-19Fig. 6 Actual irrigation water (IW) in right main canal
Water supply volume

Irrigation water demand( 106 m3.month-1)

20
15
10
5
0

Jun-18

Sep-18Dec-18Mar-19Jun-19Sep-19Fig. 7 Actual irrigation water (IW) in left main canal
Shortage rate at Right Main Canal

Shortage rate at Left Main Canal

Shortage rate (%)

100
80
60
40
20
0

Jun-18
Sep-18Dec-18Mar-19Jun-19Sep-19Fig. 8 Water shortage rate in right and left main canals

Evaluation of Actual IW Use
The actual volume of water supply from the Sangker River and the IW demand are shown in Figs. 6
and 7 for the right and left main canals, respectively, as monthly averages from June 2018 to October
2019. Figure 8 shows the water shortage rate in each month for both main canals.
The results for the actual water supply and demand balance for the right main canal (Fig. 6)
show a maximum supplied water volume of around 57 × 106 m3⋅month−1 in 2018, decreasing to 40Ⓒ ISERD

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× 106 m3⋅month−1 in 2019. Meanwhile, the highest IW demand was approximately 60 × 106 m3⋅
month−1, corresponding to the extreme lack of water in 2019. As shown in Fig. 8, large water
shortages occurred in both 2018 (70-78% from September to November) and 2019 (over 77% from
April to June). In those 2 years, the water supply was always less than the water demand in September
and October, with the shortage rate exceeding 50%. However, the IW demands in every July andAugust during this study period were met. Overall, we conclude that the right main canal can supply
around 59% of the required monthly IW.
Figure 7 shows that the maximum supplied water volume by the left main canal was around 5 ×
106 m3⋅month−1 in 2018, increasing to 17 × 106 m3⋅month−1 in 2019. Meanwhile, the maximum water
demand was around 16 × 106 m3⋅month−1. Similar to the right main canal, water was also scarce in
September and October for those 2 years, with shortage rates of 56-74% (Fig. 8). Figure 7 also showsthat water deficiency occurred between June and November, with 50-78% in 2018 and over 80%
from April to June in 2019. By 2018, the water supply in July and August was inadequate, with ashortage rate of around 51%. However, compared to that in 2019, it is increased at 7 × 106 m3⋅
month−1 and 12 × 106 m3⋅month−1, thereby meeting the water demands of those 2 months. The water
volume of the left main canal can supply only 48% of the requirement.
Cropping Pattern
To minimize the water deficiency, an analysis is conducted by modifying the existing cropping
pattern against the available IW. In the revised cropping pattern, the cultivated area is changed asfollows: (i) early-wet-season rice is reduced from 40 to 10%; (ii) transplanting-wet-season rice isincreased from 60 to 80%; and (iii) direct sowing-wet-season rice is increased from 10 to 20%. The
results of this analysis show that the water shortage rate was reduced from 20 to 100% between April
and May (2014–2017), which is at the beginning of the irrigation season.
CONCLUSION
The results of this study show that SWAT performed well in predicting the streamflow by providing
good agreement between the observed and simulated monthly flow, which is based on the statistics
of NSE = 0.58 and NSE = 0.69 in the calibration and validation periods, respectively. With such
good performance, SWAT can be used to estimate the river discharge at the Kang Hot headwork
point as needed.
The balance between the available volume of stream water and the IW demand at the Kang Hot
headwork indicates that the water shortage rate was 50-99% of the monthly requirement, which
occurred mostly in April–June, and the excess water was in August–November. This surplus should
be controlled to compensate for the deficiencies in other months. By revising the current croppingpattern calendar, the water deficits could be diminished from 20 to 100% of the monthly shortage
rate in those shortfall months.
The actual IW use during the study period was 52% and 41% have experienced the lack of waterin the left and right main canals, respectively. Also, water deficits occurred in September and Octoberin both 2018 and 2019.
The results of this study will be useful for policy makers and field technicians for irrigation
planning and for developing and managing the water resources in the river basin, in particular for thesustainability of agricultural development.
ACKNOWLEDGMENT
The authors would like to thank the Ministry of Water Resources and Meteorology of Cambodia forproviding data and the Japan International Cooperation Agency (JICA) for its support of this study.

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