IJERD – International Journal of Environmental and Rural Development (2023) 14-1

Research article
erd

Research
Clarification of Water Use under the Complicated Irrigation
articleSystem and its Modeling in the Boribo-Bamnak River Basin,
Cambodia

HENG SALPISAK
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
Received 14 February 2023

Accepted 8 May 2023(*Corresponding Author)

Abstract Effective, equitable, and sustainable management of water resources in river basinsrequires clarification of the current river basin situation and potential water use and waterbalance challenges. This study performs a water balance calculation involving an analysis of
the appropriate water distribution of the Boribo-Bamnak River basin in Cambodia. This river
basin covers the Boribo and Bamnak basins, both of which are interlinked by the Bamnak
headworks. Together, this constitutes an integrated irrigation and drainage system. The Soiland Water Assessment Tool (SWAT) model was used to simulate rainfall-runoff in the BoriboBamnak basin over a study period of nine years (2010-2018), whereupon a simulated daily
streamflow was applied to the water balance computation in the river basin according to thewater distribution ratio at Bamnak headworks. The SWAT model yielded NSE, PBIAS, andRSR values of 0.55, 9.70, respectively, and 0.67 for model calibration. It also obtained valuesof 0.51, 3.70, and 0.70 for model validation of the three quantitative statistics, NSE, PBIAS,
and RSR. The results of the calculated water balance indicate that although the Bamnak and
Boribo Rivers have abundant water in the wet season, they have faced water shortages in thedry season during every year of the study period except 2016. Irrigation safety during the dry
season is always low (approximately 10%). Attaining a desirable safety level of 80% in all
existing irrigation areas would require additional water resources of approximately 6.0 × 106
m3 for the Boribo River basin and 107 m3 for the Bamnak River basin. In light of the abovefindings, this study provides recommendations for coordination strategies to improve water
resource management and development plans in the river basin.
Keywords water balance, water distribution, SWAT model, water shortage, irrigation,
safetyINTRODUCTION
Irrigation is a mechanism for increasing agricultural production, thereby increasing the income ofrural poor regions, and developing the economy. In this regard, it is a focus of the Royal Governmentof Cambodia as described in the Rectangular Strategy (CSIRO, 2013), a planning and development
document. As a result, Cambodia has witnessed the planning and implementation of many water
resources and irrigation development in recent years, many of which have focused on river basins(Masahiko, 2013). However, the rapidly increasing demand for water has intensified the competition
for water resources, highlighted concerns about the equity of water allocation, the sustainability ofwater usage, social friction among water-user communities, and the long-term sustainability of water
resources and environmental impact of irrigation (MOWRAM and JICA, 2014). Indeed, the BoriboBamnak River basin (MOWRAM, 2018) has been affected by a large increase in water use demandⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2023) 14-1

due to the expansion of irrigated areas, as well as competition for water allocation between upstream
and downstream users.
Thus, it is crucial to address water use by assessing water supply and water demand forappropriate management and planning of water resources in the basin. Researchers have used manymethods to assess and predict the intra- and interannual availability of water resources within acatchment. Among these methods, decision support tools, such as hydrologic models, can help
researchers to develop better management strategies for local and regional water resources (Chea andOeurng, 2017). Hydrological models have been developed to help calculate water discharge moreaccurately, easily, and quickly than when using traditional measurement methods. One such method
is the SWAT (Soil and Water Assessment Tool) model, which is a basin-scale model integrated withArcGIS to improve the accuracy of simulated streamflow from rainfall and the physical propertiesof a basin (Ang and Oeurng, 2018). Regrettably, conventional water supply-oriented simulation
models are often insufficient for addressing contemporary water resource management problems(Yates et al., 2005). Therefore, this study uses the SWAT model to simulate streamflow in acatchment and then performs the computation to describe the water balance and amount of water
available for allocation under current conditions.
OBJECTIVE
This study aimed to (1) predict water balances involving different scenario evaluations of water
distribution at Bamnak headworks in the Boribo-Bamnak River basin, and (2) clarify current water
management problems by studying water distribution at Bamnak headworks.
METHODOLOGY
Study Area
The Boribo-Bamnak River basin is located within two provinces of Cambodia, namely Kampong
Chhnang, and Pursat, and has a total catchment of 1258 km2 (Fig. 1). It covers the Boribo andBamnak River basins, both of which are interlinked at Bamnak headworks (located on the Bamnak
River, an upstream tributary of the Boribo River) and constitutes an integrated irrigation and drainage
system (Fig. 2). The climate in the basin is influenced by tropical monsoon systems with distinct wet
and dry seasons. The wet season, from May to November, receives approximately 90% of the total
annual rainfall, ranging from 1200 mm to 1800 mm, and the dry season, from December to April, ischaracterized by the prevalence of hot and dry air with high potential transpiration demands. In thebasin, agriculture is the predominant water-consuming sector and twelve main irrigation schemes
have been developed; these have been classified into six schemes using water from the Bamnak River(Pursat Province) and the other six schemes using water from the Boribo River (Kampong Chhnang
Province) as shown in Table 1.

Fig. 1 Study area

Fig. 2 Schematic Bamnak headworksⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2023) 14-1

Table 1 Identified irrigation schemes in the Boribo-Bamnak River basin
No

Scheme Name

Total Command Area (ha)I. Bamnak River (Pursat Province)
1 Bamnak
2 Chamkar Krouch
3 Kampong Lor
4 Thlea Maom
5 Tram Mneash
6 Trapeang Khley
Sub-total
II. Boribo River (Kampong Chhnang Province)
1 Achang (1)2 Dambok Krahorm
3 Kamping Tatao
4 Lum Hach
5 O Rolus
6 O Chenhchraim
Sub-total
Total

Total Farmland Area (ha) Using Water
Irrigated from the Target Rivers
EW
W
D

6,000
350
130
3,500
1,200
300
11,480

600
100
30
500
120
100
1,450

2,400
100
100
2,000
1,080
200
5,880

1,122
70
30
1,222

10,300
150
700
3,289
640
170
15,24926,729

1,450

1,400
300
3,289
640
170
5,799
11,679

750
150
700
1,600
2,822

(1) Including four sub-schemes: Banteay Thlok, Kum Reab, O Sanlang, and Svay Ka’er

EW: Early wet season rice, W: Wet season rice, D: Dry season rice
Sources: (1) MEIHO Engineering Inc. 2018. Final report, Survey on basic information in river basins (phase 3),
JICA; (2) Field study in Cambodia, April 2022.

SWAT Modeling Approach
The SWAT model is a physically based, semi-distributed, agro-hydrological, continuous hydrologicalmodel developed to enable water resource managers to determine the most appropriate strategy or
solution by considering the impact of different management practices on streamflow and non-point
source pollution (Arnold et al., 1998).
In this study, the SWAT model was used to simulate, using available data, daily streamflow inthe catchment of Boribo-Bamnak. Running a comprehensive SWAT model requires spatial input
data including a Digital Elevation Model (DEM), land use, soil type, and meteorological data. Eachdata input was obtained from different sources, such as a DEM with a resolution of 30 m downloaded
from ASTER GDEM2. Soil-type data with a resolution of 250 m was retrieved from the FAO soil
map, and land use data were obtained from the Ministry of Water Resources and Meteorology(MOWRAM) of Cambodia. Daily meteorological data from 2007 to 2018 were provided by theProvincial Department of Water Resources and Meteorology (PDWRAM) of the Kampong Chhnang
and Pursat Provinces, respectively. The observed daily streamflow at Boribo River station was
required by SWAT-CUP to be calibrated between 2010 and 2015 and validated from 2016 to 2018using the SUFI-2 method.
Irrigation Water Requirements
The irrigation water requirement (IWR) is the amount of water needed to fulfill crop waterrequirements after effective rainfall, thereby ensuring a disease-free crop in large fields under nonrestricting soil and water conditions and adequate fertility (Solangi et al., 2022). In this study, theaverage five-day water requirements for proposed cropping patterns of each crop variety in the
Boribo-Bamnak River basin were estimated using the following equation Eq. (1)𝐼𝑊𝑅 = (𝐸𝑇0 × 𝐾𝑐 + 𝑃𝑅 + 𝐿𝑃 − 𝐸𝑅)⁄𝐼𝐸(1)

where IWR is the irrigation water requirement for a division unit, ET0 is reference evapotranspiration,
calculated using the FAO Penman-Monteith method (FAO, 1998), Kc is the crop coefficient defined
based on FAO guidelines (for the case of paddy) since there is no observed data in the basin, PR is
the percolation rate (3.0 mm per day in this study, referring to JICA (2012)), LP is the land
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IJERD – International Journal of Environmental and Rural Development (2023) 14-1

preparation requirement, estimated by assumption using two methods of direct sowing under dry soil
and transplanting under wet soil, ER is effective rainfall, calculated following JICA (2012), and IE
is irrigation efficiency, obtained from water losses during conveyance and application to the fieldand defined following JICA (2012).
Water Balance Study
The water balance was computed using a simplified diagram of the present condition of water
resource facilities and irrigation water uses in the river basin (Fig. 3). The fundamental formula fora water surplus or deficit at a major point where a large amount of water is taken from the river isexpressed as follows Eq. (2)
𝑄𝑠𝑑 = 𝑄 + 𝑄𝑟𝑓 − 𝑄𝑞 − 𝑄𝑟𝑚𝑓

(2)

where Qsd is the surplus or deficit of water at the calculation point, Q is the river runoff from the relevant
catchment area, Qrf is the return flow from paddy fields to the river (17% of the irrigation demand wasused in this study after JICA, 2012), Qq is a summation of irrigation water demand, and Qrmf is the river
maintenance flow, which was supplied to the downstream part of the river in order to preserve the river
as a part of the landscape (0.1 m3/s/100 km2 was applied, based on the JICA, 2012).
The water balance calculation was executed from upstream to downstream following thenatural flow direction for a time unit of five days for nine years (2010–2018). To confirm the securityof the irrigation water supply to the Boribo-Bamnak River basin, this calculation was performed asfollows: (1) identification of the impacts of existing irrigation schemes in the basin, (2) assessmentof impacts from the viewpoint of the safety levels of irrigation water supplies, and (3) assessment of
the impacts of water distribution at Bamnak headworks.
An evaluation of the safety level of the irrigation water supply is made as follows (1) a waterbalance was performed at each scheme outlet in terms of deficit or surplus, i.e., the net available flow
subtracting the irrigation water requirement (IWR), such that a deficit occurs when the net available
flow is less than the IWR, (2) for each year, when the deficit was more than or equal to fourcontinuous five-day units (20 days), this year is judged to be a deficit year, and (3) each irrigation
scheme is successful if the safety level is less or equal to 1/5 (80% dependability). The formula todefine the safety level at each irrigation scheme is expressed as Eq. (3)𝑆𝑎𝑓𝑒𝑡𝑦 𝐿𝑒𝑣𝑒𝑙 = (𝑥 + 1)⁄𝑛(3)where x is the number of years with twenty-day successive deficits (irrigation failures) and n is thetotal number of simulated years. In this study, for nine years of successful irrigation, the 1/5 term canbe stated as 2/9. Consequently, if the safety level is less than or equal to 2/9, the scheme is deemed
successful, and we can determine that water resources are secured with 80% dependability.
To calculate the probability of the deficit/storage required for a safety level 1/5, we use Eq. (4)𝑦 = 𝑐 × ln(𝑥) + 𝑏(4)

where y is the probable deficit in x% non-exceedance, c and b are constants, ln is the natural logarithm,
and x is the percentage of exceedance.
Water distribution at Bamnak headworks is an important point of the water balance calculationin the Boribo-Bamnak River basin. In this study, water distribution at Bamnak Headworks wasincorporated into the water balance calculation in two ways. (1) Water was allocated equally to Pursat
and Kampong Chhnang (i.e., 50% each) during the wet season (May–October) and allocated 75% toPursat Province, and 25% to Kampong Chhnang Province during the dry season (November toApril); refer to the field study in Boribo-Bamnak River basin, March 2022. (2) Water was allocatedequally to Pursat and Kampong Chhnang Provinces (i.e., 50% each) either in the wet season or dry
season (refer to the regular rule of fair and sustainable water distribution).
Additionally, water management was considered through a study on actual water distribution
using existing data for discharge at Bamnak headworks from 2018 to 2019.

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IJERD – International Journal of Environmental and Rural Development (2023) 14-1

Boribo River

Boribo River

Bamnak River

Bamnak River

CA6

CA7

CA1

CA3

124

234

388

135

O Chenhchraim irr.
170 haLum Hach3289 haBamnak Right irr.
500 haBamnak HW

CA9

143

Bamnak Left irr.
5,500 haCA8

Lum Hach HW

CA2
CA4

14

110
Chamkar Krouch irr.
350 haTram Mneash HW

Tram Mneash irr.
1,200 ha

38
CA5

43
O Rolus irr.
640 haAchang HW

Trapeang Khley irr.
300 haThlea Maorm irr.
3,500 haAchang irr.
10,300 ha

CA10

26

Thlea Maorm HW 1&2

Kampong Lor HW

Kampong Lor irr.
130 haTonle Sap Lake

Kamping Tatao irr.
1,000 haCA11

Dambok Krahorm irr.
150 haDambok Krahorm HW

3

Irrigation systems;- O' Sanlang- Kum Reab
- Svay Ka'er- Banteay Thlok

LegendCA i ; Catchment area, the i is a serial number given by the study performer
xxx ; Catchment area in square kilometers (km2); River flow
; Return flow or drain; Headworks/Intake; Irrigation〇〇〇 irr.
; Name of irrigation area
; Province Boundary
xxxx ha; Command area in ha

Tonle Sap Lake

Fig. 3 Diagram of present-day water use in the Boribo-Bamnak River basin
RESULTS AND DISCUSSIONSWAT Calibration and Validation for Streamflow
Figure 4 represents the simulated streamflow for the daily time step (calibrated from 2010 to 2015
and validated from 2016 to 2018) compared with observed data. The model performance is shown tobe satisfactory in terms of calibration and validation with NSE values of 0.55 and 0.51 according to theevaluation criteria of Moriasi et al. (2007). We noted, however, that the hydrological simulations did
not perform well during the dry season (November-April) due to inaccuracies in the estimateddischarge at the observation station. The estimated discharge was calculated via water level
observation by applying H-Q curves produced by the River Basin Water Resources Utilization
project of JICA in 2018. Because these water discharge measurements were conducted only duringhigh-water level periods, they did not cover the entire range of water levels experienced during
periods when the relationship between water level and discharge is known to be stable. This
relationship was, therefore, not defined for low flow conditions.

Fig. 4 Daily streamflow during the calibration and validation periodsⒸ ISERD

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Referring to the information about water management in the Boribo-Bamnak River basin
obtained through a field study conducted in March 2022, we found that the water level in the river
was very low in the upstream region, drying up in the downstream region during the dry season. Thisvariation is known to be challenging for the farmers in the river basin and has been reported bynumerous farming households located in the upstream and downstream parts of the river basin (27households in Pursat Province and 6 households in Kampong Chhnang Province). Simulated
streamflow from the SWAT model was used for the water balance study.
Water Balance Study
The water balances of the Bamnak River in Pursat Province and the Boribo River in Kampong
Chhnang Province were calculated taking into consideration the ratios of water distribution at
Bamnak headworks. Fig. 5 shows the results of the first case of water distribution at Bamnak
headworks (wet season: Pursat = Kampong Chhnang = 50%; dry season: Pursat = 75%, Kampong
Chhnang = 25%). Fig. 6 illustrates the results of the second case (wet season and dry season: Pursat= Kampong Chhnang = 50%). Water resources are sufficient during the wet season; however, water
shortages frequently occur in the dry season. Indeed, a water deficit occurred annually during the dry
season every year between 2010 to 2018, except for 2016. Thus, the irrigation safety is approximately
10% either in Pursat and Kampong Chhnang Province under these two ratio cases. This indicates thatthe Bamnak and Boribo Rivers lack sufficient water to guarantee 80% irrigation safety for existingareas of irrigation. By calculating the probability of the deficit/storage required for a safety level of
1/5, the water deficit is estimated in both cases as approximately 6 million cubic meters in Kampong
Chhnang Province and 10 million cubic meters in Pursat Province.
The Bamnak and Boribo Rivers have abundant water during the wet season. The irrigation of
wet season paddy was examined without dry season rice taken into the calculation; as a result, theirrigation safety during the wet season paddy was 80% in both provinces by virtue of the natural river
flow alone. Under these conditions, water distribution at Bamnak headworks is not considered to be
an issue during the wet season (May to October).

Fig. 5 Hydrograph showing surplus and deficit water between 2010 and 2018
Water allocated at Bamnak headworks: Pursat = Kampong Chhnang = 50% during the wet season; andPursat = 75%, Kampong Chhnang = 25% during the dry season.

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Fig. 6 Hydrograph showing water surplus and deficit between 2010 and 2018
Water allocated at Bamnak headworks: Pursat = Kampong Chhnang = 50% during the wet anddry seasons

According to our calculations, although 100% of the water was distributed to Pursat during thedry season, it was insufficient to maintain the irrigation water required for existing dry-season ricein this province (1,222 ha). Moreover, river maintenance flow in both provinces could be secured incase equitable water distribution at Bamnak was made and dry season rice in both provinces was nottaken into calculation.
Water Distribution at Bamnak Headworks
The actual water distribution at Bamnak headworks was estimated based on available data for 2018and 2019. Which were obtained from Pursat PDWRAM (recorded water level) and the JICA River
Basin Water Resources Utilization project (H-Q curve). The water distribution at Bamnak headworks
was found to be similar to the distribution ratio of the first case study in the water balance calculation(Fig. 7). On average, the ratios of water distribution were made as 40% and 30% to Kampong
Chhnang Province, respectively, and 60% and 70% to Pursat Province, respectively, in the wet anddry seasons.

Fig. 7 Water distribution at Bamnak headworks in 2018 and 2019
To assist water management in the basin, the Bamnak headworks were constructed in 2010 by
MOWRAM with the base idea of allocating 50% of the water available to Pursat Province and 50%
to Kampong Chhnang Province. In practice, this concept has been difficult to realize because thetotal width of four intake gates to Pursat Province is 8.8 m (=2.2 × 2 × 2) when the total width of twoⒸ ISERD

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spillway gates to Kampong Chhnang Province is 3.0 m (=1.5 × 2) (Fig. 2). Therefore, fair, andsustainable water distribution at Bamnak headworks must be made based on a consensus between
province stakeholders.
CONCLUSION
This study has shown that the SWAT model is an appropriately sensitive model for predicting daily
streamflow in the Boribo-Bamnak River basin. It provides insights into the importance of better
analysis relationships, which depends mostly on the quality of available data and the necessity for
additional hydrological observation.
The results of water balance studies in the Boribo-Bamnak River basin clearly indicate that theavailability of water in the river is not sufficient to supply all water demands for existing irrigation
schemes in Pursat and Kampong Chhnang Provinces. Water shortages occurred in the dry season
almost every year during the study period (2010-2018). Therefore, to ensure sustainable water use in
the river basin, a study of the possibility of constructing reservoir dams in the upstream region ofboth the Bamnak and Boribo Rivers should be considered.
In terms of water management at the Bamnak headworks, this study suggests that Pursat
PDWRAM should make efforts to communicate with Kampong Chhnang PDWRAM and related local
authorities, for example, the Farmer Water User Communities, in order to appropriately implement
water flow operations that ensure sustainable water use while avoiding conflicts among water users.
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