Water diversion systems play crucial roles in assuaging flood risk by diverting and redistributing water within and among basins. For flood and drought assessments, including investigations of the effects of diversion systems on river discharge worldwide, the explicit inclusion of these systems into global hydrological models (GHMs) is essential. However, such representation remains in the pioneering stage because of complex canal operations and insufficient data. Therefore, we developed a regionalized canal operation scheme and implemented it in the H08 GHM for flood diversion in the Chao Phraya River basin (CPRB), Thailand, which is a complex river network with several natural and artificial diversion canals and has been subject to severe flooding in the past, including recent years.
Region-specific validation results revealed that the enhanced H08 model with the regionalized diversion scheme could effectively simulate the observed flood diversion pattern in the CPRB. Diverted water comprises approximately 49 % of the annual average river discharge in the CPRB. The simulations further confirmed that the presented canal scheme had the potential to reduce flood risk in the basin by significantly reducing the number of flooding days. A generalized canal scheme with simple input data settings was also constructed for future global applications, providing insights into the maximum level of discharge reduction achievable with diversion of nearly 57 % of the annual average river discharge of the CPRB. Overall, the enhanced H08 model with canal schemes can be adapted and applied to different contexts and regions, accounting for the characteristics of each river network by maintaining the basic principles unaltered.
Floods are among the most severe natural hazards, and flooding occurs naturally when river discharge exceeds the channel carrying capacity. Extreme flood events cause serious damage to human life, property, and agricultural systems, with disrupted economic activity (Padiyedath Gopalan et al., 2020; Yin et al., 2018). In past decades, annual losses caused by extreme flood events have been approximately tens of billions of U.S. dollars;
moreover, thousands of people have lost their lives in floods each year (Hirabayashi et al., 2013). Currently, almost 13 % of the global population (around 1 billion people) live in floodplains (Di Baldassarre et al., 2013); this population is at risk of exposure to river flooding caused by extreme weather events (Alfieri et al., 2017). Climate change may increase the intensity and frequency of extreme flood events and the global
population exposed to flooding, thereby enhancing the magnitude of losses and fatalities (Dankers et al., 2014; Hirabayashi et al., 2013, 2021; Jongman et al., 2015; Prein et al., 2017). The current flood issues have been addressed in many areas to a greater extent through the construction of
Among the different water infrastructures, water diversion systems can be an influential driver for flood management by diverting water from surplus areas to deficit ones in various river basins (ICID, 2007). Concurrently, they also play a vital role in the sustainable development of water resources by distributing the diverted water for various purposes like irrigated agriculture, municipal, and industrial water supply
(Shumilova et al., 2018). Water diversion schemes accounted for about 14 % of total global water withdrawal (540
Only a few attempts have been made to assess the effects of diversion canals on flood risk reduction although water diversion plays an important role in controlling floods of the water supply areas. For instance, the effectiveness of diversion canals in decreasing floodwater levels was evaluated using the HEC-RAS hydraulic model in Indonesian river basins (Indrawati et al., 2018; Nugroho et al., 2018). In addition, Wang et al. (2019) examined the impacts of interbasin water diversion on flood control and drainage processes in a water-receiving lake basin in China using the MIKE model. Subsequently, Mel et al. (2020b) identified the optimal floodgate operation rules for diverting floodwater in the Bacchiglione–Brenta river network in Italy, using the 2DEF hydrodynamic model; they also evaluated the applicability of those rules (Mel et al., 2020a). Although the use of hydrodynamic models to simulate floodwater diversion has greatly clarified the effects of such operations on flood risk reduction, these studies have mainly considered the relationships between regional flood characteristics and associated diversion canal operation schemes, which have limited global applicability as well in other regions. This further signifies the integration of water diversion schemes into global hydrological models (GHMs) to assess their impacts on global and regional flood control that are relatively unexplored.
For the impact assessment of human interactions on flood discharge worldwide, their explicit inclusion into GHMs is necessary (Bierkens, 2015; Boulange et al., 2021; Nazemi and Wheater, 2015a, b). GHMs have been progressively updated to include reservoir operations, water withdrawal and supply for irrigation, domestic, and industrial uses, groundwater dynamics, and seawater desalination (Burek et al., 2020; Ehsani et al., 2016, 2017; Hanasaki et al., 2018; Müller Schmied et al., 2021; Pokhrel et al., 2015; Sutanudjaja et al., 2018). However, the representation of water diversion systems remains in the pioneering stage because of complex canal operations and insufficient data. Among the different GHMs, the H08 GHM is a pioneering model that explicitly considers numerous human interactions with the hydrological cycle (Hanasaki et al., 2008a, b, 2018). The H08 model has been successfully applied at global and regional scales (Masaki et al., 2017; Masood et al., 2015; Mateo et al., 2014). However, floodwater diversion has not been implemented thus far because of the challenges involved in its implementation. Therefore, the focus of this study was the development and implementation of a diversion canal operation scheme into the H08 GHM that was carefully designed to adapt for future floodwater management in complex river networks worldwide, using a case study in the Chao Phraya River basin (CPRB), Thailand.
Thailand is the second-largest economy in Southeast Asia, and 58 % of all disasters in that country are due to floods (JICA, 2015). Floods in Thailand have caused 27 % of the total disaster-related deaths; the greatest proportion can be attributed to the 2011 flood disaster in the CPRB, the largest basin in Thailand. The estimated economic losses from the 2011 flood were approximately USD 46.5 billion (JICA, 2015; Komori et al., 2012; World Bank, 2012). The CPRB has been subject to severe flooding in the past, including in recent years (Komori et al., 2012; Kotsuki et al., 2014; Kure and Tebakari, 2012; Watanabe et al., 2014). Recent studies have evaluated the applicabilities of various adaptation strategies, including afforestation (Takata and Hanasaki, 2020) and reservoir operation (Padiyedath Gopalan et al., 2021), for reducing flood risk in the CPRB; they concluded that further adaptation measures are needed to address future floods. Although the CPRB has several natural and artificial diversion canals, their adaptation potential remains largely unknown because these diversion systems have not been integrated into hydrological models. Hence, in this study, flood control was considered as being the primary feature of the implemented diversion canal scheme because of severe flooding issues in the CPRB, although water diversion systems could function as both floodways and irrigation channels.
The aim and novelty of this study were the construction of a regional floodwater diversion scheme for the CPRB that is flexible and easily modifiable for future application in any region worldwide. Based on the aforementioned discussions, this study is composed of the following objectives:
Develop a floodwater diversion scheme for the CPRB and enhance the H08 GHM by incorporating this scheme into the model. The water diversion scheme includes diversion canals and retention areas to divert and store water, respectively. Validate the effectiveness of the enhanced H08 model for reproducing the water diversion process in the CPRB. Analyse the impacts of diversion canals and retention areas on flood management in the CPRB.
H08 is an integrated global water resources model that consists of six modules, namely land surface hydrology, river routing, crop growth, reservoir operation, environmental water, and anthropogenic water withdrawal (Hanasaki et al., 2008a, b) to quantify natural and anthropogenic water availability and use in the past, present, and future. Each of the modules can run separately with standard spatial and temporal resolutions of
The latest version of the H08 model enables water abstraction from various water sources (e.g. surface water and groundwater). It explicitly includes groundwater recharge, groundwater abstraction, aqueduct water transfer, local reservoirs, irrigation return flow and delivery loss, and seawater desalination schemes (Hanasaki et al., 2018). These changes transform H08 into one of the most detailed GHMs for attributing different water sources available to human society. The aqueduct water transfer scheme was implemented into the H08 model to provide water supply to the grid cells that are farther from the river channel to meet their water demand (agricultural, industrial, and domestic). If there is a water demand to meet, then the scheme assumes that the water could be transferred until the river flow at the aqueduct origin falls below the environmental flow because the information regarding the aqueduct carrying capacity was not available for most cases (Hanasaki et al., 2018). The operation strategy transfers water only when the water demand is positive. However, the aqueduct scheme does nothing for excess water availability (floodwater), and the model still lacks the representation of floodwater diversion systems. The implementation of this representation will provide viable adaptation scenarios towards a warming climate.
To overcome this limitation, the H08 model was enhanced by implementing diversion canal schemes. Although the primary objective of diversion canals assessed in this study is flood control, the implemented canal schemes could function as both floodways (offering different routes for excess water flow) and water supply canals (redistributing water from one region to another region that is experiencing water shortage) in future global applications. If flood control is the primary purpose of a canal, then the river flow is diverted to the floodway (flood diversion canals) only during the wet season. In contrast, river flow is diverted to water supply canals during the dry season to supply water for various purposes, including irrigation. The schematic diagram of canal operation for the wet and dry seasons is shown in Fig. 1. For multipurpose canals, river flow is diverted throughout the year to provide water supply during the dry season and flood control during the wet season. Because this study included flood diversion, water supply, and multi-purpose canals in the implemented diversion scheme, this scheme can be applied to any region based on the prevalence of each canal type and the context in which canal operations are needed.
Schematic diagram of the canal diversion scheme of the
enhanced H08 model during the
The dry season is characterized by low rainfall that causes consequent water shortage for an extended period. To alleviate the water scarcity issues, water supply should be provided from the river channel to the neighbouring areas through diversion canals by preserving the environmental flow in the river channel. Based on this aspect, the operation of diversion canal systems during the dry season leads to the emergence of three cases concerning the environmental flow (
The wet season is the period during which most of the annual rainfall is received. This high rainfall will eventually cause flooding in the neighbouring areas whenever river discharge exceeds the river channel carrying capacity. The diversion canals can divert this floodwater from the river channel and restore the river water level below the carrying capacity. From this perspective, five relevant cases can be identified for the
operation of diversion canals during the wet season, as shown below:
Flow that is diverted from the main channel into canals eventually splits into four major components, as shown in Fig. 2. The first component is “returned discharge” (A), which is the canal flow returning to destinations such as the downstream section of the same river or nearby rivers or tributaries within the basin. This returned discharge increases flow at the destination and reduces flow at the origin. The second component is “supply to nearby grids” (B), which is the portion of the diverted water supplied to each of the grid cells through which the canal passes and to the immediate lateral neighbouring grid cells of the canal. The immediate lateral neighbouring grid cells are decided upon based on the presence of croplands in these grid cells. This supply to nearby grids component is enabled only during the dry season to meet the water demand. For simplicity, 10 % of diverted water is supplied to each of the nearby grid cells in this modelling approach that was further utilized for irrigation. If the demand is less than the water supplied to the local grid, then the surplus water after meeting the demand is added to the discharge of the corresponding grid cell. This river discharge finally returns to the river channel, as shown in Fig. S1a and b, and thereby closes the water balance. The remaining diverted water after supply will move to the subsequent downstream grid cells. This process is repeated until the diverted flow is fully depleted or reaches its destination. At some point, the diverted water flows to nearby river basins or drains into the sea. This component is designated as “flow out of the basin” (C); it alters the water balance of the river system in question and should be included in water budget analysis to avoid unexplained water imbalance. A portion of the diverted canal flow drains into the retention areas, and the floodwater stored in these areas is called the “retention storage” (D). Once the retention areas are full, the remaining canal flow will move to the next grid. This process is repeated in each grid cell along the canal flow route until flow either diminishes to zero or reaches its destination (either within the basin or out of the basin). The storage of diverted floodwater in retention areas is allowed to supplement flood control capacity alongside diversion systems only during the wet season. In addition to the diverted water storage during the wet season, the retention areas are modelled in such a way that they receive runoff generated from precipitation in each grid based on their areal fraction during both dry and wet seasons. This runoff constitutes a part of retention pond storage and thereby affects the land surface processes. Only the remaining storage capacity is available for the storage of diverted floodwater during the wet season. All these components can be customized (enabled/disabled) for each basin under consideration.
Schematic diagram of the four major components of the diverted canal flow. The green area denotes the river basin.
To conduct canal operations, the following data are needed:
A canal diversion scheme that utilizes the observed values of all input data (described in Sect. 2.3) specific to a particular region is described hereafter as the regionalized canal scheme. However, when the model is applied to data-scarce regions and periods, the implementation of the diversion system using the input data described in Sect. 2.3 becomes strenuous. In addition, when this diversion scheme is implemented in GHMs for global analysis, extensive data are needed for each river basin, further hampering the application of GHMs. To overcome these difficulties and support the future global application of the canal diversion scheme, the H08 model was tested with simple input data settings that can be derived from river discharge alone (hereafter designated as the generalized canal scheme).
The generalized canal scheme is regarded as the preliminary survey for future global applications. In the generalized scheme, the geographic location of the canal, and the locations and areal extents of retention areas, should be estimated for the study region in a manner similar to the regionalized scheme. Information regarding canal systems can be collected from peer-reviewed articles, official websites, reports from governmental and non-governmental organizations, and newspapers (Shumilova et al., 2018). Based on this information, the geographic locations of the canal origin, flow path, and destination can be extracted using the Google Earth application. Similarly, low-lying areas along rivers (floodplains) and canals, ponds, and lakes can be recognized as retention areas, if present.
The input data for the river carrying capacity, canal carrying capacity, and minimum flow of diversion is difficult to collect for all canal systems to support the global application of the model. The river channel carrying capacity (
The diversion canal system was developed for the CPRB, which is the largest river basin in Thailand, with an area of approximately 158 000
The left panel shows the topographic map of Thailand, where the rectangle shows the location of the Chao Phraya River basin (CPRB). The right panel shows the topographic map of the CPRB with the major tributaries and the locations of the Nakhon Sawan (C.2 station) and Bangkok.
The CPRB is divided into an upper and a lower basin at Nakhon Sawan (C.2 station), as shown in Fig. 3 (Komori et al., 2012). The upper basin includes four major tributaries (i.e. Ping, Wang, Yom, and Nan) that have headwaters originating from the northern part of the country. The Wang joins with the Ping River, and the Yom joins with the Nan River; the subsequent confluence of the Ping and Nan rivers at Nakhon Sawan is the beginning of the Chao Phraya River. From Nakhon Sawan, the river flows to the lower basin through Bangkok and finally drains into the Gulf of Thailand (Tebakari et al., 2012). Other tributaries (i.e. Pasak and Sakae Krang rivers) join the Chao Phraya River in the lower basin.
Thailand experiences two seasons, i.e. a wet season (May–October) and a dry season (November–April). The rainfall distribution over the basin significantly varies, ranging from 1000 to 2000 mm. Nearly 90 % of total annual rainfall is received during the wet season, leading to increased flood risk (Bond et al., 2018). The lower basin and downstream parts of the Yom basin have gentle slopes (Fig. 3) and reduced channel carrying capacity; this makes the region highly prone to flooding (Komori et al., 2012). The severe flooding that occurred during the 2011 monsoon season inundated the lower basin and downstream parts of the Yom and Nan rivers (Mateo et al., 2014), causing over 800 deaths and substantial economic losses of USD 46.5 billion (World Bank, 2012). To minimize flooding caused by reduced channel carrying capacity and gentle slopes at various locations, the Thai government has implemented measures to protect the CPRB by constructing dams and numerous diversion canals (both natural and artificial).
The CPRB is a complex river network with several natural and artificial diversion canals, as shown in Fig. 4. Figure 4 also shows the observed river and canal carrying capacities at various locations in the CPRB (right-hand side) that are solely determined by their cross sections. The carrying capacity values were collected by literature review (JICA, 2013; Tamada et al., 2013) and from the Royal Irrigation Department, Thailand. Whenever the river discharge exceeds river carrying capacity, flooding will occur. Therefore, a certain amount of floodwater should be diverted from the river to the canal systems, subject to a maximum value of canal carrying capacity. These canals are either for flood diversion or multipurpose; they were implemented to mitigate extreme flood events, protect high-density business and residential areas, and provide irrigation water supply. Water diversion into these canal systems is controlled by barrages installed on the Chao Phraya River. Based on the literature review (JICA, 2013; Tamada et al., 2013) and maps collected from the Royal Irrigation Department (RID) of Thailand, 11 canal systems were identified in the CPRB with direct intakes from the main river channel (Fig. 4). Details of these canal systems are provided in Table 1. Of these 11 canal systems, 2 are in the Yom basin; flooding is common in that basin because (i) large-scale flow regulating structures are absent, (ii) the downstream area exhibits a gentle slope, and (iii) various locations have low channel carrying capacities. To alleviate these issues, two diversion canals have been implemented to divert water from the Yom River to the Nan River, downstream reaches of the Yom River, and nearby low-lying paddy fields during floods (Fig. 4a).
The left-hand side shows the schematic diagram of different existing water infrastructures in the
Origin and destination points of the canal systems in the CPRB with their regulating structures and purpose.
The remaining nine canal systems are in the lower CPRB, where flood hazards are frequent because the channel's carrying capacity progressively decreases from Nakhon Sawan (C.2 station) to Ayutthaya (C.35 station), as shown in Fig. 4b. To reduce downstream flooding near Ayutthaya, the Chao Phraya diversion
dam on the Chao Phraya River allocates water to multiple canals (i.e. Chainat–Pasak, Makhamthao–Uthong, Tha Chin, Noi, and Chainat–Ayutthaya) immediately upstream of the diversion dam (canals three to seven, respectively, in Fig. 4b). Among these five canals, the Tha Chin River is a major distributary of the Chao Phraya River that flows through Bangkok and drains into the Gulf of Thailand. This river is considered as a canal system
because of its ability to carry floodwater away from the Chao Phraya River. Similarly, the Noi River, which diverts water from the main channel, is considered as a diversion canal. Together, these five canals limit flow to approximately 2840
There are four canal systems in the basin that are solely responsible for flood control, i.e. the Yom–Nan diversions in the upper CPRB and the Lopburi and Bang Kaeo canals in the lower CPRB. The remaining seven canals in the lower CPRB serve both flood control and irrigation water supply purposes. Among all canals in the basin, two remove water from the basin, namely the Makhamthao–Uthong (providing irrigation water to areas that lie outside the basin) and Tha Chin (emptying into the Gulf of Thailand, similar to the Chao Phraya River) canals. The diversion of river discharge from the main channel to canals is controlled by the operation of diversion dams and other water regulation structures (names of structures are provided in Table 1). However, the gravity transfer of water occurs at the origins of the Phong Pheng and Bang Ban canals.
Timely diversion of flood flows from the main channel is essential for controlling flooding downstream of the lower CPRB, which is a flood-prone area affected by strong tides. Therefore, two questions arise at the time of diversion, and they are when to divert and how much water to divert. Whenever river discharge exceeds the channel carrying capacity at the origin of a diversion canal, an amount no greater than the canal carrying capacity should be diverted to control flooding. Furthermore, to enable irrigation water supply because canals in the CPRB are solely used to provide water for irrigation and not for other sectors, a minimum flow amount is diverted during the dry season. Table S1 in the Supplement shows the observed river channel carrying capacity, canal carrying capacity, and minimum flow of diversion at the origins of the 11 canal systems (Tamada et al., 2013).
Several dams have been constructed in the CPRB since 1950 to store water from the wet to the dry season, providing flood control and water supply (Bond et al., 2018; Tebakari et al., 2012). Therefore, the Chao Phraya River is highly regulated by existing reservoirs. The Global Reservoir and Dam Database (GRanD; Lehner et al., 2011) lists 39 dams in Thailand, in which Bhumibol and Sirikit are the two major reservoirs in the Chao Phraya River. Together, the Bhumibol and Sirikit control approximately 22 % of runoff from the basin, with a combined storage capacity of
Prior to beginning the model set-up and simulation, the input data required for simulations in the CPRB were finalized for both regionalized and generalized canal schemes, as follows:
In both schemes, the area of retention ponds in each grid cell was obtained by multiplying the area of that grid cell with the areal fraction of rainfed cropland. Then, this area was multiplied with the depth of water storage (assumed to be 1 m in this study based on recommendations from the RID) to calculate the storage capacity of the retention ponds. The diverted canal flow was assumed to be temporarily stored in these rainfed croplands until their storage capacity was reached. Further details regarding the inclusion of canal systems and retention areas in the H08 model are provided in
Sect. S3 in the Supplement.
A detailed comparison of the two schemes in terms of input data is provided in Table 2, and the corresponding values are given in Table S1.
The differentiation of regionalized and generalized canal schemes in terms of the input data (JICA, 2013; Tamada et al., 2013).
We previously adapted the H08 model to the calculation domain of
13–20
For simulations in the present study, the same model set-up for the CPRB was maintained, except for model calibration. The H08 model was recalibrated for the CPRB because the present study uses an upgraded version of the H08 model that includes four additional parameters (a total of eight parameters) in the land surface hydrology module, representing groundwater recharge and abstraction (Hanasaki et al., 2018). The eight key parameters of the land surface hydrology module were derived through calibration of the model at Nakhon Sawan. The calibrated parameters and their values are shown in Table S4 in the Supplement. Furthermore, five types of discharge simulation were conducted (Table 3) from 1980 to 2004 (total duration of 25 years) as follows: (i) naturalized simulation (NAT), where all types of water infrastructure were disabled, (ii) dam simulation (DAM), where the reservoir operation module was enabled, (iii) irrigated simulation (IRG), where both the reservoir operation module and irrigation water abstraction module were enabled, (iv) regionalized simulation (REG), which is irrigated simulation with the regionalized canal scheme enabled, and (v) generalized simulation (GEN), which is irrigated simulation with the generalized canal scheme enabled. Equilibrium for all model state variables was achieved using spin-up calculations.
The different simulations carried out in this study.
Initially, model calibration and validation were conducted using the pristine flow, and the efficacy was evaluated based on Nash–Sutcliffe efficiency (NSE; Nash and Sutcliffe, 1970). For model validation, daily river discharge data at various locations in the CPRB (green colour gauging stations in Fig. 4) were collected from the RID. In addition, the capability of the H08 model to reproduce observed discharge at Nakhon Sawan using the calibrated parameters was examined. Regionalized canal operations were calibrated for each diversion canal system to mimic the available observed data provided by the RID. Calibration was conducted by controlling the fraction of river flow to be diverted into the canals during the wet and dry seasons, with reference to the observed fraction of water diverted. Although the wet season ends in October (Sect. 3.1), May–December was regarded as the wet season in this study, based on observed reservoir release (Padiyedath Gopalan et al., 2021) and canal diversion operations. Similarly, the dry season was redefined as January–April for simplicity of simulation.
The H08 model was calibrated for pristine flow at Nakhon Sawan and further validated at various stations in the CPRB. In addition to the pristine flow, the ability of the calibrated parameters in reproducing the observed discharge at Nakhon Sawan was assessed (the results are shown in Sect. S4 in the Supplement).
The observed and simulated fractions of river flow diverted into the canals with respect to a reference gauging station are listed in Table 4 for the wet and dry seasons. For each canal system, the nearest upstream gauging station was used as the reference gauging station (column two of Table 4 and Fig. 4) to determine the fraction of water diverted. The REG simulation closely agreed with the observed diverted fraction for all canal systems considered during both dry and wet seasons. In the GEN simulation, the diverted fraction was reasonably reproduced at many stations; it substantially differed from the observations at some stations during both seasons. On average, the REG and GEN simulations diverted 8.78 % and 22.55 % of river discharge, respectively, compared with the observed dry season discharge diversion of 9.25 %. The average values of observed, REG, and GEN diverted fractions during the wet season were 11.24 %, 11.90 %, and 11.71 %, respectively.
The observed and simulated fractions of diverted river flow to the canals under regionalized (REG) and generalized (GEN) canal schemes during the wet and dry seasons.
Simulated monthly mean canal flows (REG and GEN) at 11 canal origins are shown in Fig. 5, along with observed diverted canal flow. Monthly mean values were used to validate canal operations because observed canal operation data are available for a different period from the simulation time frame (1980–2004). The REG canal flow simulation showed good agreement with the observed flow at all stations, except the Lopburi and Bang Kaeo canals. The observed data at these two stations included missing values,
which reduced the predictive accuracy of flow diversion. From a seasonal perspective, the model exhibited good reproduction of canal flows during both dry and wet seasons. However, the GEN canal flow simulation exhibited disparities compared with the observed flow at Makhamthao–Uthong, Tha Chin, Chainat–Ayutthaya, Lopburi, Bang Kaeo, and Phong Pheng stations. These disparities can be attributed to uncertainties in the
Simulated monthly mean canal discharge (REG and GEN) at 11 canal origins (in the order shown in Fig. 4) compared with the observed flow.
The H08 model accurately reproduced canal operations in the CPRB under the REG canal scheme. Although the primary purpose of the implemented canal scheme was flood control during the wet season (May–December), the model was also able to replicate the diversion pattern during the dry season (January–April). This result reveals that the REG canal scheme can also be successfully used to provide water supply during the dry season, in addition to the flood control during the wet season.
The four components of diverted canal flow are returned discharge, supply to nearby grids, flow out of the basin, and retention storage, as described in Sect. 2.2. Figure 6a and c show the amount of flow transferred from the canal systems to each of these components under the REG and GEN schemes,
respectively, from 1980 to 2004. The maximum and minimum values of diverted annual canal flow were approximately 22 and 7
Amount of flow transferred from the canal systems to each of the components (from 1980–2004) and their percentage contribution to the annual canal flow averaged over 1980 to 2004 under the regionalized scheme
Although the total flow diverted to the canal systems was similar under both schemes, the percentage of canal flow transferred to each component differed, as shown in Fig. 6b and d. The annual average flow diversion from the river channel to the canal system under the REG scheme was nearly 13
In practice, water stored in low-lying retention areas of the CPRB (paddy fields) drains into canals and rivers after the flood, thereby creating space for water from upcoming floods in the same year. This process was not considered in the simulations, and the retention areas were assumed to accommodate floodwater only once per year until they became full. Because of this simplification, neither diversion canal scheme was able to reach the observed potential storage capacity (1701 MCM; Table S3). In addition, in the enhanced H08 model, retention areas are modelled such that the retention areas receive runoff in each grid cell; this runoff constitutes a portion of retention pond storage. Therefore, the maximum storage capacity of retention ponds (615 and 935 MCM; Table S3) cannot be utilized for the storage of diverted floodwater. The storage capacity of the retention areas can be increased by changing the maximum water depth based on regional characteristics.
Initially, the impact of canal systems and retention areas on reducing the annual average, wet season, and dry season discharges of the CPRB was analysed. Figure 7 shows the annual, wet season, and dry season discharges in the CPRB for various simulations averaged from 1980 to 2004. The maximum annual average discharge under the NAT simulation was approximately 850
Effect of canal systems on annual average (top panel), wet season (middle panel), and dry season (bottom panel) discharges in the CPRB for the NAT
The discharge reduction under various simulations during the wet season was very similar to the annual average flow reduction pattern, except for the DAM simulation. Under the DAM simulation, remarkable discharge reduction was observed in the Ping, Nan, and Chao Phraya rivers (Fig. 7b2) due to the operation of upstream dam reservoirs of Bhumibol and Sirikit with an outlet discharge deduction of nearly 15 %. The subsequent discharge simulations
of IRG, REG, and GEN ones illustrated nearly 34 %, 10 %, and 26 % reduction in the outlet discharge of the CPRB (Fig. 7c2–e2), which further revealed the dominance of wet season irrigation water abstraction and canal operations in the annual pattern. During the dry season, the outlet discharge was nearly 202
Although water diversion occurred from the Yom River to the Nan River, the impact of this diversion on the annual average and wet season discharges was negligible. Moreover, the effect of this water diversion in the upper basin was nullified at C.2 station, where the Yom and Nan rivers join. This finding elucidates the need for (i) water diversion systems to divert floodwater from the upper to lower basin, (ii) more retention areas to store floodwater, and (iii) other structural and non-structural measures. Jamrussri et al. (2018) also suggested that new retention areas adjacent to the Yom and the Nan rivers are necessary to overcome severe flooding in the upper basin. The effect of water diversion on discharge is more pronounced in the lower CPRB.
Furthermore, the effect of canal operation on flood control was examined by calculating the number of flooding days (days during which daily discharge exceeded the channel carrying capacity) at various locations, as shown in Fig. 8. The number of flooding days are identical for NAT and DAM simulations at Yom River stations because no dams are present in the Yom basin. The results from the Yom basin showed that the number of flooding
days became zero at Y.4 and Y.17 stations in the REG simulation. Both schemes produced similar results for Y.16 station because of its small channel carrying capacity of 207
The number of days the daily discharge exceeded the channel carrying capacity from 1980–2004 (25 years) at various locations (refer to Fig. 4 for the channel carrying capacity of each station) under different simulations.
Overall, both canal schemes reduced the flood risk, bringing the number of flooding days to nearly zero. Therefore, the incorporation of diversion canals in combination with reservoir operation could mitigate historic floods. However, climate change will aggravate the risk of flooding in the basin (Padiyedath Gopalan et al., 2021). In light of future climate change, new approaches combining structural and non-structural measures must be adopted in the CPRB.
Although the GEN scheme showed some differences from the observations and REG scheme, it was able to identify the general pattern of flow diversion. The combined effect of canal systems in the GEN scheme was identical and comparable with the observations and REG scheme during the wet season, even though the effect of individual canals differs (Fig. 5 and Table 4). However, it overestimated the diverted river flow values during the dry season. Considerable overestimation of canal flow was observed mainly for the Makhamthao–Uthong and Chainat–Ayutthaya canals (Fig. 5) during the wet season. This result was obtained because the main purpose of these canals is irrigation water supply, and they have low observed canal carrying capacities (Table S1). However, the GEN scheme considers the primary purpose
of all canals to be flood control (because flood control is the primary objective of this study) and fixes the canal carrying capacity at the
The GEN canal scheme was able to produce an annual diversion pattern similar to the annual diversion pattern of the REG scheme (Fig. 6); diverted water comprised approximately 57 % of the annual average river discharge in the CPRB. Although this result represents a slightly exaggerated discharge reduction scenario compared with the REG scheme (wherein 49 % of annual average river discharge is diverted), it provides insight into the maximum level of discharge reduction that is achievable by maintaining environmental flow levels in the river channel. The GEN scheme showed a greater reduction in basin-wide annual average discharge, which can be attributed to the (i) increased carrying capacity of canals flowing out of the basin (Fig. S3 in the Supplement) and (ii) greater retention area for the storage of floodwater (Table S3). This finding reveals that the GEN scheme provides more control over the annual average discharge reduction, compared with the REG scheme (Fig. 7). Furthermore, the GEN scheme has a reasonable effect on the flood risk reduction, considerably reducing the number of flooding days (Fig. 8); thus, it can be regarded as the preliminary survey of future global applications using simple input data settings that can be derived from river discharge alone.
To our knowledge, this study is the first attempt to include flood diversion canals and retention areas into GHMs for controlling flood risk. The enhanced H08 model with the REG canal scheme successfully reproduced the observed flood diversion scenario in the study region. The GEN canal scheme exhibited performance comparable with the REG scheme for floodwater diversion. Using existing canal systems, both schemes could divert at least half of the annual average discharge in the CPRB, although some of the diverted discharge gradually returns to the river network. This return flow will increase the subsequent flood risk in downstream areas of the river, although the retention areas will reduce the magnitude of flooding. This finding calls for additional water diversion systems that divert water out of the basin and into nearby basins or the Gulf of Thailand, thereby reducing overall flood risk in the basin (JICA, 2018).
Impact assessment of both canal schemes in terms of river discharge revealed that the number of flooding days could be considerably reduced. Therefore, the enhanced H08 model provides a new tool for assessing future adaptation possibilities of the diversion canal systems. However, several uncertainties and limitations remain to be addressed in future studies. The major
uncertainties and limitations of this study can be summarized as follows:
Pumping stations were constructed in the CPRB to drain the floodwater stored in the retention areas (paddy fields) to the canals immediately after the floods and further operated to drain water from canals to main rivers (JICA, 2013). This pumping process will prepare the paddy fields for cultivation and further storage of water if floods occur within the same year. However, this pumping process was not modelled in this study due to the challenges involved. Instead, the retention storage is assumed to evaporate fully at the end of a normal year and retention storage at the beginning of the following year is set at zero. The evaporation of retention storage at the end of every year is an assumption for simplicity at this stage, and further research will be pursued into this area by considering the pumping process in the canal operation scheme. An assumption was also made for the canal operations that the water transfer and retention storage occur without any loss and delay. The evaporation and conveyance loss while canal water transfer was assumed to be trivial and the delay in delivery at the canal destination is not taken into account. In addition, part of the retention storage might be lost through percolation. However, due to limitations in the availability of such data, the percolation loss was assumed to be negligible in the highly saturated paddy fields at the time of floods. The canal water is assumed to be supplied to the grid cells through which the canal passes and to the immediate lateral neighbouring grid cells of the canal for meeting the irrigation demand. Therefore, the spatial resolution of the simulation will affect the results. In this study, for simplicity, only 10 % of diverted water is supplied to each of the nearby grid cells because our primary concern was flood control. Therefore, this fraction of supply to nearby grids should change if the simulation is performed on a finer/coarser resolution. One alternative way to overcome this issue is that the supply to nearby grids can be finalized based on the water demand in each of the grid cells through which the canal passes and the in the neighbouring grid cells. In such instances, it can be confirmed that the supplied water will be completely utilized. The assumption of rainfed croplands as retention areas will affect the crop yield because the cropland is not available for cultivation until the beginning of the subsequent year due to the storage of floodwater. This is one of the limitations this study currently poses, and that will be addressed in our future studies by considering the actual pumping process in the canal operation scheme. However, this is not a universal assumption for future global applications. The most important land use for potential retention areas is the low-lying areas along rivers (floodplains) and canals. Historically, such lowland areas are used for paddy cultivation in warm Asian countries. Being paddy is not the required condition for retention areas. In addition, although the lakes/ponds could be partially filled with water during the wet season, they can also be used as retention areas based on available free space. The geographic locations of such possible retention areas along with their depth and areal extents available for storage of floodwater specific to each area can be extracted from remotely sensed data such as digital elevation models (e.g. MERIT DEM), satellite imagery (MODIS/LANDSAT), and radar altimetry, as well as from the literature, although it is strenuous. Another limitation mainly arises from the assumptions made for river channel carrying capacity, canal carrying capacity, and minimum flow of diversion in GEN canal scheme simulations.
Acquisition of observations and accurate representation of these variables in GHMs remain challenging tasks. However, this study represents a first step toward developing and incorporating a diversion scheme into the H08 GHM. The robustness of this scheme can be improved by adapting and applying it to different contexts and regions, accounting for the characteristics of each river network while maintaining the basic principles unaltered.
Assessing the impacts of flood diversion canals on flood risk reduction worldwide remains challenging. To overcome this issue, a flood diversion canal operation scheme was developed for the CPRB, a complex river network in Thailand with several natural and artificial diversion canals. The developed scheme was carefully designed and implemented into the H08 GHM for future floodwater management in complex river networks worldwide. A
generalized canal scheme was also introduced with simple input data settings that can be derived from river discharge alone for application to data-scarce regions and periods. The major conclusions from this study can be summarized as follows:
A regionalized water diversion scheme was developed for the CPRB and incorporated into the H08 GHM to evaluate the effects of floodwater diversion on water donating and its downstream areas, with the primary purpose of flood management. The validation results show that the diversion scheme can effectively simulate the observed flood diversion scenario in the CPRB. The generalized water diversion scheme with simple input data settings also exhibited comparable performance, especially during the wet season. The major share of the diverted water gradually returns to the destination points under both canal schemes. This indicates that although flood risk is reduced at the point of diversion, some risk remains in the destination area. If retention areas were able to hold more water, flood damage to residential and commercial areas of the lower CPRB could be reduced. This change can be achieved by (i) considering retention areas in irrigated and non-irrigated croplands simultaneously, (ii) assuming that the water from retention areas can be drained into canals and rivers after the floods using pumps (this makes the retention areas available for future floods in the same year), and (iii) increasing the depth of water stored in the retention areas (a depth of one m was assumed in this study). Water diversion led to a marked reduction in the annual average discharge of the CPRB, which was much greater under the generalized canal scheme than the regionalized scheme. Furthermore, both canal schemes reduced the number of flooding days to nearly zero at most of the gauging stations considered. The overall simulation results indicate that currently implemented canal schemes have the potential to reduce flood risk in the upper and lower CPRB, where many industrial and residential areas are located. However, their abilities to overcome severe flooding should be further evaluated in the context of climate change.
Therefore, we emphasize that the regionalized canal scheme described herein was successfully applied to the CPRB, whereas the generalized scheme requires further validation to evaluate its applicability in other regions worldwide with slight modification. The water diversion rules implemented in the H08 model (during wet and dry seasons) can be easily adapted to different GHMs and should be examined for their applicability. In future research, we will expand upon these schemes for future applications by considering various adaptation scenarios.
The source code of this work is available at
The meteorological data used in study are available from IMPAC-T project website (
The supplement related to this article is available online at:
SPG and NH conceived the idea. AC and TS acquired the data. SPG, NH, and SN analysed and interpreted the data. SPG developed the model code and performed the simulations. SPG prepared the paper, with contributions from all co-authors. NH, AC, TS, and SN reviewed and edited the paper.
The contact author has declared that neither they nor their co-authors have any competing interests.
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This study was carried out as a part of the research project entitled “Advancing Co-Design of Integrated Strategies with Adaptation to Climate Change in Thailand (ADAP-T)”.
This research has been supported by the Science and Technology Research Partnership for Sustainable Development (SATREPS) program of the Japan Science and Technology Agency (JST) (grant no. JPMJSA1502) and the Japan International Cooperation Agency (JICA).
This paper was edited by Elena Toth and reviewed by Fang Zhao and one anonymous referee.