Impacts of dike systems on hydrological

Impacts of dike systems on hydrological regime in Vietnamese Mekong Delta Nguyen Van Xuana, Nguyen Ngoc Long Gianga, Tran Van Tyb, Pankaj Kumarc, Nigel K Downesd, Nguyen Dinh Giang Namd, Nguyen Vo Cha[.]

© 2022 The Authors Water Supply Vol 22 No 11, 7945 doi: 10.2166/ws.2022.333 Impacts of dike systems on hydrological regime in Vietnamese Mekong Delta Nguyen Van Xuana, Nguyen Ngoc Long Gianga, Tran Van Tyb, Pankaj Kumarc, Nigel K Downesd, Nguyen Dinh Giang Namd, Nguyen Vo Chau Ngand, Lam Van Thinhd, Dinh Van Duyb, Ram Avtare d, * and Huynh Vuong Thu Minh a Mien Tay Construction University, Vinh Long Province, Vietnam College of Technology – Can Tho University, Can Tho City, Vietnam c Institute for Global Environmental Strategies, Hayama 240-0115, Japan d Department of Water Resources – College of Environment and Natural Resources, Can Tho University, Can Tho City, Vietnam e Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan *Corresponding author E-mail: hvtminh@ctu.edu.vn b HVT, 0000-0002-0851-4757 ABSTRACT This paper examines the impact of the dike systems on river flows in the Vietnamese Mekong Delta (VMD) The study combined a hydrological change index method and the Mann–Kendall test to assess the temporal dynamics of both discharge and water levels along the main rivers of the VMD Results highlight that the system of rivers and canals helps facilitate waterway traffic and drainage during the flood season However, the low elevation of the delta has created conditions suitable for saline water to increasingly penetrate upstream during the dry season Observed changes in the hydrological indicators at the upstream stations of Tan Chau (Mekong River) and Chau Doc (Bassac River) are not only due to the dike system but also upstream alterations to the flow regime More research is needed to consider the various drivers of flowregime change associated with natural and human activities both inside and outside of the study area Key words: flow regime, hydrological change index, irrigation construction, Mann-Kendall test, Vietnamese Mekong Delta (VMD) HIGHLIGHTS • • • • • Investigates the water resource management system in the Vietnamese Mekong Delta (VMD), rice basket of south-east Asia Assesses the impact of the dike system on the hydrological system in the VMD Integrated approach using hydrological change index method and the Mann–Kendall test Results from this study will play a big role for decision-makers to design robust management plans Methodology can be replicable to other parts of the world INTRODUCTION The Vietnamese Mekong Delta (VMD) as a key production landscape plays a critical role in the nation’s food security and economic development In recent years, the Government of Vietnam has introduced policies and promoted solutions to develop the region to foster socio-economic and sustainable development Essential to the national economy, the region has undergone remarkable growth with significant recent achievements in agricultural and rural development sectors (Mekong Delta Plan (MDP) 2013) The VMD’s location renders it highly vulnerable to the impacts of climate change, including rising sea levels, flood events during the rainy season and water shortages during the dry season (Wassmann et al 2004; Khang et al 2008) Changes to the flow regime, reduced sedimentation riverbank erosion and salinity intrusion caused by anthropogenic interventions or as a result of climate change remain grave challenges Since the 1990s, extensive dike systems and irrigation infrastructures have been incrementally constructed to improve crop yields and reduce the impact of annual flood on the region (Nguyen et al 2017; Minh et al 2019a, 2019b) Concurrently, alongside economic development and transformation, new and expanded residential areas, transport infrastructure and services have gradually reduced the area of forests, agricultural land, and water surfaces Furthermore, the expansion of cultivation areas, industrial zones, and hydropower dam construction in upstream countries has reduced water and sediment This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/) Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7946 sources downstream, while increasing erosion, surface water pollution, and loss of fertile sediments (Dat et al 2011) Previous studies in the VMD’s upstream provinces have found surface water pollution directly caused by agricultural runoff (Chau et al 2015; Thu Minh et al 2020) The surface water quality monitoring system in the VMD is primarily concentrated along the main rivers and canals However, the secondary and tertiary water bodies, which act as the primary source of water supply for adjacent households, often suffer from the highest levels of pollution Anthropogenic activities have impacted discharge volumes greatly Hydropower dams, reservoirs, sluice-gates, dikes, and embankments have all been developed to retain water and protect and serve lives and livelihoods The negative effects of hydropower and irrigation projects are well documented and increasing in significance, magnitude and frequency (Fujihara et al 2016; Nguyen et al 2017; Minh et al 2019a, 2019b) Several studies have investigated the cumulative effects of Chinese dams in the Mekong River Basin Lu et al (2014) investigated the water flow regime at Chiang Saen in the lower Mekong, whilst Hecht et al (2019) studied a range of hydrological impacts from hydropower dams Gunawardana et al (2021) went further and considered the multiple drivers of hydrological alteration in the transboundary Srepok River Basin of the Lower Mekong Region The fine sediment and geological structure of the riverbanks are changing the regime of the water–soil pressure balance (Liu et al 2017; Van Tho 2020) Many studies have also shown that sand-mining activities and the presence of clay in riverbank soils have increased both the frequency and severity of erosion events (Liu et al 2017; Marchesiello et al 2019; Van Tho 2020) Some studies have used modelling such as a study on an ANFIS-based approach for predicting sediment transport (Azamathulla et al 2012) and have used linear genetic programming to scour below a submerged pipeline (Azamathulla et al 2011) Other studies suggest that vertical force transmissions from riverbank construction works may also contribute to riverbank collapse (Coumou 2017; Van Tho 2020) However, very few or none of these studies have comprehensively addressed the impacts of the development of dike systems on the hydrological regime in the region with the exception of Binh et al (2020) Several studies have used the Mann–Kendall test to analyse long-term trends in climate variables for different climatic regions (Mehta & Yadav 2022) Perera et al (2020) used the Mann–Kendall test to detect the magnitude of trends for well-known climate gauges in Trinidad and Tobago Monthly climatic data including cumulative rainfall and average of the minimum and maximum atmospheric temperatures were all processed to identify the trend analysis using the above-stated non-parametric tests The objective of this study is to examine the impacts of the construction of irrigation-related infrastructures on river flow in the VMD The study utilised a hydrological change index method and the Mann–Kendall test to assess temporal changes in both water levels and discharge along the main river channels of the delta The study is expected to reveal evidence on the extent to which the dike system has altered the hydrological regime METHODOLOGY Study area Located on the lower reaches of the Mekong River, the VMD covers an area of approximately 40,816 km2 (GSO 2016) It is an area of relatively flat terrain, highly susceptible to flooding, with the majority of land below m above mean sea level (Fujihara et al 2016), intertwined by tidally influenced natural rivers and man-made channels (Figure 1) The VMD is made up of two main river systems, namely the Mekong River and the Bassac River The region exhibits the highest density of inland waterways in Vietnam (up to 0.61 km/km2) (VIWA 2008) The northeast and southwest monsoons both have an impact on the climate in the Mekong Delta in Vietnam Typically, the rainy season lasts from May to November, while the dry season lasts from December to April The flood season begins one to two months later than the rainy season and ends nearly simultaneously with it During the flood season, the delta facilitates water transport and drainage Conversely, the delta frequently suffers from saline intrusion during the dry season due to the typically low flow rates, which are approximately only 2% of that of the rainy season, and which is insufficient to prevent large-scale and long-term saline intrusion (Nguyen 2016) In the dry season, the water levels rise and recede slowly, with an average amplitude of 5–7 cm/day, yet during the rainy season, flood waters can rise rapidly by 20–30 cm/day The hydrological regime in the VMD is directly influenced by the upstream flow, rainfall, and the East and West Seas tidal regimes Heavy upstream rainfall, hydroelectric dam construction, deforestation, irrigation canals, dikes and urban development are the primary causes of floods in the VMD An Giang Province (10°110 to 10°580 N, 104°460 to 105°350 E) is located in the upstream section of the VMD with a natural area of 354  103 ha, of which 283  103 are designated as agricultural areas (79.9%) The province had a total population Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7947 Figure | Map of the dense river and canal system in the VMD of 1.9  106 in 2019, distributed over 11 administrative units; two cities (Long Xuyen and Chau Doc), one town (Tan Chau) and eight districts (An Phu, Phu Tan, Cho Moi, Chau Phu, Chau Thanh, Thoai Son, Tinh Bien and Tri Ton) The study area is bordered by Cambodia to the north, Can Tho City to the south, Dong Thap Province to the east and southeast, and Kien Giang Province to the west and southeast An Giang province exhibits an interlaced system of rivers and canals, with two main rivers, Mekong (Tien) and Bassac (Hau), which during the rainy season provide abundant surface water for irrigation-fed agricultural production The hydrological regime divides into two distinct seasons: the flood season (from September to November) and the dry season (from February to May) The maximum discharge to the Mekong Delta is about 35,000 m3/s in the middle flood years and up to 44,000 m3/s in the large flood years, in which at Tan Chau station on the Mekong River it is 29,000 m3/s (accounting for 66%), on the Hau River at Chau Doc it is 8,200 m3/s (accounting ` for 18%) and the total overflows across the border to the wetland Plain of Reeds (Ðơng Tháp Mu’ị’i) and Long Xuyen Quad3 rangles are about 7,000 m /s (16%) The benefits of floods for agricultural production in recent years are such as bringing in sediment, flushing fields, improving soil quality, water quality, replenishing groundwater sources, bringing aquaculture resources and creating jobs for farmers in the flood season However, floods also affect socio-economic activities such as causing loss of life, damaging property, and resulting in increasing infrastructure investment and maintenance costs Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7948 This study used indicators of hydrologic alteration (IHA) and the Mann–Kendall test to assess the changes in water level and discharge under dike system development in the upper section of the VMD Because it does not require normally distributed datasets and has a low sensitivity to sudden breakdowns owing to inhomogeneous time series, the nonparametric Mann– Kendall technique has become highly utilised for trend analyses in recent years (Yue & Wang 2004; Shahid 2010) Data analysis was undertaken using XLSTAT software (Addinsoft, Paris, France) version 2019 Evaluation of flow-regime dynamics The Nature Conservancy’s IHA software, which Richter et al (1996) developed, was used to assess flow-regime change by calculating 33 hydrologic variability parameters as summarised in Table (all parameters were based on daily measurements) This method has been widely used in assessing the impacts of climate change and anthropogenic interventions on flow regimes (Ty et al 2011; Liu et al 2018; Zhang et al 2019; Wang et al 2021, 2022; Guo et al 2022) In this study, flow data was collected from the two hydro-meteorological stations of Tan Chau on the Mekong River and Chau Doc on Bassac River for the period of 1986–2019, and Vam Nao station for the period of 1997–2019 Data was then divided into three phases, consisting of phase 1: 1986–1996, phase 2: 1997–2010, phase 3: 2010–2019 (Tan Chau and Chau Doc stations); and phase 1: 1997–2004, phase 2: 2005–2010, phase 3: 2011–2019 (Vam Nao station) These phases correspond to: (i) before the construction of the dike system (1986–1996); (ii) during and after the construction of the dike system (1997–2010); and (iii) after completion of the dike system (2011–2019) The level of flow change analysis results were compared with each other by period and between stations The hydrologic alteration (HA) was determined by Equation (1): HA(%) ¼ (observed expected)  100 expected (1) where HA denotes the hydrological change in flow (%) When the observed frequency of the post-impact annual values that fall within the range of variability approach (RVA) target range equals the expected frequency, HA equals zero, whereas a positive deviation value indicates that the annual Table | Hydrological parameters according to IHA Group Characteristics Indicators Group 1: Monthly flow magnitude (12 parameters) Magnitude, timing Average monthly discharge (12 months) Group 2: The magnitude and duration of the annual extreme discharge values (12 parameters) Magnitude, timing – 1, 3, 7, 30, 90 the smallest consecutive day of the year (Qmin1, Qmin3, Qmin7, Qmin30, Qmin90); – 1, 3, 7, 30, 90 biggest consecutive day of the year (Qmax1, Qmax3, Qmax7, Qmax30, Qmax90); – Base flow (Qbase) (7 minimum day divided by mean annual discharge); – Number of days without discharge Group 3: Time of occurrence of annual extreme flow values (two parameters) Timing – The value appeared date Qmax1 in year (Tmax1); – The value appeared date Qmin1 in year (Tmin1) (order date appeared in the year) Group 4: Frequency of occurrence of high and low flow (four parameters) Magnitude; frequency; period – High number of pulse occurrences per year; – Low number of pulse occurrences per year; – High pulse duration per year; – Low pulse duration per year Group 5: Rate and frequency of flow variation (three parameters) Frequency; rate of change – Rate of flow value increase between consecutive days; – Rate of flow value decreases between consecutive days; – Number of times the flow changes in the opposite direction (FRC) Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7949 parameter values fall within the RVA target range more frequently than expected; and a negative value indicates that the annual values fall within the RVA target less frequently than expected ( Jiang et al 2014) The RVA was used to investigate flow variability by determining the frequency of natural flows as the basis for considering variation The value of the IHA parameters at the post-impact stage should be as close as possible to the initial value, i.e., the RVA should be as close to zero as possible The RVA is divided into three grades ranging from 17% to median values: (i) the lowest category, which includes all values less than or equal to 33%; (ii) the mean category, which includes all values between 34% and 67%; and (iii) the highest category, which includes all values greater than 67% Although the RVA level boundaries can be adjusted, using 33% and 67% ensures that the pre-impact value falls into each category in the majority of cases, making the results easy to understand and analyse Mann–Kendall test and Sen’s slope To enable further data interpretation, the Mann–Kendall test is often applied to piezometric time-series data (Hirsch & Slack 1984) The Sen’s slope test was used to determine the magnitude of the slope (Hirsch et al 1982; Van Ty et al 2021) In this study, the Mann–Kendall test and Sen’s slope were conducted to test and determine the magnitude of the slope of annual water level and annual discharge at the three hydrological stations along both the Bassac River and Mekong River as shown in Equations (2)–(8): Sk ¼ n X Si (2) i¼1 Based on the range of Mann–Kendall statistics (between and 1) for each pair of data points, the Mann–Kendall statistic indicates if a trend exists and whether the trend is positive or negative The following is the Mann–Kendall equation, which is based on the S statistic: S¼ n n X X sgn(xj xi ) (3) iẳ1 jẳiỵ1 where xi and xj are sequential data values, n is the length of time series, and sgn(xj < xi ) ¼ : if (xj if (xj if (xj xi ) xi ) ¼ xi ) , (4) When n  8, S is almost normally distributed with the following mean and variance (Mann 1945; Kendall 1975): E(Sk ) ¼ (5) S around indicates that there is no discernible upward or descending trend S having a positive or negative value indicates an upward or downward temporal trend The trend’s strength is measured by the size of S It is worth noting that if the absolute value of S exceeds the critical value of S, trends are statistically significant n(n 1)(2n ỵ 5) n P Var(Sk ) ẳ ti (ti 1)(2ti ỵ 5) 18 (6) i¼1 where ti is the number of ties of the extent i, and n is the number of observations in the set The standard Zsk statistic is calculated as follows: Zsk S > > pffiffikffiffiffiffiffiffiffiffiffiffiffiffiffiffi > > > < Var(Sk ) ¼ > > > Sk þ > > : pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Var(Sk ) Sk Sk ¼ Sk , Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest (7) Water Supply Vol 22 No 11, 7950 where Zsk aims for a typical normal distribution with and The magnitude of the trend was determined using Sen’s slope The Z statistic is calculated and then compared with a crucial point in the standard normal distribution Furthermore, in simple non-parametric approach invented by Sen, Ti between any two values of a time series (which produces a total of n(n 1)=2 pairs of data), was used to determine the size of a time series trend: Ti ¼ xj j xi i (8) where xj and xi are values at time j and i (1 , i , j , n), and n is the amount of data The median of these n values of Ti is represented as Sen’s estimator of the slope The positive and negative values of Ti indicate the upward and downward trends in the time series, respectively RESULTS AND DISCUSSION Dike development in An Giang province Dike development in An Giang province commenced in the 1970s and by 1987 was already strongly developed During this period, dikes were built to protect the two-crop rice protection areas (summer–autumn and winter–spring rice crops) By 1996, the first 800 of land in Cho Moi district were thoroughly protected by full dikes Thereafter the area of agricultural lands protected by full dikes continued to increase Figure highlights the current dike system in An Giang province The province has a total semi-dike protected area of 53,259 (15% of the province) and a full-dike protected area of 190,768 (54% of the province area) Full dikes are concentrated in Thoai Son, Cho Moi, Phu Tan, Chau Thanh and Chau Phu districts, whilst semi-dikes are mainly located in Tinh Bien and Tri Ton districts and Long Xuyen City Figure shows the dike-protected-area statistics by district for the year 2018 It is clearly indicated that the ratio of the area protected by the full dikes is much higher than by semi-dikes among districts Thoai Son and Chau Phu districts account for over 80%, Thoai Son district has up to 80.5%, Chau Phu district also has up to 67%, Phu Tan district also has up to 75% and Chau Thanh district is 70% protected by full dikes In contrast, Tinh Bien district and Long Xuyen City have the lowest dikeprotected areas (about 41% of the natural area) This is due to the semi-mountainous geography of Tinh Bien, whilst Long Xuyen is the largest urban area with residential and industrial land uses The full-dike system meets the requirements of protecting the production landscape for the summer–autumn and autumn– winter rice crops, while also protecting the main transport routes During the 23 years (1997–2018) of dike construction and development, the area protected by the dike system has increased substantially over time, as can be seen in Figure (left vertical axis represents annual protected area) In particular, the periods 2001–2004 and 2010–2013 had the fastest increase in dike-protected area (with the rate from 10,000 to 30,000 ha/year, respectively) The dike-protected area clearly shows two important milestones in 2004 (89,435 ha) and 2011 (167,149 ha) From 2012 to 2018, the area protected by dikes continued increase, but alas at a much slower rate (approximately 23,620 ha, with an average rate of 3,374 ha/year) Changes in flow regimes The results showed that the change in flow regime (discharge) at Tan Chau in phases and was 68.2% and 76.6% respectively, whereas at Chau Doc, the change in flow for phases and was 71.2% and 66.6%, respectively The least was at Vam Nao, where the changes in flow for phases and were 49.8% and 60.7%, respectively In general, Chau Doc station showed the greatest flow alteration (71.2%) during the dike system construction (1997–2010), followed by Tan Chau (68.2%) and Vam Nao (49.8%) However, when considering phase (2011–2019), when the dike system was relatively complete and no more dikes were constructed, the change in flow regime at both Tan Chau and Vam Nao stations was seen to increase significantly to 76.6% and 60.7%, respectively Change to the five main components/groups of the flow regime The Nature Conservancy (2009) states that abnormal changes in five groups (Table 1) will have a negative impact on ecosystems The results of the IHA’s five-group change analysis revealed that the change in flow regime at Chau Doc, Tan Chau, and Vam Nao stations was primarily in group (Figure 5) Specifically, the changes at Chau Doc, Tan Chau, and Vam Nao stations were 126.2%, 104.8%, and 26.7%, in phase 1: 1997–2010; and 133.3%, 122.2%, and 82.7%, respectively during Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7951 Figure | Map of semi- and full dike protected areas in An Giang province in the year 2018 Figure | Area protected by dikes per district in 2018: (top) full-dike-protected area; (bottom) semi-dike-protected area Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7952 Figure | The development of fully protected dike areas for the period 1997–2018 phase 2: 2011–2015 As a result, after the dike development slowed (phase 3), the change in flow regime in group increased significantly, in particular in Vam Nao from 26.7% to 82.7% The change in hydrological indicators at Tan Chau and Chau Doc stations may be directly attributed to the Mekong River’s upstream flow Due to a lack of upstream data, this study only considered the assumption that the flow alteration seen at the stations was the result of dike development Figure shows the detailed changes in flow by the 33 parameters at (a) Tan Chau, (b) Chau Doc, and (c) Vam Nao stations The flow-regime-change assessment results show that the flow change was high (.67%) at both Chau Doc and Tan Chau stations during phases and 2; and 49.8% and 60.7% at Vam Nao during phases and In general, Chau Doc station showed the greatest change (71.2%) during the dike system’s construction (1997–2010), followed by Tan Chau (68.2%) and Vam Nao (the least) (49.8%) However, during phase (2011–2019), after the dike system was relatively complete and further dike development was minimal, the change in flow regime at Tan Chau and Vam Nao remained at 76.6% and 60.7%, respectively When the three indicators of group (31 – flow rate increase, 32 – flow rate decrease, 33 – amount of reverse flow) are examined in detail, all three indications highlight significant changes (Figure 6) The highest change was seen in flow rate increase and was seen at all three stations Meanwhile, indicators 32 and 33 at Vam Nao station experienced changes over both phases According to The Nature Conservancy (2009), changes of indicators in group (rate and frequency of flow variation) directly influence ecosystem functioning and may cause drought stress in plants and desiccation stress in low-mobility stream-edge organisms The dike system development to aid intensive rice production has significantly contributed to water infrastructure development and water supply However, dike development in the VMD has also resulted in environmental pollution as the flushing of highly contaminated water from protected fields releases water with high pollutant loads that harm downstream Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7953 Figure | Changes in the flow regime (five IHA groups) at (a) Chau Doc, (b) Tan Chau, and (c) Vam Nao stations ecosystems This in combination with upstream hydropower dam development leads to a series of cumulative impacts on fish numbers and sediment supply Therefore, the benefits (e.g., ecosystem services) and the costs (such as the sediment reduction, biodiversity losses, and land degradation) of the system should be taken into consideration (Binh et al 2022) Hecht et al (2019) reviewed the hydrological impacts due to hydropower dams in the Mekong River Basin and strongly recommended that dam impact studies should consider hydrological alteration in conjunction with fish-passage barriers, geomorphical changes and other contemporaneous stressors The flow-regime alteration was assessed in the upstream province of the VMD by calculating 33 hydrologic variability parameters in five groups including magnitude, duration time, timing of extreme flow, and frequency which impact on sediment and nutrition transport, and thus effect the diversity of downstream flora and fauna The results from this study would provide an assessment of the scale of change and further research on the impacts of flow-regime changes on aquatic diversity going forward is needed Naturally, such large-scale alteration to the hydrological regime can create significant threats to both plant and animal species and lead to undesirable environmental impacts The above-mentioned issue has been recently discussed in detail by several studies such as Li et al (2017), Hecht et al (2019) and Gunawardana et al (2021) According to Li et al (2017), human activities, such as dam construction, significantly altered the flow regimes in the Mekong River, particularly after the completion of two large dams, namely Xiaowan and Nuozhadu in the years 2010 and Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7954 Figure | Changes in flow parameters Detailed changes in the five groups of hydrological flow regimes (33 parameters) at (a) Tan Chau, (b) Chau Doc, and (c) Vam Nao stations Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7955 Table | Mann–Kendall test results Series/test Kendall’s tau Sen’s slope Tan Chau_Qave_Dry/Wet 0.042/0.019 16.214/10.520 Tan Chau_Qmax_Dry/Wet 0.589***/0.095 133.909/61.504 Tan Chau_Qmin_Dry/Wet 0.147/ 0.114 24.323/ 78.390 Chau Doc_Qave_Dry/Wet 0.146/ 0.029 8.876/ 6.676 Chau Doc_Qmax_Dry/Wet 0.415*/0.029 25.559/5.594 Chau Doc_Qmin_Dry/Wet 0.556**/ 0.352* 21.281/ 53.197 Vam Nao_Qave_Dry/Wet 0.006/ Vam Nao_Qmax_Dry/Wet 0.579**/0.171 78.713/37.055 Vam Nao_Qmin_Dry/Wet 0.544**/ 0.162 40.566/ 28.638 0.010 0.550/ 10.009 Note: Significance levels are denoted as follows: *p , 0.05, **p , 0.01, ***p , 0.001 Figure | Observed seasonal discharges between 1997 and 2019 The significant trend of the average of dry seasonal discharges at p less than 0.05 was observed at (a) Tan Chau, (b, c) Chau Doc, and (d, e) Vam Nao Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7956 2014, respectively The construction and operation of dams clearly has significant impacts on low-flow duration It has been observed that climate change dictated the changes in the annual streamflow during the transition period 1992–2009, whereas human activities contributed more in the post-impact period 2010–2014 Hecht et al (2019) reviewed the hydrological impacts due to hydropower dams in the Mekong River Basin and concluded that the basin’s hydropower reservoir storage, which may rise from ∼2% of its mean annual flow in 2008 to ∼20% in 2025, is attenuating seasonal flow variability downstream of many dams with integral powerhouses and large storage reservoirs In addition, tributary diversions for off-stream energy production are reducing downstream flows and augmenting them in recipient tributaries According to Gunawardana et al (2021), among the IHA parameters, the fall rate has the most significant effect on hydrological alteration of all drivers Hydropower development in the basin mostly affects the fall rate and reversal Identifying the connection between these multiple drivers and hydrological alteration could help decision-makers to design more efficient and sustainable water management policies Figure | Observed seasonal discharges between 1997 and 2019 The significant trend of average of the wet seasonal discharges is at p less than 0.05 at Chau Doc Figure | Observed seasonal discharges between 2000 and 2018 The non-significant trend of average seasonal discharges is at p greater than 0.05 at Can Tho Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7957 From all the above discussion, therefore, the change in hydrological indicators at Tan Chau and Chau Doc stations may be due not only to the province’s dike system and climate change, but also to upstream changes in flow As a result more research is needed to fully understand the factors that have led to the observed changes in flow regime Results of Mann–Kendal test and Sen’s slope Results of the Mann–Kendal test and Sen’s slope are presented in Table Figure illustrates observed dry seasonal discharges between 1997 and 2019 (the significant trend of the average of the dry seasonal discharges at p less than 0.05 was observed at Tan Chau, Chau Doc, and Vam Nao); Figure depicts observed wet seasonal discharges between 1997 and 2019 (the significant trend of the average of the wet seasonal discharges at p less than 0.05 is at Chau Doc) All three stations showed an increase in discharge during the dry season, whereas only one station, Chau Doc, showed a significant decreasing trend during the rainy season at a significance level less than or equal to 0.05 The discharge reaching the main stream increased (at Tan Chau station on the Mekong River), causing the discharge at Vam Nao station to increase Meanwhile, the measurement station on the Bassac River in Chau Doc showed a downward trend In comparison, the downstream data at Can Tho station is plotted in Figure Figure shows the downward trend in the wet season and upward trend in the dry season between 2000 and 2018 as observed in the annual discharge at Can Tho station This suggests that there was no significant trend for both seasons More observational data is required in order to establish a scientific basis for the flow variance at Can Tho station as a result of natural and/or human influences CONCLUDING REMARKS Upstream An Giang province in the VMD has strongly developed protection dikes, especially during the periods of 1997– 2004 and 2007–2010 By 2011, the dike system regulated 69% of the province’s total area, of which semi-dikes covered 15% and full dikes 54% The results of the flow-regime-change assessment showed that the flow alteration at both Chau Doc and Tan Chau stations in periods and was at a high level (over 67.0%); and in Vam Nao during periods and it was 49.8% and 60.7%, respectively In general, during the construction period of the dike system (1997–2010), Chan Doc station was the most affected (71.2%), followed by Tan Chau (68.2%) and Vam Nao was the least (49.8%) However, when considering period (2011–2019) after the relative completion of dike system construction, the change in flow regime at both Tan Chau and Vam Nao stations was seen still to increase significantly, by 76.6% and 60.7%, respectively Of the five IHA groups, group showed the biggest change at all three stations In particular, indicator 31 changed at a very high rate in both Chau Doc and Tan Chau; meanwhile, indicators 32 and 33 of group at Vam Nao station had a significant change in both phases The change in hydrological indicators at Tan Chau and Chau Doc stations were due not only to the province dike system and climate change, but also to the change in upstream flow of the Mekong River More research is needed to consider the various drivers of flow-regime alteration associated with natural and human activities both inside and external to the study area FUNDING The research received no funds DATA AVAILABILITY STATEMENT All relevant data are included in the paper or its Supplementary Information CONFLICT OF INTEREST The authors declare there is no conflict REFERENCES Azamathulla, H M., Guven, A & Demir, Y K 2011 Linear genetic programming to scour below submerged pipeline Ocean Engineering 38 (8–9), 995–1000 Azamathulla, H M., Ghani, A A & Fei, S Y 2012 ANFIS-based approach for predicting sediment transport in clean sewer Applied Soft Computing 12 (3), 1227–1230 Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7958 Binh, D V., Kantoush, S A., Saber, M., Mai, N P., Maskey, S., Phong, D T & Sumi, T 2020 Long-term alterations of flow regimes of the Mekong River and adaptation strategies for the Vietnamese Mekong Delta Journal of Hydrology: Regional Studies 32, 100742 Binh, N T., Tien, L V T., Tang, L T., Tu, N M., Dung, T D & Quan, N H 2022 Resilience of various innovative water management practices: the case of rice production in the Vietnamese Mekong Delta floodplains Agricultural Water Management 270, 107739 Chau, N D G., Sebesvari, Z., Amelung, W & Renaud, F G 2015 Pesticide pollution of multiple drinking water sources in the Mekong Delta, Vietnam: evidence from two provinces Environmental Science and Pollution Research 22 (12), 9042–9058 Coumou, L 2017 Relating Land Subsidence to Land Use through Machine Learning Using Remote-Sensing Derived Data: A Case Study in the Mekong Delta, Vietnam Master’s thesis, Utrecht University, Utrecht, Netherlands Dat, T Q., Likitdecharote, K., Srisatit, T & Trung, N H 2011 Modeling the influence of river discharge and sea level rise on salinity intrusion in Mekong Delta In: The 1st EnvironmentAsia International Conference on ‘Environmental Supporting in Food and Energy Security: Crisis and Opportunity’, Thai Society of Higher Education Institutes on Environment Thailand, pp 685–701 Fujihara, Y., Hoshikawa, K., Fujii, H., Kotera, A., Nagano, T & Yokoyama, S 2016 Analysis and attribution of trends in water levels in the Vietnamese Mekong Delta Hydrological Processes 30 (6), 835–845 General Statistics Office of Vietnam (GSO) 2016 Statistical Yearbook of Viet Nam 2016 General Statistics Office of Vietnam (GSO), Hanoi, Vietnam https://www.gso.gov.vn/en/data-and-statistics/2019/10/statistical-yearbook-of-vietnam-2016/ Gunawardana, S K., Shrestha, S., Mohanasundaram, S., Salin, K R & Piman, T 2021 Multiple drivers of hydrological alteration in the transboundary Srepok River Basin of the Lower Mekong Region Journal of Environmental Management 278, 111524 Guo, W., Zhou, H., Jiao, X., Huang, L & Wang, H 2022 Evaluation of hydrological regime alteration and ecological effects in the middle and lower of the Yangtze River, China Water Supply 22 (6), 5957–5973 Hecht, J S., Lacombe, G., Arias, M E., Dang, T D & Piman, T 2019 Hydropower dams of the Mekong River basin: a review of their hydrological impacts Journal of Hydrology 568, 285–300 Hirsch, R M & Slack, J R 1984 A nonparametric trend test for seasonal data with serial dependence Water Resources Research 20 (6), 727–732 Hirsch, R M., Slack, J R & Smith, R A 1982 Techniques of trend analysis for monthly water quality data Water Resources Research 18 (1), 107–121 Jiang, L., Ban, X., Wang, X & Cai, X 2014 Assessment of hydrologic alterations caused by the Three Gorges Dam in the middle and lower reaches of Yangtze River, China Water 6, 1419–1434 Kendall, M G 1975 Rank Correlation Methods, 4th edn Charles Griffin, London, UK Khang, N D., Kotera, A., Sakamoto, T & Yokozawa, M 2008 Sensitivity of salinity intrusion to sea level rise and river flow change in Vietnamese Mekong Delta – impacts on availability of irrigation water for rice cropping Journal of Agricultural Meteorology 64 (3), 167–176 Li, D., Long, D., Zhao, J., Lu, H & Hong, Y 2017 Observed changes in flow regimes in the Mekong River basin Journal of Hydrology 551, 217–232 Liu, J P., DeMaster, D J., Nguyen, T T., Saito, Y., Nguyen, V L., Ta, T K O & Li, X 2017 Stratigraphic formation of the Mekong River Delta and its recent shoreline changes Oceanography 30 (3), 72–83 Liu, Y., Cao, S., Yang, Y & Zhang, X 2018 Assessment of hydrologic regime considering the distribution of hydrologic parameters Water Science & Technology: Water Supply 18 (3), 875–885 Lu, X X., Li, S., Kummu, M., Padawangi, R & Wang, J J 2014 Observed changes in the water flow at Chiang Saen in the lower Mekong: Impacts of Chinese dams? Quaternary International 336, 145–157 Mann, H B 1945 Nonparametric tests against trend Econometrica 13 (3), 245–259 Marchesiello, P., Nguyen, N M., Gratiot, N., Loisel, H., Anthony, E J., Dinh, C S., Nguyen, T., Almar, R & Kestenare, E 2019 Erosion of the coastal Mekong delta: assessing natural against man induced processes Continental Shelf Research 181, 72–89 Mehta, D J & Yadav, S M 2022 Long-term trend analysis of climate variables for arid and semi-arid regions of an Indian State Rajasthan International Journal of Hydrology Science and Technology 13 (2), 191–214 Mekong Delta Plan (MDP) 2013 Long-term Vision and Strategy for a Safe, Prosperous and Sustainable Delta Dutch Government and Government of Vietnam https://www.deltares.nl/app/uploads/2014/01/Mekong-delta-plan-Long-term-vision-and-strategy.pdf Minh, H V T., Kurasaki, M., Ty, T V., Tran, D Q., Le, K N., Avtar, R., Rahman, M M & Osaki, M 2019a Effects of multi-dike protection systems on surface water quality in the Vietnamese Mekong Delta Water 11 (5), 1010 Minh, H V T., Avtar, R., Mohan, G., Misra, P & Kurasaki, M 2019b Monitoring and mapping of rice cropping pattern in flooding area in the Vietnamese Mekong Delta using Sentinel-1A data: a case of An Giang province ISPRS International Journal of Geo-Information (5), 211 Nguyen, Q T M 2016 Vietnam – Mekong Delta Water Management for Rural Development Project: Audited Financial Statement – Year Ending December 31, 2015 The World Bank, Washginton, DC, USA Nguyen, V K T., Nguyen, V D., Fujii, H., Kummu, M., Merz, B & Apel, H 2017 Has dyke development in the Vietnamese Mekong Delta shifted flood hazard downstream? Hydrology and Earth System Sciences 21, 3991–4010 Perera, A., Mudannayake, S D., Azamathulla, H M & Rathnayake, U 2020 Recent climatic trends in Trinidad and Tobago, West Indies Asia–Pacific Journal of Science and Technology 25 (2), APST-25-02-05 Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest Water Supply Vol 22 No 11, 7959 Richter, B D., Baumgartner, J V., Powell, J & Braun, D P 1996 A method for assessing hydrologic alteration within ecosystems Conservation Biology 10 (4), 1163–1174 Shahid, S 2010 Rainfall variability and the trends of wet and dry periods in Bangladesh International Journal of Climatology 30 (15), 2299–2313 The Nature Conservancy 2009 Indicators of Hydrologic Alteration: User’s Manual The Nature Conservancy Thu Minh, H V., Avtar, R., Kumar, P., Le, K N., Kurasaki, M & Ty, T V 2020 Impact of rice intensification and urbanization on surface water quality in An Giang using a statistical approach Water 12 (6), 1710 Ty, T V., Sunada, K & Ichikawa, Y 2011 A spatial impact assessment of human-induced intervention on hydrological regimes: a case study in the upper Srepok River basin, Central Highlands of Vietnam International Journal of River Basin Management (2), 103–116 Van Tho, N., 2020 Coastal erosion, river bank erosion and landslides in the Mekong Delta: causes, effects and solutions In: Geotechnics for Sustainable Infrastructure Development (P Duc Long, & N T Dung, eds), Springer Singapore, Singapore, pp 957–962 Van Ty, T., Minh, H V T., Avtar, R., Kumar, P., Van Hiep, H & Kurasaki, M 2021 Spatiotemporal variations in groundwater levels and the impact on land subsidence in CanTho, Vietnam Groundwater for Sustainable Development 15, 100680 VIWA 2008 Revised Strategic Development Plan for Vietnam’s Waterways up to 2020 Vietnam Inland Waterway Administration https://en viwa.gov.vn/ (accessed April 9, 2022) Wang, H., Zhu, Y., Zha, H & Guo, W 2021 Quantitative assessment of water level regime alterations during 1959–2016 caused by Three Gorges Reservoir in the Dongting Lake, China Water Supply 21 (3), 1188–1201 Wang, H., Zhu, Y., Jin, Y & Guo, W 2022 Quantitative assessment of hydrological alteration over multiple periods caused by human activities at the Jingjiang Three Outlets, China Water Supply 22 (1), 264–277 Wassmann, R., Hien, N X., Hoanh, C T & Tuong, T P 2004 Sea level rise affecting the Vietnamese Mekong Delta: water elevation in the flood season and implications for rice production Climatic Change 66 (1–2), 89–107 Yue, S & Wang, C 2004 The Mann–Kendall test modified by effective sample size to detect trend in serially correlated hydrological series Water Resources Management 18 (3), 201–218 Zhang, J., Zhang, X & Xiao, H 2019 Study of the dynamic evaluation model of overall hydrological alteration degree based on the RVA and set pair analysis–Markov chain methods Water Supply 19 (5), 1515–1524 First received 14 April 2022; accepted in revised form 14 September 2022 Available online 12 October 2022 Downloaded from http://iwaponline.com/ws/article-pdf/22/11/7945/1145491/ws022117945.pdf by guest