WATER RESOURCE VARIATION IN THE HAU RIVER MOUTH

WATER RESOURCE VARIATION IN THE HAU RIVER MOUTH Le Anh Tuan 1, Hoang Thi Thuy 2,Vo Van Ngoan 3 ABSTRACT The Hau River mouth is the largest estuary of the Mekong River Delta and is strongly influenced by both discharge from the upstream Mekong River and the semi-diurnal tides of the East Sea. This aquatic ecosystem is the interface between upstream freshwater flow and saltwater tidal flow, creating unique aquatic habitats with significant socio-economic importance. The area is densely populated, with many people highly reliant on aquatic ecosystem productivity and related aquaculture cultivation. This study examines changes to water resources in the Hau River mouth based on hydrological variation analysis. There is evidence that, within the last two decades, precipitation is decreasing in the early season but increasing in magnitude towards the end of the wet season. Average water level within the Hau River estuary has increased at about 0.77 cm/year with rising sea level, with concurrent increases in salinity intrusion. This is resulting in serious water supply limitations and negative impacts on cultivation. Further, the underground water table is declining at a rate of 0.39 m/year, increasing the threat of land subsidence in the region. These factors combine to pose great threats to Mekong River Delta in general and the Hau River mouth area in particular, where water resources face challenges due to climate change and sea level rise, expansion of cultivated areas and water trans-boundary related problems. Keywords: Hau River mouth, Mekong River Delta; trend analysis; water resources variation I. INTRODUCTION The Mekong River Delta, located in southern Vietnam, is an area of young geologic origin formed by alluvial deposits from the Mekong River that drains much of Southeast Asia. The Hau River, or so-called Bassac River or Hậu Giang in Vietnamese, is one of two major branches of the Mekong River that divides in Phnom Penh, Cambodia, before crossing the border of Vietnam and continuing to branch into smaller tributaries prior to discharging in the East Sea (Figure 1). Together, the Hau River in the south and the Tien River in the north form the important waterway and water-use corridor of the Lower Mekong River Delta Region. The Hau River flows through seven provinces, including An Giang, Can Tho, Vinh Long, Tra Vinh and Soc Trang. Two provinces in the Hau River mouth, Soc Trang and Tra Vinh, have the highest poverty rate in Vietnam (Vuong, 2011). The Mekong River Delta is an essential part of daily life to the people of this region, who use the river for their daily drinking, irrigation, domestic use, fish production, and transportation. Conversely, the Mekong River Delta can be impacted by human activities, which threaten the sustainable development of the region (Trieu et al., 2007). 1 Corresponding author. Research Institute for Climate change Can Tho University, Tel: +84.913.619.499; Fax: +84.7103.838.474; E-mail: latuan@ctu.edu.vn 2 Faculty of Natural Resources and Environment - Nong Lam University of HCM City 3 Office for Climate change DONRE Ben Tre province 50 NHA TRANG UNIVERSITY

Fig. 1. Location of the Hau River mouth and data-survey points The Hau River has a 225-km length in River has the greatest water discharge of all Vietnam and accounts for 5.17% the total rivers in Vietnam. In October, the Hau River length of the main river. The Hau River s width can drain about 90% of the peak floodwater is about 60 300 m when upon entering of the Mekong River, and the total annual flow Vietnam, and widens gradually as it flows to can reach nearly of 215 billion m 3. the sea. Channel braiding within the Delta The Hau River is currently facing many forms many elongated islands, the largest of challenges, including changes in hydrological which is Cu Lao Dung with the area of 249.4 flow characteristics, declining water quality, km². At the river mouth in Cu Lao Dung district lowering water table and associated land of Soc Trang province, the average river width subsidence, riverbank and coastal erosion, is approximately 2 km, and the largest distance narrowing of natural lowlands during the between two riverbanks is up to 18 km. The urbanization processes, expansion of agriculture average depth of Hau River is 10 20 m, and and fisheries production activities, and effects the maximum depth is over 40 m, although of climate change, such as sea level rise and depth tends to decrease closer to the sea due further saltwater intrusion. During the dry to deposition of sedimentation. The land season, saltwater intrusion can reach more surrounding the Hau River mouth (9.33 - than 60 km inland, leaving about 2.1 million 10.12 N, 105.71-106.42 E), including Tran hectares of coastal area in the Mekong Delta De, Cu Lao Dung and Vinh Chau districts, is affected by saline waters (Tuan et al., 2007; very low with mean land surface elevation of White, 2002). Increased salinity changes the about 1.0 m above mean sea level. The Hau chemical environment, causing substantial NHA TRANG UNIVERSITY 51

changes in species composition of the local ecological communities. Lack of adequate surface freshwater resources, combined with increasing demand for water for agricultural production and domestic utilities, has resulted in increased reliance on groundwater extraction. This trend has aggravated saline intrusion into the coastal aquifers and increased land subsidence in this region (Hagenvoort and Tri, 2013; Erban et al., 2014; Esther et al., 2015). In some areas, high salinity water has contaminated near surface aquifers, to the point at which it is unfit for consumption. Further, current and planned hydropower dams in the mainstream and tributaries of the Mekong River threaten to reduce sediment deposition to the Delta s floodplain areas, exacerbating the land subsidence effect (Cook and Tu, 2013). Sea level rise and land subsidence, combined with rainy season storms and wind driven waves, will pose a major threat to local populations in the decades to come (Rohkohl, 2014). In the current study, the objective is to survey the variation in water resources at the Hau River mouth area, including rainfall, surface flow and groundwater sources. II. RESEARCH APPROACHES AND METHODOLOGY This study used data collected from An Giang, Can Tho and Soc Trang provincial hydro-meteorological stations, and data downloaded from the website of Mekong River Commission (MRC) to detect temporal trends in water resources characteristics. Trend lines were fit using Microsoft Excel. This study also used the regional climate model, PRECIS, for downscaling coarse scale Global Circulation Models to derive climate change scenarios for the Mekong River Delta (Jones et al., 2004). PRECIS is a regional climate model developed by the Hadley Centre for Climate Prediction and Research. It can be used as a downscaling tool that adds fine scale (high resolution) information to large-scale projections of a Global Circulation Model (Tuan and Supparkorn, 2011). III. RESULTS AND DISCUSSION 1. Variation of weather patterns Generally, there are two distinct seasons in the Mekong River Delta: rainy season (from May to October) and dry season (the rest of the year). In the coastal areas, the starting dates for rainy seasons may come one-to-two weeks earlier than in the inland areas. Annual average rainfall in the Delta historically ranges from about 1,400 to 2,200 mm, with more than 90% the rainfall occurring during the rainy season. In the Hau River mouth area in Soc Trang Province, average annual rainfall is commonly higher than the Delta, ranging from 1,660 to 2,230 mm. However, in last three decades there is evidence that rainfall has become more variable, with higher rainfall during the wet season and lower rainfall during the dry season. Also, timing of the onset of rainy season and its duration has become more variable, with significant effects on rainfall amount. The annual rainfall totals in 1998 and 1999 were well above historical averages, up to over 2,750 mm/year; however, in 1990, 1991, 1992, 2004, and 2006, rainfall totals were near historical lows at less than 1,600 mm/year (Figure 2). PRECIS regional climate modeling predicts that, by the 2030s, Soc Trang province will experience a decrease in annual precipitation of 10-20% and the starting time of median rainy seasons may be two weeks later compared to the 1980s-1990s (Figure 3). During the dry season, the model predicts a significant increase in air temperature, approximately 0.6 C in the duration of 1990-2013, a trend which has already become apparent over the past 25 years (Figure 4). 52 NHA TRANG UNIVERSITY

Fig. 2. Variations and trendline of annual rainfall amounts in Soc Trang (1985 2012) Fig. 3. Accumulated rainfall projection in Soc Trang under PRECIS model (Tuan, 2009) Fig. 4. Variations and trend line of 5-year average temperature in Soc Trang (1990 2013) 2. Variation of river flow characteristics The hydrological interaction of outgoing river flow and incoming tides in the Hau River estuary is extremely complex and is influenced by many factors, including topography, seasonal currents, wind directions, water temperature, and constructed human activities. However, rising sea level is likely to greatly impact this area over the coming decades. Water level within the Hau River estuary has already begun to rise considerably, with mean river level increasing at about 0.77 cm/year since 1985 (Figure 5). Given that most of the surrounding land is only 1.0 2.0 meters above mean sea level, this rise in water level threatens millions of Vietnamese people in the Mekong River Delta. Fig. 5. Variations and trend line of water level amplitudes (Max Min) in Dai Ngai with standard error bars in each calculated point (1985 2008) NHA TRANG UNIVERSITY 53

3. Variations of seasonal salinity intrusion The lower Hau River experiences severe salt intrusion annually, usually reaching peak salinity in March/April (Figure 6) corresponding to the end time of the dry season and the lowest flows in the Mekong River system. In April 2010, the maximum salt intrusion into the Hau River reached 40 km (4 g/l) to nearly 70 km (1 g/l) inland from the river mouth. As sea levels rise and flows decrease, this annual salt intrusion will reach further inland at higher salt concentrations, with significant consequences to the aquatic community and the human communities within this region. Fig. 6. Variations of maximum monthly salinity recorded in Dai Ngai (2002-2014) 4. Variation of groundwater level and land subsidence risk 2012, with an average rate of decline of 0.39 m/year (Figure 7). By June 2013, the water Reduced precipitation combined with table at this site had reached -10.17 m. The increased reliance on groundwater resources is depleting shallow aquifers in the Mekong Delta. For example, the water table in Soc Trang City dropped more than 5 m from 1996 similar declines in the Pleistocene aquifer layer were reported at sites in and around Tran De, a coastal district of Soc Trang province in the Hau River mouth (Figure 8). Fig. 7. Variations and trend line of underground water table monitored in Soc Trang in the periods of Dec. 1996 Dec.2012 (Data source: NAWAPI, 2012) Fig. 8. Variations of groundwater level (Aquifer Pleistocene middle above) monitored in the periods of 2001 2012 in in Tran De district (data 2004 is unavailable) 54 NHA TRANG UNIVERSITY

Code and Coordinator (X,Y) of Province showed light organic matter monitoring points: QST10A (629300, 1053050); QST12B (614450, 1053900); QST13 (612100, pollution with high suspended solid and total iron contents (Table 1). In addition, suspended 1051100); G8 (618500, 1051540); G25 solids, chlorine and coliform counts exceeded (611160, 1056000) (Data source: Soc Trang the Vietnamese Standard in 2006. In the Hau DONRE) River mouth, the interface between fresh and 7. Changing the water environment Unpublished water quality data collected in 2006 by the Soc Trang Department of Natural Resources and Environment (DONRE) in the My Thanh River, Vinh Chau District, Soc Trang salt water varies with the tidal fluctuations (Duc, 2008), but may serve as an effective nutrient trap in which many minor chemical contaminants become concentrated. Table 1. Surface-water quality of My Thanh estuary, Vinh Chau District Results No Parameter Unit 2002 2003 2004 2005 2006 1 ph - 7.54 8.00 7.31 7.40 6.99 2 Turbidity NTU 88 304 108 19.40 622 3 DO mg/l 5.38 5.30 1.22 2.42 3.13 4 20 BOD 5 mg/l ND ND 6.00 8.00 16 5 COD mg/l ND 18.10 27.60 26.00 20 6 SS mg/l 121 - - 631 539 7 - NO 3 mg/l 0.40 0.60 5.20 30 6.4 8 Fe mg/l 1.29 0.02-0.268 23.5 9 - NO 2 mg/l - - - - 3.0 10 + NH 4 mg/l - - - - 0.1 Notes: ND: Non-detection (-): No data (Data source: Soc Trang DONRE, 2006, unpublished) IV. CONCLUSION The data presented here and within supporting literature provide strong evidence of changing water resources in the Mekong River Delta in general and in the Hau River mouth in particular. Due to the combination of factors related to climate change and human water extraction activities, the Mekong River Delta is currently facing many challenges, which will only worsen in the future. Increasing salinity intrusion, varying water flow regimes, and general reduction of freshwater resources threaten human populations in the Mekong Delta. More serious yet may be the rising sea level and subsequent inundation of much of the Delta. However, there are still many unknowns regarding these effects. The establishment of a water quality monitoring network for the Delta is an urgent need, and standardization of protocols is required to establish a database for sharing information. Further, the Mekong Delta Plan has been developed to provide feasible action plans to help deal with the variation of water sources and climate change. NHA TRANG UNIVERSITY 55

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