1. VERIFYING THE INCREASE OF SALINITY INTRUSION AT INTERANNUAL SCALE IN THE VIETNAM MEKONG RIVER
Im going to talk about my research is verifying the increase of salinity intrusion at inter-annual scale in the Vietnam Mekong River
Nguyen Hong Van (M2)
Master Program in Environmental Sciences
School of Life and Environmental Sciences
19 May 2019

2. Introduction
Seawater intrusion in the MKD causes: Crops damage Freshwater shortage Productive aquaculture loss Disease outbreaks Especially in the dry season 2016: 208,394 households lacked freshwater 400,000 ha cropland affected
Destroyed paddy field
Saltwater intrusion is one of the most serious natural phenomena occur in the Vietnam Mekong Delta. Seawater intrusion caused crops damage, freshwater shortage, productive aquaculture loss and disease outbreaks. Especially in the dry season of 2016, salinity intruded further with 1ppt saline water reach to 90km from estuary to upstream, and it hit a new historical record in this issue by its extensive damages. It affected to 11 provinces among 13 provinces of the Delta, 400,000 ha cropland and freshwater intake
Lacked of freshwater
(CGIAR, 2016)
(vnexpress.net, 2016)

3. Previous research Binh (2015): salinity trend between 1995 to 2010
Upward trend
Those researches just focus on simulate salinity by using hydraulic models and analyzing equations to study about salinity intrusion. However, there is still lack of researches which analyze the observed trend of salinity from the past until present. From 1996, there is an upward trend of salinity in almost estuaries of the MKD. Especially from the 2004 salinity has increased faster than previous years. But the causes which led to the increasing of salinity intrusion in the Mekong river have not been found yet.
Causes of the increasing trend have not been found

4. Objective
To investigate the factors which lead to the increasing trend of salinity at the long term in the Vietnam Mekong River
Therefore, my research will be conducted to investigate the factors which lead to the increasing trend of salinity in the long term at the Mekong river

5. Fig. The location and topography map of the MKD
Study area
Rainy season
Flooding season
Area: 3.9 million ha
Agricultural land: 2.6 million ha
Average elevation: 1.0 – 2.0 m
East Sea
West
Sea
CAMBODIA
Salinity
Tide
Discharge
The area of my study is the estuaries of the Mekong Delta located in the South of Vietnam with area around 4 million hectares, in which there is more than half of this area is used for agriculture. Due to the low topography, the tide from the sea have strongly effect to the delta in the whole year. Beside that in this area, there is seasonal fluctuation by the amount of freshwater from the upstream. It causes the flooding season from July to November and dry season from December to April. In dry season, due to a very little amount of freshwater comes to delta, so the delta suffer the serious sea water intrusion, with the most serious of salt water intrusion is in April.
Fig. The location and topography map of the MKD

6. Methodology _ Statistical analyses
(H.G. Savenije, 2005)
Salinity
Freshwater discharge
Tide
Topography
Wind
Chau Doc
Dai Ngai
Long Phu
Tran De
For analyzing the mechanism of saltwater intrusion, I use 1 point where measured freshwater discharge come to delta, 1 point to measure the fluctuation of tide at the river mouth, and 3 points which measured salinity during dry season. The data types I used include hourly data and daily data in dry season of 1 year, monthly data is used for the long term analysis from 2003 to 2016.
Data type:
- Hourly data 2014
- Averaged daily data 2014
- Averaged monthly data : (dry season: Feb – May)

7. Results _ hourly data Tidal fluctuation in salinity timeseries
In the short term timeseries, we can see that there is tidal signal in salinity. High tide causes the increase of salinity because the sea water is pushed up into the estuary, low tide have a decrease in salinity where brackish water withdraw to the ocean. This tidal signal is clearer with the station near the mouth, with the station farer upstream, the tidal amplitude decrease
Salinity at Tran De
Salinity at Long Phu
Salinity at Dai Ngai

8. Results _ averaged daily data
Tidal level
Freshwater discharge
Tran De
Long Phu
Dai Ngai
With the longer data, there is still tidal fluctuation in salinity along the estuary. The high peaks of tide from the middle of Feb to early March caused the high salinity. Beside that the daily salinity as well as the tidal amplitude of salinity depend on the amount of freshwater discharged from the upstream of the Mekong river. From the middle of March, the peak of tide were not high as last period, but the salinity got the highest value in this dry season due to the consequences of the strong reduced freshwater discharge.
Fig. Averaged daily discharge vs Salinity (dry season 2014)

9. Results _ averaged monthly data
Discharge
Salinity – Dai Ngai
Salinity – Long Phu
With longer time scale as monthly data, the tidal fluctuation reduced. As the graph show there are different trend between salinity and the tide. However, in the dry season 2016 we still can see the effect of tide to the salinity, high tide caused the high salinity in the river channel. Whereas, the salinity and freshwater discharge show the obviously opposite trend between them. The correlation cooefficient between freshwater and salinity showed the closer than salinity and tide. At the station near the river mouth, the effect of tide is stronger than in the upstream of the river so that it got the higher correlation.
Salinity – Tran De
Tidal level

10. Summary and Future works
Salinity variation depends on: Tide vs Freshwater
Short timescale (hourly, daily data): tidal fluctuation in salinity
Longer timescale (monthly data): salinity strongly depends on freshwater
Future works:
- Evaluate distribution of salinity in horizontal direction along estuary
- Examine topography and wind stress to salinity variation
To sum up my analyzing, there is a salt wedge in in the flooding season. The analyzing long term data showed that, salinity variation depend strongly on the freshwater discharge. The relationship between them got highest correlation in March when the lowest freshwater come to the Delta. This correlation is getting weaker from the river mouth to the upstream. The salinity and tide show the negative and weak correlation. The strongest correlation between salinity and tide occurs at the station near the river mouth.

11. References
CGIAR. (2016). Assessment Report: The drought and salinity intrusion in the Mekong River Delta of Vietnam. Hanoi, Vietnam: CGIAR Research Centers in Southeast Asia.
Binh, N. T. (2015). Vulnerability and adaptation to salinity intrusion in the Mekong delta of Vietnam. Doctoral dissertation.
Savenije, H. (2005). Salinity and Tides in Alluvial Estuaries. Elsevier, New York.
Nguyen, A. D., & Savenije, H. H. G. (2006). Salt intrusion in multi-channel estuaries: as case study in the Mekong Delta, Vietnam. Hydrology and Earth Sciences Discussions,
Nguyen, H. T., Tran, D., Kanchit Likitdecharote, Thares Srisatit. (2011). Modeling the Influence of River Discharge and Sea Level Rise on Salinity Intrusion in Mekong Delta. The 1st Environment Asia International Conference on “Environmental Supporting in Food and Energy Security: Crisis and Opportunity” (pp ). Bangkok, Thailand: Thai Society of Higher Education Institutes on Environment.