Study area

The study was conducted in six provinces: Tien Giang, Ben Tre, Tra Vinh, Soc Trang, Bac Lieu, Ca Mau and Kien Giang, covering the areas of 10 rivers: Cua Tieu, Co Chien, Hau, K Nhu Gia, K Maspero, Cua Cung Hau, Ham Luong, Phung Hiep, Ganh Hao, and Song Doc. The research was carried out from 1996 to 2018, and the sampling points of this study are depicted in Figure 1.

Data

The salinity measurement data from 20 stations located at river mouths and along rivers were provided by the Southern Region Hydro-Meteorological Centre. These stations have provided the highest salinity data over the years from 1996 to 2018, as presented in Table 1.

Methodology

This study was conducted following the research framework outlined below Figure 2:

The salinity trends at the stations from 1996 to 2018

Figure 3 shows the characteristic variation of salinity at two monitoring stations: Vam Kenh and Hoa Binh. The results indicate that the highest salinity (maximum salinity) recorded at Vam Kenh is 30.9‰, while the lowest salinity (minimum salinity) at Hoa Binh is 7.8‰. This reflects a distinct difference in salinity levels between the two areas.

ANOVA testing with an alpha level of 0.026 < 0.05 at the Vam Kenh station shows a confidence level of 95%. Similarly, at the Hoa Binh station, an alpha level of 0.001 < 0.005 also indicates high reliability, ensuring statistical significance for both stations in the study area.

Analysis of the trend from Figure 3 shows a decreasing salinity trend at Vam Kenh with a rate of 0.1459‰/yr, while at Hoa Binh, the rate of decrease is faster, reaching 0.6506‰/yr. This indicates that Hoa Binh is experiencing a significantly faster reduction in salinity compared to Vam Kenh. The cause of this trend may be related to the northeast monsoon (northeast winds and easterly winds throughout the dry season) blowing directly into the Mekong River delta region, especially the coastal area from Can Gio to Ben Tre and Tra Vinh. This wind may push seawater further into the river, increasing the salinity of the water.

In summary, both Vam Kenh and Hoa Binh show a trend of decreasing salinity over time, with the rate of decrease at Hoa Binh being significantly faster. This highlights the importance of studying and managing water resources to mitigate the impact of saltwater intrusion on the environment and socioeconomic conditions.

The calculations in Figure 4 show the characteristics and trend of salinity at 6 stations (Binh Dai, Ben Trai, Loc Thuan, An Thuan, Huong My, Son Doc) in Ben Tre Province. The highest salinity is recorded at the An Thuận station with 31.5‰, while the lowest salinity is 2.3‰ at the Huong My station. At the Binh Dai station, the salinity trend shows a slight decrease over the years with a reduction coefficient of approximately 0.1484‰/yr, indicating stability in salinity in this area. The Ben Trai station has a very slight increasing trend in salinity, only 0.033‰/yr, reflecting negligible fluctuations and maintaining stability over time.

The An Thuan station shows a slight increasing trend in salinity over the years, with a linear trend indicating an increase of 0.1053‰/yr. Although there is an increase, it is not significant. The Huong My station exhibits a slight increasing trend with a rate of 0.291‰/yr, indicating a faster increase compared to other stations in the area due to its location at the large Co Chien River branch and its proximity to the sea, only about 20 km away, subject to various factors such as water shortage from upstream, flow, waves, and tides.

At the Loc Thuan station, salinity shows an increasing trend with a rate of 0.1087‰/yr. The fluctuations in salinity levels in this area are not very pronounced, indicating that salinity remains relatively stable. Meanwhile, the Song Doc station has a trend of increasing salinity over the years, with a rate of 0.265‰/yr. This trend reflects a relatively stable salinity level at Song Doc.

The significance of these trends indicates changes in natural environmental conditions, the impact of climate change, and the geographical location of the stations. Ben Tre Province has 4 river branches flowing into the sea through 4 main river mouths: Dai River Mouth, Ba Lai River Mouth, Ham Luong River Mouth, and Co Chien River Mouth. Notably, Ben Tre Province has a coastline directly exposed to the monsoon winds (northeast monsoon and easterly winds), leading to severe coastal erosion and more intense saltwater intrusion during the dry season.

From the above analysis results, although an increasing trend in salinity is observed at the stations, the Song Doc station has the highest amplitude of variation in the province. The Song Doc station is located on the Hau River branch, about 20 km from the sea, with relatively high and complex salinity levels, specifically showing the most significant salinity peaks in the years 1999 (18‰) and 2016 (27.4‰). At the Huong My station, at the Cung Hau mouth, salinity varied significantly during the period 2014-2016, from 5.2‰ to 19.5‰. Overall, the analysis of linear trends shows a clear increasing trend in salinity at the stations, with varying levels of increase.

The calculations in Figure 5 show the spatial characteristics of salinity levels at 4 stations (Huong My, Tra Vinh, Tra Kha, Cau Quan) in Tra Vinh Province. The highest salinity recorded was at the Tra Kha station with 22.8‰ in 2005, while the lowest salinity was 6.0‰ at the Tra Vinh station in 2014. In terms of trends, the Huong My station shows a slight increase over the years, with a rate of 0.08‰/yr. The linear trend at this station indicates an increase in salinity but with significant fluctuations during the period 1996-2005. At the Tra Vinh station, a slight increase over the years is observed, with a rate of 0.0259‰/yr. The linear trend at the Tra Vinh station shows an increase in salinity, but the changes are not very pronounced, ensuring spatial stability.

At the Tra Kha station, there is a slight decreasing trend in salinity, with a rate of only 0.008‰/yr, indicating a negligible reduction compared to other stations in the area. Meanwhile, at the Cau Quan station, a slight increasing trend is observed with a rate of 0.0542‰/yr. The fluctuations in salinity at this station are not very pronounced, especially during the period 2004-2009.

The significance of these trends indicates that three stations (Huong My, Tra Vinh, Cau Quan) show an increasing trend, while the Tra Kha station shows a decreasing trend. The cause of these changes may be related to the El Niño phenomenon during the 2015-2016 period, which resulted in very low rainfall combined with the impact of upstream dams, leading to severe drought in the Mekong Delta region.

Figure 6 shows the trend in salinity at monitoring stations in Soc Trang Province, where the highest salinity (maximum salinity) is greater than 33‰ at the Tran De station and the lowest salinity (minimum salinity) is only 1.5‰ at the Soc Trang station. Salinity at the Tran De station shows the most significant decrease over the years. The linear trend indicates a reduction rate of -0.4511‰/yr, reflecting a considerable decrease in salinity in this area, which may be due to the station's location in a small canal, with the area and riverbed topography having a gentle slope.

At the Dai Ngai station, the salinity trend shows a slight increase over the years, with an increase of only 0.01‰/yr. The linear trend indicates relative stability but still shows a slight upward trend. Meanwhile, at the Long Phu station, a significant increasing trend is observed, with the linear trend showing an increase of 0.0503‰/yr, reflecting a high level of stability in salinity. At the Soc Trang station, there is a slight decreasing trend over the years with a rate of -0.0071‰/yr, indicating that the changes in salinity levels over the years are not significant compared to other stations in this area.

Based on the calculated salinity trends of the 4 stations, 2 stations show an increasing trend (Long Phu 0.0503‰/yr, Dai Ngai 0.0099‰/yr), while the other 2 stations show a decreasing trend (Tran De -0.4511‰/yr, Soc Trang -0.0071‰/yr). Overall, the significance of these trends indicates a reduction in salinity at the stations. However, salinity intrusion indices in Soc Trang still show an increasing trend, particularly in the area along the Co Chien River. Soc Trang Province is one of the regions at high risk due to climate change, with erratic weather changes and a shortage of main upstream flow being the two main factors directly affecting changes in salinity over time and across different spatial areas.

Figure 7 shows that the highest salinity (maximum salinity) exceeded 33‰ at the Ganh Hao station in 2015, while the lowest salinity (minimum salinity) was 3.7‰ at the Phuoc Long station in 1999, located in Bac Lieu Province. Analysis of the trend from this figure indicates that the trend rate at both stations is increasing, with the most significant increase at the Phuoc Long station, which has a rate of 0.4482‰/yr. The linear trend for Phuoc Long shows that SI fluctuates slightly over the years, indicating stability in the Phuoc Long area, due to the station's location in a small, deep inland river branch.

At the Ganh Hao station on the Phung Hiep River, the salinity trend shows a slight increase over the years, with an increase rate of 0.0221‰/yr. Salinity levels fluctuate continuously over the years; although there are variations, these changes are not significant. However, salinity fluctuated significantly during the 1999-2000 period due to the complex structural changes in the province's production, with a simultaneous shift in economic development to a higher structural scale. As a result, the coastal water area also changed due to the resonant nature of the complex riverbed topography, leading to widespread salinity intrusion along the river throughout the province.

The analysis results show that the salinity trend at the Phuoc Long station fluctuates slightly, indicating stability in this area. Meanwhile, at the Ganh Hao station, although there is a slight increasing trend, the salinity levels vary insignificantly over the years, reflecting a relatively stable salinity level in this area. Climate change and changes in production structure have significantly affected the level of salinity intrusion in the region.

Figure 8 presents the results of the study and analysis of the salinity time series data from 1996 to 2018 for the two stations Ca Mau and Song Doc in Ca Mau Province, combined with ANOVA testing to assess SI in the province. The highest salinity (maximum salinity) exceeded 33‰, and the lowest salinity (minimum salinity) was 27.3‰.

At both Ca Mau and Song Doc stations, the trend in salinity changes over the years shows a slight increase, with rates of 0.0476‰/yr at Ca Mau and 0.0108‰/yr at Song Doc. This trend indicates that the variation in salinity is not very pronounced, suggesting that salinity at these two stations remains relatively stable.

The research results show that the salinity trends at both stations are similar, particularly during the dry season of 2016 when the El Niño phenomenon caused severe drought in the MKD, the Central Highlands, and South Central Coast. At that time, the Mekong Delta faced historic salinity intrusion, with salinity in the river recorded at 4‰ and intruding up to about 50km. In the study area, salinity intrusion exceeding 33‰ was most distinctly observed at the Cà Mau and Song Doc stations.

With the aforementioned complex characteristics, the impact of salinity intrusion has spread across all coastal provinces in the MKD, including Ca Mau Province. This is clearly demonstrated by the data and salinity trends analyzed at the monitoring stations in the study.

The calculation results for the method and the R are presented in Table 2.

Salinity Characteristics by Frequency

The study has constructed a salinity frequency equation as shown in Table 3, indicating that after calculating the frequencies, the highest salinity of greater than 33‰ with a frequency of 95% occurred at the Phuoc Long station on the Hau River in Bac Lieu province. The lowest salinity of 0.78‰ with a frequency of 5% was observed at the Soc Trang station in Soc Trang province.

The authors used the PIII to conduct a comprehensive assessment of salinity intrusion for 20 stations in the study area. The results, based on two evaluation criteria, are shown in Figure 9, which illustrates the frequency of salinity exceeding 33‰ at the Hoa Binh, Tran De, An Thuan, Phuoc Long, Ca Mau, and Song Doc stations. This chart shows the relationship between the frequency of salinity occurrences and the salinity values at these stations.

The chart in Figure 9 illustrates the cumulative salinity frequency at the Hoa Binh, Tran De, An Thuan, Phuoc Long, Ca Mau, and Song Doc stations. From the chart, we observe that the Hoa Binh station has the highest and most variable salinity among the stations, especially at lower frequencies, indicating a high level of salinity intrusion and significant variation at this station. The Tran De and An Thuan stations have relatively high salinity but show a decreasing trend as frequency increases, although the variability is not as large as at Hoa Binh. Meanwhile, the Phuoc Long and Ca Mau stations have relatively stable and lower salinity compared to Hoa Binh, Tran De, and An Thuan, with slight fluctuations and maintaining stability as frequency increases. The Song Doc station has quite high salinity but is more stable compared to Hoa Binh, Tran De, and An Thuan, with less variation and remaining at a relatively high level as frequency increases. Overall, all stations show a trend of decreasing salinity with increasing frequency, indicating a relative stability of salinity values as salinity intrusion conditions occur more frequently. This chart clarifies the differences in the levels and variability of salinity intrusion among the stations in the study area, highlighting the importance of managing and monitoring salinity intrusion to protect water resources and ecosystems in these areas.

The chart in Figure 10 illustrates the cumulative frequency of salinity at the Vam Kenh, Long Phu, Son Doc, Binh Dai, Ben Trai, and Ganh Hao stations. Salinity at the Vam Kenh and Ganh Hao stations remains at the highest levels compared to the other stations and shows a slight decreasing trend as frequency increases but still remains high. The Long Phu and Son Doc stations have salinity ranging from average to high, with the frequency curve indicating a gradual decrease in salinity as frequency increases, showing that the level of saline intrusion varies moderately. Salinity at the Binh Dai and Ben Trai stations remains at lower levels compared to the other stations and shows a gradual decrease as frequency increases, indicating stability and less fluctuation.

In particular, the frequency curves at four stations (Vam Kenh, Long Phu, Son Doc, Binh Dai, Ben Trai) show that 90%-95% do not exceed the threshold of 33‰. However, at the Ganh Hao station, the frequency curve shows that 90%-95% of salinity levels have exceeded the 33‰. The general trend at all stations is that salinity decreases as frequency increases, indicating that at higher frequencies, the level of saline intrusion gradually decreases and reflects the relative stability of salinity values when saline intrusion conditions occur more frequently.

From Figure 11, the 90% and 95% frequency curves show that salinity at the Tra Kha, Dai Ngai, Loc Thuan, and Huong My stations does not exceed of 33‰. Overall, at a 95% frequency, salinity at all four of these stations is higher compared to the 90% frequency, indicating the stability and lack of significant fluctuation in salinity at these stations.

The results fromFigure 12 show that the salinity frequency curves at the Hung My, Tra Vinh, Cau Quan, and Soc Trang stations are the lowest, below the 20‰, in the dataset of 20 stations. The salinity at these four stations also does not exceed the 30‰ threshold, indicating a stable and low level of saline intrusion at these stations.

The analysis results are consistent with several previous studies in the Mekong Delta region. The study of Nguyen Van Thuong (2015) study shows that the highest salinity at the Vam Kenh station also tends to increase during the period of 1984-2014 (30 years)25, 26. In Ben Tre province, another study calculated the salinity trends at 6 stations (An Thuan, Ben Tre, Binh Dai, Huong My, Loc Thuan, Song Doc), and the results indicate that salinity at the Loc Thuan station increased, while the other stations decreased in the trend period of 2000-2016 (16 years)21. In Tra Vinh province, another study shows that salinity in this province has an increasing trend, especially at the Huong My and Tra Vinh stations27. In the provinces of Bac Lieu and Soc Trang, another author calculated that the average annual temperature increased during the period of 1980-2017 (37 years), thus concluding that the level of saline intrusion in this area is increasing28.

The summary of the research results shows that the trend of salinity changes at the study stations in the Mekong Delta is complex and diverse, reflecting the impact of climate change and local environmental factors.

However, the correlations are based on linear correlation with R² > 0.8, so there will be errors when forecasting future years; this represents the uncertainty in this study. The more historical values there are, the more reliable the correlation curve and the empirical frequency curve will be29, 30. This study is based on salinity values from 1996-2018, which is a sufficiently long period to ensure the reliability of the data series.