The contribution ratio of river water discharged into the red tide areasin the upper Gulf of Thailand based on numerical simulation
Anukul Buranapratheprat 1 and Akihiko Morimoto2
1

Department of Aquatic Science, Faculty of Science, Burapha University, Thailand

2

Center for Marine Environmental Studies (CMES), Ehime University, Japan

Introduction
Red tide and hypoxia resulting from eutrophication in the upper Gulf of Thailand havebeen reported. Nowadays, they are of interest and widely known in the Thai community.
Morimoto et al. (2021) reported the development of hypoxia in the upper Gulf of Thailand(UGoT) from August to November 2014, with the most severe event occurring from September
to November 2014. Hypoxia in the upper Gulf of Thailand can always occur with favorable
environmental conditions during the wet season when water is stratified from low-salinity
waters containing inorganic nutrients and organic matter in the water mass in large quantities.
Although we do not understand their mechanism well, we do understand their seasonalvariations with the intensity of the problem in the wet than in the dry season. The increase in
freshwater and nutrient supply during the same period of the southwesterly wind makes thenortheastern part of UGoT the most affected area. Discharge water from four main rivers, oneof which is The Chao Phraya River, the largest river in Thailand, plays a vital role in enhancing
water stratification and transporting materials from the land into the sea. In thi s research, we
are interested in the contribution ratio of water and material from each river in the red tide areain the northeast of UGoT from June to August between 2022 and 2023. Both periods areselected because strong bloom occurred in 2023 not in 2022. The ratio is expected to change
between years and may influence nutrient conditions and the red tide of green Noctiluca, adominant plankton, mostly blooms.

Procedure
The Princeton Ocean Model (POM) (Mellor, 1998) is used for current simulation inUGoT (Figure 1). It is coupled with passive tracer subroutines to simulate the distribution of aconservative dissolved material released from 4 river mouths including the Thachin River, The
Maeklong River, The Chaopraya Ricer and the Bangpakong River. POM is forced by monthlywinds (ASCAT) (http://remss.com), water elevation at the open boundary (Harmonic Analysis)
and monthly river discharge (Royal Irrigation Department). It is operated in diagnostic mode

2with climatology temperature and salinity data from World Ocean Atlas 2001. The tracers arecontinuously released at the sea surface at 4 river mouths with an initial concentration of 100
units and operation time of 30 days. The contribution of the tracer, referred to as the influenceof river water, from each river in the focused plankton-bloom area (red rectangular in Figure1) is investigated.

Figure 1 The upper Gulf of Thailand
showing water depth in meters, and thefocused eutrophic area in red rectangular.
Results and discussion
The river discharge in both years was clearly different (Figure 2). It is noted that there
was a high volume of water in 2022 and a low volume in 2023. The peak water levels
occurred between September and November. In October, the water levels were highest for theBang Pakong River, Tha Chin River, and Mae Klong River. However, for the Chao Phraya
River, the highest water levels were observed in November, approximately one month later
than the other rivers.
Wind patterns also varied each year. In 2022, the winds were lighter compared to2023 (Table 1). The wind directions showed no significant difference between June and July,but there was a noticeable difference in August. In 2023, the southwest winds were oriented
more towards the east compared to 2022 (Figure 3).

River Discharge (m3/s)
3,000

2,500
2,0001,500
Chao Phraya

1,000Bangpakong

5000Thachin
Maeklong

Figure 2 Monthly average river discharge from the four main rivers flowing into the upper
Gulf of Thailand in 2022 and 2023.

3Figure 3 Monthly average winds over the upper Gulf of Thailand in June, July, and Augustof 2022 and 2023.

Table 1 Average wind speed and standard deviation over the upper Gulf of Thailand's
northern region in June, July, and August of 2022 and 2023.
MonthJune
July
AugustAverage wind speed over UGoT (m/s)
2022
2023
5.68 ± 0.07
6.23 ± 0.04
5.60 ± 0.03
6.62 ± 0.37
5.94 ± 0.07
6.20 ± 0.08

Regarding water currents, there was a reasonable difference between the years (Figure4). It was observed that the water flow velocity in 2023 was higher than in 2022. Particularly,
the flow downstream towards the eastern side of UGoT, near Sichang Island, was stronger in2023. This led to a greater dispersion of tracers towards the south along the eastern coastlineof UGoT compared to 2022 (Figure 5). When considering the proportion of water volume
flowing into the area prone to plankton blooms, it was found that the Bang Pakong River had
the highest proportion consistently across both years. Following it were the Chao Phraya
River, Mae Klong River, and Tha Chin River, respectively. However, during the dry year of2023, the proportions from the Bang Pakong and Chao Phraya Rivers decreased, while thosefrom the Mae Klong River and Tha Chin River increased (Figure 6). This difference may
have contributed to the varying occurrences of plankton blooms between the two years,
which will require further investigation in the future.

4

Figure 4 Depth-averaged currents based on monthly average in the upper Gulf of Thailand inJune, July, and August of 2022 and 2023.

Figure 5 Dispersion of tracer from the four main rivers in the upper Gulf of Thailand on the30th day after release at the river mouths in June, July, and August of 2022 and 2023.

5Contribution (%) of Tracer from the River Mouths
7060
50403020100

Jun-22

Jul-22

Maeklong

Aug-22ThachinJun-23Chao Phraya

Jul-23Aug-23

Bangpakong

Figure 6 Proportions of tracer from the four main rivers entering the area where frequent
occurrences of red tide, particularly in the eastern region of the upper Gulf of Thailand(Figure1), in June, July, and August of 2022 and 2023.

Perspectives in the future
We will improve the simulation results by focusing on the sensitivity analysis of windand river discharge on the contribution of discharged water from the main rivers into thefocusing area.

References
Mellor, G.L. (1998). User’s Guide for a Three-Dimensional, Primitive Equation, Numerical
Ocean Model. Program in Atmospheric and Oceanographic Sciences Report,
Princeton University, Princeton, N.J.
Morimoto, A., Mino, Y., Buranapratheprat, A., Kaneda, A., Tong-U-Dom, S.,
Sunthawanic, K., Yu, X., Guo, X. (2021). Hypoxia in the Upper Gulf of Thailand:
Hydrographic observations and modeling. Journal of Oceanography 77, 859 -811.
https://doi.org/10.1007/s10872-021-00616-3