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Weekly Dry Season Situation Report for the Mekong River Basin - Prepared at: 19/11/2019, covering the week from 12th to 18th November 2019

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General weather patterns:

From 12th to 18th November 2019, there was no rainfall in the LMB. Based on the weather outlook bulletins and maps issued by the Thailand Meteorology Department (TMD) were used to verify the weather condition in the LMB. They stated that for the next 3 months from November 2019 to January 2020, the total rain of northern, northeastern, eastern and central parts including with that of Bangkok Metropolitan and Vicinity would be about40% below normal condition. Figures 1 & 2 presented the weather map for 12th and 18th November 2019.

General behavior of the Mekong River:

This week from 12 to 18 November 2019, water levels from Chiang Sean were decreased slightly due to the inflow from China, varied from 0.03 m to 0.18 m and still stay below its historical minimum level. However, water level trend at Luang Prabang and Chiang Khan are likely impacted by hydropower dam at Xayaburi, in which they fluctuated, varied from 0.02 m to 0.36 m. Water levels at stations at the middle part of LMB from Vientiane to Pakse decreased and stay below their minimum levels. Follow the same trend of water levels from upstream, stations at Stung Treng, Kratie, Chaktomuk on the Bassac, Phnom Penh Port and Neak Luong were also below minimum levels. For the 2 tidal stations at Tan Chau and Chau Doc, water levels decreased the same trends as minimum levels in November 2019. The actual water levels were stayed below their LTAs.

For stations from Chiang Saen and Luang Prabang

Water levels from 12 to 18 November 2019 at Chiang Sean station were slightly decreased, due to the decreased flow from Jinghong ( as observed during the wet season from June to October). At this station water levels decreased from 0.03 m to 0.18 m. However, at Luang Prabang station, water levels were fluctuated and stay above its LTA. Water levels at this station sometime increased rapidly in 0.66 m, due to the reservoir operation of upstream and downstream at Xayaburi. It was observed that the Luang Prabang stations is likely nominated by hydro power dam operation upstream (tributaries) and downstream (Xayaburi) in which water levels always fluctuated above their LTAs, during the impounding reservoir at Xayaburi from end of October 2018 to May 2019.

For stations from Chiang Khan, Vientiane-Nong Khai and Paksane

Water levels from 12 to 18 November 2019 at Chiang Khan station were likely also nominated by upstream hydropower dam of Xayaburi, which was noted that water levels fluctuated from -0.76 to 0.44 m (see its hydrograph in Annex B). The current observed water levels at Chiang Khan, Vientiane/Nong Khai and Paksane stations are lower than their historical minimum levels.

For stations from Nakhon Phanom to Pakse

Water levels from 12 to 18 November 2019 at Khong Chiam to Pakse stations were continued to slightly decrease, followed the same trend as inflow from upstream. The water levels decreased from 0.01 m to 0.08 m. The current water levels at these stations below their minimum historical levels.

For stations from Stung Treng to Kompong Cham/ Phnom Penh to Koh Khel/Neak Luong

Water levels from 12 to 18 November 2019 at Stung Treng, Kratie and Kompong Cham were continued to decrease, varied from 0.02 m to 0.27 m. The current water levels at Stung Trend, Kratie, Kompong Cham, Chaktomuk Koh Khel, Phnom Penh Port, Neak Luong and Prekdam on the Tonle Sap were below their historical minimum levels, while at (1980-2018).

Tan Chau and Chau Doc

Water levels from 12 to 18 November 2019 at the 2 tidal stations at Tan Chau and Chau Doc were slightly decreased, follows the tidal effect from the sea. Water levels were kept the same trends as minimum levels in November 2019 and the actual water levels were stayed below their LTAs According to the Japan Meteorological Agency (JMA), Sea surface temperature (SST) variability in the tropics Neutral, which has no major impact.

Discussion and Conclusion

From 12 to 18 November 2019, the trend of water levels at Chiang Sean were slightly decreased due to the inflows from Jinghong and no rainfall. Water level at Chiang Sean is relied from inflow from Jinghong Hydropower Station on Lancang and its catchment rainfall. The impact could obviously see the gradually decreasing water level to downstream to Vientiane/Nong Khai. Based on a hydrological phenomenon, the inflow contribution of water from the upstream of Lancang-Mekong in China to the Mekong mainstream is about 16% in total during the Dry season from Nov to May, while 24% in the Wet season (Adamson. 2010). The whole inflow of water into the lower Mekong basin is influenced more by inflow from tributaries and the direct rainfall catchment.

The low inflows from upstream and less rainfall in catchments, resulting water levels from Paksane to Pakse are drastically dropped below their minimum levels. However, from Stung Treng, Kratie and Kompong Cham stations water levels are followed the same trend from upstream which stay below their minimum levels. Based on the Exceedance Recurrence of the Minimum Flow Volumes at Chiang Sean, Chaing Khan and Kratie referred to historical data availability and the flows volume up to October 2019, it showed the current situation at these stations are considered as critical low flows in between 50 to 100 year of return period of low flow condition. Figure 2 showed the Exceedance Recurrence Flow Volume with the table of probability condition of highlighted the low flows condition at Chiang Sean, Chiang Khan and Kratie