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Annual Mekong Hydrology, Flood and Drought Report 2019: Drought in the Lower Mekong Basin, December 2021

Countries
Cambodia
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Sources
Mekong River Commission
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1. EXECUTIVE SUMMARY

The year 2019 will go down in history as extreme in various ways:

  • Unprecedented flow conditions were recorded in the Mekong mainstream.

  • Baseflow from the tributaries was negligibly low.

  • Erratic climatic fluctuations were observed ranging from extreme dry to extreme wetwithin short periods of time.

The year gives evidence of what is attributable to climate change: high climatic fluctuations, a shift towards more extreme events within short periods of time and a considerable departure from historic observations. All is in line with predicted effects of climate change. For example, there were heat waves in Viet Nam, with a slow onset of the rainy season followed by extreme rainfall intensities. Impediments for water management are on the rise, and time-proven habitsare seriously challenged. However, not all can be attributed to climate change; there are alsoongoing, increasingly visible changes that cannot be explained by it.

Unprecedented flow conditions in the Mekong mainstream were not only caused by a lack of rain, but also by reservoir operation. The flow records at Chiang Saen unmistakably reveal the impact of upstream dam releases, investing the dry and wet season. In contrast, Luang Prabang is affected by backwater conditions from a dam downstream, and the stage-discharge curve becomes unusable and overestimates flow.

The baseflow in the Mekong mainstream was at unprecedented low levels. Low upstream flow rates affect conditions further downstream and lead to insufficient baseflow. However, the low baseflow cannot be explained solely by dam operation in Mekong mainstream. Indeed, the baseflow was exacerbated by two other factors: an anomaly of rain in the tributaries that should be the water tower of the Lower Mekong Basin ( LMB); and dams in the tributaries. While an anomaly of rain is visible from the observations, there was a lack of data on impact of dams in the tributaries. Provided that dam development continues in the LMB, it is very likely that dam releases will affect the tributaries and thus flow in the Mekong mainstream, above all during a dry spell.

Rainfall anomaly and flow volume anomaly along mainstream Mekong are illustrated below; the comparison helps to better understand their interrelationship. The flow volume anomaly shows surplus (right) and benefit (left) in horizontal bars in billion cubic metres (BCM). The rainfall anomaly shows the differences in mm.

A particular dry situation is not obvious until April. Drier conditions prevail in the north from May onwards reaching the climax in August and continue in September. The south did not significantly tend to be dry. Higher rainfall causing the extreme but short peak at Pakse and downstream are visible in August and September.

The year did not start with dry conditions. On the contrary, releases from the dams brought above-average flow volume. This changed as of June, and a deficit accumulated with a short interruption in September. In conclusion, the year 2019 signals the need to consider the Mekong mainstream as a key element for cooperation given the upstream-downstream dependencies. Moreover, a closer look into the tributaries is required as well to foresee detrimental effects and to launch countermeasures.

A seasonal river flow forecast is central to the ability of water managers to take precautionary measures in time. However, the year 2019 revealed that a prediction of rainfall, soil moisture and flow runoff alone are not sufficient for an adequate river flow forecast. Dam operation becomes a crucial component and has the potential to render river flow forecasts useless if ignored. Data exchange between forecasters and dam operators is therefore essential.

Cooperation between the countries within the LMB is required, but it should also embrace China as a key player. Transboundary cooperation will undoubtedly intensify in order to allocate water resources wisely.Concerning economic consequences arising from a dry spell, an analysis of drought impact on the agricultural sector was carried out, and crop water requirements and relative crop yields were evaluated. The relative crop yield or the inverse, relative crop loss, can be used to estimate the potential economic damage. The higher the losses in production of crops due to adverse conditions, the higher the financial burden. This is illustrated below, in which the estimation of losses for a selection of crops is given. The analysis used climate and soil conditions representative for each location and applied the crop water requirements approach of Food and Agricultural Organization of the United Nations ( FAO) based on the assumption of rainfedconditions. Large red graph areas indicate high potential losses (see section 2.6).