MEKONG RIVER COMMISION SECRETARIAT PRELIMINARY REPORT ON LOW WATER LEVEL CONDITIONS IN THE MEKONG MAINSTREAM

The report provides information on current situation of extremely low water levels on the Mekong upstream at Stung Treng and in northern Lao PDR and Thailand.


Introduction
This preliminary report has been prepared to provide information with regard to the current situation of extremely low water levels on the Mekong upstream at Stung Treng and particularly in northern Lao PDR and Thailand. The analysis is based upon the data currently available which are compared to historical hydrological and meteorological conditions. Weekly reports of water levels at mainstream water level monitoring stations are updated every Monday at http://ffw.mrcmekong.org/ Figure 1 shows the rainfall data obtained from NOAA at Jinhong, Lincang, Simao and Lancang stations in the Upper Mekong. Figure 2 shows the comparison between the observed monthly rainfall from January to December, 2009 and the long-term monthly average (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) at each site.

Rainfall in the Upper Mekong River (Lancang)
There is a consistent pattern of below average rainfall in each month between August and December. The January 2010 rainfall was also below average.   Figure 3 shows the mainstream hydro-meteorological stations in northern Lao PDR and Thailand. Data at the four rainfall stations at Chiang Saen, Luang Prabang, Chiang Rai and Vientiane were selected for analysis.

Fig. 3 Mainstream hydro-meteorological stations in northern Lao PDR and Thailand
The graphs in Figure 4 show a comparison of the monthly rainfalls observed between July and December, 2009 with the long-term monthly average over the last 50 years ). In the upper LMB, the rainfall from July 2009 to February 2010 is comparable to the long-term average. However, from September onwards rainfall in this northern region of the Basin was considerably less than normal. Rainfall to date in 2010 has been minimal as it expected at this time of the year.
From these preliminary rainfall data, the indications are that the 2009 SW monsoon ended early. The average date for monsoon withdrawal at Chiang Saen is the first week of November and at Vientiane the first week of October. The early withdrawal of the monsoon in 2009 meant that the discharges on the Mekong and its northern tributaries started to recede early in the season, drawing on what natural catchment and groundwater resources there were. Natural and groundwater storage in the northern parts of the basin are not large so a deficit situation would have arisen relatively quickly, particularly on the large tributaries in northern Lao PDR, leading to considerably reduced flow contributions to the mainstream.

The 2009 flood season
The general weakness of the 2009 SW monsoon meant that flows during the flood season were well below normal, particularly in these northern parts of the Mekong Basin ( Figure 5). The peak and the total volume of the 2009 flood at Vientiane, for example, were the 5 th lowest over the last 98 years. Thus the natural catchment storage in northern Lao PDR, in particular, would be expected to be significantly below normal at the end of the wet season with the follow-on effect that the subsequent dry season flows would also be below the seasonal average.   At Chiang Saen water levels up to the end of January 2010 were above those of January 1993. These higher levels appear to be a result of releases from dam operations upstream. Between 24 January and 23 February 2010, the levels then fell by 1 metre, which is equivalent to a decrease in discharge in the order of 250 cubic metres per second. This reduction over a period of 3 to 4 weeks is steeper than in previous years and may be explained by drought conditions upstream, meaning that flow releases through the hydropower operations that had been evident earlier in This decrease in upstream water levels is reflected at Luang Prabang and Vientiane from late January onwards. Clearly the contributions from the large northern Lao tributaries such as the Nam Ou and Nam Khan were already low due to the drought conditions and observations confirm that these rivers are currently very low.
Further downstream at Kratie the decrease in water levels during February remains quite apparent with those of 2010 being half a metre higher than those of 1993 during late February. The key feature is that water levels at Chiang Saen from November 2009 onwards were higher than those in 1992-93. At Luang Prabang and Vientiane, the opposite is the case. This suggests that the water levels at Chiang Saen were kept artificially high by upstream reservoir releases until late January when they receded. The levels at Luang Prabang and Vientiane being lower than 1992-93 reflect the regional drought conditions from September 2009 onwards and the very low contributions to the mainstream by the large tributaries in northern Lao PDR. The situation represents serious regional hydrological drought conditions. 1/1/09 to 23/2/10 1/1/92 to 23/2/93 average Figure 8 confirms the severity of the regional drought conditions provided by an analysis of the flows so far in 2010 on two large Mekong tributaries the Nam Ou and Nam Khan in northern Lao PDR. Discharges on the Nam Ou are amongst the lowest recorded, while those on the Nam Khan are unprecedented, falling well below anything observed over the last 50 or so years.

Preliminary Conclusions
The main causes of low water levels being experienced in the 2010 dry season in the Mekong mainstream are a combination of an early end to the 2009 wet season, low monsoon rainfall and very low rainfall in the dry season which together have led to regional drought conditions. Based on the available information it appears that flows from tributary rivers in Lao PDR and northern Thailand are at levels that are amongst the lowest recorded in recent decades. This situation represents a regional hydrological drought affecting all countries in the Basin. The levels are expected to drop further before rising slowly in mid to late April. The higher than natural levels in the Mekong River experienced at Chiang Saen in early to mid-January resulted from hydropower operations upstream. These levels then reduced to to levels closer to those of the usual conditions in late January as reservoir storage levels upstream fell in response to the drought. Further analyses and discussion with China are planned.