Following the significant flood conditions of 2011, the situation during 2012 proved to be quite the opposite. The flood peak and volume were between 30 and 40% below the long term average. The flood season was a month shorter than usual. All in all 2012 turned out to be one of the most deficient annual floods of the last 25 years, matching the situation during 1992, 1998 and 2010.
The deficient hydrological conditions reflected a meteorologically dry year overall. Geographically, rainfall was variable during the monsoon season with only limited areas of the Basin recording monthly totals of any significance. In general total precipitation was well below average, in places by as much as 40%. Locally, however, rainfall was close to average, due largely to mesoscale storm events that are events which extend over no more than 1,000 km2 . The onset of the SW Monsoon took place during late April / early May as is normal but ended up to a month early in mid to late September over the greater part of the region. This early ended to the Monsoon combined with low seasonal rainfall in general provided the combination that has defined some of the lowest annual floods over the last two decades.
Maximum water levels across the Cambodian floodplain and the Delta reflected the hydrological conditions further upstream and were 1m and more less than the long term average. On the Tonle Sap at Prek Kdam the maximum flood level during the year was the fourth lowest that has been observed since 1960. This would lead to the depth and areal extent of the Great Lake that were at low levels.
Such annual flood deficits are not, however, uncommon. The question that arises, though, is whether they are becoming more common. A parallel query might be whether the inter-annual variability of the flood regime is increasing in any significant way. During the last three years, 2010 was exceptionally dry, 2011 exceptionally wet and 2012 exceptionally dry again. Such a pattern is unusual. Wet years tend to follow wet years and drier conditions tend to replicate themselves according to a semi-periodic pattern. These aspects are considered here.
There is a growing body of evidence that the Chinese hydropower cascade on the mainstream in Yunnan is having an impact the downstream low flow regime. This shows itself in “spikes” or spates of flow at Chiang Saen which are difficult to explain since between there and the upstream dams there are no large tributaries that would generate such events naturally. These spates are still evident at Vientiane, but dissipate further downstream.
All in all, 2012 was a “dry year” and the flood was comparable to events over the last 20 years that were classified as “extreme”, in the sense of being much below average.
The theme of the Report is “Flash Floods” and particularly their relation with Tributary and Mainstream Floods. The distinction between these and riverine floods is emphasized, while various impacts upon their incidence and severity, such as land use changes and climate change, is discussed. It is emphasized that given their very fast response times, forecasting is difficult. The strategy adopted in the Lower Mekong Basin has been to develop a Flash Flood Guidance System, which maps the areas at risk.