Palaeoclimatological perspective on river basin hydrometeorology: case of the Mekong Basin

Globally, there have been many extreme weather events in recent decades. A challenge has been to deter- mine whether these extreme weather events have increased in number and intensity compared to the past. This challenge is made more difficult due to the lack of long-term instru- mental data, particularly in terms of river discharge, in many regions including Southeast Asia. Thus our main aim in this paper is to develop a river basin scale approach for assessing interannual hydrometeorological and discharge variability on long, palaeological, time scales. For the development of the basin-wide approach, we used the Mekong River basin as a case study area, although the approach is also intended to be applicable to other basins. Firstly, we derived a basin-wide Palmer Drought Severity Index (PDSI) from the Monsoon Asia Drought Atlas (MADA). Secondly, we compared the basin-wide PDSI with measured discharge to validate our ap- proach. Thirdly, we used basin-wide PDSI to analyse the hy- drometeorology and discharge of the case study area over the study period of 1300-2005. For the discharge-MADA com- parison and hydrometeorological analyses, we used methods such as linear correlations, smoothing, moving window vari- ances, Levene type tests for variances, and wavelet analyses. We found that the developed basin-wide approach based on MADA can be used for assessing long-term average condi- tions and interannual variability for river basin hydrometeo- rology and discharge. It provides a tool for studying interan- nual discharge variability on a palaeological time scale, and therefore the approach contributes to a better understanding of discharge variability during the most recent decades. Our case study revealed that the Mekong has experienced excep- tional levels of interannual variability during the post-1950 period, which could not be observed in any other part of the study period. The increased variability was found to be at least partly associated with increased El Ni ˜ no Southern Oscillation (ENSO) activity.