4. Analysis of Drought Character in the Mekong River Basin

Drought is among the most costly natural disasters in the Mekong River Basin (MRB). To understand the spatial and temporal characteristics of drought can largely facilitate scientific drought management and risk mitigation. Using a set of long-term (1901–2016) global monthly precipitation data and nearly 30-year (1985–2016) in situ daily observed streamflow, this chapter investigates the long-term trend and inter-annual variability of meteorological and hydrological droughts in MRB, respectively, through estimations of multi-scale Standardized Precipitation Index (SPI) and Standardized Runoff Index (SRI). Results indicate that while with slight upward trend from basin-scale perspective, the SPI is found with obvious downward trend over the northeastern Thailand, most of Cambodia and Myanmar, suggesting these regions are overall subject to intensified drying during the past half-century. The occurrence frequency of drought is over 25% across much of MRB, particularly for southern Cambodia and Mekong delta where the occurrence of extreme drought is around 10%. The hydrological drought analysis show that the trend of SRI greatly varies by in situ station location over the past 30 years. The 12-month SRI (SRI-12) for the Upper station (Chiang Saen) exhibits obvious decreasing trend, and mostly falls into the negative range, suggesting the recent frequent below-normal streamflow in Chiang Saen station. The occurrence of hydrological drought in the Middle portion is found with downward trend, as indicated with the long-term increasing SRI-12 at NongKhai and Mukdahan stations. When it comes to the downstream station (Stung Treng), while the trend of SRI seems little change, the SRI-12 value is mostly below zero since 2005, suggesting that this portion is susceptible to hydrological droughts recently. These analysis provide us an overview insight into changes in meteorological and hydrological droughts, and advance our understanding on drought variations and its long-term trend in MRB.