This study investigates the variation of seasonal streamflow and streamflow extremes in five catchments of the Mahaweli River Basin (MRB) Sri Lanka from 1990 to 2014, and the relationship between streamflow and seasonal rainfall in each catchment is then examined. Furthermore, the influence of Indian Ocean Dipole (IOD) and El Nino and Southern Oscillation (ENSO) on the seasonal rainfall and streamflow in the upper (UMRB) and lower reaches (LMRB) of MRB are explored. It’s found that the rainfall amount in southwest monsoon (SWM) season contributes 29.7% out of annual total rainfall in the UMRB, while the LMRB records 41% of the total rainfall during the northeast monsoon (NEM) season. The maximum streamflow of upper (lower) Mahaweli catchments is observed in the SWM (NEM) season. Catchments in the UMRB (LMRB) recorded strong interannual variability of seasonal overall flow (Q50), Maximum 10-day, and 30-day flows during the SWM (NEM) season. It’s further revealed that the catchment streamflow in the UMRB is closely correlated with the SWM rainfall in the interannual time scale, while streamflow of catchments in the LMRB is closely associated with the NEM rainfall. The effects of ENSO and IOD on streamflow are consistent with their impacts on rainfall for all catchments in MRB, with strong seasonal dependent. These suggested that the sea surface temperature anomalies in the both Indian Ocean and tropical Pacific Ocean are important factors affecting the streamflow variability in the MRB, especially during the SWM season.

Understanding the spatiotemporal characteristics of drought at the river basin scale is vital for water resources management. In this study, the interdecadal variation of drought characteristics over the Mahaweli River Basin (MRB) in Sri Lanka was investigated for the 1985-2015 period, using the Standardized Precipitation Evapotranspiration Index (SPEI). Remarkable interdecadal change of yearly drought characteristics between 1985-1999 and 2000-2015 periods in the wet region of MRB can be found for mediumterm and long-term drought, with indications of that more frequent and severer drought events as well as longer duration of total drought months, occurred during 2000-2015 period. Furthermore, interdecadal enhancement of yearly drought in the wet region can be attributed to changes of seasonal drought in Southwest Monsoon (SWM) season, and this is coherent with the interdecadal shift of the SWM rainfall amount from wet to dry situations since the year 2000. However, no significant interdecadal change of drought was found in the intermediate and dry regions of MRB, as well as the short-term drought in the wet region. The interdecadal difference of atmospheric circulation demonstrates that the South Asian monsoon was weakened after 2000, which is accompanied by the weakening of monsoon trough, reduced cross-equatorial flow from the southern hemisphere from wind circulation at 850 hPa, and positive anomalies of geopotential height at 500 hPa over South Asian region. The weakening of South Asian monsoon leads to lesser moisture transport from the northwest Indian Ocean to Sri Lanka, inducing net moisture divergence anomalies in Sri Lanka, and ultimately results in more drought events during SWM season in the region since the year 2000.