Abstract
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 medium-term 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 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.
Highlights
In recent years, nearly one-quarter of all damages and losses in the agricultural sector in developing countries are caused by climate-related disasters such as floods and droughts (FAO, 2015)
Potential evapotranspiration can contribute to drought occurrence frequency, intensity, and severity (Sheffield and Wood, 2008b; Dai, 2011; Sheffield et al, 2012), this leads to the proposition of Standardized Precipitation Evapotranspiration Index (SPEI) index, which can incorporate the effect of potential evapotranspiration on drought characteristics (Vicente-Serrano et al, 2010)
To understand the possible reason why SPEI-3NEM exceeds SPEI-4SWM in its contribution to the interdecadal variation of drought characteristics in the dry region, while in the wet region SPEI-4SWM dominates the interdecadal variation, we present the temporal variation of total rainfall anomalies for Southwest Monsoon (SWM) and Northeast monsoon (NEM) seasons in the wet and dry regions (Figures 8A–D)
Summary
Nearly one-quarter of all damages and losses in the agricultural sector in developing countries are caused by climate-related disasters such as floods and droughts (FAO, 2015). Interdecadal Change of Drought Characteristics the associated water scarcity (Sheffield et al, 2012), an increase in the risk of wildfires (Littell et al, 2016), intensified land degradation, and desertification are some of the significant issues associated with prolonged drought in a different part of the world. Identification of drought characteristics such as intensity, magnitude, duration, and spatial extent, and quantifying their related mechanisms are challenging tasks among the climate community, because the drought is associated with complex interactions amongst the climate system, including atmospheric processes, land-based processes such as precipitation, evaporation, runoff, and ocean processes (Dai, 2011; Vicente-Serrano et al, 2016; Spinoni et al, 2017)
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