Abstract
This study investigated the temporal occurrence of dry conditions in the seven climatic sub-regions of Vietnam during the 1980–2017 period. This assessment was performed using the Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) at 1 to 24 months timescales. Results show that the main periods of extreme drought occurred simultaneously throughout the country in 1992–1993 and 2003–2004, except for 2015–2016, when it was not identified in the southern region. In addition, a slight temporal lag was identified latitudinally (north–south) at the beginning of dry conditions, revealing the largest difference between the northern and southern regions. A positive trend in the time series of both indices (SPEI and SPI) prevailed in all sub-regions, with the SPEI minus SPI difference always being negative, suggesting the importance of temperature and evapotranspiration for this trend. Further detailed analyses were then performed using SPEI at 1-month and 12-months timescales for all climate sub-regions, as well as the main indicators to characterize duration and severity. Results show that the number of drought episodes did not vary much between regions, but they did vary in duration and severity at the annual scale. Moreover, changes in the soil root zone are largely associated with dry and wet conditions not only from season to season, but also in longer accumulation periods and more strongly in the northern regions of Vietnam. Indeed, a study of the most severe drought episodes also revealed the occurrence of negative anomalies of the root-soil moisture in the subsequent four or more months. Dynamic atmospheric conditions associated with the peak of most severe drought episodes show the crucial role of subsidence of dry air in the middle and high atmosphere, which prevents convection in the lower troposphere. Finally, the linkages between drought conditions in Vietnam and large-scale atmospheric and oceanic teleconnection patterns were revealed to be quite different among northern and southern sub-regions. During the positive phase of El Niño–Southern Oscillation (ENSO), drought episodes at different timescales were identified in the southern climate sub-regions, while the negative phase was associated with drought conditions in the northern regions.
Highlights
Droughts are part of the natural climate system and can be defined as a complex phenomenon that occurs when water availability is significantly less than regular levels for an extended period and the available water cannot satisfy the demand [1,2,3,4]
The Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) at timescales from 1 to 24 months were used to investigate the temporal incidence of extreme dry conditions in the seven climate sub-regions of Vietnam during the 1980–2017 period
Results showed that the main periods of drought occurred simultaneously throughout the country in 1992–1993 and 2003–2004, with the exception of 2015–2016, which was not identified for the southern part of the country
Summary
Droughts are part of the natural climate system and can be defined as a complex phenomenon that occurs when water availability is significantly less than regular levels for an extended period and the available water cannot satisfy the demand [1,2,3,4]. The temporal and spatial variability of droughts as well as the large number of systems that it can affect make this phenomenon difficult to quantify. Meteorological droughts are defined as below-average amounts of precipitation for a certain period of time, and may be combined with increased potential evapotranspiration (PET) [16,17,18]. Efficient monitoring of meteorological droughts is necessary in order to provide early warnings and perform risk management in regions with limited water resources or high agricultural production dependent on irrigation
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