Search

We evaluated the response of the Earth land biomes to drought by correlating a drought index with three global indicators of vegetation activity and growth: vegetation indices from satellite imagery, tree-ring growth series, and Aboveground Net Primary Production (ANPP) records. Arid and humid biomes are both affected by drought, and we suggest that the persistence of the water deficit (i.e., the drought time-scale) could be playing a key role in determining the sensitivity of land biomes to drought. We found that arid biomes respond to drought at short time-scales; that is, there is a rapid vegetation reaction as soon as water deficits below normal conditions occur. This may be due to the fact that plant species of arid regions have mechanisms allowing them to rapidly adapt to changing water availability. Humid biomes also respond to drought at short time-scales, but in this case the physiological mechanisms likely differ from those operating in arid biomes, as plants usually have a poor adaptability to water shortage. On the contrary, semiarid and subhumid biomes respond to drought at long time-scales, probably because plants are able to withstand water deficits, but they lack the rapid response of arid biomes to drought. These results are consistent among three vegetation parameters analyzed and across different land biomes, showing that the response of vegetation to drought depends on characteristic drought time-scales for each biome. Understanding the dominant time-scales at which drought most influences vegetation might help assessing the resistance and resilience of vegetation and improving our knowledge of vegetation vulnerability to climate change. [ABSTRACT FROM AUTHOR]

Abstract: The historical region of Fertile Crescent (FC) was recently hit by an intense and prolonged drought episode during the two hydrological years spanning between 2007 and 2009. Here, we characterize the temporal and spatial extents of this extreme drought at the monthly and seasonal scales and perform a first assessment on the associated impact in the hydro-meteorological fields, as well as the consequent influence on vegetation dynamics and cereal productions. This episode corresponds to the driest two-year case for the FC area since 1940, although just slightly drier than the 1998–2000 drought. Precipitation decline was mostly noticeable over Iraq (up to 70%), with the suppression of rainfall particularly acute during the first hydrological year (2007–2008). From the meteorological perspective, winter and transition months were dominated by high pressures that inhibited synoptic activity entering from the eastern Mediterranean and favoured relative north-easterly winds and drier air masses with low convective instability. The impact of the 2007–2009 drought in vegetation was evaluated with Normalized Difference Vegetation Index (NDVI) obtained from VEGETATION instrument. It is shown that large sectors of south-eastern Turkey, eastern Syria, northern Iraq and western Iran present up to six months of persistently stressed vegetation (negative NDVI anomalies) between January and June 2008. During the following dry year (2008–2009) dry areas are restricted to northern Iraq with up to five months of stressed vegetation. Finally we looked at the impacts on cereal production (wheat and barley) in the region and it is shown that the major grain-growing countries in the area (Syria, Iraq and Iran) were significantly affected by this drought, particularly in the year 2008. [Copyright &y& Elsevier]

Dry lands are expected to cover about half of the terrestrial surface in the near future due to climate change. Drought events, which are recurrent over dry lands, are also projected to increase in both frequency and severity. There is a strong need to better monitor droughts over dry regions, and satellite-based indicators such as the Vegetation Health Index (VHI) have been operationally used worldwide in the last two decades. VHI is traditionally defined as the simple average of two components, the Temperature Condition Index (TCI) and the Vegetation Condition Index (VCI) respectively derived from information on the thermal and visible bands. However, the weights of VCI and TCI depend on landcover because of the different contributions of moisture and temperature to the vegetation cycle. By systematically comparing VHI with the Standardised Precipitation-Evapotranspiration Index (SPEI), a multi-scalar drought index, we demonstrate that is possible to disentangle the role played by VCI and TCI on vegetation health. Here we propose a methodology that allows estimating optimal weights for the two components of VHI and we show that VHI is persistently dominated by VCI over dry lands. Results obtained indicate that severe drought episodes over dry lands are better identified when using the proposed methodology. This may be an asset for operational monitoring, paving the way to more efficient social and political responses aiming to mitigate drought impacts. This work is also expected to contribute to the development of optimal sets of VCI and TCI weights that take into consideration expected changes in the land surface based on information from future climate scenarios. • Drought monitoring with VHI is historically estimated as the plain average between the VCI and TCI terms; • It is shown that over global drylands the importance of VCI is larger than TCI; • It is shown that drought events are more likely to be detected when using VHI with larger weights given to VCI. [ABSTRACT FROM AUTHOR]

We evaluated the response of vegetation's photosynthetic activity to drought conditions from 1998 to 2014 over Romania and the Republic of Moldova. The connection between vegetation stress and drought events was assessed by means of a correlation analysis between the monthly Standardized Precipitation Evaporation Index (SPEI), at several time scales, and the Normalized Difference Vegetation Index (NDVI), as well as an assessment of the simultaneous occurrence of extremes in both indices. The analysis of the relationship between drought and vegetation was made for the growing season (from April to October of the entire period), and special attention was devoted to the severe drought event of 2000/2001, considered as the driest since 1961 for the study area. More than three quarters (77%) of the agricultural land exhibits a positive correlation between the two indices. The sensitivity of crop areas to drought is strong, as the impacts were detected from May to October, with a peak in July. On the other hand, forests were found to be less sensitive to drought, as the impacts were limited mostly to July and August. Moreover, vegetation of all land cover classes showed a dependence between the sign of the correlation and the elevation gradient. Roughly 60% (20%) of the study domain shows a concordance of anomalously low vegetation activity with dry conditions of at least 50% (80%) in August. By contrast, a lower value of concordance was observed over the Carpathian Mountains. During the severe drought event of 2000/2001, a decrease in vegetation activity was detected for most of the study area, showing a decrease lasting at least 4 months, between April and October, for more than two thirds (71%) of the study domain. [ABSTRACT FROM AUTHOR]

This work aims at characterizing the exceptional drought that affected Madeira Archipelago (Portugal) during the 2011–2012 hydrological year while including some major impacts but also the main atmospheric circulation mechanism behind the event. Precipitation records from six meteorological stations are used to assess the extreme drought episode by means of a decile classification. The assessment of the drought duration and severity is further corroborated by the Standardized Precipitation Index (SPI) computed for the 3, 6, 12 and 24 months’ time scales which confirmed that the 2012 drought event was one of the events with higher drought intensity at all scales, being classified as extreme at the 6-month time sale (SPI6 < −1.65) from December 2011 until May 2012 on the majority of the meteorological stations analysed. Long-term precipitation data available since 1865 for the station of Funchal confirm the exceptional dryness of this episode, particularly during the winter season (December to March) corresponding to the driest winter in 150 years. Vegetation activity is assessed through anomalies of NDVI (Normalized Difference Vegetation Index), confirming several large sectors of Madeira under vegetative stress. The southern sector of Madeira Island suffered up to seven months (out of nine) of extremely negative anomalies. From an operational point of view, obtained results reveal the ability of the developed methodology to monitor vegetation stress and droughts in Madeira. The extreme dryness of the Island favoured an unusually intense summer fire season of 2012 (between June and September) in Madeira being the year with highest number of fires in the last decade (with robust data). Furthermore, the main fire hotspots of the 2012 fire season are mostly coincident with the areas affected by drought. The large-scale atmospheric circulation responsible for the setting and intensification of the drought is evaluated using reanalysis data. An extensive high pressure anomaly (maximum above 12 hPa in December) persisted over the North Atlantic during the extended winter months (October to April), centred between the Azores Islands and the UK. This feature is in agreement with a positive NAO index (2.25 in December), a negative EA index (−1.76 in January and −1.73 in February) and compatible with enhanced north-easterly trade winds over the region. As a consequence of this uncharacteristic dynamics there was a deficit of moisture availability over the region as evaluated by the vertically integrated horizontal water vapour transport with a negative anomaly up to −120 kg m −1 s −1 . [ABSTRACT FROM AUTHOR]