Characterization of Droughts in Iberian Peninsula
Different studies have analyzed the occurrence of droughts in the IP over the centuries using dendrochronological (Tejedor et al. 2015), documentary (Martfn-Vide and Barriendos 1995; Vicente-Serrano and Cuadrat 2007a), and geological (Corella et al. 2016) records. These studies show a high incidence of droughts, providing further evidence that drought is a general climate feature of the region. For instance, Domfnguez-Castro et al. (2012) analyzed drought occurrence using ecclesiastical records (pro-pluvia rogations) obtained from 17 archives during the eighteenth and nineteenth centuries. They not only found that droughts were more frequent in some decades (e.g., 1750s, 1780s, and 1820s) but also showed important spatial differences in the occurrence and severity of drought events. The consequences of droughts in the preindustrial period, with an economy based on agriculture and livestock, were devastating, with frequent famine and mortality episodes associated with dry conditions (Cuadrat et al. 2016).
The high frequency of droughts is also identified in the instrumental records available from the second half of the nineteenth century. A regional series for the entire IP based on the 12-month Standardized Precipitation Index (SPI) since 1901 shows different major episodes (Figure 23.1). The decades beginning in 1910, 1920, and 1930 showed low-severity drought periods. In contrast, after 1940 the variability of the SPI increased noticeably.
Evolution of the 12-month standardized precipitation index for the entire Iberian Peninsula between 1901 and 2015.
The decades beginning in 1940 and 1950 were dominantly dry and the 1960s and 1970s were dominantly humid. Two of the most extreme drought events since the beginning of the twentieth century were in 1945-1946 and 1949-1950. After 1980 there is a dominance of dry periods in the series. Thus, 1981-1984 and 1992-1995 were two periods affected by long and severe droughts, and the driest year of the series was recorded in 2005.
Various studies have shown that spatial variability of droughts in the IP can be very important, even at the regional scale (Vicente-Serrano and Cuadrat 2007b; Vicente-Serrano et al. 2004; Vicente-Serrano and Lopez- Moreno 2006). It is not uncommon to find a region in drought conditions while other areas of the IP show normal or even humid conditions. Thus, few historical episodes have affected more than 75 percent of the total surface area of the IP (Vicente-Serrano 2006a). Using climate drought indices and multivariate techniques, it is possible to identify some homogeneous regions in which the temporal variability of the droughts has been similar. In the IP, six main regions that demonstrated a particular temporal evolution in the drought occurrence were identified (Vicente-Serrano 2006b).
The strong spatial variability of droughts in the IP is mostly a consequence of the different atmospheric circulation patterns that control the precipitation of this region (Rodriguez-Puebla et al. 1998). Atmospheric circulation anomalies during winter and spring usually determine water availability conditions some months in advance (Lorenzo-Lacruz et al. 2011; Vicente-Serrano et al. 2016). The North Atlantic oscillation (NAO) is the atmospheric mechanism that mostly controls the interannual variability of precipitation during the cold season in the IP (Trigo et al. 2002), causing important agricultural, hydrological, and socioeconomic impacts (Vicente-Serrano and Trigo 2011). Severe drought periods as observed in the 1980s and 1990s have been associated with positive phases of the NAO. Nevertheless, the problem is much more complex, because although large areas of the IP show stronger precipitation by the NAO, other regions show the influence of other atmospheric circulation patterns (Martin-Vide and Lopez-Bustins 2006; Vicente-Serrano et al. 2009). Even so, an explanation of the main drought episodes is not possible with simple approaches based on atmospheric circulation indices since atmospheric conditions that trigger a drought episode may strongly vary from one episode to another (Garcia- Herrera et al. 2007; Trigo et al. 2013).
Possible trends in drought frequency and severity worldwide and in Europe have recently been discussed (Trenberth et al. 2014). Studies based on precipitation data have suggested a reinforcement of drought conditions in the Mediterranean area (Hoerling et al. 2012). Nevertheless, the strong spatial variability of droughts makes it difficult to establish a general pattern of drought trends. The SPI series since the beginning of the twentieth century shows a significant increase in the severity and duration of droughts in the southwest and northeast IP (Vicente-Serrano 2013). In other areas (e.g., northwest, southeast, and central regions), the trend is toward lower magnitude and duration of the drought episodes. As a consequence of the strong temporal variability of the index, the average SPI series for the IP from 1901 to 2015, pictured in Figure 23.1, does not show a significant trend toward more negative SPI values. Nevertheless, studies based on objective hydrological drought metrics (e.g., streamflows) show a noticeable increase in the frequency and severity of drought events in the past decades. Lorenzo-Lacruz et al. (2013a) analyzed the evolution of hydrological droughts in the IP between 1945 and 2005 and showed a clear increment of hydrological droughts in most of the basins of the south and east IP. This is also observed, although with less severity, in other basins of the northeast and the northwest.
It is difficult to establish a distinction between climatic and anthropogenic drivers of hydrological drought severity. Current management and use of water has affected the duration and severity of hydrological droughts. Thus, water management significantly alters the relationship between climatic and hydrological droughts, both in the magnitude of the relationship and in the timescale of the response (Lopez-Moreno et al. 2013; Lorenzo-Lacruz et al. 2013b). Water management may also alter drought duration and severity. Lopez-moreno et al. (2009) showed how the development of large reservoirs on the Spanish-Portuguese boundary has resulted in an intensification of the water scarcity episodes downstream from the dams in comparison to upstream sectors. In addition, increased water demands by irrigation may cause an important accentuation of the severity of the hydrological droughts downstream from the reservoirs and irrigation polygons. Recently, Vicente-Serrano et al. (2017) analyzed the evolution of climatic and hydrological drought events in the Segre basin, a highly regulated basin in northeast Spain, and showed that intensification of irrigated lands has increased the severity of hydrological drought events downstream in comparison to the observed evolution of the climate droughts.
In any case, the recent warming processes identified in the IP have caused a noticeable increase of the atmospheric evaporative demand (AED), with an average of 25 mm decade-1 since 1960 (Vicente-Serrano et al. 2014a). Evidence suggests that this increase has reinforced drought severity in past decades. Vicente-Serrano et al. (2014b) analyzed the possible impact of the AED on drought severity comparing two drought indices: the SPI, based on precipitation, and the Standardized Precipitation Evapotranspiration Index (SPEI), based on the climatic balance between precipitation and the AED (Vicente- Serrano et al. 2010). The results showed a clear influence of the increased AED on the climatic drought severity and also on the current availability of water resources. Thus, although precipitation trends are mostly not significant in the IP, a reinforcement of hydrological drought conditions has been identified in the majority of the Iberian basins, including natural nonregulated basins, which are not influenced by water regulation and consumption, and mostly respond to climate variability (Figure 23.2).
Percentage of surface area affected by streamflow drought from 1961 to 2009, based on the standardized streamflow index for the natural, regulated, and highly regulated basins of the entire Iberian Peninsula.