*3.1. Prolonged Drought*

By applying the methodology presented in Section 2.2.1, the PDI was calculated for the Portuguese part of the Minho and Lima river basins for the period from October 1980 to September 2017. The corresponding time evolution is depicted in Figure 6. In this figure, for each basin (Lima and Minho, respectively in Figure 6a,b), in addition to the evolution of the PDI in the Portuguese part, the indicator for the Spanish part of the basins is also presented.

**Figure 6.** Prolonged Drought Indicator (PDI) for the period October 1980–September 2017 in (**a**) the Portuguese part (Lima) and the Spanish part (Limia) of the Lima river basin; and (**b**) in the Portuguese part (Minho) and the Spanish part (Miño-Bajo) of the Minho river basin.

Table 1 highlights the periods of months in which the situation of prolonged drought was signaled in each part of the two basins, as well as the total number of months of prolonged drought between October 1980 and September 2017.

From Table 1, it immediately follows that more months signaling prolonged drought occurred in the Spanish parts of the Minho and Lima river basins than in the respective Portuguese parts.

In general terms, it can be said that, except for the lengthy period of 16 months elapsed between 2004 and 2006, the Portuguese parts of the Minho and Lima river basins were marked by the occurrence of relatively short-term drought events (1 to 6 months), whose frequency of occurrence increased in recent years.


**Table 1.** Prolonged drought events in the Portuguese and Spanish parts of the Lima and Minho river basins (period October 1980 to September 2017).

In the Portuguese parts of the Minho and Lima river basins, the PDI values (Figure 6) show that the most severe drought episode in intensity and duration occurred between late 2004 and the beginning of 2006.

In the Spanish part of both basins, two large prolonged drought events were signaled, namely the one that occurred between the end of 2004 and the beginning of 2006 and the one that occurred between the end of 2011 and the end of 2012. In the Lima river basin (Limia), the first (2004–2006) episode was the longest, lasting 16 months. However, the later (2011–2012) event was the one with the greatest intensity. In the Minho river basin, the two events presented similar intensities and durations.

In general, despite differences in intensity and duration, there is correspondence between the periods signaled as suffering prolonged drought in the Portuguese and Spanish parts of both basins between 1980/1981 and 2016/2017.

By the observation of Figure 6, it can also be inferred that the major drought events, namely those occurring in 2004/2005–2005/2006, 2011/2012 and 2016/2017, affected both international basins. It should be noted that, in the most recent occurrence, the duration and intensity of the drought events were similar in both parts of the Minho and Lima river basins.
