2.2.1. Prolonged Drought

In the Portuguese part of the Minho and Lima river basins, as considered in the Spanish part of the basins, two variables were considered in the definition of the PDI: (i) average monthly precipitation in the whole basin; and (ii) natural monthly runoff in one hydrometric station of the river basin hydrographic network.

Precipitation and runoff data recorded at stations of the Portuguese National Water Resources Information System (SNIRH in Portuguese acronym) monitoring network were used.

The monthly average precipitation in each Portuguese basin region was calculated through the Thiessen method, using the available data of the existing meteorological stations.

The hydrometric station chosen to represent the natural monthly runoff in the Portuguese part of the Lima river basin was "Pontilhão de Celeiros", located on Vez river (tributary at the right bank of the Lima river), its section having a sub-basin area of about 170 km2. For the Minho hydrographic basin, the selected hydrometric station was "Segude", which corresponds to a sub-basin with an area of nearly 130 km<sup>2</sup> located on the Mouro river (tributary at the left bank of the Minho river) (Figure 4). To fill the missing data in the hydrometric stations, hydrological modeling using HEC-HMS software in a continuous simulation mode based on daily precipitation was performed.

**Figure 4.** Relative location of the sub-basins at the sections of the hydrometric stations of "Pontilhão de Celeiros" (in the Lima river basin) and "Segude" (in the Minho river basin).

The two required variables (precipitation and natural runoff) were computed through the accumulated monthly records from the previous 12 months. Based on these, the two variables were transformed and translated into standardized values, namely, the Standardized Precipitation Index (SPI12) for the variable precipitation, and the Standardized Runoff Index (SRI12) for the variable natural runoff. For this purpose, the SPI and SRI were calculated by adjusting the precipitation and runoff data, respectively, to a gamma distribution [30].

After the selection and treatment of the variables, the SPI and SRI values were rescaled into dimensionless values ranging from 0 to 1. This rescaling process was achieved based on the following monthly reference values for each of the 12 months of the year:


After rescaling, the two variables were aggregated, in a weighted way (considering 60% for the SPI and 40% for the SRI), resulting in the PDI. The referred threshold and the weighing values were the same used by Spain in the 2018 PES-MS [20], defined considering the comparison with the historical drought events in the Spanish part of the two basins. In accordance, values below 0.3 indicate the existence of a prolonged drought situation, whereas values above 0.3 indicate a situation of normality.
