2.2.2. Scarcity

As for the PDI, the first step was the selection and treatment of the variable, or sets of variables, in each basin, that were considered the most representative of the availability of the water resources required to satisfy the different water demands. This selection was made considering the characterization and location of the most significant water demands and also the location of the water abstraction sources to meet those demands, namely, those related to urban water supply, agriculture, industry and tourism. In the Spanish part, the main water demands considered were relative to urban water supply and agriculture. In the Portuguese part of the Lima and Minho river basins, all demands (including agricultural) were not relevant compared to urban water supply, thereby only urban water demands were considered. In the two basins, although water is captured from surface and groundwater bodies, the main sources are surface water bodies.

In the Portuguese part of the Lima river, inflow to the Touvedo dam, located on the Lima river (Figure 5) was chosen as the variable representative of water availability. The Touvedo reservoir serves as the water abstraction source for the urban supply of a large cluster, including the main counties of the Lima river basin, namely Arcos de Valdevez, Ponte da Barca, Ponte de Lima, Viana do Castelo, as well for two counties of the Minho river basin, Caminha and Vila Nova de Cerveira.

**Figure 5.** Counties of the Minho and Lima river basins and locations of the Touvedo dam (Lima) and hydrometric station N015—Rio Miño en Salvaterra do Miño (Minho).

As the Touvedo reservoir was only built in 1993, for the Lima river basin the SI analysis was carried out for the period from October 1993 to September 2017.

In the Minho river basin, the most important water abstraction source to urban supply is the Troporiz abstraction, which supplies Melgaço, Monção and Valença counties. Therefore, the variable selected to assess scarcity situations in the Portuguese part was the Spanish hydrometric station "N015–Rio Miño en Salvaterra do Miño", located in the Minho river (Figure 5), just 5 km upstream of the Troporiz abstraction.

In addition to the urban water supply demands, environmental flows associated to the Touvedo dam and to the Minho river abstraction section were also considered. Regarding the Minho river, the stretch where Spanish hydrometric station N015 is located was found to have two different environmental flow regimes definitions once compared of the two countries' definitions.

In fact, Portugal and Spain have adopted two different approaches in the development of the RBMPs, namely for the shared river basins parts: Spain defined environmental flows for all river stretches, while Portugal defined environmental flow regimes only for "heavily modified" water bodies located downstream of reservoirs [27,31–33]. Each country also uses different methods to define environmental flows, which, in addition to the different approaches mentioned above, could lead (namely as referred for Minho river) to a singular transboundary river body having two different environmental flow regime definitions.

In this study, the environmental flow regime considered was the one defined in the Spanish RBMP for the Minho river stretch where the water abstraction is located, since it is larger and thus consequently more challenging to satisfy in situations of scarcity.

The different scarcity situations considered were: Normality (absence of scarcity), Pre-alert (moderate scarcity), Alert (severe scarcity) and Emergency (serious scarcity). The corresponding monthly thresholds were defined for each of the months of the year based on the urban water supply demands considered for both territorial units and also considering the corresponding environmental flow regime, as follows:


Once the scarcity thresholds were defined, the variables were rescaled into dimensionless values between 0 and 1, based on the following reference values:


Since, in this case, only one variable was chosen to characterize scarcity situations (i.e., weighting factor equal to one) for each of the two basins, the rescaling of the variable resulted in the definition of the SI, to which the different situation range limits corresponded: values between 1 and 0.5 represented a situation of normality; values between 0.5 and 0.3 corresponded to a Pre-Alert situation; values between 0.3 and 0.15 represented an Alert situation; and values below 0.15 corresponded to an Emergency situation.

### **3. Results and Discussion**
