*2.3. Framework for Assessing Future Water Resources Allocation*

To define some potential situations, the representation of reservoir water contribution depends on the rainfall of the sub-basins over each dam. Outcomes include average trends of the models for each scenario, hypothetical extreme values, and the quantification of a possible number of times that reservoir systems could encounter warning events. Reservoir volumes relate to the ACA's 1999–2018 historical data and since 2006 climate model forecasting. Therefore, the historical data range was 1999–2005 and projections cover 2006 to 2100. Rainfall time-series projections from 9 distinct climate models (see Section 3.2) were employed to simulate reservoir input volumes' behavior. The outcomes of these nine models were averaged to find a single trend for each system obtaining four trends, two for each reservoir (RCP4.5 and RCP8.5).

#### **3. A Case Study for Barcelona City**

#### *3.1. The Study Area*

The Llobregat and Ter rivers and other small basins (defined as the Ter–Llobregat system) supply the Barcelona metropolitan area with drinking water. Llobregat river provides about 38%, and the Ter river supplies 55% of raw water for water drinking treatment plants for Barcelona [22,23]. The coupled basins' total drainage area is 4957 km2, with a surface elevation variation from almost 2500 m (pre-Pyrenean mountain range) to the sea level, as Figure 1 shows.

A seasonal rainfall variability phenomenon in the two river basins led to water demand-supply fluctuations. Despite this infrastructure, water resource management is complex and involves groundwater extraction from aquifers and seawater desalination in extraordinary drought events [24].

**Figure 1.** Location of the involved reservoirs, rivers, catchments, and morphology of the study area.

Some key aspects of each sub-basin over the Llobregat and Ter river basins (see Figure 2) can be listed as follows:

