*2.1. Hydrological Model Background: Description and Setup*

Drought, under a hydrological viewpoint, as usual, indicates below-normal levels of flow from lakes, streams, and reservoirs or groundwater with generally accepted indicators, such as the standardized runoff index (SRI), the surface water supply index (SWSI), the groundwater resources index (GRI), among others [4]. Precipitation patterns and changes in the precipitation–runoff relationship as essential variables to define drought which allows the assessment and simplification of climate change impact on urban water availability [3].

The study used precipitation pattern analysis to gain insight into rainfall influence over reservoirs' water availability. Simulations of the water volume at each reservoir applied HBV, an integrated hydrological modeling system, developed at the Swedish Meteorological Hydrological Institute. The model relies on three different reservoir modules: one that simulates the behavior of the soil; the second, the upper reservoir and, finally, the lower reservoir that accounts for the groundwater base flow [18].

Some researchers have highlighted [19,20] how the HBV model is accurate, reproducing present and future water processes in the Llobregat basin. Thus, the HBV model is suitable to simulate the reservoirs' contributions over the Llobregat and Ter's basin. The model requires the physical properties of the basin as well as the climatic inputs, including precipitation, temperature, and potential evapotranspiration. The time scale for the input data was day-to-day. Detailed discussion on the hydrological model's internal functioning falls outside the scope of this paper.

Using the Thornthwaite formula (ETPraw) gave us an estimation of potential evapotranspiration. Then, we applied a correction to get a better adjustment to the results obtained according to two parameters, using the Penman evapotranspiration as a reference. The evapotranspiration calibration process with two meteorological stations data in Llobregat and Ter basins computed Penman evapotranspiration (ETP) equation according to data availability.
