**1. Introduction**

Climate change estimates to affect all spheres of human activity in the natural environment, including water resources. Defined as a shortage in rainfall over an extended period, a season or more, drought affects both human activities and the environmental balance [1]. A significant proportion of the human population is currently experiencing restrictions on access to drinking water due to drought events, a vulnerable component of the natural and social action chains [2].

Increases in drought events' frequency and severity are forecast under the impact of climate change [3], examples include events in China (1991–1996), East Africa (2010–2011), United States of America (2011–2012), Australia (2013–2016), and Catalonia, Spain (2006–2008) [4]. Water resource availability shortage threatens urban areas due to factors such as rapid urbanization, increased water use, lack of climate change adaptation policies, and repeated drought events [5]; drought has historically affected 35% of the population hit by natural hazards [1].

Globally, the drinking water requirement for cities has increased due to rapid population growth in cities, pollution of water sources, stress on groundwater sources, and the impact of extreme weather conditions [5]. Researchers concur that drought events will be intense due to lower rainfall and higher evapotranspiration in some areas of Europe, above all impacting Spain [6,7]. Therefore, understanding the complexity of drought events is essential for the city of Barcelona in facing the next century, associated with rainfall shortage. However, drought is a slow process of shortage accumulation, and the sharpness of drought episodes is not only related to current rainfall, but much more related to the cumulative impact of previous hydrological balances.

One of the criteria for proper reservoir management as part of a water supply system is the rigorous design and implementation of the guidelines of reservoir operation along with environmental planning, allowing management to have the tools to cope with growing climate change influence on water scarcity [8]. Reservoir operation is "a large-scale multi-objective optimization problem" [9]. Therefore, this paper contributes to the understanding of the hydrological process in the Barcelona water supply reservoir system under climate change influence, as one of the factors involved in this process.

Barcelona and its metropolitan area dependent on the Ter and Llobregat reservoir system to provide the water demand throughout the year. According to the Catalonian river basin management plan [10], a document from Catalan Water Agency (ACA, Catalonian water resources administrator), Barcelona uses mainly Llobregat river water (38%), and Ter river water (55%), while for the remaining 7%–8% it employs groundwater. Both basins have their higher part controlled by reservoirs which modulate the required water resources. Barcelona is far from these reservoirs, but the drought situation depends on their stored water volumes.

When these volumes are lower than threshold levels (less than 30% of water stocks in the reservoirs) as set by the Drought Plan from the ACA [11], a drought contingency triggers concerning water use restrictions for activities such as irrigation, leisure, industrial purposes, etc., as happened in 2007 and 2008. Beyond environmental and social impacts generated by Barcelona 2006 and 2008 extreme drought events, a study estimated drought impacts valued at 1605 million euros, half a point of Catalonia's GDP [12].

Catalonian droughts' knowledge is most of all based on drought events' variability studies, either historically avoiding any future SPI (standardized precipitation index) and SPEI (standardized precipitation–evaporation index) indicators projection [13], or assessing climate change effects without considering the representative concentration pathways (RCPs) presented in the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report [14].

Gallart et al. [15] estimated trends for the rivers' discharge in the Ter–Llobregat system analyzing their historical records. However, this approach did not attempt to consider the contextual factors that influence the availability of water resources in the future for the Ter Llobregat system. The research would have been relevant if a forecast had been considered, introducing a future water resources scenario.

The Drought Plan (Alert and eventual drought exceptional action plan) developed by the Catalonian government (Generalitat de Catalunya) and the ACA in 2016, proposes a Ter–Llobregat water resources evolution. This research applied the SIMGES hydrological model with the multivariate periodic autoregressive (MPAR) stochastic model, based on 68 years of historical monthly flow contribution contributions series and comparing them with the generated synthetic series for a 500-year return period.

Likewise, a synthetic series was designed to analyze critical episodes of drought and estimating probabilities of occurrence by extrapolating historical climatic conditions [11]. Therefore, this study focused on understanding how climate change plays a role in Barcelona's drought events as one of the significant nature-based concerns for the next century [10,16,17].

Our research aim is broadening future drought events' knowledge, considering climate change impacts. We defined the design and implementation of a model for water amounts reservoir balance at a month scale, analyzing basins rainfall. This paper, as a first of its two-fold aim, represents observed reservoir water levels implementing the HBV (Hydrologiska Byråns Vattenbalansavdelning) hydrological model and studied the application and validation of the SIMGES model and the

HBV model as appropriate tools to forecast drought frequency for Barcelona's case. After the historical model calibration and validation process, we obtained rainfall projections using nine Earth system models (ESM) and two representative concentration pathways' (RCP) scenarios—RCP4.5 and RCP8.5—belonging to the fifth Coupled Model Intercomparison Project (CMIP5), provided by the Spanish Climate Research Foundation (FIC, accordingly to the Spanish acronym).

Second, we integrated these rainfall outputs within the hydrological model to simulate reservoir volumes as watershed responses, developing 30 different storage patterns. Outcomes of the models were analyzed to get average trends and extreme values for each scenario to estimate a single water availability trend for both reservoirs, to understand and analyze the water resource availability in Barcelona in the near future under different climate change situations. Our study outcomes provide additional support to plan water utility improvements, to evaluate extreme case scenarios, and to assess hazards related to water scarcity in further research.
