*4.2. Calibration and Validation of Hydrological Parameters*

Figure 4 presents the response hydrographs at the locations where stream gauging stations provide records. These stations include EA078 in the la Baells sub-basin, EA087 and EA021 in the La Llosa del Cavall sub-basin and EA033 in the Sau sub-basin.

**Figure 4.** Observed (obs) and computed (sim) response hydrograph: (**a**) La Baells station; (**b**) La Llosa del Cavall station.

Figure 4a shows how response hydrograph performs a good base flows simulation but decreases accuracy in peak flows simulation. Nevertheless, the Nash-coefficient equal to 0.42 was enough to reproduce the behavior of the A78 sub-basin. Peak flows were undershot, but it was preferable to underestimate these contributions as a safety factor, considering weather station scarcity in this area, which directly influences the rainfall time series. Analyzing the response, the approximation was precise enough to reproduce the behavior of this sub-basin. In Figure 4b, the Nash coefficient in this basin was 0.30, which returns a satisfactory outcome. The main differences were gathered from 1990, when the available records had quality issues, as the frequency of the measurements increased to 4–7 days.

In the rest of the basins, two verifications reviewed the results. We compared monthly contributions for each season, and the water contributions were correlated applying a linear regression of the HBV and SIMGES models. Evaluation through the R<sup>2</sup> coefficient as shown in Figure 5 verifies whether the distribution is similar among HBV and SIMGES volumes. In addition, we checked the Nash-coefficient with each reservoir's computed contributions for all seasons. Table 3 presents the calculated Nash-coefficients.

**Figure 5.** (**a**) La Baells monthly reservoir contribution during winter months, comparison of Hydrologiska Byråns Vattenbalansavdelning (HBV) and SIMGES outputs; (**b**) Comparison analysis of the two contributions. The black line corresponds to a simple linear regression.


**Table 3.** Nash-coefficient for each reservoir at a monthly time scale, for each season and considering the historical analyzed records (1980–2013).

Subsequently, the total contributions from 1980–2013 were analyzed to check if the total volume of water available to serve the demand was the same, regarding the distribution by year at each reservoir unit for the HBV, and the SIMGES model used as a reference, as Figure 6 illustrates.

**Figure 6.** Complete Ter–Llobregat system: available water volumes comparison in years 1980–2013: (**a**) Water contributions data distribution for HBV and SIMGES models. The closer the lines are, the more similar they are. (**b**) To assess how similar they are, this shows the correlation between the two distributions. As the values are distributed along the reference line (slope 1.00), their correspondence is valid.

The outcomes from both models were similar. The main difference came from the distribution over the year, in particular during summer months, when SIMGES contributions were critical. One of the main reasons to explain this difference may come from the two time-series input models. Recognizing that both datasets are different, the results cannot be the same. Outcomes were similar, ensuring the representation created with the HBV model achieved a reasonable resemblance with the historical dataset.

#### *4.3. Simulations under Future Rainfall Conditions*

Reservoir volumes results were from historical data provided by ACA. Results of the projections indicated that not all the climate models were adversely predictive: some of them forecasted rainfall increase (volume), while others estimated a reduction. Figure 7 shows some models forecasting severe drought situations in the RCP8.5 scenario, compared with the RCP4.5 scenario, which showed water scarcity, according to a few models for Ter and Llobregat reservoirs. According to projections, it is expected that in both systems for the RCP8.5 scenario, at least one drought episode is expected with the 20-years return period.

**Figure 7.** Results summary figure showing range and average water volume outcomes for every representative concentration pathway (RCP) scenario, for the Llobregat and Ter joint system towards the 21st century. The red-spotted line provides a linear trend estimation. (**a**) Results for the RCP8.5 scenario, (**b**) Results for the RCP4.5 scenario.

Model trends and extreme values were plotted to represent the magnitude of future situations and to consider all the possible climate model variables evolution during the 21st century. An implication of the negative trend, as for the RCP8.5 in the reservoir system, is the possibility that a long-term alternative resource will be necessary to preserve reservoir equilibrium. Regression analysis predicts the combined behavior of both systems as a joint water reservoir system. In particular, the analysis of the joint system allows studying the link among the most challenging climate change scenario (RCP8.5) and the predicted water resources availability and bypass any analysis bias.

Figure 7 displays the intercorrelations of the nine performed models, providing a behavior trend-line of the reservoir system water volume. An average trend of these model outputs forecasts an 11.1% decrease in the system water availability, applying the RCP8.5 scenario for the year 2100. Such water availability variations will have city-scale consequences for social-economic conditions and ecosystems.

#### **5. Discussion**

Turning now to the assessment on the applicability of the SIMGES and HBV models as appropriate tools to forecast drought frequency, some factors play a role in determining why the HBV model underestimated the water contribution, as shown in Figure 4, for the first calibration and validation step. The main reason that can be argued to explain this may come from the precipitation records. At high areas, such as this one, convective storms may occur over a localized area not covered by any meteorological station. In any case, the HBV contributions were on the security side. SIMGES model contributions apply in ACA Water Management Plan were closer to HBV computed values. Thus, the response of the HBV model is reasonable.

Analyzing Table 3 outcomes for each season, it is observed how the winter and autumn volume contributions present the best correlation to the SIMGES model. Throughout the spring and summer periods, the dispersion of the results was high. However, the Nash-coefficients were satisfactory for the most significant reservoirs: La Baells, Sau, and Susqueda. La Llosa del Cavall and Sant Ponç reservoirs could not have been simulating with the same precision, notwithstanding, its contribution to the total water reserves was 13% and 4%. Hence, their contribution to the entire system was small in comparison with the other dams.

On the question of understanding and analyzing the water resource availability in Barcelona in the future under different climate change situations, this study found that the water availability would drop in this 21st century for the reservoir system. By 2019–2050, the models average predicts a 9% decrease in surface water volume availability over the reservoir system. However, by 2019–2100, due to precipitation reduction and warming-enhanced evaporation, Climate Change effects, the models average predicts an 11% decrease with a remarkably high consensus among analyzed models for the RCP8.5 scenario, as shown in Figure 7.

The results obtained herein are consistent with Barcelona regional and Barcelona city council results [10], estimating in the year 2050, a 12% surface water resources decrease. Table 4 compares the summary statistics for the water resources availability forecast, comparing other studies for the reservoir system.


**Table 4.** Comparison of the summary statistics for the forecast of the water resources' availability for the Ter and Llobregat reservoir system.

These results are consonant with related studies [17], finding that according to valid data control models and detailing low-heterogeneity results, Llobregat river discharges will decrease, in a 2% scale for years 2070–2100.

Our approach in this paper explored the water availability of the principal sources providing about 92% of contemporary water demand. Due to practical constraints, a full discussion of adaptation measures coping with water availability shortage lies beyond the scope of this study. Catalonian Drought Plan documents designate some current adaptation measures [11,37]. They relate an increase in alternative water sources and a decrease in drinking water consumption, such as the planning and implementation of water reclamation and reuse, desalination as a technical option to increase the drinking water availability, to increase groundwater extraction, and to decrease consumption (stronger for agriculture, breeding, and recreative uses).

However, these solutions have significant technical, legal, political, and economic hurdles. In the case of the existing desalination plant, its maximum potential for water production is 60 Hm3/year, an amount that could meet the current and future water shortages set in this study, nevertheless, the average unit cost of desalinated water production (€ 0.58/m3) cost four times more than the cost of potable water production in the drinking water treatment plant (€ 0.14/m3) [24,38].
