*2.2. Model Calibration, Validation, and Sensitivity Analysis*

Table 1 describes the seven calibrated parameters applied to characterize each rainfall event introduced into the model sub-basin. These are all conceptual parameters, not easy to estimate from basin physical properties. The choice of the seven calibration parameters followed a preliminary data analysis, checking their values' availability and validity for a monthly time-step; in addition, according to local conditions, snow-related parameters were discarded.


**Table 1.** The calibrated Hydrologiska Byråns Vattenbalansavdelning (HBV) model parameters for the Llobregat and Ter river basins.

A Montecarlo simulation was conducted examining 10,000 combinations where there was an available gauge station to calibrate the parameters at each basin, setting the established objective function as accurately as possible (Nash-coefficient, Equation (1)), at a daily timescale if conceivable, in addition, relating with monthly volumes used as a reference, based on the obtained hydrographs when the information was available. Validation of water volume contribution data used ACA's water contribution estimations from the Aquatool SIMGES module developed by IIAMA [21].

$$Nash\,\overline{coefficient} = 1 - \frac{\sum\_{t=1}^{T} \left(Q\_m^t - Q\_o^t\right)^2}{\sum\_{t=1}^{T} \left(Q\_o^t - \overline{Q\_0}\right)^2},\tag{1}$$

where *Qt <sup>m</sup>* = *simulated discharge*, *Qt <sup>o</sup>* = *observed discharge*, *and Q*<sup>0</sup> = *mean observed discharges*. Nash-Sutcliffe efficiency ranges from −∞ to 1. The closer to 1 the coefficient is, the more accurate the model is. An efficiency equal to 0 means that the approximation is as good as the mean of the observed data. Results are acceptable when positive values are higher than 0.2.
