## 5 Hydrology of the Hoyos and Papuos Rivers  
Low to moderate rain drives a similar return period in discharge unless changes in basin characteristics disturb ordinary behavior. The peak flow values for different return periods were estimated for Q2, Q25, Q100, and Q500 years, which are the typical intervals used in water resources planning and civil protection planning in Spain. The results of the rational method show theoretical discharges of Q2 of 10 and 14 m\({}^{3}\) s\({}^{-1}\) for Hoyos and Papuos, respectively. The peak flows derived from the rational method come from the application CauMAX [20], where a runoff coefficient for unburned conditions were used; the runoff coefficient value for Hoyos and Papuos was 0.28. Furthermore, the initial abstraction values were 27.24 and 24.93 mm for Hoyos and Papuos, respectively. No infiltration is considered via the CauMAX application. This theoretical value should correspond to the real value if there is a correspondence between rainfall and flow. Two hydrological analyses were conducted in order to obtain the real discharge of the 2017 flash flood and test it against the theoretical discharge of Q2. The first was performed using the HEC-HMS program (where the pre- and post-fire situations were assessed for CN value change), and the second involved an estimation of discharge by means of HWM--mainly flotsam associated with hydraulic drops. The results of both analyses and a comparison with the theoretical data are given in Table 4.  
The HEC-HMS model for the Hoyos basin divides it into three sub-basins, which were not affected to the same extent by burning. Estimated pre- and post-fired CN mean values for the basin increased from 65.36 to 82.56 (Table 4), respectively, although the changes due to fire were more intensive in the two upper sub-basins (with CN mean value changes from 62 to 84). The CN values for burned areas follow the proposed values of Yochum and Norman [21]. These changes in CN values give rise to changes in outflow rates, so the peak flow value in the pre-fire scenarios does not reach a value of 2 m\({}^{3}\) s\({}^{-1}\), while in the post-fire scenario this value increased to 12.5 m\({}^{3}\) s\({}^{-1}\) (an increase of 658%; see Figure 7). In the Papuos basin model, up to 11 sub-basins were defined, and the CN mean values for the whole basin changed from 71.18 to 78.08 when the post-fire conditions were considered. As for the Hoyos basin, there were major changes in some of the sub-basins in terms of the mean CN value (with some cases in which the CN value rose from 59 to 79). For the Papuos basin, the increase in the peak flow value was not as pronounced as in the Hoyos basin, with an outflow rate for the pre-fire scenario of 11.1 m\({}^{3}\) s\({}^{-1}\), while this value rose to 19.3 m\({}^{3}\) s\({}^{-1}\) for the post-fire scenario (an increase of 174%; see Figure 7). The results obtained with the HEC-HMS model showed that the effects of the fire not only caused an increase in the peak flow value associated with the 2017 rainfall event, but there was also an increase in flow velocity, directly associated with the increase in peak flow. Thus, the maximum flow velocities estimated for the study area in the Hoyos basin increased from 1.9 m s\({}^{-1}\) to 3.75 m s\({}^{-1}\)--they practically doubled. In the case of the Papuos basin, the variations were not so important, and in both cases, the flow velocities reached were significant. In this case, the pre-fire situation showed maximum flow velocity values of 5 m s\({}^{-1}\); in the case of the post-fire situation, this value increased to over 6 m s\({}^{-1}\).