**3.5 Hydrodynamic influence on estuarine residence time distribution**

During the warm season (late spring and summer), the Alvo sluices are almost closed (scenario RT1). For this operational condition, the RT values near Pranto mouth station can quintuplicate when compared with those resulting from a Pranto river flow discharge of 15 m3.s-1 (scenario RT6), both under dry-weather conditions (low river Mondego inflows). Figure 22 shows this sensitive increase on flushing capacity of the Mondego estuary south arm due to Pranto river discharges from Alvo sluices opening.

Fig. 22. Effect of Pranto river discharge on RT values distribution (scenarios RT1 and RT6)

For the other hand, when the Alvo sluices remain closed the salinity and the RT values inside the southern arm are strongly influenced by tidal regime. Figure 23 illustrates the gradient of RT spatial distribution, which was mapped applying the *TemResid* module computing availability for the simulation of management scenarios RT2 and RT3.

Simulation results for these two tidal scenarios showed a RT values increase of 50% for a neap tide, when compared with a spring tide, both in the south arm and in the north arm reach, between N1 and N2 control points. This increase is smoothed in the northern arm inner areas, with the lowest increase (only 17%) at the Mondego estuary mouth. The minimum RT values (3.2 days) occurred in the Mondego estuary mouth (N0) and in the mesotrophic wetland zone of the south arm (near station S2). The maximum RT values (9.5 days) were obtained for the zone (near station S3) with higher eutrophication vulnerability. Concerning the periodicity of tidal regime recurrence, its effect could be very relevant for estuarine biochemical processes with a time scale lower than 6 days.

Fig. 23. Effect of tidal regime on RT values distribution (scenarios RT2 and RT3)
