## 4 Results
### Hydrology
#### 4.1.1 Hydrological Regime  
The hydrological regime for the pre- and post-dam periods was evaluated at each flow gauging station. In addition, we calculated the changes in monthly precipitation within the basins of each project in order to identify whether the changes in discharge are influenced by the precipitation change within the basins.  
In Figure 6, the hydrological regime for both periods in two gauging stations downstream of each dam is shown. In Appendix C, the test results (Student's \(t\)-test and [PERSON]'s \(F\)-test) applied on the average monthly discharge and precipitation within the basins in the pre-dam and post-dam periods are summarized. According to the statistical results presented in Appendix C, no significant changes in the mean or variance values of the monthly precipitation within the basins were observed, except for some specific months. Therefore, those precipitation changes do not explain the monthly mean flow changes in the downstream gauging stations.  
The hydrological effect due to dam construction is similar in the four selected study sites: increased low discharges during the dry period and decreased high discharges during the rainy period (see Figure 6). Downstream of the Urra I Dam (see Figure 6a and Table 4), the differences between the pre- and post-dam periods occur in the means during the dry period, while differences in the variances are more significant in the rainy season.  
Figure 5: Geological conceptual map of the Betania–Prado Dam area (geology from the Servicio Geologico Colombiano and DEM from the SRTM mission/2000).  
Downstream of the Salvajina Dam (see Figure 6b and Table 4), the discharges decrease significantly in the rainy period compared to the other dams; this is reflected in the lower values (\(t\)-test) before and after dam construction. We also observed a considerable increase in discharges during the dry season, from August to September (see Figure 6b).  
The hydrological effects due to the Betania Dam construction are reflected by the discharge series at the Puente Santander and Angostura stations, while the hydrological effects observed downstream of the Purificacion station reflect the combined effects of the Betania and Prado Dams. It was impossible to identify the affectation of only the Prado Dam, which started operation in 1972, since the gauging station records are only available from 1975, so the combined effect of both dams was considered. In the area affected only by the Betania Dam (until the Angostura station), attenuated discharges are observed during the rainy season and increased discharges are present during the dry period (January to April). The discharges during the same dry period at the Purificacion station were higher in the post-dam period than the ones observed before dam construction. The percentages of the difference between the low discharges pre- and post-dam in this station were larger than those observed in the gauging stations that are only influenced by the Betania Dam. Of the three studied areas, this one has the slightest changes in the mean and variance, which implies a lower impact of these dams on the hydrological regime.  
Figure 6: Mean monthly discharge in downstream stations for the pre-dam (black line) and post-dam (red line) periods. Left and right columns show the nearest and the furthest station to the dam, respectively. (**a**) Urrá I project, (**b**) Salvajina project, and (**c**) Betania and Prado projects.