RESULTS

SWAT model calibration and validation

The SWAT model successfully simulated both streamflow and sediment regime of the watershed (Fig. 3 and 4). For the streamflow, the calibration results showed a very good agreement between simulated and observed data, with high R2, NSE, and good PBIAS in all available hydrometric stations (Fig. 4). Similarly, the statistical indices for the validation procedure were within the range of a “very good” model performance according to the model’s performance criteria established by Moriasi et al. (2007) (Table S3). Considering the sediment load calibration, despite using randomly distributed sediment data, simulated and observed data showed a “very good” performance match for calibration and validation in both Monte Da Ponte (Fig. 4a) and Oeiras (Fig. 4b) stations. Regarding the streamflow simulation, the calibration/validation phases showed more than 70% of data bracketed by the 95PPU (P-factor ≥ 0.70) with R-factors ranging from 0.15 to 0.3. For sediment, both Monte da Ponte and Oeiras reached a P-factor ≥ 0.6 with an R-factor of 0.32 and 0.41, respectively. These results remarked a high model performance and efficiency for both variables.
A global sensitivity analysis was implemented to identify those parameters that strongly influenced the simulated flow and soil losses within those listed in Table 1. The significance of the sensitivity test was evaluated using the statistical index “p-value”, automatically generated through the application of SUFI-2 algorithm. The results of the sensitivity analysis (Table 1, where the bold parameters represent the sensitive ones), indicated that the streamflow simulation was strongly dependent from DEEP_IMP, GW_DELAY, RCHGR_DP, and ALPHA_BF, while SLSUBBSN, USLE_K, and USCLE_C were the main parameters influencing the sediment loss simulation.