Understanding the rainfall-runoff relationships is one of the requirements and necessities in flood modeling, forecasting and grasping its contribution to the annual runoff. This study aims to evaluate the use the hydrological modeling and flood frequency analysis (FFA) in investigating the magnitude and the occurrence of floods in a complex terrain mountainous watershed and the effects of the dams on downstream floods.

The case study area, the N’fis sub-basin is located in the High Atlas of Morocco, drains a total area of 1700 km², and is characterized by an arid to semi-arid climate in plains and sub-humid in mountains. The flood modeling in this watershed constitutes a true challenge due to the lack of an adequate amount of spatial and temporal flood data for FFA. Therefore, the SWAT (Soil and Water Assessment Tool) which is a physically based, the continuous model was used to simulate and reproduce the hydrological behavior of the N’fis upstream. The model parameters were calibrated and validated with data collected from 2000 to 2016 and performed satisfactorily with statistical values of Nash-Sutcliffe for the calibration period 0.52 and validation 0.69. Finally, the daily flood data (1982-2016) was used to carry out FFA using the L-moment methods (Gumbel Normal and Log Pearson III). Furthermore, the comparison of the goodness-of-fit for Gumbel, GEV, and LP3 distributions for flood frequency analysis in the N’fis watershed highlights that the GEV distribution shows good results and appears to be the more suitable one. This study allowed for a better assessment of floods and can assist water managers and decision-makers in adequately planning and managing flood protection.