Unraveling flood dynamics at sub-daily time scales in semi-arid to arid basins in south Morocco

Floods are posing significant threats but also play a crucial role in recharging groundwater and surface water storage in arid and semiarid regions. To improve flood knowledge in these areas, this study in South Morocco considers hourly river discharge records from 20 basins, with sparse vegetation or bare land and sandy soils, to identify over 1000 flood events. Several flood event characteristics were computed: peak discharge, rising time, flood volume, and duration. The seasonal frequency, trends, and the characteristics of different types of flood events were analyzed using a peaks-over-thresholds sampling. Results indicated that floods are mostly observed during the wettest months, from September to February depending on the basins, but also in summer with comparable magnitudes, demonstrating the high variability in these regions. No significant trends in flood characteristics over time have been detected. An envelope curve that relates unit peak discharge and net runoff volume to catchment areas has been developed for this region. Examining climatic and physiographic factors, the catchment area and topographic wetness index (TWI) were strongly correlated with peak discharge, but on the opposite no relationships were found with antecedent soil moisture conditions prior to floods. There is a majority of flash-floods (34%) with rising times less than 5 h. While two-thirds of episodes have rising time less than 10 h, these floods also have a greater magnitude on average. Computing flood quantiles with daily data can be highly biased in these basins, underestimating flood quantiles by a factor of 2 or beyond, indicating the need for instantaneous data to analyze floods in semi-arid and arid basins. These findings provide insights for better water resource management, flood control strategies and infrastructure design in this type of basins.