Abstract
Humid tropics areas have generally limited hydro-climatic data in a context of rapidly changing urbanization and increasing pressures on water resources. The objective of this work is to develop an approach for understanding hydrological functioning in a poorly gauged context tropical basin in order to assess the impact of urbanization for a better water resource and flood management. A mixed instrumentation-modelling "tailor-made" approach is carried out at different spatial scales (nested basins) and temporal scales (from hour to year). The applications are carried out in the highly urbanized 421 km² Méfou basin (Yaoundé, Cameroon). The thesis is structured in three parts. The first part presents the instrumentation we conducted from 2017 to 2019 (16 rain gauges and 7 limnimeters) on 6 sub-basins with different land uses (from natural to urban). These hourly data complement the scattered sparse historical data (1950-2015) and recent global data (e.g. satellite data, re-analysis products). Data analysis highlighted overflows during high floods and quantified the terms of the annual water balance and flood events, as well as the impact of urbanization on the water balance: a 50% decline in forest areas and an increase in the proportion of urban areas from 10% in 1980 to 35% in 2017 are associated with a 53% increase in annual flow. To take into account the evolution of urbanization in a poorly gauged context, hydrological modelling must be "tailor-made", adapted to the specificities of the environment, simple, semi-distributed, non-stationary and parsimonious. The second part presents the new semi-distributed annual model that we developed on the basis of Ponce and Shetty's (1995) models, taking into account the non-stationarity of the processes. This model made it possiblelet to quantify the contribution of 8 major sub-basins, to reconstruct the historical series (1950-2017; RMSE = 99 mm) and to simulate the impact of climate change and urbanization scenarios on water resources. The third part presents the development of a continuous hourly model to simulate flood hydrographs. Two new distinct models are being developed by adapting the MHYDAS model to the poorly gauged urban tropical context: i) A global model that simulates the impact of urbanization on water resources and floods at one point in the basin; ii) A semi-distributed model that simulates the impact of developments on the sections channel network or urbanization on certain parts of the basin. The performance of the models was evaluated: (i) at different time steps from hour to year; (ii) on the continuous data series and event scale. The components of the water balance are simulated for both the current state and various climatic and development scenarios: evapotranspiration, soil stock change, surface runoff, base flow and overflow. While limiting the calibration phase, these sparse parsimonious models provide satisfactory performance for both the simulation of hydrological budgets and the simulation of flood events (NSE = 0.85). This mixed multi-scale "tailor-made" instrumentation-modelling approach seems promising for a better understanding of hydrological processes in poorly gauged environments subject to territorial modifications, and for providing managers with simple and effective tools for water resource and flood management.