Fully integrated and physically-based approach for simulating water flows in a large-scale, heavily-agricultural and low-instrumented watershed

Abstract

Recent advances in fully integrated and physically-based hydrologic models, as well as computational resources, have increased interest for these models, which seem most realistic for assessing the impacts of anthropogenic activities and climate change on water resources. However, their applications, for most of the cases, are carried out on relatively small-scale (<1,000 km2) and well-instrumented watersheds. The present study uses the fully integrated and physically-based approach to evaluate surface and subsurface hydrodynamics, in the Boubo river watershed (BRW), located in the southern part of Côte d'Ivoire. The BRW is a large-scale area (4,957 km2), lacking data and where local and transversal issues impacts' on the hydrosystem are simultaneous and significant. The conceptual model uses an equivalent porous medium for the 3D subsurface domain and the integrated modelling approach necessitated only three boundary conditions: precipitation, potential evapotranspiration and surface water outflow. The simulations were conducted using two main calibration phases: quasi-steady-state and transient conditions. Minor calibration effort was required and overall, the results obtained seem quite plausible and would reflect the field reality. However, abnormally high surface water depths (>15 mm), observed at the land surface and the underestimation of simulated surface water flows rates, compared to measured ones, seem rather problematic. These problematic results could be explained by a defect of drainage in the model. This could be due to the weak representation in the model of particularly complex local surface features and low mesh resolution, in relation to the large-scale of the study area. The transient phase simulation allowed observation of model response with respect to changes in forcing data. This is encouraging in the context of assessing the possible impacts of climate change on water resources.