Spatially Explicit Hydrological Modelling for Water Accounting under Climate Change in the Volta River Basin in West Africa

Competition for scarce water resources in the Volta River Basin (VRB) of West Africa will increase in the near future due to the combined effects of rapid population growth and climate change. Residents are dependent on subsistence, mainly rainfed agriculture that is sensitive to climate variabilities. Recurrent floods and droughts damage properties and take lives. Information on water resources and their future trends is fundamental for water actors, as the basis for proper management and implementation of adequate measures to bolster resilience to water scarcity and foster water security.

This PhD thesis proposes a clear and novel demonstration of combining the Water Accounting Plus (WA+) framework with hydrological modelling and climate change scenarios to report on the current and future states of water resources in the VRB. WA+ is a standardized framework that provides a comprehensive view of the water resources in terms of water availability and consumptive uses with respect to different land uses.

The adopted methodological framework addresses key challenges posed by large-scale hydrological modelling in data scarce environments such as the VRB. These challenges include the issue of missing data in of streamflow records, the reliability of satellite and reanalysis data for forcing or calibrating hydrological models as an alternative to in-situ measurements, and the accuracy of the spatial and temporal representation of hydrological processes with spatially explicit models. A novel multivariate model calibration strategy is proposed to improve the representation of hydrological flux and state variables simulated with the fully distributed mesoscale Hydrologic Model (mHM). The proposed calibration strategy relies on the use of multiple satellite and reanalysis datasets from various sources. Then, a large ensemble of climate models are used to assess the impacts of climate change on water resources under various scenarios. The outputs of the mHM model are used to feed the WA+ framework to comprehensively report on the current and future conditions of water resources in the VRB.

The results show a clear increase in the projected exploitable water fraction while a decrease is expected in the available water fraction in the near future (2021-2050). Consequently, there is a clear need for adaptation measures to increase the water storage capacity in the VRB to facilitate a good exploitation of the projected increase in the net inflow, which would be beneficial for agriculture production and hydropower generation.