Physics-based modeling of the water within the critical zone of West African inland valleys under natural and anthropized conditions

Global changes severely affect food production through the world. In West-Africa, small-scale subsistence farming and rainfed food crops are the dominant agricultural system, concerning about 43% of the population and accounting for about 30% of the GDP (FAOSTAT, 2015). Reinforcing population resilience and food security in a sustainable way implies local agro-ecological intensification. Inland valleys are seasonally waterlogged headwater wetlands, which keep moisture during the dry season. Inland valleys have been identified as small-scale (few ha) geomorphological features allowing for local intensification of food production, through water management (e.g. dry season irrigation from surface water or shallow groundwater). The massive development of the inland valleys raise questions of the impacts on the quality and quantity (timing and volumes) of groundwater recharge and surface streamflow at the inland valleys outlets and downstream hydraulic regimes. However, inland valleys are complex critical zone systems, particularly in low transmissivity hard-rock basement contexts, where the water table is tightly linked to surface streamflow and evapotranspiration. Anticipating for the impacts of both inland valleys intensification and global changes on the downstream and downward water transfers requires a complete modeling of the inland valleys critical zone. *

The primary goal of this work is to reinforce previous modeling studies of inland valleys by evaluating the ability of a physics-based critical zone model, ParFlow-CLM (www.parflow.org) to reproduce the behaviors of a set of four inland valleys in northern Benin (Hector et al., 2018, Danvi et al., 2017). The work will consist in synthetizing the forcing data and model parameters over each of the four catchments, and adapt the general framework proposed by Hector et al., 2018 to each of the four catchments (mainly slopes, vegetation, hydrodynamic parameters). The MSc will then evaluate model performance with respect to available hydrological observations on the catchments (streamflow, water table, evapotranspiration, soil moisture) but also with respect to previous modeling approaches (e.g. ArcSWAT, Danvi et al., 2017), in an intercomparison approach to identify the range of applicability of each model. The final target is to evaluate to what extent such a general critical zone modeling framework can be adapted to any ungauged inland valley based on data available from remote sensing or global products (slopes, vegetation, surface pedology…).

References :
Danvi, A., Giertz, S., Zwart, S. J. and Diekkrüger, B. : Comparing water quantity and quality in three inland valley watersheds with different levels of agricultural development in central Benin, Agricultural Water Management, 192, 257–270, doi:10.1016/j.agwat.2017.07.017, 2017.
FAO : FAOSTAT, [online] Available from : http://faostat3.fao.org/, 2015.
Hector, B., Cohard, J.-M., Séguis, L., Galle, S. and Peugeot, C. : Hydrological functioning of West-African inland valleys explored with a critical zone model, Hydrol. Earth Syst. Sci. Discuss., 2018, 1–35, doi:10.5194/hess-2018-219, 2018.

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