Hydraulic Performance Evaluation of Rainwater Harvesting Techniques to face climate changes in dry environments

In sub-Saharan Africa, climate and land use/land cover changes are strongly affecting the hydrological behaviour of land surface enhancing runoff and decreasing water hold capacity. The present study examines the possibility to use rainwater harvesting techniques (RWHT) as flood mitigation approach and evaluates their performance in runoff reduction. Focusing on Niger, the most commonly applied micro-catchment WHT are pits (tassa), half-moon (demi-lunes) and contour stone bunds (cordons pierreux). In addition to agricultural benefits, they have the dual function of slowing down and collecting overland flow increasing the concentration-time of the whole basin. With this twofold aim, RWHT can be adopted in the framework of the climate-smart agriculture to deal with the increase of extreme events that characterize west Africa. To evaluate the runoff reduction efficiency different hydraulic analyses have been conducted. Firstly, rainfall-runoff simulations were carried out on synthetic catchments (rectangular shape and 1% constant slope), in case of plain surface and with shaped RWHT. At this stage, the best design was evaluated and alternated half-moon configuration was chosen for an application on a real topography. 2D hydrodynamic simulations were implemented using the software HEC-RAS. Consequently, the hydraulic simulation of an intense rainfall event (measured in the wet season 2012) on a real small watershed (26 ha), located next to the village of Toure (Niger), was performed. The outcomes show a significative reduction of outlet hydrograph (10% of the flood peak) having manipulated an agricultural area that is less than 4% of the watershed surface. Thus, implementation of RWHT on a wider scale can blend flood hazard prevention and agronomic principles improving the adaptation to climate change.