Groundwater in Africa supports livelihoods and poverty alleviation 1,2 , maintains vital ecosystems, and strongly influences terrestrial water and energy budgets 3. However, hydrologic processes governing groundwater recharge sustaining this resource, and their sensitivity to climatic variability, are poorly constrained 4,5. Here we show, through analysis of multi-decadal groundwater hydrographs across sub-Saharan Africa, how aridity controls the predominant recharge processes whereas local hydrogeology influences the type and sensitivity of precipitation-recharge relationships. Some humid locations show approximately linear precipitation-recharge relationships with small rainfall intensity exceedance thresholds governing recharge; others show surprisingly small variation in recharge across a wide range of annual precipitation. As aridity increases, precipitation thresholds governing initiation of recharge increase, recharge becomes more episodic, and focussed recharge via losses from ephemeral overland flows becomes increasingly dominant. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by largescale climate controls. Intense precipitation, even during lower precipitation years, produces substantial recharge in some dry subtropical locations, challenging the 'high certainty' consensus that drying climatic trends will decrease water resources in such regions 4. The likely resilience of groundwater in many areas revealed by improved understanding of precipitation-recharge

The peculiarities of karst environment make it highly vulnerable to a number of geohazards: As concerns the natural hazards, the main categories are sinkholes, slope movements, and floods. To these, anthropogenic hazards have to be added, as pollution events, land use changes resulting in loss of karst landscape, destruction of karst landforms, etc. Even carrying out engineering works without taking into the due consideration, the peculiar aspects of karst can be extremely dangerous and cause risk to the natural environment, as well as to the man-made infrastructures and buildings. In the second half of last century, man has definitely become one of the most powerful factors that can cause changes in the karst environment, produce direct damage, predispose the territory to threatening events, and increase with mismanagement actions the negative effects deriving from natural hazards. In this chapter, the main hazards in karst are briefly described with particular focus on the natural hazards and with the help of some case studies

The problem of a groundwater model calibration is posed as a multiextremum (global) optimization problem, rather than the more widely considered single-extremum (local) optimization problem. Several algorithms of randomized search incorporated in the global optimization tool GLOBE are considered (including the canonical genetic algorithm and more recently developed adaptive cluster covering), and applied to the calibration of the groundwater model TRIWACO. The results show the usefulness of global optimization algorithms in the automatic calibration of even complex models having considerable running times. Calage automatique de modèles d'écoulement souterrain utilisant des techniques d'optimisation globales Résumé Par opposition à l'approche classique qui considère le problème du calage des modèles d'écoulement souterrain comme un problème d'optimisation à un extremum (optimisation locale), nous posons ce problème ici en termes d'optimisation à extremums multiples (optimisation globale, OG). L'application de divers algorithmes de recherche aléatoire (recherche aléatoire contrôlée, algorithme génétique canonique et couverture adaptative) au modèle d'écoulement souterrain TRIWACO montre l'intérêt de l'utilisation des algorithmes OG pour le calage de modèles qui, en raison de leur complexité, exigent des temps de calcul considérables.

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