Global change encompasses changes in the characteristics of interrelated climate variables in space and time, and derived changes in terrestrial processes, including human activities that affect the environment. As such, projected global change includes groundwater systems. Here, groundwater is defined as all subsurface water including soil water, deeper vadose zone water, and unconfined and confined aquifer waters. Potential effects of climate change combined with land and water management on surface waters have been studied in some detail. Equivalent studies of groundwater systems have lagged behind these advances, but research and broader interest in projected climate effects on groundwater have been accelerating in recent years. In this paper, we provide an overview and synthesis of the key aspects of subsurface hydrology, including water quantity and quality, related to global change. Adaptation to global change must include prudent management of groundwater as a renewable, but slow-feedback resource in most cases. Groundwater storage is already over-tapped in many regions, yet available subsurface storage may be a key to meeting the combined demands of agriculture, industry, municipal and domestic water supply, and ecosystems during times of shortage. The future intensity and frequency of dry periods combined with warming trends need to be addressed in the context of groundwater resources, even though projections in space and time are fraught with uncertainty. Finally, potential impacts of groundwater on the global climate system are largely unknown. Research to improve our understanding of the joint behaviors of climate and groundwater is needed, and spin-off benefits on each discipline are likely.

We analyse the threats of global environmental change, as they relate to food security. First, we review three discourses: (i) 'sustainable intensification', or the increase of food supplies without compromising food producing inputs, such as soils and water; (ii) the 'nexus' that seeks to understand links across food, energy, environment and water systems; and (iii) 'resilience thinking' that focuses on how to ensure the critical capacities of food, energy and water systems are maintained in the presence of uncertainties and threats. Second, we build on these discourses to present the causal, risks and options assessment for decision-making process to improve decisionmaking in the presence of risks. The process provides a structured, but flexible, approach that moves from problem diagnosis to better risk-based decision-making and outcomes by responding to causal risks within and across food, energy, environment and water systems.

The coastal aquifer of Bou-Areg (Morocco) has been studied to identify the main processes causing groundwater salinization, using a multi tracer (general chemistry and isotopes - delta H-2, delta O-18, delta C-13, delta N-15(NO3), delta O-18(NO3)) geochemical approach. Groundwater is characterized by the widespread occurrence of brackish waters (TDS < 500 mg L-1) with high cation contents, which are balanced by elevated dissolved NO3- (reaching a maximum of 208 mg L-1) and Cl-. Lagoon samples represent a mixture of fresh water and sea water, showing a Na/Cl ratio in agreement with that of sea water and an excess of Ca. The high Ca values represent the main peculiarity of the groundwater-lagoon water system. Two types of groundwater could be identified: (i) freshwater, separated from the whole system and located at the limit of the irrigated area, characterized by low TDS, depleted isotopic composition and relatively high quality; and (ii) water mainly recharged by mountain runoff, interacting with local recharge and acquiring salinity from different sources, thus creating a complex system of dilute waters. Hydrochemical results confirm that the high salinity of the aquifer is caused by the coexistence of dissolution of evaporate rocks and carbonates from Miocene strata, water-rock interaction, and human impacts due to agricultural return flows. The latter represents the main contribution to groundwater salinization, especially in the central part of the aquifer, as well as one of the main causes of the general increase in NO3- concentrations. Only locally, in the southern part of the aquifer, close to the city of Kariat Arkmane, the high salinization observed may be attributed to the presence of lagoon water intrusion. The isotopic composition of dissolved NO3- indicates manure and septic effluents, especially in urban areas and in the central part of the plain, and synthetic fertilizers in the agricultural zone as the main drivers for human induced pollution. The study shows that agricultural return flow has significantly modified the chemistry of the system and it is a prime example of the human-induced changes over coastal environments. Saline water intrusion from the lagoon in the shallow aquifer is negligible, while discharge of polluted groundwater into the lagoon has been found to partially alter its qualit

Transboundary aquifers are as important a component of global water resource systems as are transboundary rivers; yet, their recognition in international water policy and legislation is very limited. Existing international conventions and agreements barely address aquifers and their resources. To rectify this deficiency, the International Association of Hydrogeologists and UNESCO's International Hydrological Programme have established the Internationally Shared (transboundary) Aquifer Resource Management (ISARM) Programme. This multiagency cooperative program has launched a number of global and regional initiatives. These are designed to delineate and analyze transboundary aquifer systems and to encourage riparian states to work cooperatively toward mutually beneficial and sustainable aquifer development. The agencies participating in ISARM include international and regional organizations (e.g., Organization of American States, United Nations Environment Programme, United Nations Economic Commission for Europe, Food and Agriculture Organization, and South African Development Community). Using outputs of case studies, the ISARM Programme is building scientific, legal, environmental, socioeconomic, and institutional guidelines and recommendations to aid sharing nations in the management of their transboundary aquifers. Since its start in 2000, the program has completed inventories of transboundary aquifers in the Americas and Africa, and several ISARM case studies have commenced.

One of the most important issues for water resource management is developing strategies for groundwater modelling that are adaptable to data scarcity. These strategies are particularly important in arid and semi-arid areas where access to data is poor and data collection is difficult, such as the Lake Chad Basin in Africa. In the present study, we establish a numerical groundwater flow model and evaluate the effects of dry and wet periods on groundwater recharge in the Chari-Logone area (96 000 km 2 ) of the Lake Chad Basin. Boundary conditions, flow direction, sources, and sinks for the Chari-Logone local model were obtained by revising and remodelling the Lake Chad Basin regional hydrogeological model (508 400 km 2 ) developed by the BRGM (Bureau de Recherches Géologiques et Minières) in the 1990s. The simulated aquifer water level showed good agreement with observed levels. Aquifer recharge is primarily determined by river-aquifer interactions and mostly occurs in the southern section of the study area. In wet years, groundwater recharge also occurs in the N'Djamena area. The approach we adopted provided relevant results and was useful as an initial step in more detailed modelling of the area. It also proved to be a useful method for groundwater modelling in large semi-arid and arid regions where available data are scarce.