10As the world's largest distributed store of freshwater, groundwater plays a central role in 11 sustaining ecosystems and enabling human adaptation to climate variability and change. 12The strategic importance of groundwater to global water and food security will intensify 13 under climate change as more frequent and intense climate extremes (droughts, floods) 14 increase variability in soil moisture and surface water. Here we critically review recent 15 research assessing climate impacts on groundwater through natural and human-induced 16 processes as well as groundwater-driven feedbacks on the climate system.

In Africa, groundwater is the major source of drinking water and its use for irrigation is forecast to increase substantially to combat growing food insecurity. Despite this, there is little quantitative information on groundwater resources in Africa, and groundwater storage is consequently omitted from assessments of freshwater availability. Here we present the first quantitative continent-wide maps of aquifer storage and potential borehole yields in Africa based on an extensive review of available maps, publications and data. We estimate total groundwater storage in Africa to be 0.66 million km 3 (0.36-1.75 million km 3 ). Not all of this groundwater storage is available for abstraction, but the estimated volume is more than 100 times estimates of annual renewable freshwater resources on Africa. Groundwater resources are unevenly distributed: the largest groundwater volumes are found in the large sedimentary aquifers in the North African countries Libya, Algeria, Egypt and Sudan. Nevertheless, for many African countries appropriately sited and constructed boreholes can support handpump abstraction (yields of 0.1-0.3 l s −1 ), and contain sufficient storage to sustain abstraction through inter-annual variations in recharge. The maps show further that the potential for higher yielding boreholes (>5 l s −1 ) is much more limited. Therefore, strategies for increasing irrigation or supplying water to rapidly urbanizing cities that are predicated on the widespread drilling of high yielding boreholes are likely to be unsuccessful. As groundwater is the largest and most widely distributed store of freshwater in Africa, the quantitative maps are intended to lead to more realistic assessments of water security and water stress, and to promote a more quantitative approach to mapping of groundwater resources at national and regional level.

As one article in a four-part PLoS Medicine series on water and sanitation, Paul Hunter and colleagues argue that much more effort is needed to improve access to safe and sustainable water supplies.

across the full extent of the IGB. The aquifer system is usually represented as a single category on 66 hydrogeological maps [6]. However, in practice the system is complex and heterogeneous with large 67 spatial differences in permeability, storage, recharge and water chemistry as well as having an 68 important depth dimension. This complexity strongly influences how each part of the aquifer 69 responds to stresses [7]. The IGB is home to the largest surface water irrigation system in the world, 70 constructed during the 19 th and early 20th century to redistribute water from the Indus and Ganges 71 through a canal network >100,000 km long. Leakage from this irrigation infrastructure has had a 72 profound impact on the current quantity and quality of groundwater resources and is a significant 73 factor governing its response to contemporary and future pressures. Increasing groundwater use for 74 irrigation poses legitimate questions about the future sustainability of abstraction from the basin 75 and future groundwater security of this region is a major social-political concern [8]. 76Recent discussion of water security has been dominated by interpretations of remotely-sensed 77 gravity data from the GRACE mission gathered at a coarse scale of 400x400 km are poorly constrained by ground-based observations; local field studies nonetheless provide partial 82 insight into system dynamics. These include evidence of: declining groundwater levels [11,12,13], 83 groundwater security has been introduced by forecasts of climate change and the potential for 88 significant change to precipitation, river flows and groundwater recharge [20,21,22]. 89Here we present for the first time an analysis of the status of groundwater across the IGB alluvial 90 aquifer based entirely on in situ measurements. We use a statistical analysis of multiyear 91 groundwater-level records from 3652 water-wells and a compilation and interpretation of existing 92 high resolution spatial datasets and studies within Pakistan, India, Nepal and Bangladesh to assess: 93 (1) groundwater-level variations; (2) groundwater salinity; and (3) We find that the water-table within the IGB alluvial aquifer is typically shallow (< 5 m below ground 98 surface) and the long-term trend is relatively stable throughout much of the basin, with some 99 important exceptions. In areas of high groundwater abstraction in northwest India and the Punjab in 100 Pakistan ( Figure 2) the water-table can be >20 m bgl and in some locations is falling at rates of > 1 101 m/a (Figure 3). In areas of equivalent high irrigation abstraction within Bangladesh, the average 102 water-table remains shallow (<5 m bgl) due to greater direct recharge and high capacity for induced 103 recharge. Groundwater levels are deep and falling beneath many urban areas, and particularly in 104 large groundwater dependant cities such as Lahore, Dhaka and Delhi [23]. Shallow and rising water-105 tables are found in the Lower Indus, parts of the lower Bengal basin and in places throughout the 106 IGB aqui...

Groundwater is the world's largest accessible store of fresh water and supplies 36% of the world's drinking water and ~42% of the water used for irrigation 1 . Groundwater is the only reliable source of fresh water in many semi-arid and arid regions where surface waters are seasonally or perennially absent 9 . The long-term viability of groundwater resources as well as the ecosystems and livelihoods that they sustain, depends on replenishment of groundwater by recharge. Over the past 50 years, groundwater depletion has been estimated and observed in several aquifers throughout the tropics and sub-tropics [10][11][12][13] . Such depletion not only threatens ecosystem function and the livelihoods of groundwater-dependent communities in some of the world's poorest regions but is also estimated to contribute to sea-level rise 12,13 . A conceptual understanding of the relationship between rainfall and recharge is fundamental to the development of robust estimates and projections of not only groundwater recharge and depletion but of all components of the terrestrial water balance under changing climates and increasing freshwater demand.

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

Climate change represents the most significant challenge of the twenty-first century and poses risks to water and sanitation services. Concerns for water supply include damage to infrastructure from flooding, loss of water sources due to declining rainfall and increasing demand, and changes in the water quality of water sources and within distribution of water. Sanitation concerns include damage and loss of services from floods and reduced carrying capacity of waters receiving wastewater. Key actions to reduce climate risks include the integration of measures of climate resilience into water safety plans, as well as improved accounting and management of water resources. Policy prescriptions on technologies for service delivery and changes in management models offer potential to reduce risks, particularly in low-income settings. Water and sanitation services contribute to greenhouse gas emissions. Choice of wastewater treatment technologies, improved pumping efficiency, use of renewable sources of energy, and within-system generation of energy offer potential for reducing emissions. Overall, greater attention and research are required to understand, plan for, and adapt to climate change in water and sanitation services. As with many other climate change adaptations, the likely benefits from no-regrets solutions are likely to outweigh the costs of investment.

Drought in Africa has been extensively researched, particularly from meteorological, agricultural, and food security perspectives. However, the impact of drought on water security, particularly ground water dependent rural water supplies, has received much less attention. Policy responses have concentrated on food needs, and it has often been difficult to mobilize resources for water interventions, despite evidence that access to safe water is a serious and interrelated concern. Studies carried out in Ghana, Malawi, South Africa, and Ethiopia highlight how rural livelihoods are affected by seasonal stress and longer-term drought. Declining access to food and water is a common and interrelated problem. Although ground water plays a vital role in buffering the effects of rainfall variability, water shortages and difficulties in accessing water that is available can affect domestic and productive water uses, with knock-on effects on food consumption and production. Total depletion of available ground water resources is rarely the main concern. A more common scenario is a spiral of water insecurity as shallow water sources fail, additional demands are put on remaining sources, and mechanical failures increase. These problems can be planned for within normal development programs. Water security mapping can help identify vulnerable areas, and changes to monitoring systems can ensure early detection of problems. Above all, increasing the coverage of ground water-based rural water supplies, and ensuring that the design and siting of water points is informed by an understanding of hydrogeological conditions and user demand, can significantly increase the resilience of rural communities to climate variability.