Groundwater recharge in global drylands: processes, quantification & sensitivities to environmental change

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There is no substitute for water - without it humans and other animals die, crops fail and important ecosystems suffer irreversible damage. Globally around 1 billion people lack access to clean drinking water and water is becoming scarcer in already dry parts of the world. Such drylands (places where rainfall is much less than evaporation) are expanding and now cover more than a third of the Earth's landmass, support a population of around 2 billion people and around half of the world's livestock and cultivated land, and contain globally important ecosystems. Climate change is likely to reduce the amount of rainfall in many drylands by the end of this century and so it is vital that we know how to continue to supply fresh water in these areas, to enable the expanding human population to thrive whilst also keeping dryland ecosystems healthy.Water stored in pore spaces and fractures under the Earth's surface ("groundwater") is the largest store of accessible freshwater on the planet, more than 25 times more abundant than surface water. As it constitutes such a large store of freshwater, groundwater is crucial to survival in drylands, providing a reliable water supply through long droughts when surface water stores such as lakes, reservoirs and rivers dry up due to the lack of rainfall to refill them. In order to manage water resources wisely in drylands, it is therefore crucial to know how much groundwater can be pumped for human needs, without reducing its availability for future generations i.e. without 'mining' the groundwater unnecessarily. Some important ecosystems also rely on groundwater where it discharges (flows out of the ground) naturally, through springs and other locations where the water table is close to the ground surface. In the long term we should only take out as much groundwater as is replenished. This replenishment happens naturally when rainfall infiltrates into the ground more quickly than it is removed back to the atmosphere by evaporation or transpiration in a process known as groundwater recharge. At the moment, there is not enough information about how much groundwater recharge occurs in dryland areas and it is therefore impossible to predict how the amount of stored groundwater will change in response to a changing climate.The central question I want to answer is how much groundwater recharge occurs in drylands which are already short of water and what the main factors are which control when and where this recharge happens. This will be of global societal and economic benefit by helping us manage water resources more effectively, and therefore adapt to future climate change. I plan to achieve this by collating and analysing groundwater level data from drylands globally as well as collecting and analysing new field data from study catchments in specific dryland regions of Tanzania and Australia. I will then build computer models to simulate how the dryland climate and groundwater systems interact with each other, using the new data to check the models are as accurate as possible. I will do this for the study catchments but then use the new knowledge to improve global scale hydrological models which are used for predicting and managing changes in water resources due to changes in climate. Exploring these questions is a fascinating challenge because of the number of ideas from various scientific disciplines (e.g. hydrology, climatology, geology) which need to be brought together creatively. It also gives me an opportunity to contribute to understanding both how and why humans have coped with climate change in the past, and how we can do so in the future.