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

水是不可替代的——没有水，人类和其他动物就会死亡，作物歉收，重要的生态系统会遭受不可逆转的破坏。全球约有10亿人无法获得清洁饮用水，而在世界上已经干旱的地区，水正变得越来越稀缺。这些旱地（降雨量远低于蒸发量的地方）正在扩大，现在覆盖了地球陆地面积的三分之一以上，养活了大约20亿人口和世界上大约一半的牲畜和耕地，并包含了全球重要的生态系统。到本世纪末，气候变化可能会减少许多旱地的降雨量，因此，我们必须知道如何继续向这些地区供应淡水，使不断增长的人口能够茁壮成长，同时保持旱地生态系统的健康。储存在地球表面下孔隙空间和裂缝中的水（“地下水”）是地球上可获取淡水的最大储存，比地表水丰富25倍以上。由于地下水是如此巨大的淡水储存库，它对干旱地区的生存至关重要，当湖泊、水库和河流等地表水储存库由于缺乏降雨而干涸时，地下水可以在长期干旱中提供可靠的供水。因此，为了明智地管理旱地的水资源，至关重要的是要知道可以抽取多少地下水来满足人类的需求，同时不减少后代的可用性，即不不必要地“开采”地下水。一些重要的生态系统也依赖地下水，地下水通过泉水和其他地下水位接近地面的地方自然排放（从地下流出）。从长远来看，我们应该只开采和补充的地下水一样多。当降雨渗入地下的速度比通过蒸发或蒸腾作用返回大气的速度快时，这种补充就会自然发生，这一过程被称为地下水补给。目前，关于在干旱地区有多少地下水补给的信息还不够，因此不可能预测储存的地下水的数量将如何随着气候变化而变化。我想回答的核心问题是，在已经缺水的旱地有多少地下水补给，以及控制补给发生的时间和地点的主要因素是什么。这将有助于我们更有效地管理水资源，从而适应未来的气候变化，从而带来全球社会和经济效益。我计划通过整理和分析来自全球旱地的地下水位数据，以及收集和分析来自坦桑尼亚和澳大利亚特定旱地地区研究集水区的新实地数据来实现这一目标。然后，我将建立计算机模型来模拟旱地气候和地下水系统是如何相互作用的，使用新的数据来检查模型是否尽可能准确。我将为研究集水区做这项工作，然后利用新知识改进全球尺度水文模型，这些模型用于预测和管理由于气候变化导致的水资源变化。探索这些问题是一项令人着迷的挑战，因为需要创造性地将来自不同科学学科（如水文学、气候学、地质学）的大量想法结合在一起。这也给了我一个机会，让我了解人类过去是如何以及为什么应对气候变化的，以及我们未来如何应对气候变化。

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.