Deplete and Retreat: The Future of Andean Water Towers

陆地上的生命依赖淡水。山脉就像水塔一样，通过抬升潮湿的空气来产生水，并提供临时的地表和地下储水，以便稍后释放到河流中。这些储存在经历季节性干旱的地区尤为重要，因为冰雪融化可以抵消干旱期间降雨量的减少。存在两个主要的天然储水库。雪是一种短期储存，在每日到季节性的时间尺度上延迟了降雪后水的释放。冰融化也会季节性地释放水。然而，冰川冰是一个长期的水库，可以储存几十年到几个世纪的水。在山区集水区的非冻结部分也可以观察到类似的现象。湿地、池塘和浅层地下水流等储存提供短期储存，而湖泊和深层地下水则表现出长期释放特征。这些不同过程的结合决定了山脉水塔对周边地区功能的大小和行为。这在安第斯山脉尤为重要，那里发现了世界上一些最重要的水塔。安第斯山脉附近地区的人口依赖山区水资源来饮用、生产粮食和水力发电，动植物也是如此。不幸的是，人为引起的气候变化正在改变安第斯山脉水塔的储水量。温室气体的排放意味着下雪的天气变得越来越少，耗尽了山区的积雪储备。缺乏补充的雪和不断升高的温度，导致冰川失去了它们储存的冰，退缩到山区的更高和更冷的地方。再加上气候变化对其他流域的影响，这导致了整个安第斯山脉的水资源短缺。持续的干旱正在袭击人口密集的城市，人口的集中增加了对水的需求。例如，利马和瓦拉斯（秘鲁）、拉巴斯（玻利维亚）和圣地亚哥（智利）等城市都位于集水区，冰雪融化有助于河流流动。然而，上游农村地区对气候变化的适应能力较弱，往往更直接地依赖冰雪融水。这影响了农业灌溉，使该地区的粮食安全受到威胁。为了帮助管理水供应的这些变化，该项目旨在实现两件事。首先是提供更好的监控。安第斯山脉的高海拔地区仪器设备匮乏。为了了解气候变化的地点和速度，我们将安装更多的科学仪器来测量积雪、天气和河流流量。为了将我们现在可以测量的变化置于背景下，我们需要更长的观测记录，可以追溯到过去。自1850年以来，许多冰川一直在退缩，在我们将绘制的景观中留下了印记。利用卫星图像，我们可以追踪这些冰川从20世纪70年代到现在的退缩情况。我们还将利用过去气候条件的记录，这些记录是由水手在航海日志中记录的，并储存在沉积物中。我们的第二个目标是预测未来的变化，这需要气候、冰川和河流过程的计算机模型。决策者需要这样的预测，因为他们需要可靠地了解未来可能发生的变化。我们将结合最先进的模型，模拟安第斯山脉十个集水区不断变化的水资源。为了评估我们的模型在预测方面的能力，我们将根据我们对过去条件和当前变化的观察来测试它们。在复制观测条件方面表现良好的模式将用于预测一系列可能的未来气候情景。通过结合这些观测方法和基于模型的方法，我们将改进预测水资源变化的方法，并帮助为水资源管理计划提供信息。

Life on land depends upon freshwater. Mountains act as water towers, producing water by lifting moist air, and by providing temporary surface and below-ground storage of water for later release into rivers. These stores are particularly important in regions that experience seasonal droughts, as snow and ice melt can counteract reduced rainfall during dry spells. Two main natural depots of frozen water exist. Snow is a short-term store, delaying the release of water after snowfall on daily to seasonal timescales. Ice melt also releases water seasonally. However, glacier ice is a longer-term reservoir, storing water for decades to centuries. A similar behaviour can be observed in the non-frozen part of a mountain catchment. Stores such as wetlands, ponds and shallow below-ground flow provide short-term storage, while lakes and deeper groundwater show long-term release characteristics.The combination of these different processes determines the magnitude and behaviour of a mountain range's water tower function for the surrounding area. This is particularly important in the Andes, where some of the most important water towers of the globe are found. The human population in regions neighbouring the Andes depend on mountain water resources for drinking, food production and hydropower, as do animals and plant life.Unfortunately, human-induced climate change is altering the stores of water held in the Andes water towers. Greenhouse gas emissions mean that snow-bearing weather conditions are becoming less frequent, depleting the stocks of snow held in the mountains. The lack of replenishing snow, and increasing temperatures, are causing glaciers to lose the ice they store, retreating to the higher and colder portions of the mountains. In combination with climate change impacts on the rest of the catchment, this is contributing to water shortages across the Andes. Ongoing droughts are hitting high-population cities, where the concentration of people increases the demand for water. For example, the cities of Lima and Huaraz (Peru), La Paz (Bolivia) and Santiago (Chile), are all situated in catchments where snow and ice melt contribute to river flow. However, upstream rural areas, which are less adaptable to climate change, are often even more directly reliant upon snow and ice meltwater. This impacts irrigation for agriculture, stressing the food security of the region.To help manage these changes to water supplies, this project aims to achieve two things. The first is to provide better monitoring. The high altitudes of the Andes are poorly instrumented. To work out where and how fast conditions are changing, we will install more scientific instruments to measure snow, weather and river discharge. To contextualise the changes we can measure now, we need longer observational records extending back in time. Many glaciers have been retreating since 1850, leaving behind an imprint in the landscape which we will map. Using satellite imagery, we can track the retreat of these glaciers from the 1970s to their present position. We will also utilise records of past climate conditions, recorded by sailors in ships-log books and stored in the landscape in sediments. Our second goal is to project future changes, which requires computer models of climate, glacier and river processes. Such projections are required for policy makers, who need to be reliably informed of potential future change. We will combine state-of-the-art models, to simulate the changing water resources in ten Andean catchments. To assess the skill of our models at making predictions, we will test them against our observations of past conditions and current changes. Models that perform well at replicating observed conditions will be used to project a range of possible future climate scenarios. By combining these observational and model-based approaches, we will improve the approach to projecting water resource change, and help to inform water management plans.