GLObal Suspended Sediment (GLOSS): Drivers, trends and future trajectories

该项目探讨了环境变化如何影响沉积物通过河流进入海洋的运动。了解悬浮沉积物的运动很重要，因为它是营养物质和污染物的载体，因为沉积物也形成了洪泛平原，滋养了三角洲和海滩，为沿海地区提供了弹性。为了加深我们对泥沙流动的理解，我们将量化地球上宽度大于90米的每条河流泥沙负荷的近期变化（1985年至今）。我们还将预测这些河流沉积物负荷在未来将如何变化。这些目标以前是不可能实现的，因为对河流中沉积物运输的直接测量只在极少数（全球<10%）河流上进行过。我们建议通过使用35年以上的免费卫星图像档案来避免这一困难。具体来说，我们将使用基于云的谷歌地球引擎自动分析每个卫星图像的表面反射率，这将使我们能够估计悬浮在河流表面附近的沉积物浓度。结合表征水柱中悬浮泥沙流动和混合的其他方法，这些新的地表悬浮泥沙浓度（SSC）估计将用于计算河流中悬浮泥沙的总运动。然后，我们在贝叶斯网络框架内使用新颖的机器学习技术分析我们的新数据库（相对于当前最先进的空间分辨率增加了五个数量级，其规模和全球覆盖范围将是前所未有的）悬浮沉积物运输。这种分析将使我们能够将我们对沉积物运输的估计与其环境控制（如气候、地质、水坝、地形）联系起来，通过经验分析的规模，使我们能够在我们对推动沉积物通过世界河流运动的因素的理解中获得阶梯式变化。反过来，这将使我们能够建立一个可靠的沉积物运动模型，我们将应用它来提供一套全面的未来沉积物从地球到海洋的运动预测。这种未来预测是至关重要的，因为地球表面正在经历一个前所未有的变化阶段（例如，通过气候变化、筑坝、毁林、城市化等），这可能会推动沉积物通量的大规模转变，对沿海和三角洲系统及人口产生重大和广泛的潜在影响。重要的是，我们不仅将量化变化的规模和轨迹，而且还将确定人为、气候和土地覆盖过程的相对贡献如何推动这些变化进入未来。这将使我们能够解决有关沉积物通过地球河流进入海洋的运动的基本科学问题，例如：当代从大陆到海洋的总泥沙通量是多少？这个总量在空间和季节上是如何变化的？气候、土地利用和人类活动对悬沙通量的相对影响是什么？这些作用是如何变化的？地形特征（面积、地形、连通性等）如何放大或抑制沉积物通量对外部（气候、土地利用、筑坝等）变化驱动因素的响应，从而制约世界不同地区沉积物通量的总体响应、演变和轨迹？4. 极端产流事件（如热带气旋）与更常见的低震级事件相比，在多大程度上驱动了沉积物通量？预计风暴频率和强度的变化将如何影响未来世界的沉积物通量？5. 在一系列似是而非的未来环境变化情景下，21世纪全球海洋沉积物通量将如何变化？

This project addresses how environmental change affects the movement of sediment through rivers and into our oceans. Understanding the movement of suspended sediment is important because it is a vector for nutrients and pollutants, and because sediment also creates floodplains and nourishes deltas and beaches, affording resilience to coastal zones. To develop our understanding of sediment flows, we will quantify recent variations (1985-present) in sediment loads for every river on the planet with a width greater than 90 metres. We will also project how these river sediment loads will change into the future. These goals have not previously been possible to achieve because direct measurements of sediment transport through rivers have only ever been made on very few (<10% globally) rivers. We are proposing to avoid this difficulty by using a 35+ years of archive of freely available satellite imagery. Specifically, we will use the cloud-based Google Earth Engine to automatically analyse each satellite image for its surface reflectance, which will enable us to estimate the concentration of sediment suspended near the surface of rivers. In conjunction with other methods that characterise the flow and the mixing of suspended sediment through the water column, these new estimates of surface Suspended Sediment Concentration (SSC) will be used to calculate the total movement of suspended sediment through rivers. We then analyse our new database (which, with a five orders of magnitude gain in spatial resolution relative to the current state-of-the-art, will be unprecedented in its size and global coverage) of suspended sediment transport using novel Machine Learning techniques, within a Bayesian Network framework. This analysis will allow us to link our estimates of sediment transport to their environmental controls (such as climate, geology, damming, terrain), with the scale of the empirical analysis enabling a step-change to be obtained in our understanding of the factors driving sediment movement through the world's rivers. In turn, this will allow us to build a reliable model of sediment movement, which we will apply to provide a comprehensive set of future projections of sediment movement across Earth to the oceans. Such future projections are vital because the Earth's surface is undergoing a phase of unprecedented change (e.g., through climate change, damming, deforestation, urbanisation, etc) that will likely drive large transitions in sediment flux, with major and wide reaching potential impacts on coastal and delta systems and populations. Importantly, we will not just quantify the scale and trajectories of change, but we will also identify how the relative contributions of anthropogenic, climatic and land cover processes drive these shifts into the future. This will allow us to address fundamental science questions relating to the movement of sediment through Earth's rivers to our oceans, such as: 1. What is the total contemporary sediment flux from the continents to the oceans, and how does this total vary spatially and seasonally?2. What is the relative influence of climate, land use and anthropogenic activities in governing suspended sediment flux and how have these roles changed?3. How do physiographic characteristics (area, relief, connectivity, etc.) amplify or dampen sediment flux response to external (climate, land use, damming, etc) drivers of change and thus condition the overall response, evolution and trajectory of sediment flux in different parts of the world? 4. To what extent is the flux of sediment driven by extreme runoff generating events (e.g. Tropical Cyclones) versus more common, lower magnitude events? How will projected changes in storm frequency and magnitude affect the world's sediment fluxes in the future? 5. How will the global flux of sediment to the oceans change over the course of the 21st century under a range of plausible future environmental change scenarios?