Collaborative Research: Hillslope hydrology under glass: Controlled experimental testing of hillslope-scale hydrologic transport theories at Biosphere2

由于地形的复杂性以及控制水流和运移的过程，溶质在山坡和流域中的运移很难预测。目前的方法使用从小尺度观测得出的过程描述，以使用尺度相关参数的有效值在大得多的尺度上进行预测。然而，众所周知，这种模式是有缺陷的，因为在许多情况下，参数不容易测量或预测。人们越来越频繁地呼吁建立新的理论，更好地将大规模预测与景观的可观察特性联系起来。最近发展了一种新的理论框架来描述山坡和流域尺度上的溶质运移。然而，该框架作为一个关键要素是不完整的，“年龄函数”没有从理论上加以规定，必须进行实证研究。这些函数表征了如何从系统内不同年限的水的分布中选择离开系统的水。如果这些年龄函数可以从现有的景观数据中参数化，这个框架将为模拟山坡和流域尺度上的运输提供一个严格的替代方案，从而绕过其他方法面临的与尺度相关的一些挑战。该项目将利用生物圈2的新的景观演变观测站(LEO)研究设施，通过一套新颖的示踪实验，在山坡尺度上直接观察这些年龄函数的形状。该设施可用于在三个330平方米的人工山坡上创建精确控制和复制的流动和传输现象，并使用1500多个传感器和500个土壤水分采样器以前所未有的空间和时间分辨率观察它们。使用稳定水同位素示踪剂精心设计的实验序列将被用来直接测量这些年龄函数，并使用专门为此目的开发的名为PERTH(周期示踪器等级)的新实验程序将它们的性质与流动路径的结构相关联。这个项目有四个目标：1)直接测量LEO中的年龄函数；2)使用综合建模方法来理解年龄函数和下伏山坡结构之间的关系，并将经验结果推广到更广泛的景观单元类别3)测试一系列关于年龄函数控制的假设，为建立可操作的理论铺平道路；科学家、管理者和政策制定者需要工具来预测大空间尺度上溶质在景观中的移动，例如氮素从农业流域分布到河流中的移动。目前最先进的模型无法捕捉到这种运输的许多特征，因为它们所依赖的参数无法在地形中进行足够精细的测量。该项目将提出一种新的交通模拟方法，该方法不依赖于这些细尺度参数，而是直接在山坡的尺度上进行预测。这一理论比通常用于大规模预测的模型更具一般性，后者依赖于对运输过程的限制性假设，或者没有充分考虑风暴、季节和气候变化下穿过地貌的水流变化对运输的影响。该项目将培训两名博士生和几名本科生，学习水文实验和环境建模的新方法。此外，生物圈2是一个科学教育中心，每年有100,000人参观该设施，这项工作将成为设施-S教育计划的一部分。

The transport of solutes through hillslopes and watersheds is difficult to predict due to the complexity of the landscape and the processes controlling flow and transport. The current approaches use process descriptions derived from observations at small scales to make predictions at much larger scales, using effective values of the scale dependent parameters. However, this paradigm is known to be flawed, as in many cases the parameters cannot be measured or predicted easily. Increasingly frequent calls have been made for new theories that better link large-scale predictions to observable properties of the landscape. A new theoretical framework has recently been developed to represent solute transport at hillslope and watershed scales. However the framework is incomplete as a key element, the "age functions", are not specified from theory and must be investigated empirically. These functions characterize how the water leaving the system is selected from the distribution of water of different ages within the system. If these age functions could be parameterized from available landscape data, this framework would provide a rigorous alternative for modeling transport at hillslope and watershed scales that circumvents some of the scale-related challenges other methods face. This project will use the new Landscape Evolution Observatory (LEO) research facility at Biosphere2 to directly observe the shape of these age functions at a hillslope scale using a novel set of tracer experiments. This facility can be used to create precisely controlled and replicated flow and transport phenomena in three 330 m2 artificial hillslopes, and observe them with an unprecedented spatial and temporal resolution with over 1,500 sensors and 500 soil water samplers. A carefully designed sequence of experiments using stable water isotope tracers will be used to directly measure these age functions and relate their properties to the structure of flow paths using a novel experimental procedure entitled PERTH (PERiodic Tracer Hierarchy), developed specifically for this purpose. This project has four goals: 1) to make direct measurements of the age function in the LEO; 2) to use an integrated modeling approach to understand the relationship between the age function and underlying hillslope structure, and generalize the empirical findings to a broader class of landscape units 3) to test a series of hypotheses about the controls on the age function that pave the way towards an operational theory; and 4) to propose and test functional forms of the age functions that can be encoded in operational models of watershed-scale transport.Scientists, managers and policy-makers need tools to predict the movement of solutes through the landscape at large spatial scales, such as the movement of nitrogen from fertilizer distributed across agricultural watersheds into streams. Current state-of-the-art models are not able to capture many of the features of this transport, because the parameters they rely on cannot be measured at sufficiently fine detail in the landscape. This project will advance a new approach to modeling transport that does not rely on these fine-scale parameters, but instead makes predictions directly at the scale of hillslopes. This theory is more general than the models typically used for large-scale predictions, which rely on restrictive assumptions about the transport processes, or do not adequately account for the effect on transport of changes in flow through the landscape across storms, seasons and under climate change. This project will train two PhD students and several undergraduate students in new methods for hydrologic experimentation and environmental modeling. Furthermore, Biosphere2 is a hub of science education with 100,000 people visiting the facility each year, and this work will form part of the facility?s educational program.