Collaborative Research: Dynamic Roots as the Biophysical Link Between Deep Moisture and the Atmosphere

植物在陆地表面的大气和土壤之间的水分流动中起着重要的作用。有些植物可以将根系延伸到地表以下，以利用地下水，为它们提供一个在旱季和干旱期间持续存在的湿气库。当植物利用这个水库时，它们通过叶子蒸发水分，在空气最干燥的时候为大气提供水分来源。蒸腾地下水对降水、云量和大气稳定性等气象条件的影响程度尚不清楚，其对地区、季节和其他因素的依赖性也不清楚。地下水蒸腾过程涉及一系列复杂的生物和物理过程，难以观测和模拟。但是pi已经开发了一种方案，其中这些过程的总体效应可以使用两个观测动机假设来近似。首先，植物将根系伸向地下水的程度取决于它们相对于当地地形的位置。在干燥或季节性干燥的气候下，山顶上的植物通常高于地下水位，无法有效地获取地下水，因此我们可以假设它们完全依赖于近地表的土壤水分。在谷底，地下水位可能非常接近地表，以至于植物的根必须很浅，以避免过度的盐度和内涝，所以它们也完全依赖于近地表的水分。因此，最大的地下水吸收发生在山坡中部，地下水的使用取决于最近排水的高度（HAND）。pi已经开发了一种“巨型山坡”方法，以小尺度HAND地形的五个bin表示来量化这种依赖性。其次，根系对土壤水分垂直剖面的动态响应。pi认为，根系动态可以简单地表示为假设根系主动延伸到可用的地下水，从任何水平吸收水分，以最少的努力提供最大的水分。这个假设是用一种方案形式化的，在这种方案中，水分通过根系的运输类似于电路中的电流运动：根系充当“电线”，通过“电线”，水分的“电流”从特定的土层流向植物叶片，由植物叶片和根系所触及的土层之间的“电压”差（即水位差）驱动。水分从土层到土壤表面的流动是由层与叶之间的电压降与导线电阻之比给出的，这与欧姆定律（电流等于电压除以电阻）完全相似。PIs在Noah陆面模型中实现了他们的根-地下水方案，该模型与天气研究与预报（WRF）模型耦合形成了一个耦合的陆-大气模型。该模型随后被用于测试地下水蒸腾对大陆尺度水文循环的影响。需要解决的科学问题之一是，地下水蒸腾作用在多大程度上促进了降水，既增加了降水所需的水分，又降低了大气稳定性。由于水循环对水资源的重要性，该研究具有社会价值。这项工作对于在研究陆地表面水文学、大陆尺度水文气候和植物生态学等独立但密切相关的领域的研究团体之间建立桥梁具有特别的价值。由于WRF已广泛应用于天气预报，因此在WRF模式中实施新计划将使其可用于实际工作。pi还在K-12学校开展教育和推广活动，该项目支持两名研究生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Plants play an important role in moving water across the land surface between the atmosphere above and the soil below. Some plants can extend their roots substantially below the surface to take advantage of ground water, giving them a moisture reservoir that persists through dry seasons and droughts. When plants tap into this reservoir they transpire moisture through their leaves, providing a source of moisture to the atmosphere at a time when the air may be at its driest. The extent to which this transpired groundwater influences meteorological conditions such as precipitation, cloudiness, and atmospheric stability is not known, nor is its dependence on region, season, and other factors.The transpiration of groundwater involves a complex set of biological and physical processes which are difficult to observe and simulate. But the PIs have developed a scheme in which the bulk effect of these processes can be approximated using two observationally-motivated assumptions. First, the extent to which plants extend their roots to tap groundwater depends on their position relative to the local topography. In dry or seasonally dry climates plants on a hilltop are typically too high above the water table to effectively access groundwater, so we can assume that they rely exclusively on near-surface soil moisture. At the valley floor the water table can be so close to the surface that plant roots have to be shallow to avoid excessive salinity and waterlogging, so they also rely exclusively on near-surface moisture. Thus maximum groundwater uptake occurs at mid-hillslope locations, and groundwater usage depends on the Height Above Nearest Drainage (HAND). The PIs have developed a "giant hillslope" method to quantify this dependence in terms of a five-bin representation of small-scale HAND topography.Second, roots respond dynamically to the vertical profile of soil water. The PIs argue that root dynamics can be simply represented by assuming that roots actively extend to reach available groundwater, taking up water from whatever level offers the greatest moisture access for the least effort. This assumption is formalized using a scheme in which the transport of moisture through roots is analogous to the movement of electric current in a circuit: the roots act as "wires", through which a "current" of moisture flows from a specific soil layer to the plant leaves, driven by the "voltage" difference (i.e. water potential difference) between plant leaves and the soil layer tapped by the roots. The flow of moisture from a soil layer to the surface is then given by the ratio of the layer-to-leaves voltage drop to the resistance of the wire, in exact analogy to Ohm's law (electric current equals voltage divided by resistance).The PIs implement their root-groundwater scheme in the Noah land-surface model, which is coupled to the Weather Research and Forecasting (WRF) model to form a coupled land-atmosphere model. The model is then used to test the impact of groundwater transpiration on the continental-scale hydrological cycle. Among the scientific questions to be addressed is the extent to which groundwater transpiration promotes precipitation, both by making a substantial contribution to the moisture available for precipitation, and by reducing atmospheric stability.The research has societal value due to the importance of the hydrological cycle for water resources. The work is of particular value for building bridges between the research communities concerned with the separate but closely connected fields of land surface hydrology, continental-scale hydroclimate, and plant ecology. The implementation of the new scheme in the WRF model will make it available for operational use, as WRF is widely used for weather forecasting. The PIs also conduct educational and outreach activities in K-12 schools, and the project supports two graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.