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

植物在将水输送到地表大气层和土壤之间的过程中起着重要的作用。 一些植物可以将它们的根延伸到地表以下，以利用地下水，给它们一个水分库，在干旱季节和干旱中持续存在。 当植物进入这个水库时，它们通过叶子散发水分，在空气最干燥的时候为大气提供水分来源。地下水蒸腾作用对降水、云量和大气稳定性等气象条件的影响程度尚不清楚，也不清楚其与地区、季节和其他因素的关系。地下水蒸腾作用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，其中这些过程的整体效应可以使用两个观察动机假设来近似。首先，植物延伸根系汲取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中，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，植物的根系必须很浅，以避免过多的盐分和水涝，因此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的位置，地下水的使用取决于最近的排水高度（HAND）。PI已经开发了一种“巨大的山坡”的方法来量化这种依赖性的一个小规模的HAND地形的五箱表示。第二，根动态响应土壤水分的垂直剖面。 PI认为，根系动态可以简单地表示为假设根积极延伸到可用的地下水，从任何水平的水提供最大的水分访问最少的努力。这种假设是正式使用的方案，其中通过根的水分运输类似于电路中的电流的移动：根作为“电线”，通过它的水分的“电流”从特定的土壤层流到植物叶片，由植物叶片和土壤层之间的“电压”差（即水电位差）驱动。从土壤层到地表的水分流量由层到叶的电压降与导线电阻之比给出，与欧姆定律（电流等于电压除以电阻）完全类似。PI在诺亚陆面模型中实现了其根-地下水方案，该方案与天气研究和预报（WRF）模型耦合，形成耦合的陆-气模型。 然后，该模型被用来测试大陆尺度的水文循环的地下水蒸腾的影响。 其中一个有待解决的科学问题是地下水蒸腾作用促进降水的程度，既通过对降水可用的水分做出实质性贡献，又通过降低大气稳定性。由于水文循环对水资源的重要性，这项研究具有社会价值。这项工作是特别有价值的研究社区之间的桥梁，关注独立的，但密切相关的领域，陆地表面水文，大陆尺度水文气候和植物生态学。 在WRF模式中实施新方案将使其可用于业务用途，因为WRF广泛用于天气预报。 PI还在K-12学校开展教育和外展活动，该项目支持两名研究生。该奖项反映了NSF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.