SGER: Modeling Transpiration and Water-Stress from Leaf-Level to Catchment-Scale: A Supplement for Graduate Study

本研究的科学目的是开发并证明一种新的基于过程的方法的适用性，用于在从单片叶子到整个流域的空间尺度和从几分钟到整个生长季节的时间尺度上评估植物水分胁迫。建模策略的核心是由两个预先存在的气孔导度子模型构建的蒸腾公式。一种模型是“传统的”Ball等[1987]- Farquhar等[1980]气孔导度与CO2同化耦合的生物物理模型（经Leuning[1995]修改）。第二种是由Gao等人[2002]开发的气孔导度机制模型，该模型调用了土壤-植物-大气水连续体的动力学。通过耦合这两种气孔导度模型（第一种不依赖于土壤水分，第二种受土壤水分限制），蒸腾作用可以在数量和机制上依赖于植物水分胁迫。此外，植物水分胁迫本身成为一个变量，根据模型所需的参数和其他自变量来定义；因此，它也代表了气候条件和植被状况。将这些过程扩展到不同的景观和季节，将有助于了解植物水分胁迫和蒸腾的时空异质性，并有助于在流域尺度上实现水平衡。研究地点是蒙大拿州的Tenderfoot Creek实验森林，其中的一个子集水区（Stringer Creek）目前是nsf资助的一项广泛的流域范围内二氧化碳-水关系研究的重点。实验流域被针叶林和草甸的马赛克景观所覆盖，适合检查模型对代表这些不同功能类型植被的参数的敏感性。该研究将描述2006年“水年”（2005年10月至2006年9月）期间Stringer Creek流域的蒸腾作用和植物水分胁迫的特征，并特别强调2006年的生长季节。叶片水平的光合作用测量将用于估计森林和河岸草甸植被类型的子模型参数。2005年9月获得的激光雷达测量数据和IKONOS的附加图像将用于对整个斯特林格溪流域的叶片蒸腾估算。蒸腾的估算将与估算土壤蒸发量的priestley - taylor方法相结合，产生流域尺度的蒸散发估算，该估算可以通过Stringer Creek的水预算和2005年在流域内安装的两个涡动相关系统测量的蒸散发来验证。这笔资金的广泛影响将支持一名优秀的研究生，并提供一个绝佳的机会，以建立目前涉及流域尺度的耦合水和碳循环的研究。目标是在一个经过充分研究的流域中推断基于过程的植物-水关系模型，并改进对流域尺度的陆地-大气水交换进行估计的方法和能力。

The scientific intent of this research is to develop and demonstrate the applicability of a new, process-based method for evaluating plant water-stress at spatial scales ranging from a single leaf to an entire watershed, and at temporal scales ranging from several minutes to an entire growing season. At the core of the modeling strategy is a formulation of transpiration constructed from two preexisting submodels of stomatal conductance. One model is the 'traditional' Ball et al. [1987] - Farquhar et al. [1980] biophysical model of stomatal conductance coupled to CO2 assimilation (as modified by Leuning [1995]). The second is a mechanistic model of stomatal conductance developed by Gao et al. [2002] that invokes the dynamics of a soil-plant-atmosphere continuum of water. By coupling these two models of stomatal conductance (the first independent of soil water and the second limited by soil water), transpiration may be evaluated in terms of a quantitative and mechanistic dependence upon plant water-stress. Moreover, plant water-stress itself becomes a variable defined in terms of the parameters and other independent variables required by the model; it is therefore also representative of climatic conditions and vegetation status. Scaling these processes across landscapes and seasons will provide insight into the spatial and temporal heterogeneity of plant water-stress and transpiration, and is intended to aid in water-balance closure at the watershed scale. The study site is the Tenderfoot Creek Experimental Forest in Montana, a subcatchment of which (Stringer Creek) is currently the focus of an extensive NSF-funded study of CO2 - H2O relations on a watershed-wide scale. The experimental watershed is covered by a mosaic landscape of conifer forests and meadows, appropriate for examining model sensitivities to parameters representing these diverse functional types of vegetation. The research would characterize transpiration and plant water-stress across the Stringer Creek watershed during the 2006 "water year" (October 2005 through September 2006) with particular emphasis on the 2006 growing season. Leaf-level photosynthesis measurements will be used to estimate submodel parameters for forest and riparian meadow vegetation types. LIDAR measurements obtained during September 2005 and additional IKONOS imagery will be used to scale leaf-level estimates of transpiration to the entire Stringer Creek watershed. Estimates of transpiration will be combined with a Priestly-Taylor approach for estimating evaporation from the soil, yielding a watershed-scale estimate of evapotranspiration that may be validated against both Stringer Creek's water budget and evapotranspiration measured by two eddy covariance systems installed within the watershed during 2005.The broader impacts of this funding will support an excellent graduate student and provide an outstanding opportunity to build on current research involving coupled water and carbon cycling at the watershed scale. The goal is to extrapolate process-based modeling of plant-water relations across a well-studied watershed, and improve methodologies and capabilities for making watershed-scale estimates of land-atmosphere water exchange.