Scale Dependence in Water Balance Sensitivity to Soil Moisture

0074045 Salvuci水文学中的许多问题都有一个共同的主题，即水通量和大尺度蓄水量的相互关系。产生这一问题的原因很简单：在小尺度上，迁移过程可能是相当好的理解和可预测的，但迁移变得最相关的尺度（无论是作为资源/危害还是与大气相互作用）要大得多。在大尺度上，关键参数（土壤、植被和地形）和强迫（辐射、降水、风）在空间上是不均匀的，传输机制不仅是垂直的，而且是横向的（例如，地下水再分布、边界层平流）。因此，即使是最简单的问题，即“是什么控制了降水的蒸散，径流，排水和存储变化的分割”，变得难以回答在大尺度。在这里，我们提出了一个新的和简单的方法，使用直接测量来解决相关的问题：如何水平衡的敏感性，土壤湿度的变化与规模？所提出的方法的新奇在于使用月降水量（P）测量值作为根区总流出量（蒸散量（ET）、排水量（D）和径流量（R））的替代指标。该方法的基础是，在一个时间间隔内，根区水分储存（S）的变化，这构成了差异P-ET-D-R，本身与该时间间隔内的平均储存无关。这种相关性的缺乏是根区水分统计稳定性的直接结果。这一特性的效用在于，ET、D和R与S的所有协方差都可以通过测量P与S的协方差来估计（或通过计算在给定湿度水平下有条件平均的降水率，并求导数）。因此，平稳性条件赋予有条件的平均降水效用作为诊断措施的敏感性相结合的蒸发和排水/径流通量土壤水分。由于这种简单的方法仅基于质量平衡和条件平均，因此它可以通过简单的P和S数据聚合应用于任何区域尺度，从而可以用于直接估计水分平衡敏感性的形式和尺度依赖性。因此，它可以用来量化水分的变化，由于侧表面和地下水再分布过程和土地覆盖/土壤/气象强迫异质性影响大规模的敏感性的水平衡土壤水分。

0074045SalvucciMany questions in hydrology have as a common theme the inter-relation of water fluxes and storage at large scales. The issue arises for simple reasons: Transport processes may be reasonably well understood and predictable at small scales, but the scales at which transports become most relevant (either as resources/hazards or with respect to atmosphere interaction) are much larger. At large scales critical parameters (soil, vegetation, and topography) and forcings (radiation, precipitation, wind) are spatially heterogeneous, and transport mechanisms are not only vertical, but lateral (e.g., groundwater redistribution, boundary layer advection). Thus even the simplest question, i.e. "what controls the partitioning of precipitation into evapotranspiration, runoff, drainage and storage change", becomes difficult to answer at large scale. Here we propose a new and simple approach for using direct measurement to address the related question: how does the sensitivity of water balance to soil moisture change with scale?The novelty of the proposed approach is the use of monthly precipitation (P) measurements as a surrogate measure of total outflow from the root zone (evapotranspiration (ET), drainage (D), and runoff (R)). The basis of the method is that the changes in root zone moisture storage (S) over an interval, which make up the difference P-ET-D-R, are themselves uncorrelated with the mean storage over that interval. This lack of correlation is a direct result of the statistical stationary of root zone moisture. The utility of this property is that all of the covariance of ET, D and R with S can be estimated by measuring the covariance of P with S (or by calculating the precipitation rate conditionally averaged at given moisture levels, and taking derivatives). The stationarity condition thus imparts on conditionally averaged precipitation utility as a diagnostic measure of the sensitivity of combined evaporation and drainage/runoff fluxes to soil moisture. Because this simple methodology is based only on mass balance and conditional averaging, it can be applied at any area scale through simple aggregation of P and S data, and thus can be used to directly estimate both the form and the scale-dependence of moisture-water balance sensitivity. As such it can be used to quantify how variability in moisture due to lateral surface and groundwater redistribution processes and land cover/soil/meteorological forcing heterogeneity influence the large-scale sensitivity of water balance to soil moisture.