Trees typically survive prolonged droughts by absorbing water from deeper layers. Where soils are shallow, roots may be extract water from the underlying fractured bedrocks. In dry seasons, surface-soil moisture dynamics reflect hydraulic redistribution (HR). HR is usually estimated based on the gradient of mean, or bulk, soil water potential among layers in the rooting zone (HR_B). This approach neglects the potential effect of spatial heterogeneity of water content at the millimeter scale between the rhizosphere and bulk soil. We proposed to account for the rhizosphere water balance, estimating HR to the rhizosphere (HR_R) of the dry surface soil from the underlying fractured rock. The model was evaluated using a 15-year dataset collected in Sardinia. When the typical approach, based on moisture gradients among bulk soil layers, was used for estimating HR_B, tree transpiration was underpredicted in all seasons, especially in spring and summer. Forcing the model with measured tree transpiration, HR_B decreased during spring and summer, while the contribution of the underlying rock layer to tree transpiration was threefold that estimated using HR_R-based model. The average water content of the bulk surface soil layer was very low, reaching 0.06 in the driest summers while showing little diurnal dynamics; however, concentrating water in roughly estimated rhizosphere volume, produced rhizosphere water content appreciably higher (≈0.16), and much more dynamic. Predicted HR_R dominated evapotranspiration (60% - 65%) in dry springs and summers reaching 80% of tree transpiration. Most importantly, the proposed rhizosphere-HR model correctly predicts the diurnal dynamics of tree transpiration year-round, and the grass transpiration in its active spring period. Eco-hydrological models operating at sub-daily scale should consider partitioning the soil to rhizosphere volume, thus allowing both diagnostic and prognostic estimates of diurnal biosphere-atmosphere mass and energy exchanges.

树木通常通过吸收更深层次的水分而在长期干旱中幸存下来。在土壤较浅的地方，根部可能会从下面的断裂基岩中提取水分。在旱季，地表土壤水分动态反映了水力再分配 (HR)。HR 通常根据生根区各层之间的平均或体积土壤水势的梯度 (HR _B )估算。这种方法忽略了根际和大块土壤之间毫米级含水量空间异质性的潜在影响。我们建议考虑根际水平衡，估计根际的 HR (HR _R) 来自底层裂隙岩石的干燥表层土壤。该模型使用在撒丁岛收集的 15 年数据集进行评估。当基于大块土壤层之间的水分梯度的典型方法用于估算 HR _{B 时}，所有季节的树木蒸腾作用都被低估了，尤其是在春季和夏季。强制使用测量的树木蒸腾模型，HR _B在春季和夏季下降，而下伏岩层对树木蒸腾作用的贡献是使用 HR _R估计的三倍基于模型。大块表土层的平均含水量非常低，在最干燥的夏季达到 0.06，但几乎没有昼夜动态；然而，在粗略估计的根际体积中浓缩水，产生的根际水含量明显更高（≈0.16），并且更具动态性。预测的心率_R在干燥的春季和夏季，蒸发蒸腾占主导地位（60% - 65%），达到树木蒸腾量的 80%。最重要的是，所提出的根际-HR 模型正确地预测了全年树木蒸腾的昼夜动态，以及春季活跃期的草蒸腾。在次日尺度上运行的生态水文模型应考虑将土壤划分为根际体积，从而允许对昼夜生物圈-大气质量和能量交换进行诊断和预测估计。