Heat pulse methods are a popular approach for estimating sap flow and transpiration. Yet, many methods are unable to resolve the entire heat velocity measurement range observable in plants. Specifically, the Heat Ratio (HRM) and Tmax heat pulse methods can only resolve slow and fast velocities, respectively. The Dual Method Approach (DMA) combines optimal data from HRM and Tmax to output the entire range of heat velocity. However, the transition between slow and fast methods in the DMA currently does not have a theoretical solution. A re-consideration of the conduction/convection equation demonstrated that the HRM equation is equivalent to the Péclet equation which is the ratio of conduction to convection. This study tested the hypothesis that the transition between slow and fast methods occurs when conduction/convection, or the Péclet number, equals one, and the DMA would be improved via the inclusion of this transition value. Sap flux density was estimated via the HRM, Tmax and DMA methods and compared with gravimetric sap flux density measured via a water pressure system on 113 stems from 15 woody angiosperm species. When the Péclet number ≤ 1, the HRM yielded accurate results and the Tmax was out of range. When the Péclet number > 1, the HRM reached a maximum heat velocity at approximately 15 cm hr -1 and was no longer accurate, whereas the Tmax yielded accurate results. The DMA was able to output accurate data for the entire measurement range observed in this study. The linear regression analysis with gravimetric sap flux showed an r2 of 0.541 for HRM, 0.879 for Tmax and 0.940 for DMA. With the inclusion of the Péclet equation, the DMA resolved the entire heat velocity measurement range observed across 15 taxonomically diverse woody species. Consequently, the HRM and Tmax are redundant sap flow methods and have been superseded by the DMA.

中文翻译：

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热脉冲树汁流动方法中传导与对流的重要性。

热脉冲法是估算树液流动和蒸腾作用的一种流行方法。然而，许多方法不能解决植物中可观察到的整个热速度测量范围。具体来说，热比（HRM）和Tmax热脉冲方法只能分别解决慢速和快速问题。双重方法（DMA）结合了HRM和Tmax的最佳数据，以输出整个热速度范围。但是，DMA中慢速和快速方法之间的过渡目前尚无理论解决方案。对传导/对流方程的重新考虑表明，HRM方程等效于Péclet方程，后者是传导与对流的比率。这项研究检验了以下假设，即当传导/对流或佩克利数，等于1，通过包含此转换值可以改善DMA。通过HRM，Tmax和DMA方法估算汁液通量密度，并将其与通过水压系统对15种木质被子植物113个茎上测得的重量树液通量密度进行比较。当Péclet数≤1时，HRM产生准确的结果，并且Tmax超出范围。当Péclet数> 1时，HRM在约15 cm hr -1处达到最大热速度，不再精确，而Tmax产生准确的结果。DMA能够输出在这项研究中观察到的整个测量范围内的准确数据。重量树液通量的线性回归分析显示，HRM的r2为0.541，Tmax的r2为0.879，DMA的r2为0.940。加上Péclet方程，DMA解决了在15种生物分类上观察到的整个热速度测量范围。因此，HRM和Tmax是冗余的汁液流动方法，并已被DMA取代。