A Test of the Relationship between Sap Flow and Evapotranspiration, Normalized via Leaf Area, under Non-Limiting Soil Moisture

Abstract

Sap flow is the movement of fluid within plants, whereas reference evapotranspiration (ETo) occurs external to plants as the transfer of water vapor from a hypothetical grass crop. Yet, on daily time scales, and when soil moisture is non-limiting, sap flow has a positive linear relationship with ETo. Furthermore, the E2.88 model hypothesises that sap flow (Q) is equal to ETo when parameters are normalized by leaf area (AL) via the following relationship: Q/AL = ETo/2.88. The value of 2.88 is the supposed leaf area index of the hypothetical grass in the ETo model. Therefore, the E2.88 model potentially provides a null or expected value of sap flow based on independent ETo parameters and leaf area. A test of the E2.88 model was conducted via three sap flow methods (dual method approach [DMA], heat ratio [HRM], and Tmax method) on the measurement of three woody species: Pyrus communis L. (var. Beurre Bosc Pear), Syzygium floribundum F. Muell. (Weeping Lilly Pilly), and Syzygium paniculatum Gaertn. (Lilly Pilly). A data compilation of the literature expanded the sample size to include additional species. The measured trees and data compilation found a strong, positive correlation between sap flow and ETo normalized by leaf area. However, the interpretation of the results was dependent on the sap flow method. The DMA had an average accuracy of 1.6%, whereas the HRM and Tmax significantly underestimated and overestimated sap flow, respectively. This study suggested that sap flow can be reliably estimated from accurate leaf area and ETo measurements and when other variables, such as soil moisture, are non-limiting.