Evaluation of Canopy Evapotranspiration and Photosynthesis During and Immediately After Rainfall Using Enclosed-Path Eddy Covariance Method

Abstract

 To clarify canopy evapotranspiration and photosynthesis during and immediately after rainfall, measurements of ecosystem CO₂/H₂O fluxes using enclosed-path eddy covariance method, sap flow velocity, and micrometeorological conditions were taken in a Japanese cypress forest in the Kiryu Experimental Watershed in southern Shiga Prefecture, Japan. An enclosed path gas analyzer is more suitable than open-path or closed-path gas analyzers to measure CO₂/H₂O fluxes during and immediately after rainfall. Results show that H₂O flux during and after rainfall in relation to the vapor pressure deficit was greater than those on other days. H₂O flux during and after rainfall under low sap flow velocity is regarded as interception evaporation. With radiation increasing after rainfall, the H₂O flux component is regarded as a transition from interception evaporation to transpiration because of increased sap flow velocity. However, CO₂ absorption occurred during and immediately after rainfall in response to solar radiation in the same manner as on rain-free days. Observation revealed that the abaxial side of leaf surface remained dry even during and immediately after rainfall, which explains why photosynthesis occurs immediately with increased solar radiation after a rain event.