Forest-hydrological Study on Transpiration in a Mountainous Watershed

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Attempt has been made to estimate transpiration rate from mountainous watershed using three methods in a forested watershed for detail description on forest transpiration. In the Kiryu Experimental watershed, annual and monthly transpiration rate had been estimated by the short term water budget method and intercepted rainfall observation. In this study, transpiration rate has been estimated using the Penman-Monteith model with the meteorological data observed above the forest canopy in a mixed stand of Japanese Red Pine and Japanese Cypress. Measurement of sap flow velocity using the heat-pulse method also has been carried out around the tower for meteorological observation in the Kiryu watershed.Sap flow velocity using the heat-pulse method has been measured continuously more than two years by an automatic heat-pulse velocity measuring system composed of a handheld computer and a A-D converter with relay switch output. The characteristic of the seasonal variations in transpiration ra … More te estimated by these three methods agreed well one another. Diurnal variations in transpiration estimated by the Penman-Monteith method and by the sap flow measurement also agreed well. However, the relationships between estimated rate by the Penman-Monteith method and sap flow measurement shows a non-linear curve. The reason of the non-linear relationships may caused by the non-linear relationships between the heat-pulse velocity calculated by a simple method and the actual sap flow velocity.For the accurate estimation of transpiration rate using the heat- pulse method, simultaneous observation of heat-pulse method and water absorption rate measurement for a cut tree of Japanese Cypress has been conducted. The relationships between water absorption rate and the heat- pulse velocity calculated by a simple method shows is the nearly same curve as the relationships between estimated value by the Periman-Monteith method and the heat-pulse method. This relationships can be reproduced a simulation model using numerical calculation of an advection-diffusion equation with actual heat-pulse transfer conditions. Less