Articles
NUMERICAL EVALUATION OF SAP FLOW METHOD
Article number
846_4
Pages
61 – 68
Language
English
Abstract
The performance of sap flow methods has traditionally been verified using other approaches to the estimation of plant water uptake such as micrometeorological techniques, lysimeters and numerical modelling.
Some results suggest that sap flow methods still need further development before they can be applied as routine techniques for monitoring water consumption in the field. The so-called heat dissipation method (Granier, 1985, 1987, ) has been widely applied due to its relative simplicity and operational advantages, allowing low cost and long term measurements to be made on a significant number of plants.
Due to the complexities of sap flow in plants, empirical parameters are used, although their general validity has been questioned.
A finite difference simulation model developed to simulate the heat field around a linear heater was used to visualise the heat fields for varying sap flux densities and estimate the sensitivity to a range of (1) wood thermal properties, (2) heater power dissipation rates and (3) distances between sensor probes, contributing to the interpretation of results and of algorithms used in converting sensor measurements to estimates of sap flow.
Results suggested (1) a possible effect of wood thermal properties on empirical parameters of the calibration equation; (2) no influence of the power applied on measured fluxes and (3) which is the minimum distance between sensor probes to prevent interference between them.
Specific short-term measurements were also performed illustrating aspects (2) and (3); its results compared well with the simulation results, while the influence of thermal diffusivity on the empirical parameters does not fully explain the underestimation observed sometimes under field conditions.
Some results suggest that sap flow methods still need further development before they can be applied as routine techniques for monitoring water consumption in the field. The so-called heat dissipation method (Granier, 1985, 1987, ) has been widely applied due to its relative simplicity and operational advantages, allowing low cost and long term measurements to be made on a significant number of plants.
Due to the complexities of sap flow in plants, empirical parameters are used, although their general validity has been questioned.
A finite difference simulation model developed to simulate the heat field around a linear heater was used to visualise the heat fields for varying sap flux densities and estimate the sensitivity to a range of (1) wood thermal properties, (2) heater power dissipation rates and (3) distances between sensor probes, contributing to the interpretation of results and of algorithms used in converting sensor measurements to estimates of sap flow.
Results suggested (1) a possible effect of wood thermal properties on empirical parameters of the calibration equation; (2) no influence of the power applied on measured fluxes and (3) which is the minimum distance between sensor probes to prevent interference between them.
Specific short-term measurements were also performed illustrating aspects (2) and (3); its results compared well with the simulation results, while the influence of thermal diffusivity on the empirical parameters does not fully explain the underestimation observed sometimes under field conditions.
Publication
Authors
M.I. Ferreira, A.L. Silva, A. Thomsen
Keywords
water, transpiration, Granier, heat dissipation, 2D simulation
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