MEASUREMENT OF SAP FLOW BY THE HEAT-BALANCE METHOD - NUMERICAL-ANALYSIS AND APPLICATION TO CONIFEROUS SEEDLINGS

The heat balance method has been used successfully to measure sap flow in a number of plant species. This study was conducted to determine whether the method was applicable to coniferous seedlings, which have comparatively low rates of sap flow, and to investigate aspects of the method thought to be important at low flow rates. Flow gauges were constructed and used to measure sap flow rates in coniferous seedlings. A three-dimensional, transient, numerical model was developed to examine the effects of shortwave radiation, heat conduction along sensor leads, and transient characteristics on gauge accuracy. The model performed well and indicated that, for flow rates typical of conifers, shortwave radiation does not cause a significant error in measurement of the across-heater temperature difference (DELTA-T). When no reflective covering was used, shortwave radiation produced orientation-dependent deviation in modeled radial temperature gradients of up to 50% of the mean value. Heat conduction along thermocouple wires produced a measurement error of up to 15%, which can be minimized by using small-diameter wire. Gauge accuracy is poor at low flow rates because the convective heat flux and DELTA-T are small, and because the transient response of the gauge is slow. For coniferous seedlings, the method fails at low flow rates and measurements should be rejected when the convective heat flux is less than 5% of the heater power or when DELTA-T is small (less than 2.5-degrees-C in this study). At low flow rates, the heat storage term should be included in the heat balance.