Sources of error in stem heat balance sap flow measurements

Whilst the importance of recognising and reducing the sources of error in sap flow measurements has been acknowledged, relevant information is widely dispersed in the experimental literature. This paper presents a critical review of the theory and reported sources of error for the constant power stem heat balance (SHB) method. The important influence of stem diameter and sap flow rate in determining the magnitude of errors is highlighted. The magnitude and relative importance of different errors were investigated through an analysis of data obtained from a large field experiment in Nigeria. Sap flow was measured in stems of 6-year-old trees of Acacia nilotica (subsp. adstringens) and Prosopis juliflora between July 1994 and January 1995. Commercially available constant power SHB gauges were used on different sized stems (19-50 mm diameter) under a wide range of sap flow and environmental conditions. The analysis demonstrates how stem size and night-time sap flow interact to cause error in determining the sheath conductance, the manufacturer's apparent understatement of the time constant, especially for large gauges, and how stem size, heat storage and patterns of ambient temperature variation combine to determine diurnal patterns of variation in measurement error. Results highlight the importance of considering a priori, for any given set of experimental conditions, the likelihood that underlying theoretical assumptions may be violated. Precautions to minimise potentially important sources of error are described for different sap flow rates and environmental conditions, and with respect to different experimental objectives. (C) 1999 Elsevier Science B.V. All rights reserved.