Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat Calibration of compensation heat pulse velocity technique for measuring transpiration of selected indigenous trees using weighing lysimeters C.M. Tfwala a,c, ⁎ , L.D. van Rensburg a , Z.A. Bello a , S.R. Green b a Department of Soil, Crop & Climate Sciences, University of the Free State, P.O. Box 339, 9300 Bloemfontein, South Africa b Plant and Food Research, Private Bag 11600, Palmerston North, 4442 Auckland, New Zealand c Department of Agricultural Research and Specialists Services, Ministry of Agriculture, P.O. Box 4, 204 Malkerns, Swaziland ARTICLE INFO Keywords: Sap flow calibration Tree water use Indigenous South African trees ABSTRACT The compensation heat pulse velocity (CHPV) is one of the most widely used methods to measure sap flow in woody plants. However, the accuracy of this method has not been fully explored especially for indigenous tree species of South Africa. The aim of this study was to evaluate the accuracy of the CHPV method in quantifying tree transpiration for selected tree species. Three indigenous trees sampled in a monolith form; black karee (Sersia lancea), buffalo thorn (Ziziphus mucronata) and wild olive (Olea africana) grown on weighing lysimeters (1 m × 1 m × 1.3 m) were installed with CHPV probes to measure sap flow on the stem half hourly, simulta- neously with lysimeter measurements of transpiration. The surfaces of the lysimeters were covered with a 10 cm layer of Styrofoam, overlain by a 2 cm layer of gravel to minimize evaporation to a negligible level. Both the lysimeter and CHPV measurements were divided into two sets. The first set was used to develop tree specific calibration equations as well as an equation combining the three species used, here called a combination equation. The second set of data was used for validating the equations. Transpiration rates ranged from negli- gible at night to daily peaks of 3.5, 1.7 and 1.4 L h -1 for buffalo thorn, black karee and wild olive, respectively. Good agreement indices between CHPV and lysimeters were obtained when using both the tree specific equa- tions and combination equation across species (D = 0.778–1.000, RMSE = 0.001–0.017 L h -1 , MAE < 0.001 L h -1 and MBE = -0.0007 to 0.0008 L h -1 ). It was concluded that the CHPV method can ac- curately measure tree water use, and therefore can be useful for water resources management in forested areas. 1. Introduction There are various ways of quantifying transpiration of field crops, but, for trees, sap flow measurement remains the most prominent. An array of automated thermodynamic techniques which use heat as a tracer of sap flow, have for several decades been employed to quantify water fluxes in the soil – plant – atmosphere continuum at the tree scale. Amongst the thermal based methods of sap flow measurement, the most common are the compensation heat pulse velocity (CHPV) (Huber and Schmidt, 1937; Marshall, 1958; Swanson, 1962; Swanson and Whitfield, 1981; Fernández et al., 2001; Green et al., 2003) and thermal heat dissipation (THD) (Granier, 1987; Lu et al., 2004) methods. These methods are user friendly and cause minimal damage to the tree stems (Granier, 1987; Green et al., 2003). The CHPV method also has an advantage of using less energy as it works on pulses of heat as opposed to the continuous heating principle of the THD (Fernández et al., 2006; Nourtier et al., 2011). The CHPV method has also been employed to measure transpiration of local and exotic commercial forest species of South Africa such as Eucalyptus species (Morris et al., 2004; Wildy et al., 2004) and Jatropha curcas (Gush, 2008). The accu- racy of the CHPV method in measuring sap flow in indigenous trees of South Africa has unfortunately not been explored. Lysimetry is considered as one of the standard methods to quantify plant water use (Johnson et al., 2005; Clawson et al., 2009), especially if their load cells are correctly calibrated (Misra et al., 2011) and can account for various components of the soil water balance while keeping other components under control (Dlamini et al., 2016). Even though lysimeters can accurately measure plant water use and can be applied in a number of studies such as soil nutrients, evaporation and drainage analysis, it has its own drawbacks such as high cost and limited re- plication. The indigenous trees in South Africa contribute significantly to the economy of the country in the form of goods and services (Gush et al., 2015). However, knowledge on the transpiration of most of these indigenous trees is limited (Dye et al., 2008). The objective of the present study was to evaluate the accuracy of CHPV method in mea- suring transpiration of selected indigenous tree species of South Africa. https://doi.org/10.1016/j.agwat.2018.01.005 Received 7 August 2017; Received in revised form 29 December 2017; Accepted 8 January 2018 ⁎ Corresponding author at: Department of Soil, Crop & Climate Sciences, University of the Free State, P.O. Box 339, 9300 Bloemfontein, South Africa. E-mail address: 2008004043@ufs4life.ac.za (C.M. Tfwala). Agricultural Water Management 200 (2018) 27–33 0378-3774/ © 2018 Elsevier B.V. All rights reserved. T