The determination of transpiration efficiency coefficient for common bean H.O. Ogindo * , S. Walker Department of Soil, Crops and Climate Sciences, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa Abstract A number of studies have been conducted to determine species specific transpiration efficiency coefficient. Although the value is available for some C3 legumes, no value has been determined for common beans within the semi-arid tropics. The coefficient is use- ful in modelling crop water use as it has been found to be conservative over a range of climates when differences in vapour pressure deficits are accounted for. The objective of the experiment was to determine the transpiration efficiency coefficient for common beans for use in modelling within the semi-arid region of South Africa. Common bean (Phaseoulus vulgaris L.) was grown on a weighing lysimeter during the 2000/2001 and 2001/2002 seasons. Tran- spiration was measured on hourly basis using the weighing lysimeter and the data integrated over the growing season to determine the seasonal transpiration for the crop. At the same time hourly measurement of canopy vapour pressure deficit was made using wet and dry bulb resistance thermometers housed in mini-shelters at 200–400 mm height. Wet and dry bulb temperature data was also collected at the nearby standard automatic weather station and used to normalize the transpiration efficiency. Transpiration efficiency for the common bean was 1.33 and 1.93 g kg 1 which when normalized and root adjusted, gave a tran- spiration efficiency coefficient of 3.02 and 3.51 g kPa kg 1 for the 2000/2001 and 2001/2002 seasons respectively. A mean transpira- tion efficiency coefficient of 3.26 ± 0.25 g kPa kg 1 was adopted for the two seasons. This value is fairly consistent with those obtained for other C3 legumes species, confirming the conservativeness of the coefficient and therefore its usefulness as modelling parameter. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Transpiration efficiency coefficient; Phaseoulus vulgaris 1. Introduction Common bean (Phaseoulus vulgaris L.) is an impor- tant food crop in most parts of sub-Saharan Africa. It is grown either as sole crop or in combination with cer- eal crops (Siame et al., 1997; Ayisi and Poswall, 1997; Osiru and Willey, 1972; Mukhala, 1998). In either crop- ping system it is important to determine resource cap- ture and use. The concept of resource capture and utilization is important when we are considering either radiation or water use in crops. Black and Ong (2000) provide an in-depth review of the concept in relation to radiation and water use in tropical agriculture. Water resources are particularly limiting in tropical regions compared to radiation which is almost always abundant and therefore research into its use is of direct benefit to understand crop productivity. Theoretically, the link between the amount of carbon dioxide entering the crop and the amount of water leav- ing via the stomata means that in principle seasonal transpiration can be used to estimate the carbon assim- ilation of a crop. These relationships can be explained at leaf stomatal level and were expressed by Farquahar and Richards (1984) as: 1474-7065/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2004.09.025 * Corresponding author. Tel.: +27 51 401 2656; fax: +27 51 401 2212. E-mail address: ogindoh.sci@mail.uovs.ac.za (H.O. Ogindo). www.elsevier.com/locate/pce Physics and Chemistry of the Earth 29 (2004) 1083–1089