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Sap flow was measured, with Granier‐type sensors, in a crop of field‐grown water‐stressed cassava (Manihot esculenta Crantz) in Ghana, West Africa. The main objective of this study was to examine the environmental control of canopy conductance (gc) with a view to modelling the stomatal control of water transport under water‐stressed condition. Weather variables measured concurrently with sap flow were: air temperature (Ta), relative humidity (RH), wind speed (u) and solar radiation (Rs). Relationship between canopy conductance (gc) and vapour pressure deficit (Dɛ) was curvilinear while no specific pattern was observed with Rs. Average diurnal gc decreased from 3.0 ± 0.6 to 0.7 ± 0.4 mm s−1 between 0730 and 2000 h local time ( = GMT) each day. A Jarvis‐type model, based on a set of environmental control functions, was parameterized for the cassava crop in this study. Model results demonstrated that gc was estimated with a high degree of accuracy based on Rs, Ta, and Dɛ (r2 = 0.92;F = 809.2;P < 0.0001). Dɛ explained about 90% (F = 2129.7;P < 0.0001) of the variations observed in gc, whereas both Rs and Ta contributed about 2% of the explained variance in gc. The aerodynamic conductance (ga) was very high compared to gc, leading to a daily average ratio ga/gc > 100 and a decoupling factor < 0.1. Cross‐validation analysis revealed a consistent good performance (r2 > 0.85) of the gc model with Dɛ as the only independent environmental variable. Copyright © 2007 Royal Meteorological Society [ABSTRACT FROM AUTHOR]