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Surface evaporation is an important component of the urban energy balance, and its role in arid-zone cities may significantly differ from that observed in more temperate regions. However, the quantification of evapotranspiration is difficult in a Complex urban setting, given the heterogeneity of the terrain and its various dry and wet elements. Here an open-air scaled urban surface (OASUS) is employed to quantity latent heat flux as a function of the surface area available for evaporation and the three-dimensional (3-D) urban geometry. The OASUS model consists of an extensive urban-like building/street array located in an and Negev region of southern Israel (30.8 degrees N, 480 m above sea level). Measurements were carried out during the summer month of August, and flux partitioning was analysed using evaporation pans of varying surface areas embedded in arrays of varying height. Results indicated that the increase in latent heat removal with respect to equivalent ’vegetative’ surface area was nearly linear. Increasing latent heat flux with ’vegetated fraction’ was offset in approximately equal measure by decreases in storage and turbulent sensible heat flux. The proportion of the radiant energy budget represented by evaporative heat loss was also linked to the 3-D geometry of the array, increasing linearly with the ratio between vegetative cover and the ’complete’ urban surface area. Copyright (c) 2008 Royal Meteorological Society