This paper presents some new wind-tunnel observations on flux-gradient relationships for momentum and heat just above a slightly heated rough surface, with emphasis on the implications for evapotranspiration measurement. The dimensional arguments leading to the concept of a flux-gradient relationship are shown to be valid only within a limited region, the inertial sublayer, which is separated from the surface by a roughness sublayer in which the dimensional arguments no longer apply. The wind-tunnel results show that the observed turbulent diffusivity for heat in the roughness sublayer is greater than the value expected from inertial-sublayer theory by a factor γ H which is about 2 near the surface and exceeds 1 throughout a layer of depth of about 10 h (h being the roughness element height). No such enhancement is seen for momentum.