This paper, part review and part new work, falls into three main sections. The first is a review of scale issues in both hydrology and meteorology, focusing on their origins in the water and energy conservation equations, integrated over control volumes of different scales. Several guidelines for scale translations are identified.
The second section reviews the upscaling problem in boundary-layer meteorology, setting out two ‘flux-matching’ criteria for upscaling models of land-air fluxes: the conservation requirement that surface fluxes average linearly and the practical requirement that model form be preserved between scales. By considering the effects of boundary conditions, it is shown that the combination or Penman-Monteith equation is a model for elemental energy fluxes which leads to physically consistent flux-matching rules for upscaling surface descriptors (resistances). These rules are tested, along with some other possibilities, and found to perform well.
The third section tests the hypothesis that regionally averaged energy balances over land surfaces are insensitive to the scale of heterogeneity, X. Heterogeneity is classified as microscale when X ⩽ UmT*, mesoscale when UmT* ⩽ X ⩽ UmTe, and macroscale when UmTe ⩽ X [where Um is the mean wind speed in the convective boundary layer (CBL) and T* and Te the convective and entrainment time scales, respectively]. A CBL slab model is used to show that regionally averaged energy fluxes are remarkably insensitive to X in both the microscale and macroscale ranges. Other reviewed evidence suggests that the mesoscale range behaves similarly in dry conditions. Questions remain about the consequences of clouds and precipitation for regionally averaged surface energy fluxes.