Various models have been proposed to predict changes in scalar concentrations and surface fluxes following an abrupt change in surface conditions. In most cases the boundary conditions used to describe the surface represent limiting cases such as a step change in either concentration, flux or surface resistance. Three main conclusions emerge from a review of experimental results published thus far. Firstly, a dry-to-wet transition can be treated as a step change in concentration, after which the difference between the downwind and upwind surface fluxes show a power-law dependence on distance from the leading edge with an exponent of the order of - 1 6. Secondly, wet-to-dry transitions should be treated as resulting in a step change in flux. This has not always been appreciated. Thirdly, a dry-to-moist (e.g. irrigated) transition is shown to resemble more closely a step change in flux than a step change in surface resistance. This can only be explained if stomatal resistance varies in conjunction with downwind changes in saturation deficit. We propose a simple analytical model to describe the streamwise change in surface resistance in the limit of a step change in flux. A scarcity of published data suitable for validation of this model has led us to undertake a new experimental investigation. This experiment is the subject of a companion paper.