An empirical back-trajectory model has been developed for the determination of the footprint of an eddy flux measurement in the surface layer. This model was evaluated against an analytical model which, in turn, has been shown to correctly characterize the footprint over a horizontally homogeneous surface in moderately unstable to moderately stable conditions (-0.01 <; zo/L < 0.005). The chief attraction of the model is the ability to determine the evolution of, and variations within, the footprint while a flux measurement is actually in progress. Individual short-term realizations of the footprint vary significantly from estimates given by a mean or ensemble representation of the footprint if assumptions of stationarity with respect to wind direction and stability are not satisfied. The model has applications to settings characterized by homogeneous and, more significantly, inhomogeneous distributions of surface source elements.