Previous fluid-mechanical descriptions of FACE arrays have been largely one- or two-dimensional. The efficiency parameter e=XAu/Q, which describes the overall usage of gas (Q) in terms of the size of the array (A), the concentration increment (X), and the mean wind speed (u), does not explicitly recognize the three-dimensional nature of FACE. Modeling of gas dispersion has been done on a two-dimensional basis, as have some of the data analyses of the gas distribution patterns within the array. We would expect that the system will behave differently as the crop matures over the course of a growing season; in 1989 at Maricopa, the gas injection system was modified progressively to account for the increase in canopy height.