The downward convection of salt fingers or plumes developed from the unstable boundary layer of an evaporating dry salt lake is examined using a numerical model and Hele-Shaw cell experiments. In the convecting layer the early small waves evolve into fingerlike or plumelike formations, the number of fingers or plumes decreasing with time owing to differential growth and/or coalescence. Comparison of intermediate formational stages of this pattern with the pattern generated by a two-dimensional numerical simulation shows good qualitative agreement. However, there is a significant mismatch of the growth rates at long times. In the computer simulation the plume length develops approximately twice as rapidly as it does in the experimental case. A simple numerical experiment independent of the salt-lake boundary conditions is compared to previously published laboratory-scale measures of plume development in Hele-Shaw cells which confirms the retardation of Hele-Shaw plumes by a factor of approximately 50%. This departure is attributed to the differences in dimensionality between the Hele-Shaw flow domain and the model domain. The data indicated that leading plumes develop isolated behavior at long times and may not be adequately represented in Hele-Shaw analog models, and numerical simulation provides a more accurate simulation of field-scale behavior.¿ 1997 American Geophysical Union