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The Soil Moisure-Atmosphere Coupling Experiment (SMACEX) was conducted as part of the larger Soil Moisture Experiments (SMEX) field campaign in the Walnut Creek watershed near Ames, Iowa over the period from 15 June to 11 July 2002. As part of that work, measurements of the height of the boundary layer were gathered using a vertically staring lidar, and a method was developed for calculating the value of the entrainment parameter, A, from boundary layer geometry. The entrainment heat flux is of major importance to boundary layer models, representing the amount of energy that is supplied to the boundary layer by entrainment from the air above, which contributes to the growth of the boundary layer. The entrainment parameter, A, is defined as the ratio of the virtual potential temperature heat fluxes at the surface and at the top of the boundary layer. Most of the suggested values of A range between 0.1 and 0.3, with A=0.2 considered to be a good average to use in most modeling. In this paper we calculate the value of the entrainment parameter A based on entrainment zone geometry and examine a relationship between the entrainment parameter A, the vertical potential temperature gradient, and boundary layer development. The results here indicate that during some periods of boundary layer growth, the entrainment coefficient does not maintain a constant value. It is a characteristic that changes depending on the stage of boundary layer evolution.