Eddy Covariance measurements of sensible and latent heat fluxes were made in twÏ ecosystems (tallgrass prairie and winter wheat) in north-central Oklahoma. In this presentation, we will discuss the effect of the soil moisture and leaf area on daily values of Priestley-Taylor coefficient a. We will examine variations in the Priestley-Taylor a over the entire growing season. We will also present a simple empirical model to compute actual latent heat flux using the Priestley-Taylor a , adjusted for variations in soil moisture and leaf area.