The Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model Version 5 (MM5) is used here to simulate the meteorological conditions at El Reno during the Southern Great Plains Experiment (SGP97). Two PBL parameterizations, the Blackadar high-resolution PBL and the Gayno-Seaman PBL models, are used in combination with two distinct land-surface models. MM5 simulations are evaluated against SGP-Twin Otter Aircraft data when available. The study focus is on the coupling of land-surface soil moisture to the evaporative fraction, and the diurnal cycle of mixing ratio and boundary layer depth