Seasonal and diurnal stomatal conductance, leaf transpiration, and soil water contents of two shrubs of Hippophae rhamnoides L. subsp. Sinensis Rousi and Caragana korshinskii Kom., two trees of Malus pomila Mill. and Robinia pseudoacacia L., and a forb, Artemisia gmelinii, were measured in field of the semiarid Loess Plateau, north China, during the growing season of 2002. We developed a dynamic, nonlinear semi-mechanistic model to relate stomatal conductance of these plants to soil water potential, incident photon flux density, vapour pressure deficit, and partial CO₂ pressure, on leaf surface. The model can be easily adapted to ecosystem simulation because of its mathematical simplicity. Guard-cell osmotic pressure at zero light intensity, apparent elastic modulus of guard cells per leaf area, half-saturation light intensity, maximum light-inducible osmotic pressure, soil-to-leaf resistance at zero plant water potential, sensitivity of soil-to-leaf resistance to xylem water potential, and plant body water capacitance, are independent parameters of the model. The model was fitted to the field data of the five species with a non-linear least-square algorithm to obtain the parameters. The result indicates that the model explained, on average, 88% of seasonal and diurnal variation of stomatal conductance for the five species, in comparison with 67% of variation explained by an early model without plant body water capacitance. Comparisons of the physiological parameters among the species show that the woody species exhibited more tolerance for water stresses than the forb because of their higher dark osmotic pressure, greater capability of seasonal and diurnal osmotic regulation, and stiffer guard cell structure (or smaller stomatal density or both). A decreasing trend of soil-to-leaf resistance from the trees to the shrubs to the forb was found in this study. Midday depression of transpiration and stomatal conductance may or may not occur, depending on the magnitude of body water capacitance.