Partitioning of evapotranspiration (ET) into soil evaporation and vegetation transpiration (T) is important for predicting the response of ecosystem water/energy budgets to climatic change. In this study, a new two-source model of water and energy fluxes was developed and used to partition ET for a temperate–grassland ecosystem in central Japan, throughout a growing season. The Newton–Raphson iteration scheme was employed to solve the equations governing the energy balance at the canopy and the ground surface separately. Measured energy fluxes, including latent heat flux from the eddy covariance and energy balance method, and leaf temperature were compared with model predictions. Agreement between them demonstrates good performance by the model. Sensitivity analysis suggests this model is relatively insensitive to uncertainties/errors in assigned model parameters and measured input variables. Estimated T/ET was mainly controlled by shortwave radiation through changing leaf stomatal resistance at a diurnal timescale. Also, leaf area index is the primary controlling factor at a seasonal timescale and regulates both permittivity and canopy resistance. Soil moisture at the study site was a minor factor with the moderately humid conditions. Our results emphasize that water flow paths (i.e. transpiration and evaporation) from temperate grasslands to the atmosphere are strongly controlled by the physiological responses of plants to solar radiation on a diurnal timescale and vegetation growth on a seasonal timescale. Copyright © 2012 John Wiley & Sons, Ltd.