A new approach, based in part on diffusive equilibrium, has been developed to model the dynamic H+ content of plsmaspheric magnetic flux tubes. Called the Global Plasmasphere lonosphere Density (GPID) model, its aim is to reproduce the main dynamic features of the more complete Field Line Interhemispheric Plasma (FLIP) model, with a simpler formulation that allows for more rapid computation of the ions temporal variation in a magnetic flux tube. By simultaneously modelling several thousand flux tubes, a global representation of the plasmasphere can be obtained in a realistic time frame. A global model allows the plasmaspheric contribution to the Total Electron Content (TEC), measured from Global Position System (GPS) satellite radio signals, to be determined and removed, hence permitting the underlying ionosphere to be observed. This paper presents the approach used to model a single magnetic flux tube. GPID assumes an lonosphere/plasmasphere composition of O+ and H+, and includes chemical processes and simple diffusive transport. In almost all situations, GPID reproduces the predictions of the FLIP model. Comparisons between GPID and the refilling fluxes obtained from whistler observations generally show good agreement, both in diurnal variations and during prolonged refilling of empty flux tubes. Comparisons to seasonal variations during solar maximum at L = 2.5 show good agreement with observations, but at solar minimum there is variable agreement. (C) 2001 Elsevier Science Ltd. All rights reserved.