A simple model for estimating the open flux in the polar cap was based on precipitating electron data from polar orbiting satellites. This model was applied in the growth phase of two substorms on March 27, 1979, to determine the fraction of the flux of the southward IMF which merged at the forward magnetopause, contributing to the polar cap flux. The effective merging efficiency at the forward magnetopause was found to be 0.19¿0.03 under average solar wind conditions. The westward electrojet current during the expansion and recovery phases of the same substorms was approximately proportional to the time rate of decrease of polar cap flux due to merging in the tail. An empirical model for calculating polar cap flux changes using the merging at the forward magnetopause for estimating increases and the westward electrojet for decreases was compared with observed changes in the polar cap flux. Agreement between the predicted and observed changes in the polar cap flux was tested over an interval of 8 hours. The advantages and limitations of the method are discussed.