Source charge model is reviewed and is used to analyze electric field changes caused by eight directions of lightning streamers. Moreover, an improved piecewise-linear source charge model combined with the three-dimensional radiation source mapping results to interpret electric field changes on the ground is proposed. This model is applied to simulating E-field waveform and calculating electric parameters within initial streamer and the K event in an intracloud flash. The simulation result is consistent with the measured waveform. Mean charge line densities, currents and transferred charges of two processes are estimated: for the initial streamer, they are 0.53 C/km, 188.2 A, and 3.8 C, respectively; while for the K event, they are varying with the development of the channel.