A theoretical model is developed to quantify the experimental observations of self-similar parabolic pulses in a mode-locked laser cavity with net-zero dispersion. The averaging procedure used shows the pulse amplitude to be governed by the porous media equation that has the well-known Barenblatt similarity (parabolic) solution, suggesting that it is a viable theoretical description of temporal profiles observed in mode-locked cavities. To the best of our knowledge, this is the first analytic model proposing a mechanism responsible for generating temporal parabolic pulses.