We report characterization of electromagnetically induced transparency (EIT) resonances in the $D1$ line of ${}^{87}\mathrm{Rb}$ under various experimental conditions. The dependence of the EIT linewidth on the power of the pump field was investigated at various temperatures for the ground states of the lambda system associated with different hyperfine levels of the atomic $5S_{1∕2}$ state as well as magnetic sublevels of the same hyperfine level. Strictly linear behavior was observed in all cases. A theoretical analysis of our results shows that dephasing in the ground state is the main source of decoherence, with population exchange playing a minor role.