Dense wavelength-division multiplexing (DWDM) is a promising approach to design ultrahigh-capacity fiber-optic communication systems ( >; 50 Tb/s). However, DWDM gives rise to severe physical impairments that adversely affect system performance. To mitigate various physical impairments in DWDM systems and exploit their system capacity, there is a need to develop a 2-D (time and wavelength) discrete-time input-output model of physical impairments that can become the foundation of signal processing for optical communications. This paper develops such a model based on the Volterra series transfer function (VSTF) method. We overcome the well-known triple integral problem associated with the VSTF method and reduce it to a simple integral. This model takes into account multiple channel effects, fiber losses, frequency chirp, optical filtering, and photo detection, which are ignored in the current literature. The model is in excellent agreement with results obtained by split-step Fourier simulation. Furthermore, with this model, we define coefficients that capture intersymbol interference, interchannel interference, self-phase modulation, intrachannel cross-phase modulation (XPM), intrachannel four-wave mixing (FWM), XPM, and FWM to characterize the impact of these effects individually on the system performance. We also apply this model to analyze the effects of varying system parameters and pulse shapes on the individual physical impairments.