  We present a macroscopic model of the decay of a coherent classical scalar
field into statistical fluctuations through the process of parametric
amplification. We solve the field theory (henceforth,"microscopic") model to
leading order in a Large N expansion, and show that the macroscopic model gives
satisfactory results for the evolution of the field, its conjuguate momentum
and the energy momentum tensor of the fluctuations over many oscillations. The
macroscopic model is substantially simpler then the microscopic one, and can be
easily generalized to include quantum fluctuations. Although we assume here and
homogeneous situation, the model is fully covariant, and can be applied in
inhomogeneous cases as well. These features make this a promising model in
exploring the physics of preheating.
