  We prove that a linear nonautonomous differential system with nonuniform
hyperbolicity on the half line can be expressed as diagonal system with a
perturbation which is small enough. Moreover we show that the diagonal terms
are contained in the nonuniform exponential dichotomy spectrum. For this
purpose we introduce the concepts of \textit{nonuniform almost reducibility}
and \textit{nonuniform contractibility} which are generalization of this
notions originally defined in a uniform context.
