  We analyze the role of the nonperturbative input in the Collins, Soper, and
Sterman (CSS)'s $b$-space QCD resummation formalism for Drell-Yan transverse
momentum ($Q_T$) distributions, and investigate the predictive power of the CSS
formalism. We find that the predictive power of the CSS formalism has a strong
dependence on the collision energy $\sqrt{S}$ in addition to its well-known
$Q^2$ dependence, and the $\sqrt{S}$ dependence improves the predictive power
at collider energies. We show that a reliable extrapolation from perturbatively
resummed $b$-space distributions to the nonperturbative large $b$ region is
necessary to ensure the correct $Q_T$ distributions. By adding power
corrections to the renormalization group equations in the CSS formalism, we
derive a new extrapolation formalism. We demonstrate that at collider energies,
the CSS resummation formalism plus our extrapolation has an excellent
predictive power for $W$ and $Z$ production at all transverse momenta $Q_T\le
Q$. We also show that the $b$-space resummed $Q_T$ distributions provide a good
description of Drell-Yan data at fixed target energies.
