  We present results of a fully non-local model of convection for white dwarf
envelopes. We show that this model is able to reproduce the results of
numerical simulations for convective efficiencies ranging from very inefficient
to moderately efficient; this agreement is made more impressive given that no
closure parameters have been adjusted in going from the previously reported
case of A-stars to the present case of white dwarfs; for comparison, in order
to match the peak convective flux found in numerical simulations for both the
white dwarf envelopes discussed in this paper and the A-star envelopes
discussed in our previous work requires changing the mixing length parameter of
commonly used local models by a factor of 4. We also examine in detail the
overshooting at the base of the convection zone, both in terms of the
convective flux and in terms of the velocity field: we find that the flux
overshoots by approximately 1.25 H_P and the velocity by approximately 2.5 H_P.
Due to the large amount of overshooting found at the base of the convection
zone the new model predicts the mixed region of white dwarf envelopes to
contain at least 10 times more mass than local mixing length theory (MLT)
models having similar photospheric temperature structures. This result is
consistent with the upper limit given by numerical simulations which predict an
even larger amount of mass to be mixed by convective overshooting. Finally, we
attempt to parametrise some of our results in terms of local MLT-based models,
insofar as is possible given the limitations of MLT
