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We describe the derivation and validation of redshift distribution estimatesand their uncertainties for the galaxies used as weak lensing sources in theDark Energy Survey (DES) Year 1 cosmological analyses. The Bayesian PhotometricRedshift (BPZ) code is used to assign galaxies to four redshift bins betweenz=0.2 and 1.3, and to produce initial estimates of the lensing-weightedredshift distributions $n^i_{PZ}(z)$ for bin i. Accurate determination ofcosmological parameters depends critically on knowledge of $n^i$ but isinsensitive to bin assignments or redshift errors for individual galaxies. Thecosmological analyses allow for shifts $n^i(z)=n^i_{PZ}(z-\Delta z^i)$ tocorrect the mean redshift of $n^i(z)$ for biases in $n^i_{\rm PZ}$. The $\Deltaz^i$ are constrained by comparison of independently estimated 30-bandphotometric redshifts of galaxies in the COSMOS field to BPZ estimates madefrom the DES griz fluxes, for a sample matched in fluxes, pre-seeing size, andlensing weight to the DES weak-lensing sources. In companion papers, the$\Delta z^i$ are further constrained by the angular clustering of the sourcegalaxies around red galaxies with secure photometric redshifts at 0.15<z<0.9.This paper details the BPZ and COSMOS procedures, and demonstrates that thecosmological inference is insensitive to details of the $n^i(z)$ beyond thechoice of $\Delta z^i$. The clustering and COSMOS validation methods produceconsistent estimates of $\Delta z^i$, with combined uncertainties of$\sigma_{\Delta z^i}=$0.015, 0.013, 0.011, and 0.022 in the four bins. Wemarginalize over these in all analyses to follow, which does not diminish theconstraining power significantly. Repeating the photo-z procedure using theDirectional Neighborhood Fitting (DNF) algorithm instead of BPZ, or using the$n^i(z)$ directly estimated from COSMOS, yields no discernible difference incosmological inferences.

Dark Energy Survey Year 1 Results: Redshift distributions of the weak lensing source galaxies