  Determining Lya properties of distant galaxies is of great interest for
various astrophysical studies. We examine how the strength of Lya emission can
be constrained from broad-band SED fits instead of relying on spectroscopy. We
use our SED fitting tool including the effects of nebular emission, considering
in particular Lya emission as a free parameter, and we demonstrate our method
with simulations of mock galaxies. Using this tool we analyse a large sample of
U, B, V, and i dropout galaxies with multi-band photometry. We find significant
trends of the fraction of galaxies with Lya emission increasing both with
redshift z and towards fainter magnitude (at fixed z), and similar trends for
the Lya equivalent width. Our inferred Lya properties are in good agreement
with the available spectroscopic observations and other data. These results
demonstrate that the strength of Lya emission in distant star-forming galaxies
can be inferred quantitatively from broad-band SED fits, at least statistically
for sufficiently large samples with a good photometric coverage.
