  Upcoming space-based surveys such as Euclid and WFIRST-AFTA plan to measure
Baryonic Acoustic Oscillations (BAOs) in order to study dark energy. These
surveys will use IR slitless grism spectroscopy to measure redshifts of a large
number of galaxies over a significant redshift range. In this paper, we use the
WFC3 Infrared Spectroscopic Parallel Survey (WISP) to estimate the expected
number of Halpha (Ha) emitters observable by these future surveys. WISP is an
ongoing HST slitless spectroscopic survey, covering the 0.8-1.65micron
wavelength range and allowing the detection of Ha emitters up to z~1.5 and
[OIII] emitters to z~2.3. We derive the Ha-[OIII] bivariate line luminosity
function for WISP galaxies at z~1 using a maximum likelihood estimator that
properly accounts for uncertainties in line luminosity measurement, and
demonstrate how it can be used to derive the Ha luminosity function from
exclusively fitting [OIII] data. Using the z~2 [OIII] line luminosity function,
and assuming that the relation between Ha and [OIII] luminosity does not change
significantly over the redshift range, we predict the Ha number counts at z~2 -
the upper end of the redshift range of interest for the future surveys. For the
redshift range 0.7<z<2, we expect ~3000 galaxies/deg^2 for a flux limit of
3x10^{-16} ergs/s/cm^2 (the proposed depth of Euclid galaxy redshift survey)
and ~20,000 galaxies/deg^2 for a flux limit of ~10^{-16} ergs/s/cm^2 (the
baseline depth of WFIRST galaxy redshift survey).
