  We present an empirical parameterization of the [NII]/H$\alpha$ flux ratio as
a function of stellar mass and redshift valid at 0 < z < 2.7 and 8.5 < log(M) <
11.0. This description can easily be applied to (i) simulations for modeling
[NII]$\lambda6584$ line emission, (ii) deblend [NII] and H$\alpha$ in current
low-resolution grism and narrow-band observations to derive intrinsic H$\alpha$
fluxes, and (iii) to reliably forecast the number counts of H$\alpha$
emission-line galaxies for future surveys, such as those planned for Euclid and
the Wide Field Infrared Survey Telescope (WFIRST). Our model combines the
evolution of the locus on the Baldwin, Phillips & Terlevich (BPT) diagram
measured in spectroscopic data out to z = 2.5 with the strong dependence of
[NII]/H$\alpha$ on stellar mass and [OIII]/H$\beta$ observed in local galaxy
samples. We find large variations in the [NII]/H$\alpha$ flux ratio at a fixed
redshift due to its dependency on stellar mass; hence, the assumption of a
constant [NII] flux contamination fraction can lead to a significant under- or
overestimate of H$\alpha$ luminosities. Specifically, measurements of the
intrinsic H$\alpha$ luminosity function derived from current low-resolution
grism spectroscopy assuming a constant 29% contamination of [NII] can be
overestimated by factors of ~8 at log(L) > 43.0 for galaxies at redshifts z =
1.5. This has implications for the prediction of H$\alpha$ emitters for Euclid
and WFIRST. We also study the impact of blended H$\alpha$ and [NII] on the
accuracy of measured spectroscopic redshifts.
