  We simulate the emission line galaxy properties for the Nancy Grace Roman
Space Telescope by creating a 4 deg$^{2}$ galaxy mock catalog using Galacticus,
a semi-analytic model (SAM) for galaxy formation and evolution. The simulated
galaxy properties include emission line luminosity, equivalent width (EW),
broad band photometry, and spectral energy distribution (SED). We compare this
mock catalog with the latest observational data from WISP and MOSDEF. We find
that our Galacticus model makes predictions consistent with observational data,
over a wide redshift range for multiple emission lines. We use this validated
galaxy mock catalog to forecast the photometric completeness of H$\alpha$ and
[OIII] emission lines as a function of line flux cut and H band magnitdue cut,
for both Roman and Euclid-like surveys. Our prediction for the photometric
completeness of a Euclid-like survey is in good agreement with previous work
based on WISP data. Our predictions for the photometric completeness of
possible implementations of the Roman High Latitude Wide Area Spectroscopic
Survey (HLWASS) provides key input for the planning of such a survey. We find
that at H = 24, a Euclid-like survey to the line flux limit of 2$\times
10^{-16}\,$erg/s/cm$^2$ is 97\% complete, while a Roman HLWASS to the line flux
limit of $10^{-16}\,$erg/s/cm$^2$ is only 94.6\% complete (it becomes 98\%
complete at H = 25).
