def hargreaves_ETo(tmin, tmax, tmean, Ra):
    """
    Calcultaes evapotranspiration over grass [mm day-1] using the Hargreaves
    ETo equation. Generally, when solar radiation data, relative humidity data
    and/or wind speed data are missing, they should be estimated using the
    procedures/functions outlined in the comments at the top of this file and
    then ETo calculated using the Penman-Monteith equation.
    As an alternative, ETo can be estimated using the Hargreaves ETo equation.

    tmin    - minimum daily temperaure [deg C]
    tmax    - maximum daily temperaure [deg C]
    tmean   - mean daily temperaure [deg C]
    Ra      - extraterrestrial radiation as equivalent evaporation [mm day-1]
    """
    ETo = 0.0023 * (tmean + 17.8) * (tmax - tmin)**0.5 * Ra
    return ETo