def _rnl_daily(tmax, tmin, ea, fcd):
    """Daily net long-wave radiation  (Eq. 17)

    Parameters
    ----------
    tmax : ee.Image or ee.Number
        Daily maximum air temperature [C].
    tmin : ee.Image or ee.Number
        Daily minimum air temperature [C].
    ea : ee.Image or ee.Number
        Actual vapor pressure [kPa].
    fcd : ee.Image or ee.Number
        cloudiness fraction.

    Returns
    -------
    ee.Image or ee.Number
        Daily net long-wave radiation [MJ m-2 d-1].
        Output data type will match "tmax" data type.

    Notes
    -----
    rnl = 4.901E-9 * fcd * (0.34 - 0.14 * sqrt(ea)) *
          0.5 * ((tmax + 273.16) ** 4 + (tmin + 273.16) ** 4))

    """
    return tmax.add(273.16).pow(4).add(tmin.add(273.16).pow(4)).multiply(0.5)\
        .multiply(ea.sqrt().multiply(-0.14).add(0.34))\
        .multiply(fcd).multiply(4.901E-9)