def rad2equiv_evap(energy):
    """
    Converts radiation in MJ m-2 day-1 to the equivalent evaporation in
    mm day-1 assuming a grass reference crop using FAO equation 20.
    Energy is converted to equivalent evaporation using a conversion
    factor equal to the inverse of the latent heat of vapourisation
    (1 / lambda = 0.408).

    Arguments:
    energy - energy e.g. radiation, heat flux [MJ m-2 day-1]
    """
    # Determine the equivalent evaporation [mm day-1]
    equiv_evap = 0.408 * energy
    return equiv_evap