def oudin(Ra, Tmean=None, Tmin=None, Tmax=None):
    """ Ra is the extraterrestrial solar radiation (MJ m−2 d−1) given by the Julian day and the latitude,
         T is the mean air temperature at a 2-m height (°C).
     References
    ----------
    Oudin, Ludovic, Lætitia Moulin, Hocine Bendjoudi, and Pierre Ribstein. 2010.
    “Estimating Potential Evapotranspiration without Continuous Daily Data: Possible Errors and Impact on Water Balance Simulations.”
    Hydrological Sciences Journal 55 (2): 209–22. doi:10.1080/02626660903546118.
     http://www.tandfonline.com/doi/full/10.1080/02626660903546118
     Eq. 3     """
    if Tmean == None:
        Tmean = (Tmin + Tmax) / 2.0
    if Tmean + 5 < 0:
        return 0
    else:
        return 0.408 * Ra * (Tmean + 5.0) / 100
