def turc(tmean, rs, rh, k=0.31):
    """Evaporation calculated according to [turc_1961]_.

    Parameters
    ----------
    tmean: pandas.Series
        average day temperature [°C]
    rs: pandas.Series
        incoming solar radiation [MJ m-2 d-1]
    rh: pandas.Series
        mean daily relative humidity [%]
    k: float, optional
        calibration coefficient [-]

    Returns
    -------
    pandas.Series containing the calculated evaporation.

    Examples
    --------
    >>> et_turc = turc(tmean, rs, rh)

    Notes
    -----
    Based on equation 2 and 3 in [xu_2000]_.

    .. math:: PE=k(\\frac{T_a}{T_a+15})(R_s/4.184 + 50)*4.184; for RH>50

    References
    ----------
    .. [turc_1961] Xu, C‐Y., and V. P. Singh. "Evaluation and generalization of
       radiation‐based methods for calculating evaporation." Hydrological
       processes 14.2 (2000): 339-349.
    .. [xu_2000] Xu, C. Y., & Singh, V. P. (2000). Evaluation and
       generalization of radiation‐based methods for calculating evaporation.
       Hydrological processes, 14(2), 339-349.
    """
    c = tmean / tmean
    c[rh] = 1 - (50 - rh) / 70
    et = k * c * tmean / (tmean + 15) * (rs + 2.094)
    return et