def horizontal_weight_zreda2008b(r, a1=1.311e-2, a2=9.423e-5, a3=3.2e-7, a4=3.95e-10):
    """Presented by Bogena et al. (2013, Eq. 13), fitted to Zreda et al. (2008).

    Bogena et al. (2013) fitted a polynomial function through the relationship between
    cumulative fraction of counts (CFoC) and CRP footprint radius (r) presented by
    Zreda et al. (2008).

    Parameters
    ----------
    r : float or 1d array of floats
       Distance to the CRNS probe (in meters)
    a1, a2, a3, a4 : float
       Empirical parameters

    """
    return 1 - a1 * r + a2 * r**2 - a3 * r**3 + a4 * r**4