def calc_fsoil(vwc, Tsoil):
    """implement Mary Whelan's soil COS flux model described in her email
    of 5 Feb 2015.

    INPUTS
       vwc: soil volumetric water content [fraction]
       Tsoil: soil temperature [k]

    OUTPUTS:
       fsoil: soil COS flux [pmol/m2/sec]
    """

    fsoil = (vwc * -28.77873448 + (Tsoil * 0.88867741) - 252.76497309)

    return(fsoil)