def water(target, temperature='pore.temperature', salinity='pore.salinity'):
    r"""
    Calculates surface tension of pure water or seawater at atmospheric
    pressure using Eq. (28) given by Sharqawy et al. Values at
    temperature higher than the normal boiling temperature are calculated at
    the saturation pressure.

    Parameters
    ----------
    target : OpenPNM Object
        The object for which these values are being calculated.  This
        controls the length of the calculated array, and also provides
        access to other necessary thermofluid properties.

    temperature : string
        The dictionary key containing the temperature values.  Temperature must
        be in Kelvin for this emperical equation to work

    salinity : string
        The dictionary key containing the salinity values.  Salinity must be
        expressed in g of salt per kg of solution (ppt).

    Returns
    -------
    sigma_sw, the surface tension of seawater in [N/m]

    Notes
    -----
     T must be in K, and S in g of salt per kg of phase, or ppt (parts per
        thousand)
    VALIDITY: 273 < T < 313 K; 0 < S < 40 g/kg;
    ACCURACY: 0.2 %

    References
    ----------
    Sharqawy M. H., Lienhard J. H., and Zubair, S. M., Desalination and
    Water Treatment, 2010.

    """
    T = target[temperature]
    if salinity in target.keys():
        S = target[salinity]
    else:
        S = 0
    sigma_w = 0.2358*((1-(T/647.096))**1.256)*(1-0.625*(1-(T/647.096)))
    a1 = 2.2637334337E-04
    a2 = 9.4579521377E-03
    a3 = 3.3104954843E-02
    TC = T-273.15
    sigma_sw = sigma_w*(1+(a1*TC+a2)*sp.log(1+a3*S))
    value = sigma_sw
    return value