the accuracy of scaling laws has been small. For example, Zhang et al. (2009) used a linear combination of $i$-band surface brightness and $g-r$ colour that was motivated by the Kennicutt-Schmidt law to achieve a scatter in their  estimator of $\sim$ 0.31 dex. Catinella et al. (2010) found that the near ultraviolet (NUV) NUV$-r$ colour was the single best estimator of HI mass, and determined a ‘gas fraction plane’ by combining the stellar mass surface density ($\mu_{\star}$), NUV$-r$ and HI gas mass fraction. However, the scatter in this relation remained at 0.3 dex (see also Catinella et al. 2012). Motivated by the finding that gas fractions are linked to outer disk colours (Wang et al. 2011), Li et al. (2012) add the colour gradient to their linear estimator, applied to ALFALFA and GASS samples, but again the scatter in the HI gas fraction was not improved beyond $\sim0.3$ dex. Moreover, many of these linear scaling relationships still have significant systematics relative to stellar mass, age, colour or concentration indices (e.g., Zhang et al. 2009).

Despite their current limitations, linear scaling relations and the gas fraction plane are useful as indicators of ‘HI normalcy’, which can be used to identify outliers 