  Small-angle X-ray scattering from GaN nanowires grown on Si(111) is studied
experimentally and modeled by means of Monte Carlo simulations. It is shown
that the scattering intensity at large wave vectors does not follow Porod's law
$I(q)\propto q^{-4}$. The intensity depends on the orientation of the side
facets with respect to the incident X-ray beam. It is maximum when the
scattering vector is directed along a facet normal, as a reminiscence of the
surface truncation rod scattering. At large wave vectors $q$, the scattering
intensity is found to be decreased by surface roughness. A root mean square
roughness of 0.9~nm, which is the height of just 3--4 atomic steps per micron
long facet, already gives rise to a strong intensity reduction.
