  The ultraconfined light of plasmonic modes put their effective wavelength
close to the mean free path of electrons inside the metal electron gas. The
Drude model, which can not take the repulsive interactions of electrons into
account, then clearly begins to show its limits. In an intermediate length
scale where a full quantum treatment is computationally prohibitive, the
semiclassical hydrodynamic model, instrinsically non-local, has proven
successful. Here we generalize the expression for the absorption volume density
and the reciprocity theorem in the framework of this hydrodynamic model. We
validate numerically these generalized theorems and show that using classical
expressions instead leads to large discrepancies.
