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The influence of surface enhanced covalency on the Madelung potential isexperimentally investigated using angle-resolved photoemission for (100), (110)and (111) SrTiO$_3$ surfaces after annealing in UHV at 630 {\deg}C.Deconvolution of the core level spectra (O 1s, Sr 3d and Ti 2p) distinguishesbulk and surface components, which are interpreted in terms of surface enhancedcovalency (SEC). By comparing the experimentally measured binding energies withtheoretical calculations developed in the framework of the Localized-HolePoint-Ion Model, we quantitatively determine the effective electron occupancyat bulk and surface Sr and Ti sites. Our results confirm the essentially ioniccharacter of Sr$-$O bond and the partially covalent character of Ti$-$O bond inbulk STO. The cation Ti and Sr electron occupation is greater for all the threesurfaces than in the bulk. Surface covalency shifts the Madelung potential atthe surface by {\Delta}EM. {\Delta}EM is a minimum for the (111) surface, andincreases through (100), attaining a maximum for (110). The angle-resolvedvalence band spectra and the work function values also confirm this trend. Theresults are consistent with d$-$d charge fluctuations dominating at thesurface, whereas metal-ligand charge transfers are more energeticallyfavourable in the bulk.

Surface enhanced covalency and Madelung potentials in Nb doped SrTiO$_3$ (100), (110) and (111) single crystals