ESSnuSB is a design study for a future experiment which will measure CP violation in the lepton sector at the second neutrino oscillation maximum. Since the experiment will observe muon neutrino to electron neutrino oscillations, it is important to measure interaction cross section of electron neutrinos with water. For this purpose, neutrino flux at near detector site must be precisely known. This work shows the progress done in constraining the ESSnuSB neutrino flux by considering the elastic scatterings of neutrinos on orbital electrons observed at the near water Cherenkov detector.