Technical NoteInter-annual and spatial variability of Hamon potential evapotranspiration model coefficients

•Hamon PET coefficients are calibrated for 109,951 hydrologic response units.•PET generally is underestimated using the standard Hamon PET coefficient.•Calibrated Hamon PET coefficients provide a useful way to improve PET estimates.

SummaryMonthly calibrated values of the Hamon PET coefficient (C) are determined for 109,951 hydrologic response units (HRUs) across the conterminous United States (U.S.). The calibrated coefficient values are determined by matching calculated mean monthly Hamon PET to mean monthly free-water surface evaporation. For most locations and months the calibrated coefficients are larger than the standard value reported by Hamon. The largest changes in the coefficients were for the late winter/early spring and fall months, whereas the smallest changes were for the summer months. Comparisons of PET computed using the standard value of C and computed using calibrated values of C indicate that for most of the conterminous U.S. PET is underestimated using the standard Hamon PET coefficient, except for the southeastern U.S.