The Hargreaves Samani (HS) equation is one of the most promising approaches for estimation of reference evapotranspiration under data-scarce conditions. Many modifications of the HS equation have been performed under different climatic conditions using different approaches to improve the precision of the evapotranspiration (ET 0 ) estimates for use at different locations; the results have not been consistent.The purpose of this study was to review and to evaluate the two most promising parameters used for the calibration of the HS evapotranspiration equation, under the climatic conditions of Gangtok, East Sikkim, India. The calibration was based on solar radiation and temperature difference. The equation was calibrated for solar radiation coefficient, C H first and then for temperature constant, T H and temperature exponent, E H to estimate the extent of improvement in estimation. The resulting modification shows a significant reduction in error. When only C H is modified, the percentage bias error (PBIAS) value was reduced from 23.3 to 0.8%, 26.1 to 10.19% and 30.1 to 3.6% for daily, weekly and monthly scales respectively. Consistent results for all three time spaces were observed, when, C H = 0.001 and T H = 13.9 and E H = 0.56. The PBIAS value was À1.9, 7.7 and 1.3% respectively for the three time spans, with index of agreement near to 1. The proposed equation may be an alternative to the Penman-Monteith method under limiting data conditions.