Evapotranspiration (ET) is the major component of hydrological cycle, and will affect the crop water requirement and future planning and management of water resources under changing climate scenarios. In the present study, an attempt was made to study the sensitivity of reference evapotranspiration (ETo) to different climatic variables, and effect of temperature and elevated CO2 on ETo using meteorological data (1973-2010) of the Banaras Hindu University (BHU), Varanasi. The FAO-56

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... th method was used to estimate ETo, and sensitivity of ETo was studied in terms of changes in temperature, solar radiation, wind speed and vapour pressure deficit. The combined effect of temperature and elevated CO2 levels was studied by varying the temperature from 1°C to 5°C, and the CO2 level from 330 ppm to 660 ppm. The simulation results showed that the mean temperature (Tmean) influenced the annual ETo the most followed by solar radiation (Rs), vapour pressure deficit (VPD) and wind speed (U2). There are about 18.20, 13.80, 7.90 and 5.40% increase in annual ETo with 25% increase in Tmean, Rs, VPD, and U2, respectively. Simulating the combined effect of temperature and elevated CO2 indicated that the effect of 2.5 °C rise in temperature is offset by doubling of CO2 concentration. Thus, the effect of rising temperature is moderated by the increasing CO2 concentrations, and the crop water demand may not rise significantly under the climate change scenarios.