Crop water requirement (CWR) under the projected climate change could be mediated through changes in other weather parameters including the air temperature. The present study was directed to assess the on-farm water requirement in wheat crop in future, in semi-arid Indo-Gangetic Plains of India, through field and computer simulations. Field simulation using temperature gradient tunnels shows 18% higher crop evapotranspiration (ET c) and 17% increase in root water extraction at 3.6°C elevated temperature compared to 1.5°C increase over the ambient. A time series model (ARIMA) with long-term (1984-2010) weather data of the experimental site and a global climate model (IPCC-SRES HADCM3) were used to simulate the potential ET (ET 0) of wheat for 2020-2021 and 2050-2051 years. The crop coefficient ( Kc) values for these years were generated through Kc-CGDD (Cumulative growing-degree-days) relation by using LARS-WG model-derived daily minimum and maximum temperatures. The CWR and NIR (Net Irrigation Requirement) are likely to be less in projected years even though air temperatures increase. The CWR reduces in ARIMA outputs owing to a lower reference ET (ET 0) due to decline in solar radiation. Under IPCC-SRES scenarios, the ET c-crop phenophase relation [CGDD-LGP (length of growing period) response] may offset the effect of rising temperature and a net decline in CWR is observed. It may be likely that the effect of temperature increase on CWR is manifested mostly through its relation with crop phenophase (thermal requirement to complete a specific growth stage) and not the temperature effect on ET 0 per se. This is certainly a ray of hope in managing the depleting irrigation water resources in the semi-arid wheat-growing regions of the IGP.
