Sensitivity of evapotranspiration to global warming: a case study of arid zone of Rajasthan (India)

Global warming due to greenhouse effect is expected to cause major changes in climate of some areas. The change in climate is likely to have a profound effect on hydrological cycle viz. precipitation, evapotranspiration, soil moisture etc. Evapotranspiration (ET) being the major component of hydrological cycle will affect crop water requirement and future planning and management of water resources. In the present study, an attempt has been made to study the sensitivity of evapotranspiration to global warming for arid regions of Rajasthan (India). The Penman-Monteith equation was used to estimate reference evapotranspiration, and sensitivity of ET has been studied in terms of change in temperature, solar radiation, wind speed and vapor pressure within a possible range of ±20% from the normal long-term meteorological parameters of 32 years (1971-2002). Changes in precipitation and stomatal resistance to increased CO2 concentration have not been considered in the present study. The study suggests an increase of 14.8% of total ET demand with increase in temperature by 20% (maximum 8 °C). ET is less sensitive (11%) to increase in net solar radiation, followed by wind speed (7%) in comparison to temperature. Increase in vapor pressure (20%) has a small negative effect on ET (−4.31%). A 10% increase in temperature and actual vapor pressure coupled with 10% decrease in net solar radiation could result even in marginal decrease of total ET (0.30%). Increase of 10% in temperature alone, with 10% decrease in net solar radiation, actual vapor pressure and wind velocity could also result in marginal decrease in total ET (0.36%). A marginal increase in ET demand due to global warming will have a larger impact on resource-poor, fragile arid zone ecosystem of Rajasthan. It is high time for planners/users to think in terms of expected change in water requirement due to global warming while planning for development of future water resources in the arid region of Rajasthan. A wide spectrum climate change scenario is discussed in the paper as guideline for future development of water resources.