To study the effect of global warming on evapotranspiration (ET), knowledge of the predictions/forecasts for the likely change in climate parameters is the prerequisite. In the Indian context, a warming trend of 0.57 C per 100 years is broadly consistent with the observed global warming over last century (Pant and Kumar, 1997). However, the precise forecast for the overall spatial and temporal changes in climate due to global warming is not available. Presently no global tool or model is available to predict the magnitude of likely changes in climatic parameters due to global warming for a region/place specific.
Under such circumstances, the effect of global warming in a particular region can only be studied regarding likely changes in certain climate parameters. The most visible sign of climate change is a rise in temperature affecting ET primarily by increasing the capacity of air to hold water vapour. Decreased cloudiness and increased solar radiation would increase ET. Saturation pressure increases exponentially with increase in temperature. If all other factors remain unchanged, warming will cause drier air and hence more ET. In the present study, the temperature has been raised by 1 to 2 C keeping other parameters constants, and its effects were studied on total evapotranspiration demands for the western Rajasthan.
IMPLICATIONS FOR WATER RESOURCES
Weekly reference evapotranspiration was calculated using above described Penman-Monteith equation. The sensitivity of ET has been studied by varying temperature within a range as described earlier while keeping other parameters constant. The normal average annual evapotranspiration of the western Rajasthan varied from 1502 mm for Nagaur to 2177.2 mm for Barmer. According to Irving (1993), the greatest certain threat from climatic changes is by an increase in evaporative losses and water demands caused by higher temperature. Globally evapotranspiration trends are projected for +5% o to +10% increase due to increase in temperature by +2 to +5 C (Schneider et al., 1990). Wetherald and Manabe (1981) found that evaporation changes by 3% when temperature changes by 1 C. Similarly, Budyko (1982) suggests a 5% increase in evapotranspiration demand for each degree Celsius rise in temperature.
Table 2. Evapotranspiration demand under Global warming
Table 3. Additional water requirement for irrigation under global warming by 1 C in western Rajasthan
The total available utilizable ground water for western Rajasthan is 3516.9 mcm and increase of 1% in temperature will put additional stress of 44% of existing groundwater resources based on present land use pattern Since western Rajasthan is not blessed with good perennial river systems, so any increase in water demand requires careful planning for future water resource development. More emphasis is needed to develop technologies for reducing water losses in reservoirs, conservation of rainwater and development of such crop varieties that require less water. So it is high time for the planners/users/water resources managers to think in term of expected water demand due to global warming and its likely effect on water resources of Rajasthan. The availability of water has a direct bearing on the type of crops to be grown and will determine the economy of the state.
The basic intrinsic characteristic of water dynamics in the soil cannot be altered to a large degree, but availability can be modified enormously by water conservation and water management practices. During the complete life cycle of a plant, the most dynamic property is water content in the soil. Since water that is conserved or supplemented becomes available to the plant through the soil, monitoring of water content under field condition is of paramount importance.
The whole life cycle of plant revolves around the extraction of water from the soil through its root network and transpiring through the leaves in response to evaporation demand set up by the atmosphere. Any shortage of water in the soil is ultimately reflected by the growth of the plants in some form or another. Increasing water supply available to plants or enhancing water use relative to other losses and making efficient use of limited water are the three major concepts in optimizing water use in agriculture.
INCREASING WATER AVAILABILITY TO CROP PLANTS
Since rainfall is low and erratic efforts are needed to enhance the total water supply available to crops. Some of these include tillage, mulching, soil moisture barrier, contour bunding and in-situ water harvesting. Mechanical structures like contour bunding, terracing is the pre-requisite for water conservation on sloppy agricultural lands. Off-season tillage on Alfisols helped in controlling weeds and conserving water. Such practice on arid soils may lead to soil erosion. Hence, minimum tillage was found to control 73% weeds and 43% increase in yield. Use of weed mulch increased the water availability to crops by restricting evaporation directly from the soil surface.
- Central Arid Zone Research Institute.