Potential Evapotranspiration estimation for Indian conditions : Improving accuracy through calibration coefficients

Abstract

Water foot printing is a useful tool to assess future consumption of water for production of crops and consumers based products that give a forecast of water demand on regional or national basis. The global consumption of water is doubling every 20 years, more than twice the rate of human population growth. An FAO estimate puts that 70-80 per cent of the increase in food demand between 2000 and 2030 will have to be met by irrigation (OECD, 2008). Irrigated agriculture is practiced on about 300 million hectares only or 20 per cent of the cultivable area (FAO, 2010), but contributing substantially with more than 40 per cent of world’s food production. Irrigation can reduce the risks associated with the unpredictable nature of rainfed agriculture in dry regions. It helps to insulate farming from droughts that are predicted to occur more frequently. Efficient water use can increase crop diversity, produce higher yields, enhance employment and lower food prices (IFAD, 2008). Irrigated agriculture offers great potential for economic growth and poverty reduction. Considering the dominant role of irrigated agriculture in global water use, management practices that increase the productivity of irrigation water use can greatly increase the availability of water for other human and environmental uses (Tiwari and Dinar, 2002). Evaporation demand or potential evaporation is projected to increase almost everywhere in the world in future climate scenarios (IPCC, 2008). This is because the water holding capacity of the atmosphere increases with higher temperatures, but relative humidity is not projected to change markedly. As a result water vapor deficit increases in the atmosphere as does the evaporation rate. Thus, the process of evapotranspiration (ET) is of great importance in present and future climates. The measurement of ET from a crop surface is a very difficult and time consuming task. In spite of the efforts of numerous scientists, reliable estimates of regional ET are extremely difficult to obtain mainly because of its dependence on soil conditions and plant physiology, so that advances in the knowledge of the underlined interactions and it’s all round influence have been few and far between. Because of its complexity, the concept of potential evapotranspiration (PET) has been introduced, which is largely independent of soil and plant factors but has shown dependent on climatic factors. Temporal variations of PET and quantification of its trend can serve as a valuable reference data for the regional studies of hydrological modeling, agricultural water management, irrigation planning and water resource management as demonstrated by Liang et al. (2010).