ABSTRACT
Food and Agriculture Organization (FAO). ET0 is one of the main elements in the hydrological cycle, which is affected by the changes in air temperature, sunshine duration, and wind speed and so on. Changes in air temperature can alter the saturation vapor pressure, which in turn changes the evaporation and ET0 rate. It is well known that ET0 is a nonlinear complex function of many parameters and change in any one parameter can change the other parameter(s) and therefore, the effect of such changes on ET0 is very difficult to understand. Most of the meteorological variables can influence ET0; however, sunshine duration and wind speed are the two important meteorological parameters controlling ET0 in different regions of the world (Jhajharia et al., 2009). Many investigators have studied trends in ET0 across the different regions of the world. For example, Xu et al. (2006) investigated trends in Epan and ET0 in Changjiang (Yangtze River) catchment and found decreasing trends in both Epan and ET0 for the whole catchment. Bandyopadhayay et al. (2009) also studied ET0 trends in India and reported decreases in ET0 all over India. Differently, several researchers also reported increases in ET0 trends. Hess (1998) reported an increasing trend in ET0,
1 INTRODUCTION
Having been increasingly concerned about the problem of climate changes and water resources shortage in recent years, people have begun to make a research on the influence that the climate changes plays on regional reference crop evapotranspiration (crop water requirement) (Tong Ling et al., 2004). Reasonable and accurate method to estimate evapotranspiration is significantly important to evaluate and manage the water resource and eco-system under the influence of land use changes and climate changes, especially in the arid regions. In the agricultural production, evapotranspiration is an important indicator to measure crop water consumption and to guide irrigation.
