Changes of reference crop evapotranspiration and causes in Guanzhong Region of Shaanxi Province

Changes of reference crop evapotranspiration (ET₀) and its causes were analyzed by the computed ET₀ using Penman-Monteith equation which was recommended by FAO according to weather data of 41 years for 30 weather stations in Guanzhong Region of Shaanxi Province(Central Shaanxi Plain). The results show that ET₀ presents periodical changes based on decreasing trend in the long term and increasing trend before and after 1980 respectively in the short term. The maximum ET₀ appeared on June before 1980 and on June and July after 1980. ET₀ percentage between May and August in a year after 1980 is less than the one before 1980, but still is over 50%. The tendencies for mean, maximum and minimum temperature are to increase and those for wind speed, sunlight hours and relative humidity are to decrease. ET₀ is related positively to wind speed, sunlight hours insignificantly, and is related negatively to water vapor insignificantly. The influence capacity of weather factors on ET₀ is as follows: wind speed＞sunlight hours＞water vapor＞yearly evaporation. The decline trend of ET₀ is mainly caused by the decrease of wind speed and sunlight hours.