Changing Trends of Reference Crop Evapotranspiration and Impact Factors in Yunnan Mengzhi Plain in Recent 50 Years

Based on the meteorological data of Mengzhi weather station from 1961 to 2010 and FAO Penman-Moteith formula, reference crop evapotranspiration (ET0) was calculated, and the annual and monthly changing trends of ET0 were analyzed. Using the statistics test and regression analysis methods, the main meteorological factors that impact ET0 were discussed. The results show that the annual ET0 in Mengzhi plain is decreased slowly over the recent 50 years, and reduced by 13.43mm per decade. The influence capacity of meteorological factors on ET0 is as follows: annual average wind speed > relative humidity > sunshine hours > average maximum temperature. The maximum monthly ET0 appeared in April or May each year, and the minimum monthly ET0 occurred in December. ET0 declined significantly in April and May in recent 50 years; in other months, there is no significant increase or decline trend. The most important factor affecting ET0 from January to May and December is the average wind speed, and from June to September, ET0 is affected the most by the sunshine hours and in October and November by humidity.