Comparative Analysis on Automatic and Manual Evaporation Measurements in China

Automatic Observation System (AOS) has been set up since 2002 and Manual Observation System (MOS) will be substituted gradually. The results show that data quality will be influenced by the changes of either the measurement instrument or the manner. So it is necessary to conduct AOS-MOS parallel observations to determine the discrepancy and correlation. For evaporation observations the difference is great between the manual and automatic observation. In fact manual evaporation is measured once a day at about 20:00, the difference of the two days measurements is the daily evaporation quantity. However, automatic measurements have been implemented to record continuously water surface height which is then used to calculate the hourly and daily evaporation at a given station. It is wanted by many meteorologists to know how much is the difference and the relationship between the amount of evaporation measured by automatic and manual observations and what is the main causes of the difference.There are 130 stations by 2005 conducting evaporation parallel measurements in China. The monthly or annual value is often used for evaporation analysis. Moreover the contrast coefficient is defined as the ratio between automatic and manual value and often used, too. Based on the monthly, annual value and contrast coefficient, various analyses are carried out on evaporation difference arising from two observation systems. The results are as follows: Monthly difference doesn't satisfy normal distribution pattern. The average monthly evaporation a mount observed by automatic way is 5.74 mm (or 5.51%) more than that by manual way. The standard deviation of the difference is 9.02 mm. In the 1050 evaporation comparison observations, 3.33% of data observed in the two ways is identical, 19.71% of data observed by automatic way is smaller, 76.95% of data observed by automatic way is larger. Namely, about 80% of automatic monthly data is greater than corresponding manual ones. Super sensitive of automatic ultrasonic probe and the measurement time discrepancy may be the reasons. When manual amount is small, its associated difference between automatic and manual is large, especially in the Wanyuan station in Sichuan Province. The monthly evaporation by automatic observation is obviously a linear function of that by manual one with the correlation coefficient passing 0.01 significant level test. The annual contrast coefficient is within the range of 1.0 to 1.2, with two high value regions centered at Douan and Nanxian stations in Guangxi and Hunan provinces, respectively. The change of monthly contrast coefficient is similar to bi-parabolic distribution pattern with highest and lowest value in January and June or July, respectively. Meteorological factors influencing the monthly contrast coefficient include the monthly average relative humidity and monthly average wind speed. There is a trend to merge the different kind of data to compose high quality evaporation dataset which deserves further research in the future.