Applicability evaluation of different algorithms for reference crop evapotranspiration in Yangtze River Basin based on inverse distance weighted method
In order to making true the exact calculation of reference crop evapotranspiration(ET_0) and increase the accuracy with the lack of meteorological data in the large area, the Yangtze River Basin is divided into upstream, midstream and downstream according to the altitude of the stations. A new method of space distribution based on Inverse Distance Weighted Interpolation method is raised which can present every substream, including upstream, midstream and downstream. This method can include the connection of different stations. There are 102 meteorological stations which can provide 50 years of daily meteorological data from 1963 to 2013. In this research, we used the methods of Penman-Monteith(P-M), Hargreaves-Samani(H-S), Irmark-ALLEN(I-A), Pristley-Taylor(P-T), Makkink(M-K), Penman-Van Bavel(PVB), 1948 Penman(48-PM) to calculate daily ET_0 of every stations of the Yangtze River Basin. The method of Penman-Monteith can be used to be the standard method to calculate standard daily ET_0 to evaluate other methods because of its accuracy. The coefficient of Nash-Sutcliffe, the daily relative root mean square error, the consistency coefficient of Kendall can be used to evaluate the precision index of the method. The result showed that the best method of daily ET_0 imitative effect compared with P-M is PVB in the three substreams, because the slope of the imitative equation of PVB in upstream is 0.946, the slope in midstream is 1.065, and the slope in downstream is 1.005. The method of Pristley-Taylor has a better effect in the midstream and downstream, and the slopes of the imitative equation are 1.030 and 1.201. The method of Makkink also has a good effect in the midstream and downstream, and the slopes are 0.857 and 0.936. The determination coefficient of daily ET_0 imitative equation of these six methods all achieved very significant levels(alpha=0.01) in three substreams. The methods of Pristley-Taylor and Penman-Van Bavel have high calculation accuracy in all area of the Yangtze River Basin, and the highest absolute error of monthly ET_0 is 0.55 mm/d using the method of Pristley-Taylor, at the same time the highest absolute error of monthly ET_0 is 0.48 mm/d using the method of Penman-Van Bavel. The effect of the methods of Hargreaves-Samani and Irmark-ALLEN are worse than other methods to calculate monthly ET_0 in the whole Yangtze River Basin. The method of Pristley-Taylor is the best method to calculate ET_0 in the upstream of the Yangtze River Basin, because the daily relative root mean square error is 0.341 mm/d, the coefficient of Nash-Sutcliffe is 0.886, and the consistency coefficient of Kendall is 0.829. The method of Penman-Van Bavel is the best method to calculate ET_0 in the midstream and downstream of the Yangtze River Basin. In the midstream, the daily relative root mean square error is 0.201 mm/d, the coefficient of Nash-Sutcliffe is 0.973, and the coefficient of Nash-Sutcliffe is 0.926. In the downstream, the daily relative root mean square error is 0.306 mm/d, the coefficient of Nash-Sutcliffe is 0.954, and the consistency coefficient of Kendall is 0.869. In the Yangtze River Basin, the relative error of Pristley-Taylor and Penman-Van Bavel are the lowest among these methods which are less than 35%, the relative error of is the highest among these methods which is more than 40%. In conclusion, the method of Pristley-Taylor and Penman-Van Bavel are the best methods to calculate ET_0 in the Yangtze River Basin, the calculation process are simple at the same time. The method of Pristley-Taylor and Penman-Van Bavel can be a simplified recommendation of calculating ET_0 in the Yangtze River Basin.