Comparison of the Thornthwaite method and pan data with the standard Penman-Monteith estimates of reference evapotranspiration in China

Various methods are available to estimate reference evapotranspiration (ET0) from standard meteorological observations. The Penman-Monteith method is considered to be the most physical and reliable method and is often used as a standard to verify other empirical methods. This study estimates and compares the monthly ET0 calculated by 3 methods at 580 Chinese stations over the last 50 yr. The Penman-Monteith method is used here as a reference, and its spatial and temporal differences with the Thornthwaite method and pan measurement are evaluated. The results show that: (1) in terms of spatial difference, the Thornthwaite estimates show different regional patterns, while pan measurements display a consistent regional pattern; (2) the temporal variability of ET0 is much better represented by pan measurements than by the Thornthwaite estimates. Overall, pan measurements are more useful than the Thornthwaite estimates if appropriate pan coefficients are determined. The Thornthwaite method only considers the temperature and latitude and gives unreliable results under dry conditions, e.g. in NW China. With reference to the Penman-Monteith estimates, the correction factors (pan coefficients) of pan measurements for the whole of China, and the regional averages over the 10 major drainage basins are determined. The average value lies between 0.6 and 0.8, although a seasonal and regional difference is present.