A simple linear regression method was used to calculate the slope of linear least squares regression line fit to the inter-annual variation of ET0 and other climatic variables inputted in the P-M equation. The significance of changes of these variables across China was mapped and assessed with the help of the two-tailed significance tests. The segmented regression with constraints method developed by Shao and Campbell63, which can detect both shift trends and step changes simultaneously without knowing the number of trend segments and change points and their location over time64, is employed to investigate the possible turning points of the regional average ET0 series across China. The method was summarily described as follow636465: