The Influence of Meteorological Variables on Reference Evapotranspiration Based on the FAO P-M Model—A Case Study of the Taohe River Basin, NW China

To explore the mechanisms driving variation in ET0 (reference evapotranspiration) in an arid inland region of Northwest China, daily meteorological data from 1960 to 2019 from 19 meteorological stations in the Taohe River basin and its surrounding areas were used to analyze the temporal and spatial distributions of ET0 and meteorological variables. Various qualitative and quantitative analysis methods were used to reveal the correlation between ET0 and meteorological variables. The degree of sensitivity of ET0 variations to meteorological variables and the contribution from each meteorological variable were clarified, and the mechanisms driving variation in ET0 were fully revealed. These are the results: (1) ET0 in the Taohe River basin presented a significant upward trend with a linear change rate of 0.93 mm/a, and a sudden change occurred in 1994. The spatial variation in ET0 ranged from 779.8 to 927.6 mm/a, with low values in the upper and middle reaches and high values in the lower reaches. The ET0 at 14 stations (73.68% of the total) was significantly increased (p < 0.05), and that at 5 stations (26.32% of the total) was not significantly increased (p > 0.05). (2) RH, Rn, and u2 did not change significantly, while Tmax and Tmin showed a significant upward trend. (3) Rn is a meteorological variable closely related to variations in ET0, and is the most sensitive variable for variations in ET0, followed by Tmax and u2. (4) Tmax is the meteorological variable that contributes most to the variation in ET0 (30.98%), followed by Tmin (29.11%), u2 (6.57%), Rn (2.22%), and RH (0.05%). The research results provide a scientific basis for the rational and efficient utilization of water resources and the maintenance of ecosystem health.