Comparison of Actual and Reference Evapotranspiration Between Seasonally Frozen and Permafrost Soils on the Tibetan Plateau

A comparison of evapotranspiration between seasonally frozen and permafrost soils has important theoretical value for studying land surface processes and ecological environmental evolution on the Tibetan Plateau. In this work, the actual (ETa) and reference (ET0) evapotranspiration and crop coefficient (Kc) were calculated via eddy covariance data and meteorological gradient data from sites in the Naqu Prefecture and Tanggula Mountains. The variations, differences, and factors influencing the ETa and ET0 were analysed. The results revealed that at the two sites in 2008, the annual total ETa values were 493.53 and 585.17 mm, which accounted for 83.58% and 144.39% of the total annual rainfall, respectively. The ETa at the Naqu site was affected mainly by the Tibetan Plateau monsoon (TPM), whereas the ETa at the Tanggula site was strongly affected by both the TPM and the freezing–thawing processes of the permafrost. The annual total ET0 values at the two sites were 819.95 and 673.15 mm, respectively. The monthly total ET0 at the Naqu site was greater than that at the Tanggula site. The ETa and ET0 values at the two sites were low in winter–spring, high in summer–autumn, and concentrated from May to October. When snow was present, the ETa values at the Naqu site were relatively high, and the ET0 values at both sites were very small and even negative at the Naqu site. The ETa and ET0 values at the two sites were significantly positively correlated with the net radiation (Rn), surface temperature (T0), air temperature (Ta), water vapour pressure (e) and soil water content (smc), and negatively correlated with the wind speed (ws). The correlation between the ETa and the T0 at the Naqu site was the most significant, and the coefficient of partial correlation was 0.812; meanwhile, the correlation between the ETa and the smc at the Tanggula site was the most significant, and the coefficient of partial correlation was 0.791. The Rn at the Naqu and Tanggula sites both had greater impacts on the ET0.