Actual evapotranspiration of subalpine meadows in the Qilian Mountains, Northwest China

As a main component in water balance, evapotranspiration (ET) is of great importance for water saving, especially in arid and semi-arid areas. In this study, the FAO (Food and Agriculture Organization) Penman-Monteith model was used to estimate the magnitude and temporal dynamics of reference evapotranspiration (ET0) in 2014 in subalpine meadows of the Qilian Mountains, Northwest China. Meanwhile, actual ET (ETc) was also investigated by the eddy covariance (EC) system. Results indicated that ETc estimated by the EC System was 583 mm, lower than ET0 (923 mm) estimated by the FAO Penman-Monteith model in 2014. Moreover, ET0 began to increase in March and reached the peak value in August and then declined in September, however, ETc began to increase from April and reached the peak value in July, and then declined in August. Total ETc and ET0 values during the growing season (from May to September) were 441 and 666 mm, respectively, which accounted for 75.73% of annual cumulative ETc and 72.34% of annual cumulative ET0, respectively. A crop coefficient (k(c)) was also estimated for calculating the ETc, and average value of k(c) during the growing season was 0.81 (ranging from 0.45 to 1.16). Air temperature (T-a), wind speed (u), net radiation (R-n) and soil temperature (T-s) at the depth of 5 cm and aboveground biomass were critical factors for affecting k(c), furthermore, a daily empirical k(c) equation including these main driving factors was developed. Our result demonstrated that the ETc value estimated by the data of k(c) and ET0 was validated and consistent with the growing season data in 2015 and 2016.