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 (ET 0 ) in 2014 in subalpine meadows of the Qilian Mountains, Northwest China. Meanwhile, actual ET (ET c ) was also investigated by the eddy covariance (EC) system. Results indicated that ET c estimated by the EC System was 583 mm, lower than ET 0 (923 mm) estimated by the FAO Penman-Monteith model in 2014. Moreover, ET 0 began to increase in March and reached the peak value in August and then declined in September, however, ET c began to increase from April and reached the peak value in July, and then declined in August. Total ET c and ET 0 values during the growing season (from May to September) were 441 and 666 mm, respectively, which accounted for 75.73% of annual cumulative ET c and 72.34% of annual cumulative ET 0 , respectively. A crop coefficient (k c ) was also estimated for calculating the ET c , 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 ET c value estimated by the data of k c and ET 0 was validated and consistent with the growing season data in 2015 and 2016.