Trends in reference evapotranspiration and their causative factors in the West Liao River basin, China

- We used MK test with removal of the effect of significant lag-1 serial correlation.
- We analyzed ETref influence factors by regression and partial correlation approach.
- We found a decreasing ETref and a significant air temperature increase in the basin.
- Solar radiation was the most dominant variable influencing ETref in the basin.

In this study the daily reference evapotranspiration (ETref) was estimated through the Penman-Monteith method at 15 meteorological stations in the arid and semi-arid area of the West Liao River basin of China from 1960 to 2012. The characteristics of the seasonal and annual variation and the spatial distribution of ETref through the Cokriging (spherical) interpolation methods were analyzed. The trends of seasonal and annual ETref and meteorological factors were investigated using the Mann-Kendall test after eliminating the effect of significant lag-1 serial correlation by trend-free pre-whitening. Moreover, the major meteorological factors affecting ETref using stepwise regression and partial correlation analysis were investigated. Results showed that: (1) The decreasing degree of ETref at 46.7% stations is larger than the increasing degree of ETref at 53.3% stations, which resulted in the decreasing trends of ETref during the study period. The magnitude of the negative trend in annual average ETref was 0.28 mm yr−1; (2) a significant overall increase in air temperature, a significant decrease in wind speed, solar radiation, sunshine duration, and relative humidity was observed; (3) ETref was larger for the plain area and gradually decreased toward the surrounding areas. It was smaller for the mountain area. ETref has obvious zonal effect; (4) There was a significant negative correlation between ETref and relative humidity while ETref has a significant positive correlation with other factors (p < 0.05). In annual time scale, solar radiation was found to be the most dominant variable influencing ETref, however, in seasonal time scale, average air temperature, maximum air temperature, relative humidity were the most dominant factors in spring, summer and winter, and autumn, respectively. Wind speed was the second most significant factor in all seasons. The results of this study support the conclusion that the evapotranspiration decreases over the West Liao River basin are controlled mainly by trends in the radiative component.