Estimation of Reference Crop Evapotranspiration in Northwest China

Based on the daily meteorological data from 1956 to 2011 in Northwest (NW) China and the Penman-Monteith (PM) equation, the regional reference crop evapotranspiration (ET0) is estimated. The ET0 variations in time series and spatial distributions are analyzed. The trend analysis, Mann-Kendall (M-K) test, wavelet analysis, stepwise regression and EOF analysis methods are used to investigate the spatiotemporal variability of ET0 and its contributing climatic factors, the mutation of ET0, the period of ET0, and the main influencing meteorological factors, respectively. Major conclusions are obtained as follows: (1) In the past 56 years, the trend of average annual ET0 time series in the NW China is significantly reduced, the differences exists in different seasons, i.e., the trends of ET0 in spring (-0.26mm/a), summer (-0.72mm/a) and autumn (-0.31mm/a) are decreased, respectively, the ET0 in winter is slowly increased (0.02mm/a). (2) The region which ET0 decreased most is located at the field from Kumul to Hotan (from northeast to southwest). ET0 has a sharply decrease around the 1980s, with a multiple-time scale nesting complex structure in the period. The first, second and third EOF models account 36.84%, 13.87% and 9.04% for the explained variance, respectively. The summer EOF model is the main contributor to the annual first model. (3) The upward trend of mean surface air temperature (T) and the decreased trend of sunshine duration (SD), relative humidity (RH) and wind speed at 2 m high (U2) induce ET0 to decline. The variability of annual ET0 rate is most influenced by the variations of U2, followed by SD, RH and T, which is influenced by various climatic variables. The investigation of spatiotemporal variability of ET0 and its contributor meteorological factors may help us better understand how ET0 responds to regional climate change.