Variation characteristics of reference crop evapotranspiration and its responses to climate change in upstream areas of Yellow River basin

It is practically significant for identification of different impacts of meteorological factors to study the response of reference crop evapotranspiration (ET0) to climate change. Based on the daily data of ten weather stations across a large climatic gradient in upstream areas of the Yellow River basin, China for recent 50 years, ET0 was estimated with the FAO56 Penman-Monteith (P-M) method. The P-M results and pan evaporation data were compared and the trends and response to climate change for reference crop evapotranspiration were calculated. The results show that the P-M method is suitable for study area. ET0 occurs mainly from March to October in growing season, and decreases with the increase of altitude. Significant increasing ET0 can be observed in most stations, where elevation is higher than 3000 m, and most stations in low elevation (<3 000 m) show obvious decreasing ET0. There are different change processes of ET0 in high altitude and low elevation area. Daily sensitivity coefficients to air temperature, wind speed and relative humidity exhibit large fluctuations during the year, while small fluctuations for sunshine hours. ET0 is insensitive to air temperature in winter and early spring, and the sensitivity gradually increases and achieves its maximum value in summer while opposite patterns were found for weed speed. Relatively strong negative sensitivity coefficients were obtained for relative humidity, and ET0 is more sensitive to relative humidity in growing season. The long-term trend analyses of sensitivity coefficients show that the sensitivity coefficients to air temperature increased, while decreased for relative humidity in low elevation stations. Increased air temperature, sunshine hours and reduced relative humidity led to increasing ET0 in high altitude area, whereas decreased ET0 is dominated by reduced sunshine hours, wind speed and increased relative humidity in low elevation area. So the high altitude area in upstream areas of of the Yellow River basin is taken as the 'energy-limited' system and the low elevation area as the 'water-limited' system.