Abstract: In this paper, we calculated the fractional vegetation cover (f_c) within growing season in the arid region of Northwest China based on two kinds of NDVI data (AVHRR-GIMMS and SPOT-VEGETATION). The desert vegetation area was determined according to the thresholds of vegetation cover, and which was validated in the Tarim River Basin. The ecological water requirement of desert vegetation in the arid region of Northwest China was calculated using the phreatic evaporation model with the area of desert vegetation and the reference crop evapotranspiration (ET₀) during 1982-2010. The results were expected to provide evidence for water use management in this area in future. Results showed that the thresholds of f_c were reasonable to be 0.1-0.35 to determine the desert vegetation area in this area. There was no drastic change in area of desert vegetation during 1982-2010, which showed an increasedecreaseincrease trend. The mean areas of high vegetation cover and low vegetation cover were 2.343 million and 4.482 million hm², respectively. The annual ET₀ in the Tarim River Basin was higher than that in Northern Xinjiang, and the ET₀ in eastern part was larger than that in western part. The mean annual ET₀ values in Tarim River Basin, Northern Xinjiang and QilianHexi area were 1066.9, 975.6, and 1171.6 mm, respectively. Phreatic evaporation of the desert vegetation in the arid region of Northwest China was calculated according to the data fitting of the ET₀. In the Tarim River Basin, Northern Xinjiang and QilianHexi area, the mean annual phreatic evaporation values of the lowcover desert vegetation were 75, 64 and 82 mm, respectively; the mean annual phreatic evaporation values of the highcover desert vegetation were 275, and 233 and 299 mm, respectively. The mean annual ecological water requirement of desert vegetation was 13.413 billion m³ in the arid region of Northwest China, with a reduced rate of 10.63 million m³·a^-1 during 1982-2010. In area with high vegetation cover, the mean annual ecological water requirement was 9.164 billion m³, with a reduced rate of 30.34 million m³·a^-1, while that in area with low vegetation cover was 4.249 billion m³, with an increased rate of 19.72 million m³·a^-1.

Key words: carbon footprint, wheat-maize rotation system, greenhouse gas (GHG), net global warming potential (NGWP), irrigation amount