Characterizing the evapotranspiration of a degraded grassland in the Sanjiangyuan Region of Qinghai province

The evapotranspiration(ET) of degraded grassland in the Sanjiangyuan region was investigated to further elucidate the water cycle in this region.The eddy covariance technique was used to measure evapotranspiration and sensible heat flux continuously for 3 years in this degraded grassland in Qinghai province,China.The annual precipitations(PPT) were 460.7,496.1,and 480.1 mm for the three consecutive years.The annual ETs were 452.24,474.24,and 459.57 mm,varying in accord with the annual PPT.About 75% of the annual ET occurred during the growing season from DOY121 to 300.The ratio of ET to PPT was greater than 95% during the study period,indicating that there is some balance between ET and PPT and the average in the growing season for each of the three years were 1.86,1.92,1.81 mm/d,respectively.However,the averaged ET was lower than 0.6 mm/d during the non-growing season.The annual and daily dynamics were single peak type curves.The maximum monthly ET rate was 2.0-2.2 mm/d,which occurred mainly in June or July.The daily peak appeared at noon and was 0.21 and 0.10 mm/h during the growing and non-growing seasons respectively.ET was most sensitive to net radiation during the growing season while the effect of air temperature and vapour pressure deficit were secondary.Compared with the non-degraded area of Kobresia humilis meadow,the ET of the degraded grassland was lower in the growing season but higher in the non-growing season.The lower vegetation cover was attributed to this result.