Water and carbon dioxide fluxes over an alpine meadow in southwest China and the impact of a spring drought event

Based on the eddy covariance measurements from June 2011 to December 2013, the seasonal variations and the controls of water and CO₂ fluxes were investigated over an alpine meadow in Lijiang, southwest China. The year 2012 had the largest total precipitation among years from 2011 to 2013 (1037.9, 1190.4, and 1066.1 mm, respectively). A spring drought event occurred from March to May 2012, and the peak normalized difference vegetation index (NDVI) in 2012 was the lowest. Throughout the whole year, net radiation (R ₙ), vapor pressure deficit, and air temperature (T ₐ) were the primary controls on evapotranspiration (ET), and R ₙ is the most important factor. The influence of R ₙ on ET was much more in the wet season (R ² = 0.93) than in the dry season (R ² = 0.28). In the wet season, the ratio of ET to equilibrium ET (ETₑq) (0.92 ± 0.14; mean ± S.D.) did not show a clear seasonal pattern with NDVI when the soil water content (SWC) was usually more than 0.25 m³ m⁻³, indicating that ET could be predicted well by ETₑq (or radiation and temperature). On half-hourly and daily scales, photosynthetic active radiation (PAR) and air temperature were the main meteorological factors in determining the net ecosystem production (NEP). The seasonal trends of NEP were closely related with the change of NDVI. The integrated NEP in the 2012 wet season (157.8 g C m⁻² year⁻¹) was 19.5 and 23.8 % lower than in the 2011 and 2013 wet season (207.0 and 196.1 g C m⁻² year⁻¹). The mean ET/ETₑq for each of the wet seasons from 2011 to 2013 was 0.88. The 2012 spring drought and its reduction in NDVI decreased the total NEP significantly but had little effect on the total ET in the wet season. The different response of NEP and ET to the spring drought was attributed to the high SWC and small vapor pressure deficit during the wet season.