Changes in vegetation and surface water balance at basin-scale in Central China with rising atmospheric CO₂

Elevated atmospheric CO₂ concentration alters vegetation growth and composition, increases plant water use efficiency (WUE), and changes surface water balance. These changes and their differences between wet and dry climate are studied at a mid-latitude experiment site in the Loess Plateau of China. The study site, the Jinghe River basin (JRB), covers an area of 43,216 km² and has a semiarid climate in the north and a semi-humid climate in the south. Two simulations from 1965 to 2012 are made using a site-calibrated Lund–Potsdam–Jena dynamic global vegetation model: one with the observed rise of the atmospheric CO₂ from 319.7–391.2 ppmv, and the other with a fixed CO₂ at the level of 1964 (318.9 ppmv). Analyses of the model results show that the elevated atmospheric CO₂ promotes growth of woody vegetation (trees) and causes a 6.0% increase in basin-wide net primary production (NPP). The NPP increase uses little extra water however because of higher WUE. Further examination of the surface water budget reveals opposite CO₂ effects between semiarid and semi-humid climates in the JRB. In the semiarid climate, plants sustain growth in higher CO₂ because of the higher level of intracellular CO₂ and therefore WUE, thus consuming more water and causing a greater decrease of surface runoff than in the fixed-lower CO₂ case. In the semi-humid climate, NPP also increases but by a smaller amount than in the semiarid climate. Plant transpiration (E_T) and total evapotranspiration (E) decrease in the elevated CO₂ environment, yielding the increase of runoff. This asymmetry of the effects of elevated atmospheric CO₂ exacerbates drying in the semiarid climate and enhances wetness in the semi-humid climate. Furthermore, plant WUE (=NPP/E_T) is found to be nearly invariant to climate but primarily a function of the atmospheric CO₂ concentration, a result suggesting a strong constraint of atmospheric CO₂ on biophysical properties of the Earth system.