Simulation of CO_2 and sensible/latent heat fluxes exchange between land surface and atmosphere over cropland and grassland in semi-arid region,China

A comparison between simulated land surface fluxes and observed eddy covariance （EC） measurements was conducted to validate Integrated Biosphere Simulator （IBIS） at Tongyu field observation station （44°25＇N, 122°52＇E） in Jilin Province, China. Results showed that the IBIS model could reproduce net ecosystem CO2 exchange （NEE）, sensible and latent heat fluxes reasonably, as indicated by correlation coefficients exceeding the significant level of 0.05. It was also evident that the NEE and sensible heat fluxes were characterized by diurnal and seasonal variation both in the grassland and the cropland ecosystems, while the latent heat fluxes correlated with evapotranspiration, only took on the diurnal variation during the growing season. Moreover, both sensible heat fluxes and the latent heat fluxes were larger in the cropland ecosystem than that in the degraded grassland ecosystem. This different characteristic was possibly correlated with vegetation growing situation in the two kinds of ecosystems. A close agreement between observation and simulation on NEE, sensible heat fluxes and latent heat flux was obtained both in the degraded grassland and the cropland ecosystems. In addition, the annual NEE in the model was overestimated by 23.21% at the grassland and 27.43% at the cropland, sensible heat flux with corresponding 9.90% and 11.98%, respectively, and the annual latent heat flux was underestimated by 4.63% and 3.48%, respectively.

Modeling net primary productivity of the terrestrial ecosystem in China from 1961 to 2005

Net primary productivity （NPP） is the structure and function of the ecosystem. NPP can most important index that represents the be simulated by dynamic global vegetation models （DGVM）, which are designed to represent vegetation dynamics relative to environ- mental change. This study simulated the NPP of China＇s ecosystems based on the DGVM Integrated Biosphere Simulator （IBIS） with data on climate, soil, and topography. The appli- cability of IBIS in the NPP simulation of China＇s terrestrial ecosystems was verified first. Comparison with other relevant studies indicates that the range and mean value of simula- tions are generally within the limits of observations; the overall pattern and total annual NPP are close to the simulations conducted with other models. The simulations are also close to the NPP estimations based on remote sensing. Validation proved that IBIS can be utilized in the large-scale simulation of NPP in China＇s natural ecosystem. We then simulated NPP with climate change data from 1961 to 2005, when warming was particularly striking. The following are the results of the simulation. （1） Total NPP varied from 3.61 GtC/yr to 4.24 GtC/yr in the past 45 years and exhibited minimal significant linear increase or decrease. （2） Regional differences in the increase or decrease in NPP were large but exhibited an insignificant overall linear trend. NPP declined in most parts of eastern and central China, especially in the Loess Plateau. （3） Similar to the fluctuation law of annual NPP, seasonal NPP also displayed an insignificant increase or decrease; the trend line was within the general level. （4） The re- gional differences in seasonal NPP changes were large. NPP declined in spring, summer, and autumn in the Loess Plateau but increased in most parts of the Tibetan Plateau.