Water Use Efficiency In Global Terrestrial Ecosystems And Estimate Of Heat Balance Relating To Vegetation In Artificial Areas

The carbon and water cycles are the core issue and hot topics in global change study. The interaction between carbon and water cycles can be characterized by the Water Use Efficiency(WUE). It is essential to study WUE across global terrestrial ecosystems to explore the inherent relationship between the carbon and water cycles and to help predict the influences of climate change on terrestrial ecosystems. Furthermore, due to intensive human activities, the natural vegetation has been largely replaced by the artificial surface area. Unlike vegetation, the artificial area can’t fix the solar energy, in addition, the impervious material covers the land surface and prevents heat dissipation from evapotranspiration. Therefore, part solar energy will not be directly fixed by vegetation, and some is also not taken away through evapotranspiration in these artificial areas, that is to say, both parts of the energy are relatively reserved in surrounding air, and will be eventually transformed into the heat and warms the land surface and atmosphere convection layer. It may cause and aggravate the local or even global climate change.In this study, we calculated and analyzed the fourteen-year WUE of the global terrestrial ecosystems using remotely sensed data from the MODIS NPP products(MOD17A3) and the MODIS ET products(MOD16A3) from 2000 to 2013. Also, based on the global land cover data in 2009(GlobCover 2009), we compared the WUE of different land-cover types. For the global artificial surface, we calculated the energy that was not fixed and taken away by evapotranspiration in global artificial surface area because of lacking vegetation, and assessed the climate forcing resulted from the above energy. The main results as follows:(1) The fourteen-year average WUE was 873.45 mg C m-2 mm-1. The annual mean WUE fluctuated from 849.26 to 896.59 mg C m-2 mm-1 in 2000-2013, and it increased in the whole process but non-significant. Before 2010, WUE significantly decreased(P<0.01), and rapidly increased after that year, but the variable trend was non-significant.(2) During fourteen years, pixels with significant and highly significant decreasing trends accounts for 7.51% of the total global land pixels, which mainly distributed in the Eurasian transition area, the Malay Archipelago, Australia’s central plains, Amazon Plain and so forth. Pixels with significant and highly significant increasing trends accounted for 10.96%, and they mainly distributed in Congo Basin, Gangetic Plains, Loess Plateau and Tibet Plateau.(3) WUE presents a clear spatial distribution on each continent. In Africa and Oceania, the WUE gradually increased from north to south. Similarly, an obvious increase with longitude from east to west can be observed in Europe and South America. However, the patterns in Asia and North America are more complex, in that the WUE appears to increase in a southern direction in areas above the latitude of 50° N. WUE decreases with an order of Europe, Oceania, Africa, South America, North America and Asia. WUE in North America is very nearto the fourteen-year average WUE.(4) WUE values in the 600-900 mg C m-2 mm-1 interval accounted for 31-36% of the entire land surface, followed by the interval of 900-1200 mg C m-2 mm-1(21-25%), and the least were the intervals of 0-300 mg C m-2 mm-1 and >1500 mg C m-2 mm-1 that accounted for only 5-8%.(5) WUE of nine land-cover types showed a decreasing order as Needleleaved evergreen forest, Mixed forest, Broadleaved deciduous forest, Needleleaved deciduous forest, Cropland, Shrubland, Wetland, Broadleaved evergreen forest and Grassland. During fourteen years, WUE of broadleaved evergreen forest, needleleaved deciduous forest and cropland showed significant variation(P<0.05). From 2000 to 2010, WUE values of five types including broadleaved evergreen forest, needleleaved evergreen forest, cropland, grassland and wetland varied significantly(P<0.05) or highly significantly(P<0.01).(6) The energy which was not directly fixed was 3.35×1018J and the energy that was not transformed was 2.05×1020 J. Total energy which was relatively increased was 2.08×1020 J, causing climate forcing of 15.16 W m-2 on research areas, 0.0443 W m-2 on global land areas and 0.0129 W m-2 on globe. Climate forcing induced by the heat not taken away by evapotranspiration accounted for above 98% of the total climate forcing value.