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- Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends

The ORCHIDEE model is a process-based biogeography–biogeochemistry model developed to assess the transient impacts of climate change on the transfer of water and carbon in the vegetation–soil–atmosphere system (12). The model includes parameterizations of canopy physiological processes (photosynthesis and canopy conductance) that are intimately linked to biospheric energy and hydrological balances and operated at a time scale of 30 min. Soil hydrology are modeled as described by Ducoudre et al. (26) following a semiempirical approach. There are two soil layers, and the total depth of soil considered at all land points is the root zone. The water content of each layer is updated by accounting for inputs from rainfall, which is reduced by interception losses and snowmelt, as well as by losses to soil evaporation, transpiration, deep drainage, and surface runoff. The amount of water intercepted by the foliage is controlled by the incident rainfall and LAI, and it gives rise to interception losses that depended on the prevailing meteorological conditions. Soil evaporation is calculated from the relative humidity of the air at the land surface and aerodynamic and soil resistances, where the soil resistance was a function of soil moisture. Vegetation transpiration depends on the modeled photosynthetic activity and atmospheric vapor-pressure deficit, as described by Ball et al. (27), and is mediated by soil-water availability. Surface runoff and drainage are calculated as the excess water above field capacity in both soil layers.

Vegetation productivity is calculated based on a coupled photosynthesis–water balance scheme. Plant growth based on the net plant carbon gain is allocated to six tissue pools (leaf, root, and wood, as well as reserve and reproductive organs), with a response of the relative investment into above- and below-ground structures, depending on soil temperature and moisture. Therefore, the ecosystem water balance affected plant carbon gain and structure. Leaf phenology and decomposition of litter and soil organic matter depend on temperature and water stress. Therefore, modeled water evapotranspiration is an integrator of meteorological, hydrological, and ecological processes. More detailed descriptions of the various components of ORCHIDEE can be found in Krinner et al. (12) and Ducoudre et al. (26).

The ORCHIDEE model has been widely used to assess the transient impacts of climate change on the global or regional water and carbon cycles (2832). The seasonal cycles of energy and water exchanges and carbon fluxes from the ORCHIDEE model have been extensively calibrated and validated against eddy covariance data from a number of field sites (12, 33). The global distribution of LAI and runoff (12) and satellite-derived interannual variability in LAI over the recent period are also realistically represented (31, 32).

The monthly climate data sets (temperature, precipitation, wet-day frequency, diurnal temperature range, cloud cover, relative air humidity, and wind speed), with a spatial resolution of 0.5° for 1901–1999, were provided by the Climatic Research Unit (School of Environmental Sciences, University of East Anglia, U.K.) (34). The historical changes in atmospheric CO2 concentration were taken from Rayner et al. (35). Cropland area is prescribed each year from Ramankutty and Foley (36). We combined this data set with that of Goldewijk (37) to account for the extent of pasture. The distribution of natural vegetation at each grid cell is derived from Loveland et al. (38). The extent of natural vegetation varied with time as a function of the prescribed extent of cropland and pasture.

Using average climate data from 1901 to 1910, the atmospheric CO2 concentration, and land cover of 1860, we first ran the model at a 2° resolution until the vegetation and soil carbon pools reached equilibrium. On the basis of this equilibrium status, four simulations from January 1860 to December 1999 were carried out to investigate the relative contribution of atmospheric CO2, climate change, and land-use change on the global runoff trends and patterns. Because of the lack of climate data before 1901, the average monthly climatology from 1901 to 1910 was used for the 1860–1900 run and for monthly climate data for individual years thereafter.

In simulation E1, atmospheric CO2 concentration alone was varied. In simulation E2, atmospheric CO2 and climate were varied. In simulation E3, atmospheric CO2 concentration, climate, and land use were varied. Finally, we performed an additional simulation (E4), comparable to the one carried out by Gedney et al. (1), which only considered the response of stomatal conductance to rising atmospheric CO2 and did not consider the subsequent effects on vegetation structure, notably the foliage cover. The changes in vegetation growth accompanied by environmental change were taken into account in simulations E1, E2, and E3 to comprehensively evaluate the hydrological contribution of different factors. The individual effects of climate variations were derived as the difference between simulations E2 and E1, and the effects of land use change were estimated by subtracting E2 from E3. The difference of runoff trend between simulations E1 and E4 reflected the effects of LAI changes in response to increased CO2.

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