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Reliable quantitative data on the extent and rates of soil erosion are needed to understand the global significance of soil‐erosion induced carbon exchange and to underpin the development of science‐based mitigation strategies, but large uncertainties remain. Existing estimates of agricultural soil and soil organic carbon (SOC) erosion are very divergent and span two orders of magnitude. The main objective of this study was to test the assumptions underlying existing assessments and to reduce the uncertainty associated with global estimates of agricultural soil and SOC erosion. We parameterized a simplified erosion model driven by coarse global databases using an empirical database that covers the conterminous USA. The good agreement between our model results and empirical estimates indicate that the approach presented here captures the essence of agricultural erosion at the scales of continents and that it may be used to predict the significance of erosion for the global carbon cycle and its impact on soil functions. We obtained a global soil erosion rate of 10.5 Mg ha‐1 y‐1 for cropland and 1.7 Mg ha‐1 y‐1 for pastures. This corresponds to SOC erosion rates of 193 kg C ha‐1 y‐1 for cropland and 40.4 kg C ha‐1 y‐1 for eroding pastures and results in a global flux of 20.5 (±10.3) Pg y‐1 of soil and 403.5 (±201.8) Tg C y‐1. Although it is difficult to accurately assess the uncertainty associated with our estimates of global agricultural erosion, mainly due to the lack of model testing in (sub‐)tropical regions, our estimates are significantly lower than former assessments based on the extrapolation of plot experiments or global application of erosion models. Our approach has the potential to quantify the rate and spatial signature of the erosion‐induced disturbance at continental and global scales: by linking our model with a global soil profile database, we estimated soil profile modifications induced by agriculture. This showed that erosion‐induced changes in topsoil SOC content are significant at a global scale (an average SOC loss of 22% in 50 years) and agricultural soils should therefore be considered as dynamic systems that can change rapidly. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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In this work, we address the Black Sea setup of Nucleus for European Modelling of the Ocean (NEMO), and in particular some
model enhancements associated with the most important characteristic of ocean dynamics in this semi-enclosed basin, that is
the sea-level variability and its relationship with water cycles and wind. Forcing data are presented in detail and compared
with previously used coarser-resolution data. One emphasis in this paper is on the statistical analyses of forcing data and
outputs from simulations with a focus on the sea level and its change. Numerical simulations are carried out as free run,
and alternatively, altimeter data assimilation based on displacement of water properties in the pycnocline is used. Comparisons
between the two runs identify the robustness of circulation driven by water balance and winter intensification. Problems in
the model to replicate the redistribution of water properties between the two sub-basins in free-run mode are also discussed,
which are observed during years with extreme climatic conditions. 相似文献
316.
Upscaling pore-scale processes into macroscopic quantities such as hydrodynamic dispersion is still not a straightforward matter for porous media with complex pore space geometries. Recently it has become possible to obtain very realistic 3D geometries for the pore system of real rocks using either numerical reconstruction or micro-CT measurements. In this work, we present a finite element–finite volume simulation method for modeling single-phase fluid flow and solute transport in experimentally obtained 3D pore geometries. Algebraic multigrid techniques and parallelization allow us to solve the Stokes and advection–diffusion equations on large meshes with several millions of elements. We apply this method in a proof-of-concept study of a digitized Fontainebleau sandstone sample. We use the calculated velocity to simulate pore-scale solute transport and diffusion. From this, we are able to calculate the a priori emergent macroscopic hydrodynamic dispersion coefficient of the porous medium for a given molecular diffusion Dm of the solute species. By performing this calculation at a range of flow rates, we can correctly predict all of the observed flow regimes from diffusion dominated to convection dominated. 相似文献
317.
Pasachoff Jay M. Babcock Bryce A. Russell Kevin D. McConnochie Timothy H. Diaz J. Sebastian 《Solar physics》2000,195(2):281-298
As part of a study of the cause of solar coronal heating, we searched for high-frequency (1 Hz) intensity oscillations in coronal loops in the [Fexiv] coronal green line. We summarize results from observations made at the 3 November 1994, total solar eclipse from the International Astronomical Union site in Putre, Chile, through partly cloudy skies, and at the 26 February 1998 total solar eclipse from Nord, Aruba, through clear skies. We discuss the image reduction and analysis of two simultaneous series of coronal CCD images digitized at 10 Hz for a total time of 160 s in Chile. One series of images was taken through a filter isolating the 5303 Å[Fexiv] coronal green line and the other through a 100 Å filter in the nearby K-corona continuum. We then discuss the modifications made for the 1998 eclipse, and the image reduction and analysis for those image sequences. After standard calibrations and image alignment of both data sets, we use Fourier analysis to search in the [Fexiv] channel for intensity oscillations in loops at the base of the corona. Such oscillations in the 1-Hz range are predicted as a result of density fluctuations from the resonant absorption of MHD waves. The dissipation of a significant amount of mechanical energy from the photosphere into the corona through this mechanism could provide sufficient energy to heat the corona. At neither eclipse do we find evidence for oscillations in the [Fexiv] green line at a level greater than 2% of coronal intensity. 相似文献
318.
The Dalton Minimum (1790–1830) was a period with reduced solar irradiance and strong volcanic eruptions. Additionally, the
atmospheric CO2 concentrations started to rise from the background level of previous centuries. In this period most empirical climate reconstructions
indicate a minimum in global or hemispheric temperatures. Here, we analyse several simulations starting in 1755 with the coupled
atmosphere-ocean model ECHO-G driven by different forcing combinations to investigate which external forcing could have contributed
most strongly to the reduced temperatures during the Dalton Minimum. Results indicate that on global and hemispheric scales,
the volcanic forcing is largely responsible for the temperature drop in this period, especially during its second half, whereas
changes in solar forcing and the increasing atmospheric CO2 concentrations were of minor importance. At regional scales, especially the extratropical, the impact of volcanic forcing
is much less discernible due to the large regional variability, a finding that agrees with empirical temperature reconstructions. 相似文献
319.
Earthquake activity related to seismic cycles in a model for a heterogeneous strike-slip fault 总被引:1,自引:0,他引:1
We investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike-slip fault in elastic solid. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction and dislocation creep along the fault, and 3D elastic stress transfer. The fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events, and aseismic creep cells which are characterized by high ongoing creep motion. This mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified Omori law and scale with the mainshock size. Furthermore, we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations. The temporal occurrence, however, is magnitude-dependent; in particular, the small events are clustered in time, whereas the largest earthquakes occur quasiperiodically. Averaging the seismicity before several large earthquakes, we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached. These results are characteristics of a critical point behavior. The presence of critical point dynamics is further supported by the evolution of the stress field in the model, which is compatible with the observation of accelerating moment release in natural fault systems. 相似文献
320.