Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local...Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include： （1） Waiter＇s two-layer model, （2） Moir＇s one-layer model, （3） state-and-transition theory, （4） equilibrium theory, （5） disequilibrium theory, and （6） non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.展开更多
In order to clarify how groundwater leakage and river runoff occur in a catchment under tectonic movement, the water balance was estimated in the forested (88.3% in area) Oikamanai River catchment (area, 62.6 km2), Ho...In order to clarify how groundwater leakage and river runoff occur in a catchment under tectonic movement, the water balance was estimated in the forested (88.3% in area) Oikamanai River catchment (area, 62.6 km2), Hokkaido, Japan. The catchment’s geology is early Miocene to Pliocene sedimentary bedrock of partly high permeability, accompanied by currently active faults. Daily evapotranspiration, E, in water balance was calculated by applying the one-layer model to meteorological data in the rainfall season of 2011 and 2012, with the topographic influence on heat balance of the catchment considered. The coupling with the short-term water balance method for river runoff events allows us to estimate groundwater leaking to the other catchments through the faults and bedrock. As a result, the leakage corresponded to 50% - 80% of effective rainfall (=P - E: P, rainfall) in 2011, whereas it was lower or negative in 2012. The estimate of leakage then included variability of ca. 80%. In 2012, shallow groundwater storage seems to retain high baseflow during non-rainfall.展开更多
This paper puts forth a simplified dynamic modeling strategy for the eddy viscosity coefficient parameterized in space and time.The eddy viscosity coefficient is dynamically adjusted to the local structure of the flow...This paper puts forth a simplified dynamic modeling strategy for the eddy viscosity coefficient parameterized in space and time.The eddy viscosity coefficient is dynamically adjusted to the local structure of the flow using two different nonlinear eddy viscosity functional forms to capture anisotropic dissipation mechanism,namely,(i)the Smagorinsky model using the local strain rate field,and(ii)the Leith model using the gradient of the vorticity field.The proposed models are applied to the one-layer and two-layer wind-driven quasigeostrophic ocean circulation problems,which are standard prototypes of more realistic ocean dynamics.Results show that both models capture the quasi-stationary ocean dynamics and provide the physical level of eddy viscosity distribution without using any a priori estimation.However,it is found that slightly less dissipative results can be obtained by using the dynamic Leith model.Two-layer numerical experiments also reveal that the proposed dynamic models automatically parameterize the subgrid-scale stress terms in each active layer.Furthermore,the proposed scale-aware models dynamically provide higher values of the eddy viscosity for smaller resolutions taking into account the local resolved flow information,and addressing the intimate relationship between the eddy viscosity coefficients and the numerical resolution employed by the quasigeostrophic models.展开更多
文摘Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include： （1） Waiter＇s two-layer model, （2） Moir＇s one-layer model, （3） state-and-transition theory, （4） equilibrium theory, （5） disequilibrium theory, and （6） non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.
文摘In order to clarify how groundwater leakage and river runoff occur in a catchment under tectonic movement, the water balance was estimated in the forested (88.3% in area) Oikamanai River catchment (area, 62.6 km2), Hokkaido, Japan. The catchment’s geology is early Miocene to Pliocene sedimentary bedrock of partly high permeability, accompanied by currently active faults. Daily evapotranspiration, E, in water balance was calculated by applying the one-layer model to meteorological data in the rainfall season of 2011 and 2012, with the topographic influence on heat balance of the catchment considered. The coupling with the short-term water balance method for river runoff events allows us to estimate groundwater leaking to the other catchments through the faults and bedrock. As a result, the leakage corresponded to 50% - 80% of effective rainfall (=P - E: P, rainfall) in 2011, whereas it was lower or negative in 2012. The estimate of leakage then included variability of ca. 80%. In 2012, shallow groundwater storage seems to retain high baseflow during non-rainfall.
文摘This paper puts forth a simplified dynamic modeling strategy for the eddy viscosity coefficient parameterized in space and time.The eddy viscosity coefficient is dynamically adjusted to the local structure of the flow using two different nonlinear eddy viscosity functional forms to capture anisotropic dissipation mechanism,namely,(i)the Smagorinsky model using the local strain rate field,and(ii)the Leith model using the gradient of the vorticity field.The proposed models are applied to the one-layer and two-layer wind-driven quasigeostrophic ocean circulation problems,which are standard prototypes of more realistic ocean dynamics.Results show that both models capture the quasi-stationary ocean dynamics and provide the physical level of eddy viscosity distribution without using any a priori estimation.However,it is found that slightly less dissipative results can be obtained by using the dynamic Leith model.Two-layer numerical experiments also reveal that the proposed dynamic models automatically parameterize the subgrid-scale stress terms in each active layer.Furthermore,the proposed scale-aware models dynamically provide higher values of the eddy viscosity for smaller resolutions taking into account the local resolved flow information,and addressing the intimate relationship between the eddy viscosity coefficients and the numerical resolution employed by the quasigeostrophic models.
