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41.
Large-scale atmospheric patterns are examined on orbital timescales using a climate model which explicitly resolves the atmosphere–ocean–sea ice dynamics. It is shown that, in contrast to boreal summer where the climate mainly follows the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and the Northern/Southern Annular Modes. We find that during a positive phase of the AO/NAO the convection in the tropical Pacific is below normal. The related atmospheric circulation provides an atmospheric bridge for the precessional forcing inducing a non-uniform temperature anomalies with large amplitudes over the continents. We argue that this is important for mechanisms responsible for multi-millennial climate variability and glacial inception. 相似文献
42.
The characteristics of a global set-up of the Finite-Element Sea-Ice Ocean Model under forcing of the period 1958–2004 are presented. The model set-up is designed to study the variability in the deep-water mass formation areas and was therefore regionally better resolved in the deep-water formation areas in the Labrador Sea, Greenland Sea, Weddell Sea and Ross Sea. The sea-ice model reproduces realistic sea-ice distributions and variabilities in the sea-ice extent of both hemispheres as well as sea-ice transport that compares well with observational data. Based on a comparison between model and ocean weather ship data in the North Atlantic, we observe that the vertical structure is well captured in areas with a high resolution. In our model set-up, we are able to simulate decadal ocean variability including several salinity anomaly events and corresponding fingerprint in the vertical hydrography. The ocean state of the model set-up features pronounced variability in the Atlantic Meridional Overturning Circulation as well as the associated mixed layer depth pattern in the North Atlantic deep-water formation areas. 相似文献
43.
Estimating groundwater recharge for an arid karst system using a combined approach of time‐lapse camera monitoring and water balance modelling 下载免费PDF全文
Stephan Schulz Gerrit H. de Rooij Nils Michelsen Randolf Rausch Christian Siebert Christoph Schüth Mohammed Al‐Saud Ralf Merz 《水文研究》2016,30(5):771-782
Groundwater is the principal water resource in semi‐arid and arid environments. Therefore, quantitative estimates of its replenishment rate are important for managing groundwater systems. In dry regions, karst outcrops often show enhanced recharge rates compared with other surface and sub‐surface conditions. Areas with exposed karst features like sinkholes or open shafts allow point recharge, even from single rainfall events. Using the example of the As Sulb plateau in Saudi Arabia, this study introduces a cost‐effective and robust method for recharge monitoring and modelling in karst outcrops. The measurement of discharge of a representative small catchment (4.0 · 104 m2) into a sinkhole, and hence the direct recharge into the aquifer, was carried out with a time‐lapse camera. During the monitoring period of two rainy seasons (autumn 2012 to spring 2014), four recharge events were recorded. Afterwards, recharge data as well as proxy data about the drying of the sediment cover are used to set up a conceptual water balance model. The model was run for 17 years (1971 to 1986 and 2012 to 2014). Simulation results show highly variable seasonal recharge–precipitation ratios between 0 and 0.27. In addition to the amount of seasonal precipitation, this ratio is influenced by the interannual distribution of rainfall events. Overall, an average annual groundwater recharge for the doline (sinkhole) catchment on As Sulb plateau of 5.1 mm has estimated for the simulation period. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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46.
Juan F. García-Quijano Jan Peters Liesbet Cockx Gerrit van Wyk Andrei Rosanov Gaby Deckmyn Reinhart Ceulemans Shane M. Ward Nicholas M. Holden Jos Van Orshoven Bart Muys 《Climatic change》2007,83(3):323-355
A three-step methodology to assess the carbon sequestration and the environmental impact of afforestation projects in the
framework of the Flexible Mechanisms of the Kyoto Protocol (Joint Implementation and Clean Development Mechanism) was developed
and tested using a dataset collected from the Jonkershoek forest plantation, Western Cape, South Africa, which was established
with Pinus radiata in former native fynbos vegetation and indigenous forest. The impact of a change in land use was evaluated for a multifunctional,
a production and a non-conversion scenario. First, the carbon balance was modelled with GORCAM and was expressed as (1) C
sequestration in tC ha−1 year−1 in soil, litter, and living biomass according to the rules of the first commitment period of the Kyoto Protocol, and (2)
CO2 emission reductions in tC ha−1 year−1, which includes carbon sequestered in the above-mentioned pools and additionally in wood products, as well as emission reductions
due to fossil fuel substitution. To estimate forest growth, three data sources were used: (1) inventory data, (2) growth simulation
with a process-based model, and (3) yield tables. Second, the effects of land use change were assessed for different project
scenarios using a method related to Life Cycle Assessment (LCA). The method uses 17 quantitative indicators to describe the
impact of project activities on water, soil, vegetation cover and biodiversity. Indicator scores were calculated by comparing
indicator values with reference values, estimated for the climax vegetation. The climax vegetation is the site-specific ecosystem
phase with the highest exergy content and the highest exergy flow dissipation capacity. Third, the land use impact per functional
unit of 1 tC sequestered was calculated by combining the results of step 1 and step 2. The average baselines to obtain carbon
additionality are 476 tC ha−1 for indigenous forest and 32 tC ha−1 for fynbos. Results show that the influence of the growth assessment method on the magnitude of C sequestration and hence
on the environmental impact per functional unit is large. When growth rate is assessed with the mechanistic model and with
the yield table, it is overestimated in the early years and underestimated in the long term. The main conclusion of the scenario
analysis is that the production forest scenario causes higher impacts per functional unit than the multifunctional scenario,
but with the latter being less efficient in avoiding CO2 emissions. The proposed method to assess impacts on diverse components of the ecosystem is able to estimate the general tendency
of the adverse and positive effects of each scenario. However, some indicators, more specifically about biodiversity and water
balance, could be improved or reinterpreted in light of specific local data about threat to biodiversity and water status. 相似文献
47.
Generalized equations using fractional-flow dimensions were derived to estimate the Darcy and groundwater-flow velocities
obtained from the point-dilution and the single-well injection-withdrawal field tests. Flow velocities can only be estimated
from single-well tests if the kinematic porosity or the hydraulic conductivity and hydraulic gradient are known a priori.
A pumping test performed on the boreholes will yield an estimate of the fractional-flow dimension and the extent of the flow
region by applying the generalized radial flow (GRF) model of Barker [Barker JA (1988) A generalized radial flow model for
hydraulic tests in fractured rock. Water Resour Res 24(10):1796–1804]. These parameters are used in the generalised equations
for the single-well tracer tests to estimate the flow-through area and, therewith, the Darcy or flow velocity.
The generalized procedure, described in detail in Part 1 of this paper, is applied to two boreholes on the Campus Test Site
located at the University of the Free State, Bloemfontein, South Africa, and it is shown that the two independent tests (i.e.
the point-dilution and the single-well injection-withdrawal tests) yield similar estimates of the natural-seepage velocity
in the aquifer. The estimated natural-flow velocity obtained by using fractional dimensions is about two times higher than
the velocity estimated by using the standard method (i.e. flow dimension n=2, flow thickness equal to length of the sealed-off section of the borehole).
Electronic Publication 相似文献
48.
Marius J. van Essen Johannes A. C. Heynen Adriaan H. van der Veen Gerrit H. Vonkeman 《Mineralium Deposita》1971,6(1):41-48
A portable gibbsite analyzer suited for field operation and intended for rapid determination of available alumina is described. The available alumina content is calculated from the relative weight loss caused by the dehydration of gibbsite. More than 100 samples of gibbsite-containing bauxites of different origin, were analyzed with this apparatus. The results always appeared to be correct within 10% of the actual content. In the laboratory, an accuracy of ±2.60% gibbsite, and a reproducibility of ±1.87% gibbsite, both at the 95% confidence interval, could be achieved. The apparatus can be used under tropical conditions. Its weight is 13 kg. It can be fed from a motor-driven generator weighing 18 kg. The duration of a single analysis in the field is 30–40 minutes. In the laboratory about 25 analyses can be performed within 8 hours. The apparatus can be handled by an untrained operator. One operator can run at least two analyzers at a time. The complete set-up, including the generator, costs less than US $ 1500.
Zusammenfassung Ein tragbarer Apparat für die Gibbsit Bestimmung im Freien wird beschrieben. Der Gibbsitgehalt wird berechnet aus dem Gewichtsverlust während der Entwässerung. Für Gibbsit-haltige Bauxite, die mit diesem Apparat analysiert sind, wurden Resultate gefunden, die immer weniger als 10% vom richtigen Gehalt abwichen. Im Labor konnte eine Genauigkeit von ±2.60% Gibbsit und eine Reproduzierbarkeit von ±1.87% Gibbsit, beide mit einer Wahrscheinlichkeit von 95%, erreicht werden. Der Apparat ist ausgerüstet für den Gebrauch in den Tropen. Er wiegt 13 kg und kann von einem durch einen Benzinmotor angetriebenen Generator gespeist werden. Der Generator hat ein Gewicht von 18 kg. Eine einzige Bestimmung im Freien dauert 30–40 Minuten. Im Labor können etwa 25 Bestimmungen innerhalb von 8 Stunden gemacht werden. Ein Mann kann mindestens zwei Apparate zugleich beobachten. Apparat und Generator kosteten weniger als US $ 1500.相似文献
49.
Spectral mixture analysis (SMA) is an image-processing technique used for the analysis of airborne hyperspectral remote-sensing data which consist of a large number of spectral bands, typically over 100. In this paper the possibilities are examined of using SMA for the analysis of Landsat Thematic Mapper satellite data with only six bands in the visible to shortwave-infrared wavelength. We use data from a semi-arid area in the Sanmatenga province of Burkina Faso, an area known to experience land-degradation problems. In SMA, we assume that pixels in an image consist of one or more homogeneous (uniform in character) or pure surfaces, the so called end-members. The end-members were derived from the image data on the basis of specific image characteristics. Field data was collected to describe the characteristics of these end-members in terms of land cover, soil and degradation phenomena. The end-members derived from the image analysis, although statistically pure in terms of image spectral characteristics, prove to be mixtures at a field scale. This lack of purity is explained by the nature of semi-arid areas which is more heterogeneous than that of most other areas. The SMA yielded cover percentages for the end-members from which an unsupervised classification was made. Comparison of the classification with the results of SMA shows that the latter provides information on the amount and spatial distribution of land characteristics such as land degradation. 相似文献
50.
Gerrit L. Verschuur 《Astrophysics and Space Science》1991,185(1):137-152
Large-scale morphological waves with wavelengths of order 30° and amplitude=1/8 exist in several long Hi filaments in a 540 square degrees of sky aroundl=230°,b=+40°. The extent of the longest filament is greater than 72°, the limit set by the boundaries of the area surveyed. TheHi gas appears to be controlled by magnetic fields and the motion within flux tubes, determined by analysis of velocity along the filament axes, shows the presence of wave patterns with amplitude 5 to 6 km s–1 on an angular scale similar to that seen in the spatial structure projected on the sky. 相似文献