Problems in Estimating Impacts of Future Climate Change on Midwestern Corn Yields     

Stanley A. Changnon  Steven E. Hollinger 《Climatic change》2003,58(1-2):109-118

Recent studies of trends in Midwestern precipitation show marked increases over the last 50 years of the 20th Century, and most climate models project that future rainfall in the Corn Belt will be increased further. During five years, 1988–1991 and 1994, field tests were conducted on agricultural test plots in central Illinois, an area typical of the Corn Belt, to discern how corn yields reacted to varying levels of added rainfall (+10%, +25%, and +40%) during the growing season. The best treatment over the five years was a 40% rain increase, with an average yield increase of 9%. Its yield increase was up to 34% in a hot-dry year, but below that of natural rainfall in a wet year as were the yields of the other lesser increases. The average yield changes from the three treatments were not statistically significantly different. Major interannual yield differences were found in the yields for each rain treatment, reflecting how rain timing and temperatures also have major effects on yields. A 40% summer rain increase has little influence if natural rains do not occur in the high stress period of mid summer. The plots results show that only small average increases in corn yields occur from growing season rain additions in the 10% to 40% range, except in dry years. Weather-crop yield regression models incorporating the same rain increases predict greater yield increases than found in these field tests. This suggests that future yields projected for a wetter climate using yield-weather models may be over-estimated. The plot sample size is small but conditions sampled in the five years represented 43% of all past 97 growing seasons in central Illinois and extremely good and bad weather years, which resulted in large between-year yield differences. Hence, the experimental results provide useful information about how increased rainfall may affect future corn yields, especially since the sample included three of the five types of dry growing seasons found in the area's climate since 1900.  相似文献   

Field measurement and numerical modelling of aeolian mass flux distributions on a sandy beach   总被引：6，自引：0，他引：6  

Steven L. Namikas 《Sedimentology》2003,50(2):303-326

ABSTRACT The vertical and horizontal distributions of aeolian mass flux were measured at Oceano Dunes, California, and these data were used to evaluate a numerical model of saltation. Grain‐size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population, and modelling thus focused on replicating the distributions of the mean grain size. Although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity, simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field. Both exponential and gamma distributions of launch velocity produced realistic simulations, although the latter approach required the inclusion of an additional reptation component to achieve good results. A range of mean launch angles and an equivalent sphere correction were also found to generate comparable results, providing the other input parameters could be varied freely. All the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions, and underestimated the proportion occurring at the upwind end of the horizontal distributions. No theoretical shortcoming that would account for these small, but systematic, discrepancies could be identified, and experimental error thus represents a more plausible explanation. The conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed, and that increases in the transport rate are therefore driven primarily by the ejection of additional grains. It is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation, equivalent to a plastic limit. Hence, grains of larger mass (diameter) rebound from the bed at lower speeds, and follow shorter, lower trajectories, as has been widely observed previously.  相似文献   

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa   总被引：1，自引：0，他引：1  

Steven B. Shirey  Jeffrey W. Harris  Stephen H. Richardson  Matthew Fouch  David E. James  Pierre Cartigny  Peter Deines  Fanus Viljoen 《Lithos》2003,71(2-4):243-258

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.  相似文献   

Strontium in coral aragonite: 1. Characterization of Sr coordination by extended absorption X-ray fine structure     

Adrian A. Finch  Nicola Allison  Steven R. Sutton  Matthew Newville 《Geochimica et cosmochimica acta》2003,67(6):1197-1202

Aragonite was analyzed from Porites lobata, Pavona gigantea, Pavona clavus, and Montastrea annularis corals by Sr K-edge extended absorption X-ray fine structure (EXAFS) and compared with aragonite, strontianite, and mechanically mixed standards. Bulk analyses were performed and data compared with equivalent micro-EXAFS analyses on small (∼400 μm³) analytical volumes with a microfocused X-ray beam. As a result of the architecture of the coral skeleton, the crystals within the microanalytical volume are not randomly oriented, and the microanalytical X-ray absorption spectra show orientational dependence. However, refinement of bulk and microanalytical data provided indistinguishable interatomic distances and thermal vibration parameters in the third shell (indicative of Sr speciation). The Sr K-edge EXAFS of all the coral samples refine, within error, to an ideally substituted Sr in aragonite, in contrast to previous studies, in which significant strontianite was reported. Some samples from that study were also analyzed here. Strontianite may be less widely distributed in corals than previously thought.  相似文献   

Dissolution kinetics of magnesite in acidic aqueous solution: a hydrothermal atomic force microscopy study assessing step kinetics and dissolution flux     

Steven R Higgins  Guntram Jordan  Carrick M Eggleston 《Geochimica et cosmochimica acta》2002,66(18):3201-3210

Magnesite (104) dissolution kinetics were studied in acidic aqueous solutions (2.0 < pH < 4.2) at temperatures between 60 and 90°C by atomic force microscopy (AFM). Comparison of dissolution fluxes obtained by AFM and chemical methods revealed six to seven times larger dissolution fluxes obtained by chemical analysis. Corresponding empirical activation energies were found to be 74 ±22 kJ/mol and 41 ± 4 kJ/mol (at pH 4.2) for the AFM and chemical methods, respectively. The empirical reaction order with respect to proton concentration was 0.36 ± 0.13 and 0.47 ± 0.03 for AFM and chemical methods, respectively. These comparisons suggest that the two experimental measurement methods differ as a result of the different sampling length scales associated with the methods. Negligible changes in step dissolution velocity with changes in bulk pH were found, suggesting that the principal source of increasing dissolution flux with decreasing pH is an increase in step density. However, the observed stable step orientation, which is dependent on pH, suggests that more than one proton adsorption equilibrium should be used to describe the surface chemistry of magnesite in acidic solution.  相似文献   

Cephalopods from a borehole in the type Wenlock area     

Charles Hepworth Holland 《Proceedings of the Geologists' Association. Geologists' Association》2002,113(3):207-215

  相似文献   

Estimating the windward slope profile of a barchan dune     

Hiroshi Momiji  Steven R. Bishop 《Sedimentology》2002,49(3):467-481

A new theoretical scheme is presented to model the shape of a sand dune at equilibrium that does not require iterative calculations of the interaction between the wind flow and topography. The model is constructed by incorporating theory based on aerodynamics into a grain‐scale model that estimates the shear velocity at the dune crest through the calculation of the sand trapping efficiency of the slipface in the lee of the dune. Published field data, collected in southern Peru, California and southern Morocco, show that as a dune becomes higher the windward slope becomes steeper. For the model proposed, the wind flow over a dune was first assumed to be similar to that over a Gaussian hill. By further assuming a fixed shear velocity on the level surface, the windward slope angle and migration speed of dunes in southern Peru can be explained. To comply with all available data, some aspects are still open to further investigation. However the theoretical insight presented herein implies that the upper limit of dune height may be greater in windier environments.  相似文献   

Regulation of Arsenic in Drinking Water Affects Ground Water Systems     

Charles Job 《Ground Water Monitoring & Remediation》2002,22(1):40-43

  相似文献   

Multibeam bathymetry and the depositional environments of Kongsfjorden and Krossfjorden, western Spitsbergen, Svalbard     

John A. Howe  Steven G. Moreton  Clara Morri  Peter Morris 《Polar research》2003,22(2):301-316

Kongsfjorden and Krossfjorden are two ice-proximal fjords on the western coast of Spitsbergen which have been surveyed using multibeam bathymetry, sub-bottom profiling and gravity coring. Central and outer Kongsfjorden is dominated by a 30 km² outcrop of bedrock, with a thin (<10m) sediment cover. The bedrock displays a relict sub-glacial, ice-scoured topography produced during the glacial re-advances of the Weichselian (20 Ky BP) and again during the last major Holocene re-advance of the Little Ice Age (550-200 yrs BP). Drumlins and glacial flutes are common across the floor of Kongsfjorden, with lengths of 1.5-2.5 km and widths of <100 m, rising up to 10 m in water depths of <100 m. This topography is smoothed by bottom currents from the wind-driven forcing of surface waters. The flow is counter-clockwise, matching boundary layer movement under the influence of Coriolis force. Both fjords are characterized by a variable acoustic character, based on sub-bottom profile data. The deepest basins are dominated by parallel, well-laminated reflectors and an irregular-transparent acoustic character indicating the presence of Holocene-age fine-grained sediments up to 30 m thick. A parallel, irregular-transparent acoustic character with waveform morphology in inner Kongsfjorden is interpreted as moraines, originating from the 1948 and 1869 surges of Kronebreen glacier. Mass-flows are common on the flanks of topographic highs as acoustically chaotic-transparent lensoid and wedge-shaped reflectors. The sediments of outer and central Kongsfjorden are characterized by bioturbated, gas-rich homogeneous muds interpreted as being the result of the settling of fine-grained sediment and particulate suspensions.  相似文献   

Elevation Dependence of Winter Wheat Production in Eastern Washington State with Climate Change: A Methodological Study     

Allison M. Thomson  Robert A. Brown  Steven J. Ghan  R. Cesar Izaurralde  Norman J. Rosenberg  L. Ruby Leung 《Climatic change》2002,54(1-2):141-164

Crop growth models, used in climate change impact assessments to project production on a local scale, can obtain the daily weather information to drive them from models of the Earth's climate. General Circulation Models (GCMs), often used for this purpose, provide weather information for the entire globe but often cannot depict details of regional climates especially where complex topography plays an important role in weather patterns. The U.S. Pacific Northwest is an important wheat growing region where climate patterns are difficult to resolve with a coarse scale GCM. Here, we use the PNNL Regional Climate Model (RCM) which uses a sub-grid parameterization to resolve the complex topography and simulate meteorology to drive the Erosion Productivity Impact Calculator (EPIC) crop model. The climate scenarios were extracted from the PNNL-RCM baseline and 2 × CO₂ simulationsfor each of sixteen 90 km² grid cells of the RCM, with differentiation byelevation and without correction for climate biases. The dominant agricultural soil type and farm management practices were established for each grid cell. Using these climate and management data in EPIC, we simulated winter wheat production in eastern Washington for current climate conditions (baseline) and a 2 × CO₂ `greenhouse' scenario of climate change.Dryland wheat yields for the baseline climate averaged 4.52 Mg ha^–1 across the study region. Yields were zero at high elevations where temperatures were too low to allow the crops to mature. The highest yields (7.32 Mgha^–1) occurred at intermediate elevations with sufficientprecipitation and mild temperatures. Mean yield of dryland winter wheat increased to 5.45 Mg ha^–1 for the 2 × CO₂ climate, which wasmarkedly warmer and wetter. Simulated yields of irrigated wheat were generally higher than dryland yields and followed the same pattern but were, of course, less sensitive to increases in precipitation. Increases in dryland and irrigated wheat yields were due, principally, to decreases in the frequency of temperature and water stress. This study shows that the elevation of a farm is a more important determinant of yield than farm location in eastern Washington and that climate changes would affect wheat yields at all farms in the study.  相似文献