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71.
Future development and climate change pose potentially serious threats to estuarine fish populations around the world. We
examined how habitat suitability for delta smelt (Hypomesus transpacificus), a state and federally protected species, might be affected by changes in outflow in the San Francisco Estuary due to future
development and climate change. Forty years of sampling data collected during fall from 1967 to 2008 were examined to define
abiotic habitat suitability for delta smelt as a function of salinity and water transparency, and to describe long-term trends
in habitat conditions. The annual habitat index we developed, which incorporated both quantity and quality of habitat, decreased
by 78% over the study period. Future habitat index values under seven different development and climate change scenarios,
representing a range of drier and wetter possibilities, were predicted using a model which related estuarine outflow to the
habitat index. The results suggested that each of the scenarios would generally lead to further declines in delta smelt habitat
across all water year types. Recovery targets for delta smelt will be difficult to attain if the modeled habitat conditions
are realized. 相似文献
72.
In cold regions, the response and related antecedent mechanisms that produce flood flows from rainfall events have received limited study. In 2007, a small watershed at Cape Bounty, Melville Island, Nunavut, was studied in detail during the melt season. Two rainfall events on June 30 and July 22, totalling 9·2 and 10·8 mm, respectively, represented significant contributions to seasonal discharge and sediment transport in a year with a low winter snowpack. The precipitation events elevated discharge and suspended sediment concentrations to twice the magnitude of the nival melt, and generated the only measurable downstream lacustrine turbidity current of the season. In two days, rainfall runoff transported 35% of the seasonal suspended sediment load, in contrast to 29% transported over the nival freshet. The magnitude and intensity of the rain events were not unusual in this setting, but the rainfall response was substantial in comparison with equivalent past events. Exceptional temperatures of July 2007 generated early, deep permafrost thaw, and ground ice melt. The resultant increase in soil moisture amplified the subsequent rainfall runoff and sediment transport response. These results demonstrate the importance of antecedent moisture conditions and the role of permafrost active layer development as an important factor in the rainfall runoff and sediment transport response to precipitation events. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
73.
As in seen from comparisons of Mariner 9 images obtained in 1972 and Viking Orbiter 1 images obtained in 1978, several changes have occurred in the Cerberus region of Mars. Changes in the boundary of the low albedo feature resulted in an increase of the total area of Cerberus by slightly more than 1%, although the southwestern boundary had shifted as much as 90 km. Relative darkening of Cerberus has resulted in a more uniform tone, and is accompanied by the disappearance of dark filamentary markings. Although several bright streaks within Cerberus changed in length, neither lengthening nor shortening of the streaks predominated. However, changes in streak direction indicate a clockwise rotation of mean streak azimuth between 1972 and 1978. These changes in the outline and appearance of Cerberus can best be explained by eolian redistribution and removal of bright material during major dust storms. Volcanic flow fronts which show through the albedo feature indicate that the contrast between Cerberusand the surrounding light plains is not due to a difference in lithology, but to the distribution of surficial deposits. Because of local topographic influences on the regional atmospheric circulation patterns, it is probable that Cerberus will retain a similar appearance and location. 相似文献
74.
Lewis and Clark Lake is located on the main stream of the Missouri River. The reservoir is formed behind Gavins Point dam near Yankton, South Dakota, U.S.A. The Lewis and Clark Lake reach extends about 40 km from the Gavins Point dam. The reservoir delta has been growing since the closure of Gavins Point dam in 1955 and has resulted in a 21% reduction of storage within the maximum pool of the reservoir. Among several sediment management methods, drawdown flushing has been recommended as a possible management technique. The engineering viability of removing sediments deposited in the lake should be examined by numerical modeling before implementing a drawdown flushing. GSTARS4 was used for this study and calibrated by using measured data from 1975 to 1995. Channel cross-section changes and amount of flushed sediment were predicted with four hypothetical flow scenarios. The flushing efficiencies of all scenarios were estimated by comparing the ratios between water consumption and flushed sediment during flushing. 相似文献
75.
The kinetics of heterogeneous quartz precipitation were determined at 180°C and a pH near 4 as a function of the degree of super saturation and dissolved NaCl concentrations. Stirred batch reactors were used and the changes in H4SiO4 concentrations over time were used to model the reaction rates. As the change in concentration in the batch experiment is not linear with time during all sampling intervals, the precipitation rates were retrieved by fitting the experimental observations to a forward model, i.e., a model that uses a prescribed rate law and compares the results with the experiments. Quartz precipitation occurs via an overall reaction, and therefore one can not presuppose the applicability of the predictions of Transition State Theory (TST). Therefore, different functions describing the dependence of the rate on deviation from equilibrium were examined in the forward model. The obtained precipitation rates were within error the same, regardless of the function that was used.By substituting the prediction of TST into the forward model, the change of quartz precipitation rate (at 180°C, pH 4 and ionic strength of 0.1M) with deviation from equilibrium was found to be
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79.
The surface susceptibility to erosion (erodibility) is an important component of soil erosion models. Many studies of wind erosion have shown that even relatively small changes in surface conditions can have a considerable effect on the temporal and spatial variability of dust emissions. One of the main difficulties in measuring erodibility is that it is controlled by a number of highly variable soil factors. Collection of these data is often limited in scale because in situ measurements are labour‐intensive and very time‐consuming. To improve wind erosion model predictions over several spatial and temporal scales simultaneously, there is a requirement for a non‐invasive approach that can be used to rapidly assess changes in the compositional and structural nature of a soil surface in time and space. Spectral reflectance of the soil surface appears to meet these desirable requirements and it is controlled by properties that affect the soil erodibility. Three soil surfaces were modified using rainfall simulation and wind tunnel abrasion experiments. Observations of those changes were made and recorded using digital images and on‐nadir spectral reflectance. The results showed clear evidence of the information content in the spectral domain that was otherwise difficult to interpret given the complicated interrelationships between soil composition and structure. Changes detected at the soil surface included the presence of a crust produced by rainsplash, the production of loose erodible material covering a rain crust and the selective erosion of the soil surface. The effect of rainsplash and aeolian abrasion was different for each soil tested and crust abrasion was shown to decrease as rainfall intensity increased. The relative contributions of the eroded material from each soil surface to trapped mixtures of material assisted the erodibility assessment. Ordination analyses within each of two important soil types explained significant amounts of the variation in the reflectance of all wavebands by treatments of the soil and hence changes in the soil surface. The results show that soil surface conditions within a soil type are an underestimated source of variation in the characterization of soil surface erodibility and in the remote sensing of soil. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
80.