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931.
C. Muñoz-Robles N. ReidM. Tighe S.V. BriggsB. Wilson 《Journal of Arid Environments》2011,75(10):936-945
In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha−1), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha−1) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h−1 for 30 min applied on 1 -m2 plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h−1, followed by recent pasture (8.2 mm h−1), long-established pasture (5.9 mm h−1) and open woodland (4.2 mm h−1). Total sediment production in recent pasture was 11.6 g m−2, followed by woody encroachment (9.0 g m−2), long-established pasture (7.3 g m−2) and open woodland (4.3 g m−2). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h−1 and 1.3 g m−2) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h−1and 6.5 g m−2, respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher. 相似文献
932.
Hydrogeochemical controls and usability of groundwater in the semi-arid Mayo Tsanaga River Basin: far north province,Cameroon 总被引:2,自引:0,他引:2
Wilson Y. Fantong Hiroshi Satake Samuel N. Ayonghe Festus T. Aka Kazuyoshi Asai 《Environmental Geology》2009,58(6):1281-1293
Deuterium, δ
18O, major ions and dissolved silica in groundwater from semi-arid Mayo-Tsanaga river basin in the Far North Province, Cameroon
were used to trace hydrogeochemical processes that control their concentrations and to explore for usability of the water.
Electrical conductivity ranges from 57–2,581 μs/cm with alternating low and high values along the hydraulic gradient. Waters
from piedmont alluvium show low concentrations in major cations, which peak in Mg within basalt, Na within plain alluvium,
and Ca within basalt and the sandy Limani-Yagoua ridge. The initial dominant groundwater composition is CaHCO3, which did not evolve within the basalt and piedmont alluvium, but evolved to NaHCO3 in the granite and plain alluvium. The main processes controlling the major ions composition include the following: (1) dissolution
of silicates and fluorite; (2) precipitation of fluorite and carbonate; (3) cation exchange of Ca in water for Na in clay;
(4) and anthropogenic activities. The δD and δ
18O ratios vary from −35 to 0.7 and −5.3 to 1.1‰, respectively. The lowest and highest isotope ratios are observed in groundwater
within the downstream sandy Limani-Yagoua ridge and the upstream graintes respectively. Variation in isotope ratios depends
on altitude effect of −0.48‰ per 100 m between 600 and 850 m asl, and on evaporation, which had insignificant effect on the
water salinity. Seventy percent of the groundwater shows poor drinking quality and 90% is suitable for irrigation.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
933.
R. W. Wilson N. O'Mahony C. Packham M. Azzaro 《Monthly notices of the Royal Astronomical Society》1999,309(2):379-387
We present results from a study of the seeing quality at the 4.2-m William Herschel telescope at La Palma. The median intrinsic seeing at the site is found to be excellent, and is comparable to that at the Paranal and Mauna Kea observatories. Contributions to the seeing from turbulence within the telescope dome are insignificant. Furthermore, the optical quality and tracking stability of the telescope are good, so that long-exposure image widths close to the limit imposed by the site seeing can be expected. 相似文献
934.
Mark L. Heinrichs Susan E. Wilson Ian R. Walker John P. Smol Rolf W. Mathewes Kenneth J. Hall 《International Journal of Salt Lake Research》1997,6(3):249-267
Salinity fluctuations in lakes of semi-arid regions have long been recognised as indicators of palaeoclimatic change, and
have provided a valuable line of evidence in palaeo-climatic reconstruction. In the present study, fossil remains of diatoms
and midges were used to reconstruct salinity changes at Mahoney Lake from the early postglacial, through the early, mid and
late Holocene. A transition from midges typical of a freshwater community (Protanypus, Sergentia, Heterotrissocladius, Cladopelma, Dicrotendipes) during the early postglacial, to those indicative of saline environments (Cricotopus/Orthocladius, Tanypus) occurred in the early Holocene. The midge-inferred salinity values reflected the shift from freshwater (0.031 g/L) immediately
after deglaciation, to saline water (2.4 to 55.2 g/L) in subsequent periods. A less saline period was found to have occurred
after 1000 yr BP, suggesting a cooler or wetter period. The diatom record indicates similar trends, with freshwater taxa (e.g.,Cyclotella bodanica var. aff.lemanica) dominating near the bottom of the core. Diatom-inferred salinities indicate that saline conditions (about 30 g/L) prevailed
throughout subsequent Holocene time, although relatively fresh conditions are indicated following deposition of the Mazama
Ash, and from about 1500 yr BP until the present day. Midge and diatom-inferred salinity reconstructions for Mahoney Lake
compare favorably with each other, and with climate trends inferred from earlier palynological evidence. The palaeosalinity
record thus contributes new data relevant to past climatic conditions, in a region where little data have previously been
collected. 相似文献
935.
Whitman Z. R. Wilson T. M. Seville E. Vargo J. Stevenson J. R. Kachali H. Cole J. 《Natural Hazards》2013,65(3):1849-1861
A methodology for evaluating real-time optimal reservoir releases under flooding conditions that minimizes flood damages for a river-reservoir system is described in this paper. The problem is formulated as a discrete-time optimal control problem in which reservoir releases are the control variables, and water surface elevations and discharges are the state variables. Constraints imposed on the reservoir’s water surface elevations and reservoir releases to the downstream reaches are incorporated into an objective function using a penalty function method. The optimal control model consists of the two primary interfaced components: (1) the U.S. Geological Survey Full EQuation routing model to simulate the unsteady flow dynamics of the river-reservoir system and (2) an optimization technique, simulated annealing that optimizes reservoir releases (flood control gate operations) subject to system constraints. The model solves an augmented control problem. The model was applied to the river-reservoir system of Lake Travis on the Lower Colorado River in Texas. The model application to Lake Travis revealed the usefulness of the model in improving a given operation policy, regardless of the type objective function (linear or nonlinear). The methodology and the operation model developed here are unique since they can be applied to any river-reservoir system, do not require simplification of nonlinearities, and guarantee the determination of an optimal or near-global optima. 相似文献
936.
C. D. Wilson G. R. Petitpas D. Iono A. Peck M. Krips B. E. Warren A. J. Baker M. S. Yun Y. Pihlstrom C. Mihos S. Matsushita M. Juvela P. T. P. Ho T. J. Cox L. Armus 《Astrophysics and Space Science》2008,313(1-3):297-302
Luminous and Ultraluminous infrared galaxies (ULIRGs) contain the most intense regions of star formation in the local universe.
Because molecular gas is the fuel for current and future star formation, the physical properties and distribution of the warm,
dense molecular gas are key components for understanding the processes and timescales controlling star formation in these
merger and merger remnant galaxies. We present new results from a legacy project on the Submillimeter Array which is producing
high resolution images of a representative sample of galaxies with log L
FIR
>11.4 and D<200 Mpc. 相似文献
937.
The role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the Moon: Morphology,topography, subresolution roughness,and mode of emplacement of the Sosigenes lunar irregular mare patch 下载免费PDF全文
Le Qiao James W. Head Long Xiao Lionel Wilson Josef D. Dufek 《Meteoritics & planetary science》2018,53(4):778-812
Lunar irregular mare patches (IMPs) comprise dozens of small, distinctive, and enigmatic lunar mare features. Characterized by their irregular shapes, well-preserved state of relief, apparent optical immaturity, and few superposed impact craters, IMPs are interpreted to have been formed or modified geologically very recently (<~100 Ma; Braden et al. 2014 ). However, their apparent relatively recent formation/modification dates and emplacement mechanisms are debated. We focus in detail on one of the major IMPs, Sosigenes, located in western Mare Tranquillitatis, and dated by Braden et al. ( 2014 ) at ~18 Ma. The Sosigenes IMP occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago. The floor of the pit crater is characterized by three morphologic units typical of several other IMPs, i.e., (1) bulbous mounds 5–10 m higher than the adjacent floor units, with unusually young crater retention ages, meters thick regolith, and slightly smaller subresolution roughness than typical mature lunar regolith; (2) a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness; and (3) a lower blocky unit composed of fresh boulder fields with individual meter-scale boulders and rough subresolution surface texture. Using new volcanological interpretations for the ascent and eruption of magma in dikes, and dike degassing and extrusion behavior in the final stages of dike closure, we interpret the three units to be related to the late-stage behavior of an ancient dike emplacement event. Following the initial dike emplacement and collapse of the pit crater, the floor of the pit crater was flooded by the latest-stage magma. The low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake, and CO gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface. This phase produced a very rough and highly porous (with both vesicularity and macroporosity) lava lake surface as the lake surface cooled. In the terminal stage of the eruption, dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to H2O gas exsolution. This magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds. We interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago. We attribute the unusual physical properties of the mounds and floor units (anomalously young ages, unusual morphology, relative immaturity, and blockiness) to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater. The unique physical properties of the mounds (magmatic foams) and hummocky units (small vesicles and large void space) altered the nature of subsequent impact cratering, regolith development, and landscape evolution, inhibiting the typical formation and evolution of superposed impact craters, and maintaining the morphologic crispness and optical immaturity. Accounting for the effects of the reduced diameter of craters formed in magmatic foams results in a shift of the crater size–frequency distribution age from <100 Myr to billions of years, contemporaneous with the surrounding ancient mare basalts. We conclude that extremely young mare basalt eruptions, and resulting modification of lunar thermal evolution models to account for the apparent young ages of the IMPs, are not required. We suggest that other IMP occurrences, both those associated with pit craters atop dikes and those linked to fissure eruptions in the lunar maria, may have had similar ancient origins. 相似文献
938.
Recently there has been growing interest in, and use of, the PROFILE model, principally through the role of weathering rates in determining critical loads. In many cases the accuracy of the data used to run the model is not quoted. A simple sensitivity analysis of the model has been carried out by varying the input data one at a time in a systematic fashion. The variation in weathering rate that this generates for monomineralic soil profiles and for a soil profile from an acid sensitive forested catchment in Scotland has been recorded. Variations in weathering rates of over 100% can be generated using ranges of input parameters measured in field studies. The model broadly predicts the relative ease of weathering of the different minerals. Minerals which are particularly sensitive to input variations have been identified, e.g. Kfeldspar. Some input parameters exert a larger influence on the weathering rate as calculated by the model than others. The most sensitive input data are soil temperature, moisture content and exposed mineral surface area. The least sensitive input data are cation load and precipitation rate. 相似文献
939.
A new radiochemical method for measuring the amount of water in the hydrated layer on the surface of obsidians exchanges tritiated water with the water in the layer (20 μl of 5 Ci ml?1 at 90°C for 10 days) and then back-exchanges it (in 150 ml of water at 35°C for ~ 200 hr.). The activity of the back-exchange water (F) is monitored by liquid scintillation counting of aliquots extracted at known time intervals (t). The activity so measured is then related to the thickness of the hydration rim. A sheet diffusion model shows that the thickness of the hydration shell (l) is inversely proportional to the slope of the F vs. plot. Comparison of l-values so obtained between obsidians, whose age (x) is inferred from archaeological occupation layers containing radiocarbon-dated wood and charcoal, suggests a relationship between l and x. Implications for New Zealand prehistory are briefly considered. The technique, which is non-destructive, appears particularly applicable to young glasses where the development of hydrated layers may be inadequate for accurate optical measurement. 相似文献
940.
Arthur W. Wells J. Rodney Diehl Grant Bromhal Brian R. Strazisar Thomas H. Wilson Curt M. White 《Applied Geochemistry》2007
Geological sequestration of CO2 in depleted oil reservoirs is a potentially useful strategy for greenhouse gas management and can be combined with enhanced oil recovery. Development of methods to estimate CO2 leakage rates is essential to assure that storage objectives are being met at sequestration facilities. Perfluorocarbon tracers (PFTs) were added as three 12 h slugs at about one week intervals during the injection of 2090 tons of CO2 into the West Pearl Queen (WPQ) depleted oil formation, sequestration pilot study site located in SE New Mexico. The CO2 was injected into the Permian Queen Formation. Leakage was monitored in soil–gas using a matrix of 40 capillary adsorbent tubes (CATs) left in the soil for periods ranging from days to months. The tracers, perfluoro-1,2-dimethylcyclohexane (PDCH), perfluorotrimethylcyclohexane (PTCH) and perfluorodimethylcyclobutane (PDCB), were analyzed using thermal desorption, and gas chromatography with electron capture detection. Monitoring was designed to look for immediate leakage, such as at the injection well bore and at nearby wells, and to develop the technology to estimate overall CO2 leak rates based on the use of PFTs. Tracers were detected in soil–gas at the monitoring sites 50 m from the injection well within days of injection. Tracers continued to escape over the following years. Leakage appears to have emanated from the vicinity of the injection well in a radial pattern to about 100 m and in directional patterns to 300 m. Leakage rates were estimated for the 3 tracers from each of the 4 sets of CATs in place following the start of CO2 injection. Leakage was fairly uniform during this period. As a first approximation, the CO2 leak rate was estimated at about 0.0085% of the total CO2 sequestered per annum. 相似文献