Near-infrared spectra of the leading and trailing hemispheres of Enceladus     

Anne J. Verbiscer  Dawn E. Peterson  Michael Cushing  Matthew J. Nelson  John C. Wilson 《Icarus》2006,182(1):211-223

We present individual spectra 0.8-2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH₃⋅H₂O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH₃⋅H₂O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution (R=λ/Δλ) of these CorMASS data (R∼300) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) (R=225) onboard the Cassini spacecraft.  相似文献   

Transport and fate of chloride from road salt within a mixed urban and agricultural watershed in Illinois (USA): assessing the influence of chloride application rates     

Ludwikowski  Jessica J.  Peterson  Eric W. 《Hydrogeology Journal》2018,26(4):1123-1135

In a typical winter season, approximately 471,000 tons of road salt are deposited along roadways in Illinois, USA. An estimated 45% of the deposited road salt will infiltrate through the soils and into shallow aquifers. Transported through shallow aquifers, chloride associated with the road salts has the potential to reside within groundwater for years based on the pathway, the geologic material, and the recharge rate of the aquifer system. Utilizing MODFLOW and MT3D, simulations employing various road-salt application rates were conducted to assess the net accumulation of chloride and the residence times of chloride in an agriculture-dominated watershed that originates in an urban area. A positive-linear relationship was observed between the application rate of chloride and both the maximum chloride concentration and total mass accumulated within the watershed. Simulated annual recharge rates along impacted surfaces ranged from 1,000 to 10,000 mg/L. After 60 years of application, simulated chloride concentrations in groundwater ranged from 197 to 1,900 mg/L. For all application rates, chloride concentrations within the groundwater rose at an annual rate of >3 mg/L. While concentrations increase throughout the system, the majority of chloride accumulation occurs near the roads and the urban areas. Model simulations reveal a positive relationship between application rate and residence time of chloride (1,123–1,288 days based on application rate). The models indicate that continued accumulation of chloride in shallow aquifers can be expected, and methods that apply less chloride effectively need to be examined.

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The Bloom-Forming Macroalgae, <Emphasis Type="Italic">Ulva</Emphasis>, Outcompetes the Seagrass, <Emphasis Type="Italic">Zostera marina</Emphasis>, Under High CO<Subscript>2</Subscript> Conditions     

Craig S. Young  Bradley J. Peterson  Christopher J. Gobler 《Estuaries and Coasts》2018,41(8):2340-2355

While multiple species of macroalgae and seagrass can benefit from elevated CO₂ concentrations, competition between such organisms may influence their ultimate responses. This study reports on experiments performed with a Northwest Atlantic species of the macroalgae, Ulva, and the seagrass, Zostera marina, grown under ambient and elevated levels of pCO₂, and subjected to competition with each other. When grown individually, elevated pCO₂ significantly increased growth rates and productivity of Ulva and Zostera, respectively, beyond control treatments (by threefold and 27%, respectively). For both primary producers, significant declines in tissue δ¹³C signatures suggested that increased growth and productivity were associated with a shift from use of HCO₃^? toward CO₂ use. When grown under higher pCO₂, Zostera experienced significant increases in leaf and rhizome carbon content as well as significant increases in leaf carbon-to-nitrogen ratios, while sediments within which high CO₂ Zostera were grown had a significantly higher organic carbon content. When grown in the presence of Ulva; however, above- and below-ground productivity and tissue nitrogen content of Zostera were significantly lower, revealing an antagonistic interaction between elevated CO₂ and the presence of Ulva. The presence of Zostera had no significant effect on the growth of Ulva. Collectively, this study demonstrates that while Ulva and Zostera can each individually benefit from elevated pCO₂ levels, the ability of Ulva to grow more rapidly and inhibit seagrass productivity under elevated pCO₂, coupled with accumulation of organic C in sediments, may offset the potential benefits for Zostera within high CO₂ environments.  相似文献   

Effects of Linking a Soil‐Water‐Balance Model with a Groundwater‐Flow Model     

Jennifer S. Stanton  Derek W. Ryter  Steven M. Peterson 《Ground water》2013,51(4):613-622

A previously published regional groundwater‐flow model in north‐central Nebraska was sequentially linked with the recently developed soil‐water‐balance (SWB) model to analyze effects to groundwater‐flow model parameters and calibration results. The linked models provided a more detailed spatial and temporal distribution of simulated recharge based on hydrologic processes, improvement of simulated groundwater‐level changes and base flows at specific sites in agricultural areas, and a physically based assessment of the relative magnitude of recharge for grassland, nonirrigated cropland, and irrigated cropland areas. Root‐mean‐squared (RMS) differences between the simulated and estimated or measured target values for the previously published model and linked models were relatively similar and did not improve for all types of calibration targets. However, without any adjustment to the SWB‐generated recharge, the RMS difference between simulated and estimated base‐flow target values for the groundwater‐flow model was slightly smaller than for the previously published model, possibly indicating that the volume of recharge simulated by the SWB code was closer to actual hydrogeologic conditions than the previously published model provided. Groundwater‐level and base‐flow hydrographs showed that temporal patterns of simulated groundwater levels and base flows were more accurate for the linked models than for the previously published model at several sites, particularly in agricultural areas.  相似文献   

Using δ15N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales     

Joel C. Hoffman  John R. Kelly  Greg S. Peterson  Anne M. Cotter  Matthew A. Starry  Michael E. Sierszen 《Estuaries and Coasts》2012,35(6):1453-1467

There is growing interest in applying ??¹⁵N in biota as an indicator of anthropogenic nutrient inputs to coastal environments because changes in ??¹⁵N correlate to inputs of land-based nutrients. In complex coastal receiving waters, however, land-use effects on biota ??¹⁵N may be masked by local hydrologic processes, especially exchange with coastal waters of different geochemical character. We examined ??¹⁵N differences among larval fish, a novel biotic indicator, in coastal receiving waters at both among and within watershed scales. Our goal was to characterize how hydrologic processes within coastal river mouths and embayments mediate the effect of land-based N sources on larval fish ??¹⁵N. We sampled three Lake Superior river-embayment systems from watersheds that span a large population density gradient. Over all stations, mean fish ??¹⁵N ranged from 2.7??? to 10.8???. Within each system, we found a different pattern in ??¹⁵N across the river?Clake transition zone. Correlations between fish ??¹⁵N and water quality, particularly NH ₄ ⁺ and total nitrogen, were highly significant and corresponded to known differences in sewage waste water inputs. A multivariate model that included both watershed-based population density and NH ₄ ⁺ was found to provide the best fit to the ??¹⁵N data among a series of multi- and univariate candidate models. These results demonstrate that: (1) fish larvae ??¹⁵N responded at within watershed scales, and (2) within coastal receiving waters, fish larvae ??¹⁵N was related to waste water inputs at the watershed scale; however, expression at specific locations within a coastal system was strongly influenced by local hydrologic processes.  相似文献   

Detecting Freshwater Inputs via Groundwater Discharge to Marina Lagoon, Mediterranean Coast, Egypt     

Ayman A. El-Gamal  Richard N. Peterson  William C. Burnett 《Estuaries and Coasts》2012,35(6):1486-1499

Much work in recent years has reported on the role of submarine groundwater discharge (SGD) on coastal biogeochemistry, but most of those studies have focused on temperate or tropical climates where year-round rainfall recharges surficial aquifers. The aim of this work, however, was to examine SGD behaviors in an arid setting??Marina Lagoon on the Egyptian Mediterranean coast. SGD was estimated via radon surveys and time-series measurements in lagoon waters during two campaigns (wet season in March 2009 and dry season in July 2010). Relatively higher values of radon were detected in March (maximum >30?dpm/L) compared to July (up to 16?dpm/L), which would indicate either enhanced input rates, or lower mixing/atmospheric losses during the wet season. Lower salinity waters within Marina Lagoon were characterized by higher radon and higher concentrations of dissolved inorganic nitrogen (DIN) and silica (DISi), characteristic of groundwater inputs. Based on lagoon and groundwater radon measurements and an advection-diffusion model, SGD average rates between 0.83 to 2.4?×?10⁸?L/day were estimated for both surveys. Since no seasonal pattern was detected, we suspect that either (1) SGD is derived from a regional aquifer not influenced by recharge from local rainfall or (2) local water use for irrigation and domestic purposes artificially recharges the surficial aquifer in the dry summer (tourist) months, which compensates for the lack of rainfall occurring at that time.  相似文献   

Groundwater pumping by heterogeneous users     

Alexander E. Saak  Jeffrey M. Peterson 《Hydrogeology Journal》2012,20(5):835-849

Farm size is a significant determinant of both groundwater-irrigated farm acreage and groundwater-irrigation-application rates per unit land area. This paper analyzes the patterns of groundwater exploitation when resource users in the area overlying a common aquifer are heterogeneous. In the presence of user heterogeneity, the common resource problem consists of inefficient dynamic and spatial allocation of groundwater because it impacts income distribution not only across periods but also across farmers. Under competitive allocation, smaller farmers pump groundwater faster if farmers have a constant marginal periodic utility of income. However, it is possible that larger farmers pump faster if the Arrow-Pratt coefficient of relative risk-aversion is sufficiently decreasing in income. A greater farm-size inequality may either moderate or amplify income inequality among farmers. Its effect on welfare depends on the curvature properties of the agricultural output function and the farmer utility of income. Also, it is shown that a flat-rate quota policy that limits the quantity of groundwater extraction per unit land area may have unintended consequences for the income distribution among farmers.  相似文献   

Integrating Scales of Seagrass Monitoring to Meet Conservation Needs     

Hilary A. Neckles  Blaine S. Kopp  Bradley J. Peterson  Penelope S. Pooler 《Estuaries and Coasts》2012,35(1):23-46

We evaluated a hierarchical framework for seagrass monitoring in two estuaries in the northeastern USA: Little Pleasant Bay, Massachusetts, and Great South Bay/Moriches Bay, New York. This approach includes three tiers of monitoring that are integrated across spatial scales and sampling intensities. We identified monitoring attributes for determining attainment of conservation objectives to protect seagrass ecosystems from estuarine nutrient enrichment. Existing mapping programs provided large-scale information on seagrass distribution and bed sizes (tier 1 monitoring). We supplemented this with bay-wide, quadrat-based assessments of seagrass percent cover and canopy height at permanent sampling stations following a spatially distributed random design (tier 2 monitoring). Resampling simulations showed that four observations per station were sufficient to minimize bias in estimating mean percent cover on a bay-wide scale, and sample sizes of 55 stations in a 624-ha system and 198 stations in a 9,220-ha system were sufficient to detect absolute temporal increases in seagrass abundance from 25% to 49% cover and from 4% to 12% cover, respectively. We made high-resolution measurements of seagrass condition (percent cover, canopy height, total and reproductive shoot density, biomass, and seagrass depth limit) at a representative index site in each system (tier 3 monitoring). Tier 3 data helped explain system-wide changes. Our results suggest tiered monitoring as an efficient and feasible way to detect and predict changes in seagrass systems relative to multi-scale conservation objectives.  相似文献   

Methodology for benefit–cost analysis of seismic codes     

Jomari Peterson  Mitchell J. Small 《Natural Hazards》2012,63(2):1039-1053

A number of high-profile seismic events have occurred in recent years, with a wide variation in the resulting economic damage and loss of life. This variation has been attributed in part to the stringency of seismic building codes implemented in different regions. Using the HAZUS Earthquake Model, a benefit?Ccost analysis was performed on varying levels of standard buildings codes for Haiti and Puerto Rico. The methodology computes expected loss assuming a Poisson event process with lognormally distributed event magnitude and idealized damage?Cmagnitude response functions. The event frequency and magnitude distributions are estimated from the historical record, while the damage functions are fit using HAZUS simulation results for events with systematically varying magnitudes and different seismic code levels. To validate the approach, a single-event analysis was conducted using alternative building codes and mean magnitude earthquakes. A probabilistic analysis was then used to evaluate the long-term expected value for alternative levels of building codes. To account for the relationship between lives saved and economic loss, the implicit cost of saving a life is computed for each code option. It was found that in the two areas studied, the expected loss of life was reduced the most by use of high seismic building code levels, but lower levels of seismic building code were more cost-effective when considering only building damages and the costs for code implementation. The methodology presented is meant to provide a basic framework for the future development of an economic-behavioral model for code adoption.  相似文献   

Drift currents in the southern New Zealand region as derived from Lagrangian measurements and the remote sensing of sea‐surface temperature distributions     

W. N. Jenks  J. Grindrod  J. A. Peterson 《新西兰海洋与淡水研究杂志》2013,47(3-4):359-371

Four drift bottles, cast adrift south of the Subtropical Convergence at 48°S, 156°E in November 1980, landed within 123 days of release at a short stretch of coast north of Banks Peninsula. A high degree of coherence in the responsible drift pattern is indicated. The contemporary surface circulation inferred from satellite‐derived sea‐surface temperature distributions indicates that the bottles were entrained in a meridionally‐converging flow after drifting across the southern Tasman Sea without crossing the Convergence. They were prevented from further eastward drifting because of a marked southward flexing of the Convergence east of the Southland Current during February 1981. Because of local weather and tide effects, the bottles finally beached in Pegasus Bay.  相似文献