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181.
Bidirectional stream–groundwater flow in response to ephemeral and intermittent streamflow and groundwater seasonality
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It is often assumed that the net groundwater flow direction is towards the channel in headwater streams in humid climates, with magnitudes dependent on flow state. However, studies that characterize stream–groundwater interactions in ephemeral and intermittent streams in humid landscapes remain sparse. Here, we examined seasonally driven stream–groundwater interactions in response to temporary streamflow on the basis of field observations of streamflow and groundwater on an adjacent hillslope. The direction of hydraulic head gradients between the stream and groundwater shifted seasonally. The stream gained water (head gradients were towards the stream) when storage state was high. During this period, streamflow was persistent. The stream lost water to the groundwater system (head gradients were away from the stream) when storage state was low. During this period, streamflow only occurred in response to precipitation events, and head gradients remained predominantly away from the stream during events. This suggested that mechanisms other than deep groundwater contributions produced run‐off when storage was low, such as surface and perched subsurface flowpaths above the water table. Analysis of the annual water balance for the study period showed that the residual between precipitation inputs and streamflow and evapotranspiration outputs, which were attributed to the loss of water to the deeper, regional groundwater system, was similar in magnitude to streamflow. This, coupled with results that showed bidirectionality in stream–groundwater head gradients, indicated that headwaters composed of temporary (e.g., ephemeral and intermittent) streams can be important focal areas for regional groundwater recharge, and both contribute to and receive water, solutes, and materials from the groundwater system. 相似文献
182.
183.
Nicole C. Lautze Thomas W. Sisson Margaret T. Mangan Timothy L. Grove 《Contributions to Mineralogy and Petrology》2011,161(2):331-347
Diffusive coarsening (Ostwald ripening) of H2O and H2O-CO2 bubbles in rhyolite and basaltic andesite melts was studied with elevated temperature–pressure experiments to investigate
the rates and time spans over which vapor bubbles may enlarge and attain sufficient buoyancy to segregate in magmatic systems.
Bubble growth and segregation are also considered in terms of classical steady-state and transient (non-steady-state) ripening
theory. Experimental results are consistent with diffusive coarsening as the dominant mechanism of bubble growth. Ripening
is faster in experiments saturated with pure H2O than in those with a CO2-rich mixed vapor probably due to faster diffusion of H2O than CO2 through the melt. None of the experimental series followed the time1/3 increase in mean bubble radius and time−1 decrease in bubble number density predicted by classical steady-state ripening theory. Instead, products are interpreted
as resulting from transient regime ripening. Application of transient regime theory suggests that bubbly magmas may require
from days to 100 years to reach steady-state ripening conditions. Experimental results, as well as theory for steady-state
ripening of bubbles that are immobile or undergoing buoyant ascent, indicate that diffusive coarsening efficiently eliminates
micron-sized bubbles and would produce mm-sized bubbles in 102–104 years in crustal magma bodies. Once bubbles attain mm-sizes, their calculated ascent rates are sufficient that they could
transit multiple kilometers over hundreds to thousands of years through mafic and silicic melt, respectively. These results
show that diffusive coarsening can facilitate transfer of volatiles through, and from, magmatic systems by creating bubbles
sufficiently large for rapid ascent. 相似文献
184.
Relative contribution of foliar and fine root pine litter to the molecular composition of soil organic matter after in situ degradation 总被引:1,自引:0,他引:1
Stefania Mambelli Jeffrey A. Bird Gerd Gleixner Todd E. Dawson Margaret S. Torn 《Organic Geochemistry》2011,42(9):1099-1108
The influence of litter quality on soil organic matter (SOM) stabilization rate and pathways remains unclear. We used 13C/15N labeled litter addition and Curie-point pyrolysis gas chromatography–mass spectrometry combustion-isotope ratio mass spectrometry (Py–GC–MS–C–IRMS) to explore the transformation of litter with different composition and decay rate (ponderosa pine needle vs. fine root) to SOM during 18 months in a temperate conifer forest mineral (A horizon) soil. Based on 13C Py–GC–MS–C–IRMS the initial litter and bulk soil had ∼1/3 of the total pyrolysis products identified in common. The majority was related either to carbohydrates or was non-specific in origin. In bulk soil, carbohydrates had similar levels of enrichment after needle input and fine root input, while the non-specific products were more enriched after needle input. In the humin SOM fraction (260 yr C turnover time) we found only carbohydrate and alkyl C-derived compounds and greater 13C enrichment in the “carbohydrate” pool after fine root decomposition. 15N Py–GC–MS–C–IRMS of humic substances showed that root litter contributed more than needle litter to the enrichment of specific protein markers during initial decomposition.We found little evidence for the selective preservation of plant compounds considered to be recalcitrant. Our findings suggest an indirect role for decomposing plant material composition, where microbial alteration of fine root litter seems to favor greater initial stabilization of microbially derived C and N in SOM fractions with long mean turnover times, such as humin, compared to needles with a faster decay rate. 相似文献
185.
Herold M Román-Cuesta RM Mollicone D Hirata Y Van Laake P Asner GP Souza C Skutsch M Avitabile V Macdicken K 《Carbon balance and management》2011,6(1):13
Measuring forest degradation and related forest carbon stock changes is more challenging than measuring deforestation since
degradation implies changes in the structure of the forest and does not entail a change in land use, making it less easily
detectable through remote sensing. Although we anticipate the use of the IPCC guidance under the United Framework Convention
on Climate Change (UNFCCC), there is no one single method for monitoring forest degradation for the case of REDD+ policy.
In this review paper we highlight that the choice depends upon a number of factors including the type of degradation, available
historical data, capacities and resources, and the potentials and limitations of various measurement and monitoring approaches.
Current degradation rates can be measured through field data (i.e. multi-date national forest inventories and permanent sample
plot data, commercial forestry data sets, proxy data from domestic markets) and/or remote sensing data (i.e. direct mapping
of canopy and forest structural changes or indirect mapping through modelling approaches), with the combination of techniques
providing the best options. Developing countries frequently lack consistent historical field data for assessing past forest
degradation, and so must rely more on remote sensing approaches mixed with current field assessments of carbon stock changes.
Historical degradation estimates will have larger uncertainties as it will be difficult to determine their accuracy. However
improving monitoring capacities for systematic forest degradation estimates today will help reduce uncertainties even for
historical estimates. 相似文献
186.
The debate surrounding US climate policy has been strongly influenced by concerns about the impact that placing a price on GHG emissions could have on potentially vulnerable populations, including senior citizens. This article shows that seniors would be one of the best protected groups under the climate policies recently debated in the US Congress. Moreover, low-income groups would also be protected generally under these proposals. This is due to a combination of existing government transfer programmes that automatically adjust benefit payments to account for inflation and provisions in the proposals. 相似文献
187.
Natalie C. Williams Karen A. Bjorndal Margaret M. Lamont Raymond R. Carthy 《Estuaries and Coasts》2014,37(4):986-994
The foraging ecology and diet of the green turtle, Chelonia mydas, remain understudied, particularly in peripheral areas of its distribution. We assessed the diet of an aggregation of juvenile green turtles at the northern edge of its range during winter months using two approaches. Stomach content analyses provide a single time sample, and stable isotope analyses integrate diet over a several-month period. We evaluated diet consistency in prey choice over time by comparing the results of these two approaches. We examined stomach contents from 43 juvenile green turtles that died during cold stunning events in St. Joseph Bay, Florida, in 2008 and 2011. Stomach contents were evaluated for volume, dry mass, percent frequency of occurrence, and index of relative importance of individual diet items. Juvenile green turtles were omnivorous, feeding primarily on seagrasses and tunicates. Diet characterizations from stomach contents differed from those based on stable isotope analyses, indicating the turtles are not feeding consistently during winter months. Evaluation of diets during warm months is needed. 相似文献
188.
189.
The observed size distribution of Kuiper belt objects (KBOs)—small icy and rocky Solar System bodies orbiting beyond Neptune—is well described by a power law at large KBO sizes. However, recent work by Bernstein et al. (2004, Astron. J. 128, 1364-1390) indicates that the size distribution breaks and becomes shallower for KBOs smaller than about 70 km in size. Here we show that we expect such a break at KBO radius ∼40 km since destructive collisions are frequent for smaller KBOs. Specifically, we assume that KBOs are gravity-dominated bodies with negligible material strength. This gives a power-law slope q?3 where the number N>r of KBOs larger than a size r is given by N>r∝r1−q; the break location follows from this slope through a self-consistent calculation. The existence of this break, the break's location, and the power-law slope we expect below the break are consistent with the findings of Bernstein et al. (2004, Astron. J. 128, 1364-1390). The agreement with observations indicates that KBOs as small as ∼40 km are effectively strengthless. 相似文献
190.