First‐order exchange coefficient coupling for simulating surface water–groundwater interactions: parameter sensitivity and consistency with a physics‐based approach

Distributed hydrologic models capable of simulating fully‐coupled surface water and groundwater flow are increasingly used to examine problems in the hydrologic sciences. Several techniques are currently available to couple the surface and subsurface; the two most frequently employed approaches are first‐order exchange coefficients (a.k.a., the surface conductance method) and enforced continuity of pressure and flux at the surface‐subsurface boundary condition. The effort reported here examines the parameter sensitivity of simulated hydrologic response for the first‐order exchange coefficients at a well‐characterized field site using the fully coupled Integrated Hydrology Model (InHM). This investigation demonstrates that the first‐order exchange coefficients can be selected such that the simulated hydrologic response is insensitive to the parameter choice, while simulation time is considerably reduced. Alternatively, the ability to choose a first‐order exchange coefficient that intentionally decouples the surface and subsurface facilitates concept‐development simulations to examine real‐world situations where the surface‐subsurface exchange is impaired. While the parameters comprising the first‐order exchange coefficient cannot be directly estimated or measured, the insensitivity of the simulated flow system to these parameters (when chosen appropriately) combined with the ability to mimic actual physical processes suggests that the first‐order exchange coefficient approach can be consistent with a physics‐based framework. Copyright © 2009 John Wiley & Sons, Ltd.     

Vorticity waves in a shallow basin

Observed oscillatory current patterns in the southern basin of Lake Michigan, with a distinctive peak in the energy spectrum at a period of about 90 h, are simulated using a linear potential vorticity conservation model. Solutions of the forced vorticity equation in a paraboloidal basin show rotational, oscillatory motions tuned to the low-frequency topographic modes that are very similar to the observed flow patterns. Topography-controlled vorticity waves are excited most effectively by wind episodes with frequency nearly in resonance with the topographic modes. Bottom resistance has no significant effect on the frequency equation; it simply decays the waves slowly in the open lake and more quickly near the coast. Flow patterns of both the gravest free vorticity wave and the corresponding forced wave consist of two opposite circulation cells separated by a null streamline through the center of the basin and rotating cyclonically near the free wave and atmospheric forcing frequencies, respectively. Interactions between the forced and free waves result in an apparent rotational pattern with a frequency the median of the two. A combination of elliptic—paraboloidal basin and shorter period forced modes can approximate the observed Lake Michigan response. Doppler shift, due to the persistence of cyclonic vorticity in the flow field, is also determined to be a factor in shifting the elliptical basin mode to a higher frequency.     

Super-speed centrifugation at sea using a gimballed platform

The use of a super-speed centrifuge at sea for the recovery of suspended solids or interstitial pore fluids requires a stabilizing platform. In this report, we describe a simple platform gimball and magnetic rotor stabilizer that have been fitted to a Sorvall model SS_?3 super-speed centrifuge. In principle, any centrifuge of comparable weight (86 kg) could be easily adapted for use at sea with gimbals of similar design.The magnetic rotor coupler is designed to eliminate rotor precession when the centrifuge is operated at sea. This attachment is particularly useful when the Sorvall SS_?3 (equipped with a conventional rotor) is used to recover interstitial pore fluids.     

The angular dimensions of extragalactic radio sources

The angular dimensions of compact components of extragalactic radio sources are calculated theoretically on three hypotheses: (a) magnetic field and relativistic particles energy equipartition; (b) equilibrium between synchrotron and Compton losses; (c) radiative lifetime of the same order as the source's light-travel time. Using angular measurements of 73 radio components, divided into two groups (multicomponent and single-component sources), a correcting formulae is derived to reduce the theoretical values to the observed values.This work was partially supported by the Brasilian research agency CNPq.     

The middle Pleistocene transition: characteristics, mechanisms, and implications for long-term changes in atmospheric pCO2

The emergence of low-frequency, high-amplitude, quasi-periodic (100-kyr) glacial variability during the middle Pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system. This middle Pleistocene transition (MPT) began 1250 ka and was complete by 700 ka. Its onset was accompanied by decreases in sea surface temperatures (SSTs) in the North Atlantic and tropical-ocean upwelling regions and by an increase in African and Asian aridity and monsoonal intensity. During the MPT, long-term average ice volume gradually increased by 50 m sea-level equivalent, whereas low-frequency ice-volume variability experienced a 100-kyr lull centered on 1000 ka followed by its reappearance 900 ka, although as a broad band of power rather than a narrow, persistent 100-kyr cycle. Additional changes at 900 ka indicate this to be an important time during the MPT, beginning with an 80-kyr event of extreme SST cooling followed by the partial recovery and subsequent stabilization of long-term North Atlantic and tropical ocean SSTs, increasing Southern Ocean SST variability primarily associated with warmer interglacials, the loss of permanent subpolar sea-ice cover, and the emergence of low-frequency variability in Pacific SSTs and global deep-ocean circulation. Since 900 ka, ice sheets have been the only component of the climate system to exhibit consistent low-frequency variability. With the exception of a near-universal organization of low-frequency power associated with marine isotope stages 11 and 12, all other components show an inconsistent distribution of power in frequency-time space, suggesting a highly nonlinear system response to orbital and ice-sheet forcing.Most hypotheses for the origin of the MPT invoke a response to a long-term cooling, possibly induced by decreasing atmospheric pCO₂. None of these hypotheses, however, accounts for the geological constraint that the earliest Northern Hemisphere ice sheets covered a similar or larger area than those that followed the MPT. Given that the MPT was associated with an increase in ice volume, this constraint requires that post-MPT ice sheets were substantially thicker than pre-MPT ice sheets, indicating a change in subglacial conditions that influence ice dynamics. We review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline Precambrian Shield bedrock became exposed by glacial erosion of a thick mantle of regolith. This exposure of a high-friction substrate caused thicker ice sheets, with an attendant change in their response to the orbital forcing. Marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the MPT. If this carbon came from terrigenous sources, an increase in atmospheric pCO₂ would be likely, which is inconsistent with evidence for widespread cooling, Apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith. A more likely source of organic carbon and nutrients (which would mitigate pCO₂ rise) is from shelf and upper slope marine sediments, which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the MPT. Modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long-term silicate weathering rates, in good agreement with marine Sr and Os isotopic records. We use a carbon cycle model to show that a post-MPT increase in silicate weathering rates would lower atmospheric pCO₂ by 7–12 ppm, suggesting that the attendant cooling may have been an important feedback in causing the MPT.     

Distribution and Sources of Carbon,Nitrogen and Their Isotopic Compositions in Tropical Estuarine Sediments of Mtoni,Tanzania

Ocean Science Journal - Elemental (C, N) and isotopic (δ13C, δ15N) signatures were used as proxies to identify seasonal changes in proportions of sedimentary organic matter (OM) in the...     

Desert isotope hydrology: Water sources of the Sinai Desert

The isotopic composition of groundwater sources of the Sinai Desert was surveyed. The results are characterized by a large spread in the oxygen-18 and deuterium abundances, compared to equivalent systems from less arid climates. The variability reflects differences in the altitude at which precipitation occurred, the evaporation from stagnant surface waters prior to their infiltration into the ground and admixtures of waters which are not of meteoric or recent origin. It is difficult to distinguish between water sources recharged by direct infiltration and others recharged through the intermediary of flood waters, on the basis of their isotope composition. The isotopio composition enables a clearcut distinction, however, between paleowaters and more recently recharged groundwaters. Among the conclusions: paleowaters play a central role in the deep aquifers of desert areas; direct rain recharge to aquifers is widespread; surface waters which have undergone extensive evaporation contribute their water to local perched aquifers which are found along their route.