Calibrating a Salt Water Intrusion Model with Time‐Domain Electromagnetic Data

Salt water intrusion models are commonly used to support groundwater resource management in coastal aquifers. Concentration data used for model calibration are often sparse and limited in spatial extent. With airborne and ground‐based electromagnetic surveys, electrical resistivity models can be obtained to provide high‐resolution three‐dimensional models of subsurface resistivity variations that can be related to geology and salt concentrations on a regional scale. Several previous studies have calibrated salt water intrusion models with geophysical data, but are typically limited to the use of the inverted electrical resistivity models without considering the measured geophysical data directly. This induces a number of errors related to inconsistent scales between the geophysical and hydrologic models and the applied regularization constraints in the geophysical inversion. To overcome these errors, we perform a coupled hydrogeophysical inversion (CHI) in which we use a salt water intrusion model to interpret the geophysical data and guide the geophysical inversion. We refer to this methodology as a Coupled Hydrogeophysical Inversion‐State (CHI‐S), in which simulated salt concentrations are transformed to an electrical resistivity model, after which a geophysical forward response is calculated and compared with the measured geophysical data. This approach was applied for a field site in Santa Cruz County, California, where a time‐domain electromagnetic (TDEM) dataset was collected. For this location, a simple two‐dimensional cross‐sectional salt water intrusion model was developed, for which we estimated five uniform aquifer properties, incorporating the porosity that was also part of the employed petrophysical relationship. In addition, one geophysical parameter was estimated. The six parameters could be resolved well by fitting more than 300 apparent resistivities that were comprised by the TDEM dataset. Except for three sounding locations, all the TDEM data could be fitted close to a root‐mean‐square error of 1. Possible explanations for the poor fit of these soundings are the assumption of spatial uniformity, fixed boundary conditions and the neglecting of 3D effects in the groundwater model and the TDEM forward responses.     

Storage of carbon in U.S. forests predicted from satellite data,ecosystem modeling,and inventory summaries

A plant and soil simulation model based on satellite observations of vegetation and climate data was used to estimate the potential carbon pools in standing wood biomass across all forest ecosystems of the conterminous United States up to the year 1997. These modeled estimates of vegetative carbon potential were compared to aggregated measurements of standing wood biomass from the U. S. Forest Service’s national Forest Inventory and Analysis (FIA) data set and the Carbon Online Estimator (COLE) to understand: 1) predominant geographic variations in tree growth rate and 2) local land cover and land use history including the time since the last stand-replacing disturbance (e.g., from wildfire or harvest). Results suggest that although wood appears to be accumulating at high rates in many areas of the U.S. (Northwest and Southeast), there are still extensive areas of relatively low biomass forest in the late 1990s according to FIA records. We attribute these low biomass accumulation levels to the high frequency of disturbances, which can be observed even in high production areas such as the Southeast due to frequent forest harvests. Ecosystem models like the one presented in this study have been coupled with satellite observations of land cover and green plant density to uniquely differentiate areas with a high potential for vegetative carbon storage at relatively fine spatial resolution.     

Net primary production of terrestrial ecosystems from 2000 to 2009

The CASA (Carnegie-Ames-Stanford) ecosystem model has been used to estimate monthly carbon fluxes in terrestrial ecosystems from 2000 to 2009, with global data inputs from NASA??s Terra Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation cover mapping. Net primary production (NPP) flux for atmospheric carbon dioxide has varied slightly from year-to-year, but was predicted to have increased over short multi-year periods in the regions of the high-latitude Northern Hemisphere, South Asia, Central Africa, and the western Amazon since the year 2000. These CASA results for global NPP were found to be in contrast to other recently published modeling trends for terrestrial NPP with high sensitivity to regional drying patterns. Nonetheless, periodic declines in regional NPP were predicted by CASA for the southern and western Untied States, the southern Amazon, and southern and eastern Africa. NPP in tropical forest zones was examined in greater detail to discover lower annual production values than previously reported in many global models across the tropical rainforest zones, likely due to the enhanced detection of lower production ecosystems replacing primary rainforest.     

N-15 NMR spectra of naturally abundant nitrogen in soil and aquatic natural organic matter samples of the International Humic Substances Society

The naturally abundant nitrogen in soil and aquatic NOM samples from the International Humic Substances Society has been characterized by solid state CP/MAS ¹⁵N NMR. Soil samples include humic and fulvic acids from the Elliot soil, Minnesota Waskish peat and Florida Pahokee peat, as well as the Summit Hill soil humic acid and the Leonardite humic acid. Aquatic samples include Suwannee River humic, fulvic and reverse osmosis isolates, Nordic humic and fulvic acids and Pony Lake fulvic acid. Additionally, Nordic and Suwannee River XAD-4 acids and Suwannee River hydrophobic neutral fractions were analyzed. Similar to literature reports, amide/aminoquinone nitrogens comprised the major peaks in the solid state spectra of the soil humic and fulvic acids, along with heterocyclic and amino sugar/terminal amino acid nitrogens. Spectra of aquatic samples, including the XAD-4 acids, contain resolved heterocyclic nitrogen peaks in addition to the amide nitrogens. The spectrum of the nitrogen enriched, microbially derived Pony Lake, Antarctica fulvic acid, appeared to contain resonances in the region of pyrazine, imine and/or pyridine nitrogens, which have not been observed previously in soil or aquatic humic substances by ¹⁵N NMR. Liquid state ¹⁵N NMR experiments were also recorded on the Elliot soil humic acid and Pony Lake fulvic acid, both to examine the feasibility of the techniques, and to determine whether improvements in resolution over the solid state could be realized. For both samples, polarization transfer (DEPT) and indirect detection (¹H–¹⁵N gHSQC) spectra revealed greater resolution among nitrogens directly bonded to protons. The amide/aminoquinone nitrogens could also be observed by direct detection experiments.     

Hydro-micromechanical modeling of wave propagation in saturated granular crystals

Biot theory predicts wave velocities in a saturated granular medium using the pore geometry, viscosity, densities, and elastic moduli of the solid skeleton and pore fluid, neglecting the interaction between constituent particles and local flow, which becomes essential as the wavelength decreases. Here, a hydro-micromechanical model, for direct numerical simulations of wave propagation in saturated granular media, is implemented by two-way coupling the lattice Boltzmann method (LBM) and the discrete element method (DEM), which resolve the pore-scale hydrodynamics and intergranular behavior, respectively. The coupling scheme is benchmarked with the terminal velocity of a single sphere settling in a fluid. In order to mimic a small amplitude pressure wave entering a saturated granular medium, an oscillating pressure boundary on the fluid is implemented and benchmarked with the one-dimensional wave equation. The effects of input waveforms and frequencies on the dispersion relations in 3D saturated poroelastic media are investigated with granular face-centered-cubic crystals. Finally, the pressure and shear wave velocities predicted by the numerical model at various effective confining pressures are found to be in excellent agreement with Biot analytical solutions, including his prediction for slow compressional waves.     

Phospholipid ester-linked fatty acids composition of size-fractionated particles at the top ocean surface

Surface microlayer and subsurface water samples were collected at an oligotrophic Mediterranean site during a diel cycle. The composition of phospholipid ester-linked fatty acids (PLFA) was determined in size-fractionated particles (0.2–0.7 μm; 0.7–200 μm) in order to characterize the major contributors of organic matter to different size fractions. GF/F-retained particles (0.7–200 μm) from the surface microlayer were consistently enriched in PLFA relative to the underlying water. Molecular markers revealed a substantial difference in biological assemblages in both particle sizes. The larger particles were dominated by dinoflagellates, cyanobacteria, microzooplankton and attached bacteria, whereas particles filtered through GF/F and collected on 0.2 μm porosity Durapore filters (0.2–0.7 μm) were mostly bacteria and heterotrophic flagellates. Bacterial PLFA associated with 0.7–200 μm particles were more abundant than those in the 0.2–0.7 μm particles. Specific markers in the branched series appeared more representative of bacteria of smaller particle size.     

Phytolith evidence for C4-dominated grassland since the early Holocene at Long Pocket, northeast Queensland, Australia

Preliminary phytolith analysis of ephemeral lake fill sediment at Long Pocket, near Toomba, northeast Queensland, Australia, indicates that a C4-dominated grassland with a minor woody component has been present in the region since ca. 8000 cal yr B.P. Based on the modern distribution of C4 and C3 native grasses in Australia, this suggests that mean summer temperatures of at least 14°C (ca. 10°C cooler than present) were maintained since the early Holocene. This interpretation is comparable with previous studies, which together imply that the establishment of C4-dominated grasses in central and northeast Australia occurred between the last glacial maximum (most likely after ca. 16,000 ¹⁴C yr B.P.) and ca. 7200 ¹⁴C yr B.P. (ca. 8000 cal yr B.P.). Taxonomic composition of the grassland appears relatively consistent since the early Holocene at Long Pocket and includes phytoliths comparable with those from modern Arundinoideae, Panicoideae, and Chloridoideae. Rare non-grass phytoliths are also present. A gradual decrease in abundance of saddle phytolith forms (attributed to Chloridoideae grasses) from the base of the record at ca. 6500-7000 cal yr B.P. suggests decreasing aridity throughout the Holocene. This trend could reflect a locally drawn out effect of the end of the postglacial arid period due to the well-drained basalt flow catchment maintaining a local arid habitat for the Chloridoideae grasses.     

Geomorphology as science: the role of theory

Because geomorphology is a science, it is permeated by theory. Overt recognition of this actuality is frequently resisted by geomorphologists. Earth history does not represent an alternative to earth science, it is an essential component of earth science. In its broadest sense science seeks to discover new knowledge through a two-stage activity involving the creation and justification of ideas (theory). Deduction is generally regarded as the only logically-consistent method of justifying ideas. The creation of ideas is a much more controversial topic. Some methodologists deem it beyond logic; certainly deductive arguments, which are nothing more than formal, logical expressions of theory, play no role in the conception of new ideas. Many earth scientists generate possible explanations of observed phenomena based on abductive reasoning. Others advocate reliance on purported forms of “pure” induction, such as serendipity and intuition, in which observations assume primacy over theory. Besides lacking consistency and educability, the latter posture is flawed because it mistakenly implies that becoming well-versed in theory is irrelevant to or impedes scientific discovery. Irrational or subjective factors play a role in the creation of ideas, but it is erroneous to claim that these factors are divorced from theory. Science is first and foremost a cognitive activity; thus, the primacy of observations in science is a myth. All observations are theory-laden in the sense that the act of observation inherently involves interpretation and classification, both of which can only occur within the context of theoretical preconceptions. Even discoveries based on unexpected observations require the fortunate investigator to recognize the theoretical importance of what is seen or measured.The most useful view of geomorphology as a science is one in which theory is seen as central, but fragile, and in which theory and observation are viewed symbiotically with theory providing the generative force and observation providing a vital policing role. Much of the current debate in geomorphology centers around differences in characteristics of theory, type of scientific arguments, and metaphysical perspectives among investigators working at different temporal scales. Full recognition and understanding of these differences are essential for developing a unified approach to the science of geomorphology.     

The Relationship between Geomorphic River Structure and Coarse Particulate Organic Matter (CPOM) Storage along the Kangaroo River,New South Wales,Australia

Fluvial landforms provide a physical template upon which to appraise biophysical relationships along river courses. In this study, the spatial pattern of organic matter storage along the Kangaroo River, NSW, is related to geomorphic controls that operate at a range of scales within a nested hierarchy. This snapshot study of CPOM storage found that at the catchment scale the longitudinal pattern of coarse particulate organic matter (CPOM) storage is dependent on the type and downstream pattern of River Styles. At the reach scale, CPOM storage is dependent on the geomorphic unit structure and physical heterogeneity of the river, and associated energy conditions along the reach. At the geomorphic unit scale, CPOM storage capacity is related to the position of geomorphic units relative to the thalweg (i.e. flow characteristics) and associated roughness attributes. At the hydraulic unit scale, CPOM storage capacity is related to local flow velocity and substrate characteristics (clast size and distribution).     

Microvariability in the early stages of cobble weathering by microenvironment on a glacier foreland,Storbreen, Jotunheimen,Norway

The initial stages of cobble weathering, measured as increasing percentage porosity, were calculated for sets of cobbles taken from the foreland of Storbreen, a cirque glacier in the Jotunheimen, Norway. Cobbles were taken from in front of the 1998 glacier snout, from the 1928, 1870, 1810 and 1750 moraine crests and from the ～10 000‐year‐old land surface beyond the Neoglacial foreland limit. Three microenvironments were examined within each site: (1) lichen‐free surfaces from exposed cobbles, (2) lichen‐covered surfaces from exposed cobbles and (3) buried cobbles taken from a soil depth of ～40 cm. Percentage porosity within plagioclase minerals was calculated using backscatter electron images of prepared thin sections. Porosity was calculated from five depth profiles into each cobble. Depth profiles were subdivided into a sequence of 50 µm × 50 µm rectangles running to at least a nominal depth of 500 µm within each cobble. Three cobbles from each dated land surface were sampled, except for the 1750 surface, which had five cobbles. Statistical analysis was by analysis of variance of rank‐order transformed data. Findings indicate that cobbles close to the glacier snout are largely unweathered; also, weathering is generally weak in the 1928, 1870 and 1810 positions, but statistically significantly higher in the 1750‐ and 10 000‐year‐old positions. Weathering of buried cobbles always exceeded weathering of exposed cobbles and may possibly reach a value beyond which it cannot progress while retaining surface cohesion. The degree of weathering on lichen‐free and lichen‐covered cobble surfaces is not initially distinguishable, but diverges sharply after ～250 years, when lichen‐covered surfaces experience significantly higher totals. Overall, the weathering trend in cobbles matches that found in soils at the same sites. Copyright © 2007 John Wiley & Sons, Ltd.