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At least 91 records · Page 5Linked to original sources
USGS2010

Variability and trends in dry day frequency and dry event length in the southwestern United States

Daily precipitation from 22 National Weather Service first-order weather stations in the southwestern United States for water years 1951 through 2006 are used to examine variability and trends in the frequency of dry days and dry event length. Dry events with minimum thresholds of 10 and 20 consecutive days of precipitation with less than 2.54 mm are analyzed. For water years and cool seasons (October through March), most sites indicate negative trends in dry event length (i.e., dry event durations are becoming shorter). For the warm season (April through September), most sites also indicate negative trends; however, more sites indicate positive trends in dry event length for the warm season than for water years or cool seasons. The larger number of sites indicating positive trends in dry event length during the warm season is due to a series of dry warm seasons near the end of the 20th century and the beginning of the 21st century. Overall, a large portion of the variability in dry event length is attributable to variability of the El Niño–Southern Oscillation, especially for water years and cool seasons. Our results are consistent with analyses of trends in discharge for sites in the southwestern United States, an increased frequency in El Niño events, and positive trends in precipitation in the southwestern United States.

Journal of Geophysical Research D: Atmospheres↗
USGS2012

Modeling thermal dynamics of active layer soils and near-surface permafrost using a fully coupled water and heat transport model

Thawing and freezing processes are key components in permafrost dynamics, and these processes play an important role in regulating the hydrological and carbon cycles in the northern high latitudes. In the present study, we apply a well-developed soil thermal model that fully couples heat and water transport, to simulate the thawing and freezing processes at daily time steps across multiple sites that vary with vegetation cover, disturbance history, and climate. The model performance was evaluated by comparing modeled and measured soil temperatures at different depths. We use the model to explore the influence of climate, fire disturbance, and topography (north- and south-facing slopes) on soil thermal dynamics. Modeled soil temperatures agree well with measured values for both boreal forest and tundra ecosystems at the site level. Combustion of organic-soil horizons during wildfire alters the surface energy balance and increases the downward heat flux through the soil profile, resulting in the warming and thawing of near-surface permafrost. A projection of 21st century permafrost dynamics indicates that as the climate warms, active layer thickness will likely increase to more than 3 meters in the boreal forest site and deeper than one meter in the tundra site. Results from this coupled heat-water modeling approach represent faster thaw rates than previously simulated in other studies. We conclude that the discussed soil thermal model is able to well simulate the permafrost dynamics and could be used as a tool to analyze the influence of climate change and wildfire disturbance on permafrost thawing.

Journal of Geophysical Research D: Atmospheres↗
USGS2012

Sensitivity of ground - water recharge estimates to climate variability and change, Columbia Plateau, Washington

The sensitivity of groundwater recharge estimates was investigated for the semiarid Ellensburg basin, located on the Columbia Plateau, Washington, to historic and projected climatic regimes. Recharge was estimated for predevelopment and current (1980s) land use conditions using a daily energy-soil-water balance model. A synthetic daily weather generator was used to simulate lengthy sequences with parameters estimated from subsets of the historical record that were unusually wet and unusually dry. Comparison of recharge estimates corresponding to relatively wet and dry periods showed that recharge for predevelopment land use varies considerably within the range of climatic conditions observed in the 87-year historical observation period. Recharge variations for present land use conditions were less sensitive to the same range of historical climatic conditions because of irrigation. The estimated recharge based on the 87-year historical climatology was compared with adjustments to the historical precipitation and temperature records for the same record to reflect CO 2 -doubling climates as projected by general circulation models (GCMs). Two GCM scenarios were considered: an average of conditions for three different GCMs with CO 2 doubling, and a most severe “maximum” case. For the average GCM scenario, predevelopment recharge increased, and current recharge decreased. Also considered was the sensitivity of recharge to the variability of climate within the historical and adjusted historical records. Predevelopment and current recharge were less and more sensitive, respectively, to the climate variability for the average GCM scenario as compared to the variability within the historical record. For the maximum GCM scenario, recharge for both predevelopment and current land use decreased, and the sensitivity to the CO 2 -related climate change was larger than sensitivity to the variability in the historical and adjusted historical climate records.

Journal of Geophysical Research D: Atmospheres↗
USGS2012

Dust deposition in southern Nevada and California, 1984–1989: Relations to climate, source area, and source lithology

Dust samples collected annually for 5 years from 55 sites in southern Nevada and California provide the first regional source of information on modern rates of dust deposition, grain size, and mineralogical and chemical composition relative to climate and to type and lithology of dust source. The average silt and clay flux (rate of deposition) in southern Nevada and southeastern California ranges from 4.3 to 15.7 g/m 2 /yr, but in southwestern California the average silt and clay flux is as high as 30 g/m 2 /yr. The climatic factors that affect dust flux interact with each other and with the factors of source type (playas versus alluvium), source lithology, geographic area, and human disturbance. Average dust flux increases with mean annual temperature but is not correlated to decreases in mean annual precipitation because the regional winds bring dust to relatively wet areas. In contrast, annual dust flux mostly reflects changes in annual precipitation (relative drought) rather than temperature. Although playa and alluvial sources produce about the same amount of dust per unit area, the total volume of dust from the more extensive alluvial sources is much larger. In addition, playa and alluvial sources respond differently to annual changes in precipitation. Most playas produce dust that is richer in soluble salts and carbonate than that from alluvial sources (except carbonate‐rich alluvium). Gypsum dust may be produced by the interaction of carbonate dust and anthropogenic or marine sulfates. The dust flux in an arid urbanizing area may be as much as twice that before disturbance but decreases when construction stops. The mineralogic and major‐oxide composition of the dust samples indicates that sand and some silt is locally derived and deposited, whereas clay and some silt from different sources can be far‐traveled. Dust deposited in the Transverse Ranges of California by the Santa Ana winds appears to be mainly derived from sources to the north and east.

California, Nevada↗
USGS1997

Comparing nocturnal eddy covariance measurements to estimates of ecosystem respiration made by scaling chamber measurements at six coniferous boreal sites

During the growing season, nighttime ecosystem respiration emits 30–100% of the daytime net photosynthetic uptake of carbon, and therefore measurements of rates and understanding of its control by the environment are important for understanding net ecosystem exchange. Ecosystem respiration can be measured at night by eddy covariance methods, but the data may not be reliable because of low turbulence or other methodological problems. We used relationships between woody tissue, foliage, and soil respiration rates and temperature, with temperature records collected on site to estimate ecosystem respiration rates at six coniferous BOREAS sites at half-hour or 1-hour intervals, and then compared these estimates to nocturnal measurements of CO 2 exchange by eddy covariance. Soil surface respiration was the largest source of CO 2 at all sites (48–71%), and foliar respiration made a large contribution to ecosystem respiration at all sites (25–43%). Woody tissue respiration contributed only 5–15% to ecosystem respiration. We estimated error for the scaled chamber predictions of ecosystem respiration by using the uncertainty associated with each respiration parameter and respiring biomass value. There was substantial uncertainty in estimates of foliar and soil respiration because of the spatial variability of specific respiration rates. In addition, more attention needs to be paid to estimating foliar respiration during the early part of the growing season, when new foliage is growing, and to determining seasonal trends of soil surface respiration. Nocturnal eddy covariance measurements were poorly correlated to scaled chamber estimates of ecosystem respiration ( r 2 =0.06–0.27) and were consistently lower than scaled chamber predictions (by 27% on average for the six sites). The bias in eddy covariance estimates of ecosystem respiration will alter estimates of gross assimilation in the light and of net ecosystem exchange rates over extended periods.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

A comparison of six methods for measuring soil-surface carbon dioxide fluxes

Measurements of soil-surface CO 2 fluxes are important for characterizing the carbon budget of boreal forests because these fluxes can be the second largest component of the budget. Several methods for measuring soil-surface CO 2 fluxes are available: (1) closed-dynamic-chamber systems, (2) closed-static-chamber systems, (3) open-chamber systems, and (4) eddy covariance systems. This paper presents a field comparison of six individual systems for measuring soil-surface CO 2 fluxes with each of the four basic system types represented. A single system is used as a reference and compared to each of the other systems individually in black spruce (Picea mariana), jack pine (Pinus banksiana), or aspen (Populus tremuloides) forests. Fluxes vary from 1 to 10 μmol CO 2 m −2 s −1 . Adjustment factors to bring all of the systems into agreement vary from 0.93 to 1.45 with an uncertainty of about 10–15%.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Dust deposition downwind of Owens (dry) Lake, 1991–1994: Preliminary findings

Salt‐rich dust derived from the Owens Lake playa is deposited in significant quantities to distances of at least 40 km north and south of the playa. Semiannual measurements from 1991 to 1994 of dust deposition rates (dust flux) and composition 2 m above the ground at seven sites in Owens Valley show that (1) dust in Owens Valley is derived mainly from the playa, although areas closer to the sites can also be sources; (2) south of the playa, dust flux is higher in the winter than in the summer, but north of the playa, dust flux is about the same or slightly lower in the winter; (3) on the playa, interannual variation in dust flux is large (factors of 5–10 during the 3 years), but at downwind sites, the variation is much smaller; (4) the dust typically has total soluble salt content as high as 30%, generally much higher than that of dust elsewhere in southern Nevada and California; and (5) to a distance of at least 40 km south of the playa, soluble salt flux is significantly higher than regional rates. The dust flux measurements indicate that significant quantities of salt‐rich dust are probably being added to the soils in the region around Owens Lake playa; these dust additions may affect soil pH and vegetation.

California↗
USGS1997

Accumulation and turnover of carbon in organic and mineral soils of the BOREAS northern study area

Rates of input, accumulation, and turnover of C differ markedly within soil profiles and in soils with different drainage in the BOREAS northern study area. Soil C storage increases from ∼3 kg C m −2 in well-drained, sandy soils to greater than 100 kg C m −2 in wetlands. Two modes of C accumulation were observed in upland soil profiles. Large annual C inputs (0.06–0.1 kg C m −2 yr −1 ) and slow decomposition (turnover times of 6–250 years) lead to rapid C accumulation in regrowing surface moss and detrital layers following fire. Deep organic layers that have accumulated over the millennia since the initiation of soil development, and are located below the most recent charred horizon, show slower rates of input (0.015–0.03 kg C m −2 yr −1 ) and turnover (100–1600 years) and accumulate C about 10 times slower than surface detrital layers. Rates of C input to soils derived from C and 14 C data were in accord with net primary production estimates, with highest rates of input (0.14–0.6 kg C m −2 yr −1 ) in wetlands. Turnover times for C in surface detrital layers were 6–15 years for well-drained sand soils that showed highest soil temperatures in summer, 30–40 years for wetlands, and 36–250 years for uplands with thick moss cover and black spruce trees. Long (>100 years) turnover times in upland black spruce/clay soils most likely reflect the influence of woody debris incorporated into detrital layers. Turnover times for deep organic and mineral layer C were controlled by drainage, with fastest turnover (80–130 years) in well-drained sand soils and slowest turnover (>3000 years) in wetlands. Total C accumulation rates, which account for C losses from both deep organic and surface detrital layers, are close to zero for sand/jack pine soils, 0.003–0.01 kg C m −2 yr −1 for moderately to poorly drained sites in mature forest stands, and 0.03 kg C m −2 yr −1 for a productive fen. Decomposition of organic matter more than several decades old accounts for 9–22% of total heterotrophic respiration at these sites. The rates of C accumulation derived here are decadal averages for specific stands and will vary as stands age or undergo disturbance. Extrapolation to larger regions and longer timescales, where burning offsets C gains in moss layers, will yield smaller rates of C storage.

Journal of Geophysical Research D: Atmospheres↗
USGS1998

Analysis of simulated advanced spaceborne thermal emission and reflection (ASTER) radiometer data of the Iron Hill, Colorado, study area for mapping lithologies

The advanced spaceborne thermal emission and reflection (ASTER) radiometer was designed to record reflected energy in nine channels with 15 or 30 m resolution, including stereoscopic images, and emitted energy in five channels with 90 m resolution from the NASA Earth Observing System AMI platform. A simulated ASTER data set was produced for the Iron Hill, Colorado, study area by resampling calibrated, registered airborne visible/infrared imaging spectrometer (AVIRIS) data, and thermal infrared multispectral scanner (TIMS) data to the appropriate spatial and spectral parameters. A digital elevation model was obtained to simulate ASTER-derived topographic data. The main lithologic units in the area are granitic rocks and felsite into which a carbonatite stock and associated alkalic igneous rocks were intruded; these rocks are locally covered by Jurassic sandstone, Tertiary rhyolitic tuff, and colluvial deposits. Several methods were evaluated for mapping the main lithologic units, including the unsupervised classification and spectral curve-matching techniques. In the five thermalinfrared (TIR) channels, comparison of the results of linear spectral unmixing and unsupervised classification with published geologic maps showed that the main lithologic units were mapped, but large areas with moderate to dense tree cover were not mapped in the TIR data. Compared to TIMS data, simulated ASTER data permitted slightly less discrimination in the mafic alkalic rock series, and carbonatite was not mapped in the TIMS nor in the simulated ASTER TIR data. In the nine visible and near-infrared channels, unsupervised classification did not yield useful results, but both the spectral linear unmixing and the matched filter techniques produced useful results, including mapping calcitic and dolomitic carbonatite exposures, travertine in hot spring deposits, kaolinite in argillized sandstone and tuff, and muscovite in sericitized granite and felsite, as well as commonly occurring illite/muscovite. However, the distinction made in AVIRIS data between calcite and dolomite was not consistently feasible in the simulated ASTER data. Comparison of the lithologie information produced by spectral analysis of the simulated ASTER data to a photogeologic interpretation of a simulated ASTER color image illustrates the high potential of spectral analysis of ASTER data to geologic interpretation.

Journal of Geophysical Research D: Atmospheres↗
USGS2012

Reservoir analysis of the Denver earthquakes: A case of induced seismicity

Injection of fluid wastes into the fractured Precambrian crystalline bedrock beneath the Rocky Mountain Arsenal near Denver triggered earthquakes in the 1960's. An analysis, based on the assumption that fluid flow in the fractured reservoir can be approximated by flow in a porous medium, is presented. The configuration and hydrologic properties of the reservoir are determined from two lines of evidence: (1) locations of earthquake hypocenters determined by seismic arrays installed at the Arsenal and (2) observed long-term decline in fluid levels in the injection well. Together these two sets of data indicate that a long, narrow reservoir, aligned in the direction N 60°W, exists. The reservoir is 3.35 km in width, extends 30.5 km to the northwest and infinitely to the southeast, and spans a depth interval from 3.7 to 7.0 km below land surface. It has a transmissivity of 1.08×10-5 m2/s and a storage coefficient of 1.0×10-5. Computed pressure buildup along the length of the reservoir is compared with the spatial distribution of earthquake epicenters. The comparison shows that earthquakes are confined to that part of the reservoir where the pressure buildup exceeds 32 bars. This critical value is interpreted as the pressure buildup above which earthquakes occur. The migration of earthquake epicenters away from the injection well, a phenomenon noted by previous investigators, can be accounted for by the outward propagation of the critical pressure buildup. The analysis is extended to examining the effects of rapid flow in fractures opened by high injection pressure. The results show that the effect is confined to a small region within 1 km of the injection well. The existence of a critical pressure buildup above which earthquakes occur is completely consistent with the theory on the role of fluid pressure in fault movement as presented by Hubbert and Rubey.

Colorado↗
USGS2012

Age estimates and uplift rates for late Pleistocene marine terraces: Southern Oregon portion of the Cascadia forearc

Interest in the Cascadia subduction zone has increased because recent investigations have suggested that slip along plates at certain types of convergent margins is characteristically accompanied by large earthquakes. In addition, other investigations have suggested that convergent margins can be broadly classified by the magnitude of their uplift rates. The authors generated new uranium series, amino acid, and stable isotope data for southern Oregon marine terrace fossils. These data, along with terrace elevations and two alternative estimates of sea level at the time of terrace formation, allow one to determine terrace ages and uplift rates. Uranium series analysis of fossil coral yields an age of 83 {plus minus} 5 ka for the Whisky Run terrace at Coquille Point in Bandon, Oregon. A combination of amino acid and oxygen isotope data suggest ages of about 80 and 105 ka for the lowest two terraces at Cape Blanco. These ages indicate uplift rates of 0.45-1.05 and 0.81-1.49 m/kyr for Coquille Point and Cape Blanco, respectively. In order to assess the utility of the southern Oregon uplift rates for predicting the behavior of the Cascadia subduction zone, the authors compared late Quaternary uplift rates derived from terrace data from subduction zones around the world. On the basis of this comparison the southern Oregon rates of vertical deformation are not usually high or low. Furthermore, late Quaternary uplift rates show little relationship to the type of convergent margin. In the case of the southern Oregon coast, variability in uplift rate probably reflects local structures in the overriding plate, and the rate of uplift cannot be used as a simple index of the potential for great earthquakes along the southern Cascadia subduction zone.

California, Oregon↗
USGS1999

Dark material in the polar layered deposits and dunes on Mars

Viking infrared thermal mapping and bistatic radar data suggest that the bulk density of the north polar erg material is much lower than that of the average Martian surface or of dark dunes at lower latitudes. We have derived a thermal inertia of 245–280 J m −2 s −1/2 K −1 (5.9–6.7×10 −3 cal cm −2 s −1/2 K −1 ) for the Proctor dune field and 25–150 J m −2 s −1/2 K −1 (0.6–3.6×10 −3 cal cm −2 s −1/2 K −1 ) for the north polar erg. The uniqueness of the thermophysical properties of the north polar erg material may be due to a unique polar process that has created them. The visible and near‐infrared spectral reflectance of the erg suggests that the dark material may be composed of basalt or ferrous clays. These data are consistent with the dark material being composed of basaltic ash or filamentary sublimate residue (FSR) particles derived from erosion of the layered deposits. Dark dust may be preferentially concentrated at the surface of the layered deposits by the formation of FSR particles upon sublimation of water ice. Further weathering and erosion of these areas of exposed layered deposits may form the dark, saltating material that is found in both polar regions. Dark FSR particles may saltate for great distances before eventually breaking down into dust grains, re‐mixing with the global dust reservoir, and being recycled into the polar layered deposits via atmospheric suspension.

Journal of Geophysical Research E: Planets↗
USGS2013

Accumulation of atmospheric sulfur in some Costa Rican soils

Sulfur is one of the macronutrient elements whose sources to terrestrial ecosystems should shift from dominance by rock-weathering to atmospheric deposition as soils and underlying substrate undergo progressive weathering and leaching. However, the nature and timing of this transition is not well known. We investigated sources of sulfur to tropical rain forests growing on basalt-derived soils in the Osa Peninsula region of Costa Rica. Sulfur sources were examined using stable isotope ratios (δ 34 S) and compared to chemical indices of soil development. The most weathered soils, and the forests they supported, are dominated by atmospheric sulfur, while a less weathered soil type contains both rock-derived and atmospheric sulfur. Patterns of increasing δ 34 S with increasing soil sulfur concentration across the landscape suggest atmospheric sulfur is accumulating, and little rock-derived sulfur has been retained. Soil sulfur, minus adsorbed sulfate, is correlated with carbon and nitrogen, implying that sulfur accumulation occurs as plants and microbes incorporate sulfur into organic matter. Only the lower depth increments of the more weathered soils contained significant adsorbed sulfate. The evidence suggests a pattern of soil development in which sulfur-bearing minerals in rock, such as sulfides, weather early relative to other minerals, and the released sulfate is leached away. Sulfur added via atmospheric deposition is retained as organic matter accumulates in the soil profile. Adsorbed sulfate accumulates later, driven by changes in soil chemistry and mineralogy. These aspects of sulfur behavior during pedogenesis in this environment may hasten the transition to dominance by atmospheric sources.

Journal of Geophysical Research: Biogeosciences↗
USGS1997

Variation of the 3-μm absorption feature on Mars: observations over eastern Valles Marineris by the mariner 6 infrared spectrometer

A new approach for calibration of the shortest wavelength channel (1.8 to 6.0 μm) of the Mariner 6 infrared spectrometer was derived. This calibration provides a new description of the instrument response function from 1.8 to 3.7 μm and accounts for the thermal contribution to the signal at longer wavelengths. This allows the two segments from 1.8 to 6 μm to be merged into a single spectrum. The broad water of hydration absorption spans these two segments and is examined in these merged spectra using a method of band integration. Unlike previous analyses which rely on ratios at two wavelengths, the integration method can assess the band strength independently from the albedo in the near infrared. Spectra taken over the eastern end of the Valles Marineris are examined for variations of the band-integrated value, and three distinct clusters are found. Within the estimated uncertainty, two clusters (both low and high albedo) have approximately the same integrated band depth. The third cluster (medium albedo) has an integrated band depth about 10% higher. This difference cannot be systematically attributed to either surface or atmospheric parameters and suggests variation in the amount of water either chemically or physically bound in surface materials. Approximately one-half of the high integrated band depth cluster is associated with chaotic terrain at the source of outflow channels, the other half occurs over lower inertia plains adjacent to chasmata. This suggests both surface physical properties and mineralogy as well as water in exchange with the atmosphere contribute to the 3-μm bound water absorption.

Journal of Geophysical Research E: Planets↗
USGS2008

Middle UV to near-IR spectrum of electron-excited SO 2

We investigated the electron impact–induced fluorescence spectrum of SO 2 to provide excitation cross sections for modeling Io's emission spectrum and analyzing Cassini Imaging Science Subsystem observations. The electron-excited middle-ultraviolet visible optical near-infrared (VOIR) emission spectrum of SO 2 gas was generated in the laboratory and studied from 2000 to 11,000 Å at a resolution of Δ λ ∼ 2.5 Å full width at half maximum (FWHM). The VOIR laboratory spectrum longward of 6000 Å consists entirely of S I, II and O I, II multiplets for electron impact energies above ∼15 eV. Between 2000 and 6000 Å, we find previously identified molecular bands from both SO and SO 2 . This work represents a significant improvement in spectral resolution over our earlier work done at 18 Å FWHM. From a measurement of the medium-resolution spectrum, we provide detailed 25- and 100-eV emission cross sections for spectral features from 2000 to 11,000 Å. On the basis of these data, we suggest future ground-based and satellite telescopic observations in the VOIR that are of promise for understanding Io's atmosphere.

Journal of Geophysical Research E: Planets↗
USGS1999

Air blasts generated by rockfall impacts: Analysis of the 1996 Happy Isles event in Yosemite National Park

The July 10, 1996, Happy Isles rockfall in Yosemite National Park, California, released 23,000 to 38,000 m 3 of granite in four separate events. The impacts of the first two events which involved a 550-m free fall, generated seismic waves and atmospheric pressure waves (air blasts). We focus on the dynamic behavior of the second air blast that downed over 1000 trees, destroyed a bridge, demolished a snack bar, and caused one fatality and several injuries. Calculated velocities for the air blast from a two-phase, finite difference model are compared to velocities estimated from tree damage. From tornadic studies of tree damage, the air blast is estimated to have traveled <108–120 m/s within 50 m from the impact and decreased to <10–20 m/s within 500 m from the impact. The numerical model simulates the two-dimensional propagation of an air blast through a dusty atmosphere with initial conditions defined by the impact velocity and pressure. The impact velocity (105–107 m/s) is estimated from the Colorado Rockfall Simulation Program that simulates rockfall trajectories. The impact pressure (0.5 MPa) is constrained by the kinetic energy of the impact (10 10 –10 12 J) estimated from the seismic energy generated by the impact. Results from the air blast simulations indicate that the second Happy Isles air blast (weak shock wave) traveled with an initial velocity above the local sound speed. The size and location of the first impact are thought to have injected <50 wt% dust into the atmosphere. This amount of dust lowered the local atmospheric sound speed to ∼220 m/s. The discrepancy between calculated velocity data and field estimated velocity data (∼220 m/s versus ∼110 m/s) is attributed to energy dissipated by the downing of trees and additional entrainment of debris into the atmosphere not included in the calculations.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Linking soil organic matter dynamics and erosion-induced terrestrial carbon sequestration at different landform positions

Recently, the potential for terrestrial carbon (C) sequestration by soil erosion and deposition has received increased interest. Erosion and deposition constitute a sink for atmospheric carbon dioxide relative to a preerosional state or a noneroding scenario, if the posterosion watershed C balance is increased due to (1) partial replacement of eroded C by new photosynthate in the eroded site; and (2) preservation from decomposition of at least some eroded soil organic carbon (SOC) arriving in depositional settings. Little is known, however, about differences in C dynamics at different erosional and depositional landform positions within the same eroding system. We determined the contribution of different landform positions to erosion-induced terrestrial C sequestration by measuring rates of net primary productivity (NPP), replacement of eroded C, and decomposition of organic matter (OM) at four categorically different landform positions within a naturally eroding toposequence in northern California. We found that eroded C is replaced by NPP 15 times over in the summit of the site studied and 5 times over in the slope. Profile-averaged, long-term rate constant for SOM decomposition was 2 to 14 times slower in the depositional settings compared with that in eroding slopes. As a result, the inventory of C in the depositional settings was 2 to 3 times larger than that of the eroding positions. Owing to both C replacement at eroding sites and reduced rates of OM decomposition in depositional sites, soil erosion constitutes a C sink from the atmosphere at our study site.

California↗
USGS2012

The influence of formation material properties on the response of water levels in wells to Earth tides and atmospheric loading

The water level in an open well can change in response to deformation of the surrounding material, either because of applied strains (tidal or tectonic) or surface loading by atmospheric pressure changes. Under conditions of no vertical fluid flow and negligible well bore storage (static-confined conditions), the sensitivities to these effects depend on the elastic properties and porosity which characterize the surrounding medium. For a poroelastic medium, high sensitivity to applied areal strains occurs for low porosity, while high sensitivity to atmospheric loading occurs for high porosity; both increase with decreasing compressibility of the solid matrix. These material properties also influence vertical fluid flow induced by areally extensive deformation and can be used to define two types of hydraulic diffusivity which govern pressure diffusion, one for applied strain and one for surface loading. The hydraulic diffusivity which governs pressure diffusion in response to surface loading is slightly smaller than that which governs fluid flow in response to applied strain. Given the static-confined response of a water well to atmospheric loading and Earth tides, the in situ drained matrix compressibility and porosity (and hence the one-dimensional specific storage) can be estimated. Analysis of the static-confined response of five wells to atmospheric loading and Earth tides gives generally reasonable estimates for material properties.

Journal of Geophysical Research Solid Earth↗