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1.
A peatland complex disturbed by berm construction in the 1950s was used to examine the long‐term impact of water table (WT) manipulation on peatland hydraulic properties and moisture retention at three adjacent sites with increasing depth to WT (WET, INTermediate reference and DRY). Saturated hydraulic conductivity (Ks) was found to decrease with depth by several orders of magnitude over a depth of 1–1.5 m at all sites. The depth dependence of WT response to rainfall was similar across sites: WT response increased from 1 : 1 at the surface, to 5 : 1 at 50 cm depth. While surface specific yield (Sy) values were similar across all sites, it decreased with depth at a rate of 0.014 cm?1 in hollows and 0.007 cm?1 in hummocks. Bulk density (ρb) exhibited similar depth‐dependent trends as Sy and explains a high amount of variance (r2 > 0.69) in moisture retention across a range of pore water pressures (?15 to ?500 cm H2O). Because of higher ρb, hollow peat had greater moisture retention, where site effects were minimal. However, the estimated residual water content for surface Sphagnum samples, while on average lower in hummocks (0.082 m3 m?3) versus hollows (0.087 m3 m?3), increased from WET (0.058 m3 m?3) to INT (0.088 m3 m?3) to DRY (0.108 m3 m?3) which has important implications for moisture stress under conditions of persistent WT drawdown. Given the potential importance of microtopographic succession for altering peatland hydraulic structure, our findings point to the need for a better understanding of what controls the relative height and proportional coverage of hummocks in relation to long‐term disturbance‐response dynamics. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

2.
Measures of the differential relief of adjacent feldspar and pyroxene grains provide the first clear negative correlation of Schmidt hammer rebound (R-) values and degree of weathering. However, weathering and roughness are intimately related, so the latter cannot be seen simply as a source of error limiting the utility of the Schmidt hammer. Only where surfaces had similar textures prior to weathering can R-values be compared directly. Even where surface texture is well controlled, differences in R-values should be viewed critically rather than assigned indiscriminately to differences in degree of weathering.  相似文献   

3.
Illite clay particles with adsorbed137Cs were added as a submillimeter layer to the surface of silt-clay sediments contained in rectangular Plexiglas cells stored in a temperature-regulated aquarium, in order to trace the effect of the oligochaete, Tubifex tubifex, and the amphipod, Pontoporeia hoyi, on mass redistribution near the sediment-water interface. A well-collimated NaI gamma detector scanned each sediment column (～10 cm deep) at daily or weekly intervals for six months, depicting the time evolution of radioactivity with and without added benthos. In a cell with tubificids (～5 × 104 m?2), which feed below 3 cm and defecate on surface sediments, the labeled layer was buried at a rate of 0.052 ± 0.007 cm/day (20°C). When labeled particles entered the feeding zone,137Cs reappeared in surface sediments creating a bimodal activity profile. In time, the activity tended toward a uniform distribution over the upper 6 cm, decreasing exponentially below to undetectable levels by 9 cm. In a cell with amphipods (～1.6 × 104 m?2) uniform activity developed rapidly (～17 days) down to a well-defined depth (1.5 cm). The mixing of sediments by Pontoporeia is described by a simple quantitative model of eddy diffusive mixing of sediment solids. The value of the diffusion coefficient, 4.4 cm2/yr (7°C) was computed from a least squares fit of theoretical to observed profile broadening over time. In a cell without benthos, small but measurable migration of137Cs indicated an effective molecular diffusion coefficient of 0.02 cm2/yr.  相似文献   

4.
Surface infiltration and internal drainage properties of five soil types from arid drylands of South Africa were studied under double ring infiltrometer, rainfall simulation plots (1 m2) and instantaneous drainage plots (9 m2). Changes in soil water content during 40 minute rainfall simulation for a rainstorm with average intensity of 1.61 mm min?1 and 30 day drainage period were measured at various depths by 1.5 m long capacitance soil water measuring (DFM) probe. Different (P < 0.05) mean surface steady infiltration rate ranged from 0.05 to 4.47 mm min?1 and had a negative power relationship (R 2 = 0.65) with horizon clay plus fine silt content. Power regression (R 2 ≥ 86%) described rainstorm infiltration and obtained steady rates within an average time of 15 minutes. Mean total infiltrated soil water content was lowest (P < 0.05) from surface horizons with either 47.7% clay plus fine silt content or bulk density of 1.91 g cm?3 and exchangeable sodium of not less than 44 mg kg?1. Surface horizons with lower surface bulk density and total sand fraction of more than 72% had infiltrated depth and mean total infiltrated soil water content up to 40 cm deeper and 0.55 mm mm?1 greater, respectively. Drainage rate at drained upper limit calculated from the Wilcox drainage model (R 2 ≤ 0.97%) was 0.2 mm day?1 or less were from underlying horizons with either clay plus fine silt of 45% or soft calcium carbonate. Higher drainage rate with accumulative drainage amount greater than 60 mm were from soil profile horizons with clay plus fine silt content of less than 20% and above unity steady infiltration rates. Rainstorm infiltration and drainage rates was shown to depend on permeability and coarseness of the respective soil surface and subsurface horizons; a phenomenon critical for harnessing rain and flood water to recharge groundwater. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

5.
Characteristic badlands are incised into Plio‐Pleistocene clays in Basilicata, southern Italy, creating steep, scarp slopes with knife‐edge ridges (calanchi) and small dome‐shaped forms (biancane). Erosion pin data for the period 1997–2003 give mean annual erosion rates for dome‐shaped biancane in the range 9–19 mm a?1, while rates for the calanchi scarps are lower, at 7–10 mm a?1. The erosion pin data also show a non‐linear relationship with slope angle. Maximum erosion rates coincide with a slope angle of 35°, within an envelope defined by combining the theoretical effects of both rainsplash and surface weathering. Monitoring of surface changes and erosion rates for two 0·5 m2 cleared swathes on biancane forms reveals a complex relationship between weathering and erosion. Characteristic forms can develop from large blocks of intact clay bedrock over a time period of less than 30 a. The implications of the measured erosion rates for the landform association of mountain front/pediment/domed inselberg are explored. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

6.
The thermal conductivity of a simulated Apollo 12 lunar soil sample was measured with a needle probe under vacuum. The result showed that the sample, with bulk densities of 1.70–1.85 g cm?3 held in a vertical cylinder (2.54 cm in diameter and 6.99 cm long) has a thermal conductivity ranging from 8.8 to 10.9 mW m?1 K?1. This is comparable to the lunar regolith's thermal conductivity as determined in situ. Besides the dense packing of the soil particles, an enhanced intergranular thermal contact, due to the self-compression of the sample, is necessary to raise the sample's thermal conductivity from the level of loose soil (< 5 mW m?1 K?1) to that of the lunar regolith deeper than 35 cm (～ 10 mW m?1 K?1). A model of the lunar regolith, a thin layer of loose soil resting on a compacted self-compressed substratum, is consistent with the lunar regolith's surface structure as deduced from an observation of the lunar surface's brightness temperature. Martian regolith surface structure is similar, except that its surface layer may be missing in places because of aeolian activity. Measurements of thermal conductivity under simulated martian surface conditions showed that the thermal properties of loose and compacted soils agreed with the two peak values of the martian surface's thermal inertia as observed from “Viking” orbiters, suggesting that drifted loose soil and exposed compacted soil are responsible for the bimodal distribution of the martian surface's thermal inertia near zero elevation. For compacted soil exposed to the martian surface to have the same thermal conductivity as that buried under the surface layer, a cohesion of the soil particles must be assumed.  相似文献   

7.
Data for the diffusion of cations in pyroxenes are relevant to a variety of sub-solidus processes including order-disorder and exsolution. Similar data must also be available if the reliability of geobarometers and geothermometers involving pyroxenes is to be assessed. Two types of diffusion experiment have been performed to determine cation diffusion rates in pyroxenes: (1) interdiffusion between single crystals of diopside and polycrystalline sinters enriched in Al and Fe, and (2) interdiffusion between single crystals of diopside and a glass of the same composition which was isotopically enriched in26Mg and43Ca. Following high-temperature annealing for periods up to several hundred hours, analysis of the diffusion couples, using an electron microprobe and an ion microprobe respectively, failed to show any measurable diffusion profiles. From these “null result” experiments the diffusion coefficients (D) for Al and Fe in diopside are estimated to be less than4×10?14cm2s?1 at 1200°C, and values ofD for Ca and Mg in diopside are estimated to be less than7 × 10?14cm2s?1 at 1250°C. These rates are significantly slower than published tracer-type diffusion data for Ca and Al.A review of studies of order-disorder, microstructural coarsening, and diffusion in pyroxenes suggest that activation energies for cation exchange are typically in excess of 60 kcal mol?1. Transport rates will be assisted, and activation energies lowered by sample non-stoichiometry, inhomogeneities, high dislocation densities and the presence of water.The collective data for Al, Mg and Ca diffusion in diopside indicate diffusion coefficients? 10?15cm2s?1 at 1200°C. A comparison with data for diffusion in garnet, olivine and spinel suggests that pyroxenes may have the highest blocking temperatures.  相似文献   

8.
Five representative cumulative samples of basal flows were chemically analysed. Normative composition (CIPW),DI, FI, MI, SI, f (norms) and ‘S’ indices have been calculated therefrom. It is concluded that the Linga flows are tholeiitic in nature with slightly more FeO, MgO and Al2O3 than most tholeiitic basalts. They represent eariy-middle stage basalts with DI less than 30. Thef-norms put these flows in the pyroxene field indicating an early fractionation of pyroxenes causing relative iron enrichment. Textural evidences, also, support a late fractionation of feldspar with overlapping periods of crystallisation of pyroxene and feldspar. The similarity in composition of the lower flows is more apparent than true. The trend of differentiation is characteristically tholeiitic. Oxygen and vapour fugacity may have been responsible for a shift in the general trend of differentiation. Explosive eruption of the magma, which was well mixed and which cooled rapidly, is borne by the features exhibited by these flows.  相似文献   

9.
Excess delivery of land‐based sediments is an important control on the overall condition of nearshore coral reef ecosystems. Unpaved roads have been identified as a dominant sediment source on St John in the US Virgin Islands. An improved understanding of road sediment production rates is needed to guide future development and erosion control efforts. The main objectives of this study were to: (1) measure sediment production rates at the road segment scale; (2) evaluate the importance of precipitation, slope, contributing area, traffic, and grading on road sediment production; (3) develop an empirical road erosion predictive model; and (4) compare our measured erosion rates to other published data. Sediment production from 21 road segments was monitored with sediment traps from July 1998 to November 2001. The selected road segments had varying slopes, contributing areas, and traffic loads. Precipitation was measured by four recording rain gauges. Sediment production was related to total precipitation and road segment slope. After normalizing by precipitation and slope, the mean sediment production rate for roads that had been graded within the last two years was 0·96 kg m?2 cm?1 m m?1 or approximately 11 kg m?2 a?1 for a typical road with a 10 per cent slope and an annual rainfall of 115 cm a?1. The mean erosion rate for ungraded roads was 42 per cent lower, or 0·56 kg m?2 cm?1 m m?1. The normalized mean sediment production rate for road segments that had been abandoned for over fifteen years was only about 10 per cent of the mean value for ungraded roads. Sediment production was not related to traffic loads. Multiple regression analysis led to the development of an empirical model based on precipitation, slope to the 1·5 power, and a categorical grading variable. The measured and predicted erosion rates indicate that roads are capable of increasing hillslope‐scale sediment production rates by up to four orders of magnitude relative to undisturbed conditions. The values from St John are at the high end of reported road erosion rates, a finding that is consistent with the high rainfall erosivities and steep slopes of many of the unpaved roads on St John. Other than paving, the most practical methods to reduce current erosion rates are to minimize the frequency of grading and improve road drainage. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

10.
Investigations to understand linkages among climate, erosion and weathering are central to quantifying landscape evolution. We approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate, geochemistry, and denudation rates for low sloping upland profiles. Focusing on Na as a proxy for plagioclase weathering, we quantified regolith Na depletion, Na mass loss, and the relative partitioning of denudation to physical and chemical contributions. The depth and magnitude of regolith Na depletion increased continuously with increasing water availability, except for locations with mean annual temperature < 5 °C that exhibited little Na depletion, and locations with physical erosion rates < 20 g m? 2 yr? 1 that exhibited deep and complete regolith Na depletion. Surface Na depletion also tended to decrease with increasing physical erosion. Depth-integrated Na mass loss and regolith depth were both three orders of magnitude greater in the fully depleted, low erosion rate sites relative to other locations. These locations exhibited strong erosion-limitation of Na chemical weathering rates based on correlation of Na chemical weathering rate to total Na denudation. Sodium weathering rates in cool locations with positive annual water balance were strongly correlated to total Na denudation and precipitation, and exhibited an average apparent activation energy (Ea) of 69 kJ mol? 1 Na. The remaining water-limited locations exhibited kinetic limitation of Na weathering rates with an Ea of 136 kJ mol? 1 Na, roughly equivalent to the sum of laboratory measures of Ea and dissolution reaction enthalpy for albite. Water availability is suggested as the dominant factor limiting rate kinetics in the water-limited systems. Together, these data demonstrate marked transitions and nonlinearity in how climate and tectonics correlate to plagioclase chemical weathering and Na mass loss.  相似文献   

11.
The geopotential value of W 0 = (62 636 855.611 ± 0.008) m 2 s –2 which specifies the equipotential surface fitting the mean ocean surface best, was obtained from four years (1993 - 1996) of TOPEX/POSEIDON altimeter data (AVISO, 1995). The altimeter calibration error limits the actual accuracy of W 0 to about (0.2 - 0.5) m 2 s –2 (2 - 5) cm. The same accuracy limits also apply to the corresponding semimajor axis of the mean Earth's level ellipsoid a = 6 378 136.72 m (mean tide system), a = 6 378 136.62 m (zero tide system), a = 6 378 136.59 m (tide-free). The variations in the yearly mean values of the geopotential did not exceed ±0.025 m 2 s –2 (±2.5 mm).  相似文献   

12.
Tropical rainforest canopy trees that have large projected areas of upwardly inclined branches are capable of funnelling large volumes of rainwater down their trunks. During periods of prolonged heavy rainfall on Mount Bellenden Ker in northeast Queensland, Australia, stemflow volumes were found to be as much as two orders of magnitude greater than the volume of incident rainfall expected in a rain gauge occupying an area equal to the trunk basal area. Stemflow totals ranging from 6000 to 70000 litres were generated by individual trees from 7800 mm of rainfall over two successive wet seasons. The combination of high intensity rainfall and the funnelling effect results in significant quantities of infiltration-excess at the ground surface. Stemflow fluxes as high as 31.4 cm3 min?1 per cm2 of basal area (i.e. the equivalent of 314 mm min?1) were recorded when rainfall intensity was only 2 mm min?1. The mean infiltration capacity of the topsoil was determined to be 6.2 mm min?1. The areas over which the stemflow would have had to spread in order to infiltrate were computed to be as much as 3 m2 around the bases of individual canopy trees. Approximations of the distances that the infiltration-excess would have travelled away from the tree bases were calculated by assuming that the infiltration area either expands radially outward in the form of an annulus or extends straight downslope from the tree base.  相似文献   

13.
The preatmospheric mass of Angra dos Reis is estimated to be about 80 kg, based on cosmic ray track densities of 1?3 × 106 cm?2 in feldspars, olivines, and pyroxenes. The244Pu-fission track densities in pyroxenes and adjacent olivine-feldspar track detectors, as well as in whitlockites and adjacent pyroxene-olivine detectors are about 20 and 280 × 106 cm?2 respectively. These values correspond exactly to the244Pu-fission xenon contents in the two minerals. This suggests an almost instantaneous cooling (>70 K per m.y.) of the Angra dos Reis material 4.55 b.y. ago. This fast cooling is the reason for a quantitative4He retention since that time. If the4He excess found in Angra dos Reis is a real effect, the source of this excess remains unknown.  相似文献   

14.
Runoff and sediment lost due to water erosion were recorded for 36 (1 m2) plots with varying types of vegetative cover located on sloping gypsiferous fields in the South of Madrid. 75% of the events had maximum 30‐minute intensity (I30) less than 10 mm h?1 in the period studied (1994–2005). As for the vegetative cover, maximum correlation between runoff and soil loss was found in the least protected plots (0–40% cover) during the most intense rainfall events; however, a significant positive correlation was also observed in plots with greater coverage (40–60%). If coverage exceeded 60%, rainfall erosivity declined. The average amount of sediment produced in high‐intensity events was significantly greater (approximately 7 g m?2 per I30 event >10 mm h?1) than that produced in the rest of the moderate‐intensity events (approximately 3 g m?2 per I30 event <10 mm h?1), but due to the high rate of occurrence of the latter throughout the year sediment loss during the period studied totaled 128 g m?2. By comparison, only 40 g m?2 was produced by the I30 events greater than 10 mm h?1. Even though the amount of soil lost is relatively insignificant from a quantitative standpoint, the organic matter content lost in the sediment (six times more than in the soil) is a permanent loss that threatens the development of the surface of the soil in this area when the vegetative cover is less than 40%. The soil here experiences a chronic loss of 0·02 mm annually as a consequence of frequent, moderate events, in addition to any loss produced by extraordinary events, which, though less frequent, are much more erosive. If moderate events are ignored, an important part of soil loss will be lost in the long run. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

15.
Obtaining good quality soil loss data from plots requires knowledge of the factors that affect natural and measurement data variability and of the erosion processes that occur on plots of different sizes. Data variability was investigated in southern Italy by collecting runoff and soil loss from four universal soil‐loss equation (USLE) plots of 176 m2, 20 ‘large’ microplots (0·16 m2) and 40 ‘small’ microplots (0·04 m2). For the four most erosive events (event erosivity index, Re ≥ 139 MJ mm ha?1 h?1), mean soil loss from the USLE plots was significantly correlated with Re. Variability of soil loss measurements from microplots was five to ten times greater than that of runoff measurements. Doubling the linear size of the microplots reduced mean runoff and soil loss measurements by a factor of 2·6–2·8 and increased data variability. Using sieved soil instead of natural soil increased runoff and soil loss by a factor of 1·3–1·5. Interrill erosion was a minor part (0·1–7·1%) of rill plus interrill erosion. The developed analysis showed that the USLE scheme was usable to predict mean soil loss at plot scale in Mediterranean areas. A microplot of 0·04 m2 could be used in practice to obtain field measurements of interrill soil erodibility in areas having steep slopes. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

16.
The long-term average annual soil loss (A) and sediment yield (SY) in a tropical monsoon-dominated river basin in the southern Western Ghats, India (Muthirapuzha River Basin, MRB; area: 271.75 km2), were predicted by coupling the Revised Universal Soil Loss Equation (RUSLE) and sediment delivery ratio (SDR) models. Moreover, the study also delineated soil erosion risk zones based on the soil erosion potential index (SEPI) using the analytic hierarchy process (AHP) technique. Mean A of the basin is 14.36 t ha?1 year?1, while mean SY is only 3.65 t ha?1 year?1. Although the land use/land cover types with human interference show relatively lower A compared to natural vegetation, their higher SDR values reflect the significance of anthropogenic activities in accelerated soil erosion. The soil erosion risk in the MRB is strongly controlled by slope, land use/land cover and relative relief, compared to geomorphology, drainage density, stream frequency and lineament frequency.  相似文献   

17.
Track densities in feldspar crystals from L16A14 and L16G14 (6–8 cm and 29-21 cm) range from 2.5 × 108/cm2 to > 2 × 109/cm2. No significant difference is found between the two positions. The track densities are similar to those observed in heavily irradiated samples of Apollo 11, 12 and 14 and indicate that these two positions are composed of well mixed materials from a number of sources. This is in contrast to a number of fines samples from Apollo 12 and 14 which are less irradiated and represent relatively recent additions to the lunar surface.  相似文献   

18.
Analytical modelling of heat transport was used to address effects of uncertainty in thermal conductivity on groundwater–surface water exchange. In situ thermal conductivities and temperature profiles were measured in a coastal lagoon bed where groundwater is known to discharge. The field site could be divided into three sediment zones where significant spatial changes in thermal conductivity on metre to centimetre scale show that spatial variability connected to the sediment properties must be considered. The application of a literature‐based bulk thermal conductivity of 1.84 Wm?1 °C?1, instead of field data that ranged from 0.62 to 2.19 W m?1 °C?1, produced a mean overestimation of 2.33 cm d?1 that, considering the low fluxes of the study area, represents an 89% increase and up to a factor of 3 in the most extreme cases. Incorporating the uncertainty due to sediment heterogeneities leads to an irregular trend of the flux distribution from the shore towards the lagoon. The natural variability of the thermal conductivity associated with changes in the sediment composition resulted in a mean variation of ±0.66 cm d?1 in fluxes corresponding to a change of ±25.4%. The presence of organic matter in the sediments, a common situation in the near‐shore areas of surface water bodies, is responsible for the decrease of thermal conductivity. The results show that the natural variability of sediment thermal conductivity is a parameter to be considered for low flux environments, and it contributes to a better understanding of groundwater–surface water interactions in natural environments. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

19.
Abstract

The use of the bootstrap technique to estimate the reference level of137 Cs in an uneroded site is tested. The analysis is developed using 137Cs measurements made in a small experimental Sicilian basin. In the reference area the 137Cs activity is normally distributed with a known sample mean value, m equal to 94.4 mBq cm?2. The influence of137 Cs reference site sampling was determined generating samples having a fixed size, N and six different values of the sample coefficient of variation, CV, by a Monte Carlo technique. Then, for each size N, the probability distribution of the mean μ of the sequences generated by Monte Carlo technique is defined. The soil redistribution is determined, both at morphological unit and basin scale, using the proportional method of Martz & de Jong for calculating the net soil loss. The analysis showed that the spatial distribution of the net soil loss E i, and the basin value E b are independent of the sample size, N, and the coefficient of variation, CV, of the samples drawn from the reference area, if the bootstrap technique is applied for estimating the mean μ(μ) to use as reference value. The soil redistribution is also examined using as reference value the quantiles μ2.5, μ25, μ75, μ97.5 corresponding to a frequency F(μ) equal to 2.5, 25, 75 and 97.5%, respectively. In conclusion, the analysis established that a robust estimate of the reference value can be obtained even in fields where a small number of samples was drawn (high CV of the 137Cs activity of the field samples), using the bootstrap technique for generating sequences of reference values having known mean value m (the mean value of the 137Cs activity of the drawn field samples) and large sample size (N = 50).  相似文献   

20.
Water flow in the soil–root–stem system was studied in a flooded riparian hardwood forest in the upper Rhine floodplain. The study was undertaken to identify the vertical distribution of water uptake by trees in a system where the groundwater is at a depth of less than 1 m. The three dominant ligneous species (Quercus robur, Fraxinus excelsior and Populus alba) were investigated for root structure (vertical extension of root systems), leaf and soil water potential (Ψm), isotopic signal (18O) of soil water and xylem sap. The root density of oak and poplar was maximal at a depth of 20 to 60 cm, whereas the roots of the ash explored the surface horizon between 0 and 30 cm, which suggests a complementary tree root distribution in the hardwood forest. The flow density of oak and poplar was much lower than that of the ash. However, in the three cases the depth of soil explored by the roots reached 1·2 m, i.e. just above a bed of gravel. The oak roots had a large lateral distribution up to a distance of 15 m from the trunk. The water potential of the soil measured at 1 m from the trunk showed a zone of strong water potential between 20 and 60 cm deep. The vertical profile of soil water content varied from 0·40 to 0·50 cm3 cm?3 close to the water table, and 0·20 to 0·30 cm3 cm?3 in the rooting zone. The isotopic signal of stem water was constant over the whole 24‐h cycle, which suggested that the uptake of water by trees occurred at a relatively constant depth. By comparing the isotopic composition of water between soil and plant, it was concluded that the water uptake occurred at a depth of 20 to 60 cm, which was in good agreement with the root and soil water potential distributions. The riparian forest therefore did not take water directly from the water table but from the unsaturated zone through the effect of capillarity. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

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