Modelling the spatial variation in soil moisture at the landscape scale: an application to five areas of ecological interest in the UK

A simple conceptual hydrological model that explicitly includes the lateral movement of soil water and operates efficiently at the landscape scale is outlined. It is applied to five areas of ecological interest in the UK to provide distributed mean monthly soil moisture on a 50 m grid. As the model's driving variables—daily rainfall and potential evapotranspiration—are assumed constant over each of the tracts of land, the variability in soil moisture is due to different soil types and to topographic effects. Box plots of the mean monthly simulated soil moisture clearly show the spread of values occasioned by modelling the lateral water movement down the hillslope. The general magnitude of the results are compared with published data wherever possible and there is some discussion of the form of the curve used in the model to describe the attenuation of evapotranspiration with decreasing soil moisture. Copyright © 2000 John Wiley & Sons, Ltd.     

Assessing the past thermal and chemical history of fluids in crystalline rock by combining fluid inclusion and isotopic investigations of fracture calcite

Fluid inclusion studies combined with the isotope geochemistry of several generations of fracture calcite from the Olkiluoto research site, Finland, has been used to better understand the past thermal and fluid history in the crystalline rock environment. Typically, fracture mineral investigations use O and C isotopes from calcite and an estimate of the isotopic composition of the water that precipitated the calcite to perform δ¹⁸O geothermometry calculations to estimate past temperature conditions. By combining fluid inclusion information with calcite isotopes, one can directly measure the temperature at which the calcite formed and can better determine past fluid compositions. Isotopic, petrologic and fluid inclusion studies at the Olkiluoto research site in Finland were undertaken as part of an investigation within the Finnish nuclear waste disposal program. The study revealed that four fluids were recorded by fracture calcites. From petrologic evidence, the first fluid precipitated crystalline calcite at 151–225°C with a δ¹³C signature of −21 to −13.9‰ PDB and a δ¹⁸O signature of 12.3–13.0‰ SMOW. These closed fracture fillings were found at depths greater than 500 m and were formed from a high temperature, low salinity, Na–Cl fluid of possible meteoric water altered by exchange with wallrock or dilute basinal origin. The next fluid precipitated crystalline calcite with clay at 92–210°C with a δ¹³C signature of −2.6 to +3.8‰ PDB and a δ¹⁸O signature of 19.4–20.7‰ SMOW. These closed fracture fillings were found at depths less than 500 m and were formed from a moderate to high temperature, low to moderate salinity, Na–Cl fluid, likely of magmatic origin. The last group of calcites to form, record the presence of two distinct fluid types. The platy (a) calcite formed at 95–238°C with a δ¹³C signature of −12.2 to −3.8‰ PDB and a δ¹⁸O signature of 14.9–19.6‰ SMOW, from a high temperature, low salinity, Na–Cl fluid of possible magmatic origin. The platy (b) calcite formed at 67–98°C with a δ¹³C signature of −13.0 to −6.2‰ PDB and a δ¹⁸O signature of 15.1–20.1‰ SMOW, from a low temperature, high salinity, Ca–Na–Cl fluid of possible basinal brine origin. The two calcites are related through a mixing between the two end members. The source of the fluids for the platy grey (a) calcites could be the olivine diabase dykes and sills that cut through the site. The source of fluids for the platy (b) calcites could be the Jotnian arkosic sandstone formations in the northern part of the site. At the Olkiluoto site, δ¹⁸O geothermometry does not agree with fluid inclusion data. The original source of the water that forms the calcite has the largest effect on the isotopic signature of the calcites formed. Large isotopic shifts are seen in any water by mineral precipitation during cooling under rock–water equilibrium fractionation conditions. Different calcite isotopic signatures are produced depending on whether cooling occurred in an open or closed system. Water–rock interaction, at varying W/R ratios, between a water and a host rock can explain the isotopic shifts in many of the calcites observed. In some cases it is possible to shift the δ¹⁸O of the water by +11.5‰ (SMOW) using a realistic water–rock ratio. This process still does not explain some of the very positive δ¹⁸O values calculated using fluid inclusion data. Several other processes, such as low temperature recrystallization, boiling, kinetic effects and dissolution of calcite from fluid inclusion walls can affect isotopic signatures to varying degrees. The discrepancy between fluid inclusion data and δ¹⁸O geothermometry at the Olkiluoto site was most likely due to poor constraint on the original source of the water.     

Formation of Working Surfaces in Radiatively Cooled Laboratory Jets

Whilst observations provide many examples of collimated outflows or jets from astrophysical bodies, there remain unresolved questions relating to their formation, propagation and stability. The ability to form scaled jets in the laboratory has provided many useful insights. Experiments (Lebedev et al.: 2002, ApJ 564, 113) using conical arrays of fine metallic wires on the MAGPIE generator (1MA in 240 ns) have produced radiatively cooled collimated jets in vacuum using the redirection of convergent flows by a conical shock. Here we present results of a jet produced by this method propagating through a photo-ionized, quasi-stationary gas cloud. A working surface is observed at the head of the jet. The velocity of this working surface is lower than the velocity of a jet tip in vacuum.     

Variations in shock deformation at the Slate Islands impact structure,Lake Superior,Canada

A progressive change in the level of shock deformation is documented in autochthonous rocks from the central uplift of the Slate Islands impact structure, Lake Superior. Correlation of these observations, which are based mainly on the relative frequency of planar features of specific crystallographic orientation in quartz, with experimental data is used to estimate the average shock pressures recorded in the samples studied. Recorded pressures range from 5.8 to 15.3 GPa and generally increase towards the proposed shock centre. Variations in the shock response of quartz of different grain size and texture are observed within and between samples. It is apparent that large interlocking quartz grains in eyes record approximately 15–20% higher levels of shock deformation than small grains in mosaics or large isolated phenocrysts. These variations in shock deformation are attributed to the effect of shock wave reverberations between grains and length of shock pulse duration within grains.Comparison of the Slate Islands data with similar observations at the larger Charlevoix impact structure indicates that the rate of change of recorded shock pressure with distance is greater at the Slate Islands structure. This is interpreted as due to variations in the strain rates and/or the rate of shock wave attenuation with radial distance between impact structures of different size.Contribution from Earth Physics Branch No. 626     

Patch scale aggregation of heterogeneous land surface cover for mesoscale meteorological models

Theoretical studies dealing with aggregation of surface parameters at small scale are reviewed. Finding effective parameters for surface resistance is possible for most cases by taking simple geometric or arithmetic averages of the component resistances. The use of more sophisticated techniques such as the blending height improves the calculations. Resistances for heat and water vapour behave differently in heterogeneous terrain. A simple surface energy balance model is adapted to show the behaviour of the roughness length of heat and water vapour in heterogeneous terrain. It is suggested that this simple parameterization can adequately take into account the effect of variation in surface cover on the fluxes of heat and water vapour.     

Using a simple SVAT scheme to describe the effect of scale on aggregation

Avaporation is enhanced at a dry-to-wet transition but not equally depressed at a wet-to-dry transition. Therefore, the more dry-wet edges there are per unit area, the higher the evaporation. This behaviour is predicted by the blending height; the smaller the length scale of variation, the smaller the blending height and the higher the evaporation. The blending height principle can be modelled simply for a mixture of two surfaces using a model with three resistances and applying the energy combination theory developed for sparse canopies. Results from numberical model simulations are shown to agree with the simple analytical model. The blending height is found to be too large to model correctly observations from small-scale heterogeneous terrain in the Sahel. This is assumed to be due to edge effects.The results show that a modification of the sparse canopy model can be made to represent heterogeneity at all scales.SVAT is Surface Vegetation Atmosphere Transfer.     

Impact melt rocks from New Quebec Crater,Quebec, Canada

Abstract— Approximately 1500 g of float samples of impact melt rocks have been recovered from gravel deposits ～4 km north and northeast of the rim of the 3.4 km diameter New Quebec Crater (61°17′N; 73°40′W) in northern Quebec, Canada. Previously, only two small samples of impact melt rocks were known. The newly recovered samples have cryptocrystalline to microcrystalline matrices with microlites of andesine and pigeonite. Mineral clasts of quartz and feldspar occur and, in some cases, show shock metamorphic features. The melt rocks have a normative mineralogy corresponding to ～70% quartz, orthoclase and albite and are compositionally similar. Their major element composition can be modeled as a mix of granitic gneisses that make up the target rocks. The melt rocks show enrichments, however, in Cr (21 ppm), Co (9 ppm), Ni (12 ppm) and Ir (1.5 ppb) over the target rocks. Interelement ratios suggest a chondritic impacting body, although they do not define a specific type. Assuming a C-1 chondrite, the impact melt rocks average ～2% meteoritic contamination. Stepwise ⁴⁰Ar-³⁹Ar dating using a laser on three chips from three samples give integrated ages of 0.6–2.5 Ma. From the best plateau ages, the age of the New Quebec impact is taken to be 1.4 ± 0.1 Ma, which places it before the first major northern hemisphere continental glaciation of the Pleistocene. A number of considerations suggest that the impact melt rocks were originally deposited in fractures in the crater wall and later transported to their discovery site by glacial ice and melt water.