ESTIMATING INFILTRATION PARAMETERS FROM BASIC SOIL PROPERTIES

Infiltration data were collected on two rectangular grids with 25 sampling points each. Both experimental grids were located in tropical rain forest (Guyana), the first in an Arenosol area and the second in a Ferralsol field. Four different infiltration models were evaluated based on their performance in describing the infiltration data. The model parameters were estimated using non-linear optimization techniques. The infiltration behaviour in the Ferralsol was equally well described by the equations of Philip, Green–Ampt, Kostiakov and Horton. For the Arenosol, the equations of Philip, Green–Ampt and Horton were significantly better than the Kostiakov model. Basic soil properties such as textural composition (percentage sand, silt and clay), organic carbon content, dry bulk density, porosity, initial soil water content and root content were also determined for each sampling point of the two grids. The fitted infiltration parameters were then estimated based on other soil properties using multiple regression. Prior to the regression analysis, all predictor variables were transformed to normality. The regression analysis was performed using two information levels. The first information level contained only three texture fractions for the Ferralsol (sand, silt and clay) and four fractions for the Arenosol (coarse, medium and fine sand, and silt and clay). At the first information level the regression models explained up to 60% of the variability of some of the infiltration parameters for the Ferralsol field plot. At the second information level the complete textural analysis was used (nine fractions for the Ferralsol and six for the Arenosol). At the second information level a principal components analysis (PCA) was performed prior to the regression analysis to overcome the problem of multicollinearity among the predictor variables. Regression analysis was then carried out using the orthogonally transformed soil properties as the independent variables. Results for the Ferralsol data show that the parameters of the Green–Ampt and Kostiakov model were estimated relatively accurately (maximum R² = 0.76). For the Arenosol, use of the second information level together with PCA produced regression models with an R² value ranging from 0.38 to 0.68. For the Ferralsol, most of the variance was explained by the root content and organic matter content. In the Arenosol plot, the fractions medium and fine sand explained most of the observed variance.     

A palaeoecological study of the mites (Acari) from a Lateglacial deposit at Usselo (The Netherlands)

The distribution of the remains of mites in the Lateglacial (Late Weichselian) type section at Usselo (The Netherlands) is presented. The division of the sequence into (sub-)zones, based upon combined palaeo-botanical and palaeoentomological studies, is shown, as well as the approximate age based on ¹⁴C determinations. The present habitats of the species are in agreement with the reconstruction of the local vegetational succession Four species of mites, three oribatids and one uropodid, have not previously been recorded as subfossils.     

Short‐term volume changes of the Greenland ice sheet in response to doubled CO2 conditions

This paper focuses on the rôle of accumulation and cloudiness changes in the response of the Greenland ice sheet to global warming. Changes in accumulation or cloudiness were often neglected, or coupled to temperature changes. We used model output on temperature, precipitation and cloudiness from a GCM (ECHAM4 T106). The GCM output was used to drive the Greenland model that exists of a vertically averaged ice flow model, coupled to a 1D surface energy balance model that calculates the ablation. Variables are temperature, accumulation and cloudiness. Sensitivity experiments with this model show that changes in accumulation are very important for the ice sheet mass balance, whereas cloudiness is of secondary importance. If the Greenland model is forced by the GCM output, the Greenland model is found to contribute 70% less to sea level rise after 70 years than is indicated by the results presented in the IPCC report. This large discrepancy is mainly due to the fact that the enhanced ablation is strongly compensated by increased accumulation. Comparing the result obtained here with changes in mass balance derived directly from the same general circulation model, indicates a 20% larger contribution to sea level. This increase is due to changes in ice flow, and a different method for the ablation calculation.     

Linear dynamics of hydrostatic adjustment to horizontally homogeneous heating

The process of hydrostatic adjustment to horizontally homogeneous heating in a stably stratified atmosphere of arbitrary thermal structure is investigated in the limit of small perturbations. A linear differential equation is derived for the vertical pressure distribution in the final balanced state. Solutions of this equation are compared with the time dependent solution which is found by numerically integrating the equations in time. During the process of hydrostatic adjustment acoustic‐buoyancy oscillations are generated. The amplitudes of these oscillations become so great that static instability is generated at heights above 100 km, depending on where and how abruptly the heat is added. As a crude representation of the unstable breakdown and damping of these waves, Rayleigh damping is introduced. If the associated damping coefficient in the upper atmosphere is sufficiently large (greater than the Brunt Väisälä frequency), the oscillations vanish. Below a height of about 50 km the steady state predicted by the above mentioned differential equation is reached approximately in 10 min.     

Two mid-Holocene millennia of swamp forest in the Rhine/Meuse deltaic plain

The paleoenvironmental evolution of a Dutch deltaic swamp forest area during the mid-Holocene was strongly governed by a general rise of water level (bound to postglacial sea-level rise) and by fluctuations in this water-level rise. A phasing of this 2,000 years period shows alternation of closed swamp forest on the one hand, and (fluvio-) lacustrine environment with swamp forest islands on the other. The refined scale of investigation permits bird's-view landscape reconstructions. These also show how attractive the area was for neolithic fishermen.     

Impact‐induced frictional melting in ordinary chondrites: A mechanism for deformation,darkening, and vein formation

Abstract— High speed friction experiments have been performed on the ordinary chondrites El Hammami (H5, S2) and Sahara 97001 (L6, S3) using an axial friction‐welding apparatus. Each sample was subjected to a strain rate of 10³ to 10⁴ s^?1, which generated 250 to 500 μm‐deep darkened zones on each sample cube. Thin section analyses reveal that the darkened areas are composed of silicate glass and mineral fragments intermingled with dispersed submicron‐size FeNi and FeS blebs. Fracturing of mineral grains and the formation of tiny metallic veins define the extent of deformation beyond the darkened shear zone. These features are not present in the original meteorites. The shear zones and tiny veins are quite similar to certain vein systems seen in naturally deformed ordinary chondrites. The experiments show that shock deformation is not required for the formation of melt veins and darkening in ordinary chondrites. Therefore, the presence of melt veins and darkening does not imply that an ordinary chondrite has undergone severe shock deformation. In fact, high strain rate deformation and frictional melting are especially important for the formation of veins at low shock pressures.