An event-based stochastic model of phosphorus loading into a lake

In most lakes, phosphorus (P) is the nutrient controlling the trophic state. Thus, for effective control of eutrophication, the uncertainty in P-loading should be encoded as a probability density function (pdf). Specifically, the pdf of P-loading Y from non-point agricultural sources is sought by means of an event-based stochastic model.P-loading events are triggered by precipitation events (X₁, X₂, T), in which X₁ is the rainfall amount, X₂ the duration, and T the interarrival time between events. (X₁, X₂) are dependent random variables, while T is assumed to be exponentially distributed. The precipitation event causes runoff, which carries dissolved P into the lake with a concentration C₁ and sediment yield, Z, which carries fixed or sorbed P into the lake in a fraction C₂ of Z. Seasonal loading of P is calculated by adding random numbers of random variables. The model accounts separately for dissolved P and sorbed P. Explicit expressions are given for the mean and variance of each type of P-loadings. The case study of a sub-watershed of Lake Balaton, Hungary, is used to illustrate the methodology. Precipitation data, empirical rainfall-runoff-sediment yield relationships and a small number of observations of events are used to calibrate the model and estimate the means and variances of loading per event and per season. Then a simulation method is used to estimate complete pdf of these random variables. Use of the model for alternative methods of controlling P-loading is briefly discussed, as well as the economics of control.     

Spatial Variability of Trends in the Rainfall-Runoff Relationship: A Mesoscale Study in the Mosel Basin

The spatial variability of observed trends in rainfall structure over the last 5 decades and its effects on the spatial variability of maximum daily water levels in the Grand-Duchy of Luxembourg (Europe) were investigated in 9 sub-basins of the Mosel river. Over the past 25 years, an increase in westerly atmospheric circulation types during winter months has caused an increase in winter rainfall totals, duration and intensity. More specifically, the spatial variability of trends having affected winter rainfall totals, duration and intensity have led to spatially varying positive trends in maximum daily water levels. Observed trends in rainfall characteristics and maximum daily water levels during winter show closely linked spatial patterns that are strongly related to the topography of the study area.     

Pb isotopic zoning of K-feldspar megacrysts determined by Laser Ablation Multi-Collector ICP-MS: Insights into granite petrogenesis

This study investigates Pb isotopic zoning in magmatic K-feldspar megacrysts from the Monte Capanne pluton (Elba, Italy) using Laser Ablation Multi-Collector-ICPMS. The studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open-system processes and better characterize the components involved in the genesis of complex magma systems. Earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal-derived magmas.The investigated K-feldspar megacrysts exhibit strong zoning in ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb, correlated with lead elemental variations. We interpret these variations as reflecting growth zoning, as opposed to secondary diffusive exchange. Despite a great variety of zoning patterns, we were able to correlate different events of megacryst growth, reflecting crystallization in a dynamic magma system. Our two-step model includes (1) growth of a granitic magma chamber by addition of low ²⁰⁸Pb/²⁰⁶Pb magma to a high ²⁰⁸Pb/²⁰⁶Pb magma contaminated with crustal material (i.e., the megacryst cores) and (2) recharge by mantle-derived magma (i.e., the megacryst rims). We interpret the thorogenic nature of the megacryst rims to reflect the mantle-derived component involved in the mixing process. Taking account of other data from the Tuscan Magmatic Province, the mantle source is inferred to have been metasomatized by continental material during subduction. TIMS Sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled.     

The effect of a shallow low viscosity zone on the apparent compensation of mid-plate swells

A simple model for mid-plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region. Finite element calculations have been used to determine the effects of the viscosity contrast, the layer thickness and the Rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell. As the viscosity decreases in the low viscosity zone, the effective local Rayleigh number for the top boundary layer of the convecting cell increases. Also, because the lower viscosity facilitates greater velocities in the low viscosity zone, the low viscosity layer produces proportionally greater horizontal flow near the conducting lid, causing the base of the conducting lid to appear like a free boundary. The change in the local Rayleigh number and in the effective boundary condition both cause the top boundary layer to thin. Through a Green's function analysis, we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth, whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations. By increasing the viscosity contrast, the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small. Therefore, shallow depths of compensation cannot be used to argue against dynamic support of mid-plate swells. Furthermore, we compared the distribution of the effective compensating densities, which is used to obtain the geoid, to that of Pratt compensation, which is often used to calculate the depth of compensation from geoid and topography data for mid-plate swells. For all of our calculations including those with no low viscosity layer, the effective gravitational mass distribution is more complex than assumed in simple Pratt models, so that the Pratt models are not an appropriate gauge of the compensation mechanism.     

Crustal lineaments and shear zones in Africa: Their relationship to plate movements

Many of the major lineaments in southern Africa are major ductile shear zones with large displacement, occurring within, though often bounding orogenic belts. An example is the boundary to the Limpopo belt in Botswana and Zimbabwe. However, some of these shear zones only record slight displacement when considered on a crustal scale; they are merely planes recording differential movement on much larger, flat to gently dipping, shear zones where the boundary to the orogenic belt is a low-angle thrust zone. These different types of shear zones are clearly shown in the Pan-African belt of Zambia where large ENE-trending lineaments have been recorded. Recent work has shown the northern group of shears to be large lateral ramps; for example, the rocks of the copper belt are part of an ENE-verging thrust package, the southern boundary of which is a major, oblique to lateral ramp. In southern Zambia shears are more analogous to major transform faults; they form as tear faults separating zones of different thrust vergence. A possible plate tectonic model is given for this part of Africa, showing the different relative plate movement vectors estimated from the geometry of the Pan-African shear zones.     

Turbidites from the Clew Bay Complex,Ireland: provenance based on petrography,geochemistry and crustal residence values

The Clew Bay Complex in the Irish Caledonides represents a Caledonian element separating the Irish–Scottish Dalradian to the north from the South Mayo Ordovician arc basins to the south. The petrographic, geochemical and crustal residence characteristics of the Silurian sandstones within the complex are presented. Petrological analysis indicates that the sandstones were derived from a transitional continental provenance, possibly a passive continental margin or the cratonic flank of a foreland basin. Whole rock geochemistry confirms this provenance type and demonstrates the absence of any significant ophiolite detritus. Sm–Nd model ages indicate a possible derivation from Upper Dalradian or Torridonian rocks, although a mixed provenance may also be considered. These data indicate distinct differences between these turbidites and those of the nearby North Galway succession and suggest that the Clew Bay Complex could be regarded as a suspect terrane with respect to at least parts of the South Mayo arc zone. © 1996 John Wiley & Sons, Ltd.     

Observation of a flux transfer event on the earthward side of the magnetopause

The medium energy particle spectrometer (electrons of energy > 20 keV, protons > 25 keV) on board ISEE-2 has measured very similar pitch angle distributions and intensities during “flux transfer” events in the magnetosheath and events previously designated as “inclusion” events in the magnetosphere on a single pass through the magnetopause. This is interpreted as strong evidence that magnetic field lines in the magnetosphere can connect to field lines in the magnetosheath, at least locally and for brief times, allowing the same population ofparticles to be observed on both sides of the boundary. In addition, a simple mathematical model is provided incorporating a time constant for the process re-supplying particles to the open flux tube. The observed data are satisfactorily reproduced using a time constant of 46 s, which is comparable to the half-bounce time of protons at this position.     

Thermal response of Saturn's ring particles during and after eclipse

We present infrared (20 μm) observations of Saturn's rings for a solar elevation angle of 10° and phase angle of 6°. Scans across the rings yield information about the cooling of particles during eclipse and the subsequent heating along their orbits. All three rings exhibit significant cooling during eclipse, as well as a 20-μm brightness asymmetry between east and west ansae, the largest asymmetry occuring in the C ring (the brightest ring). The eclipse cooling is a simple and adequate explanation for 20-μm brightness asymmetries between the ansae of Saturn's rings. The relatively large C ring asymmetry is thought to be primarily due to the short travel time of the particles in that ring from eclipse exit to east ansa. We compare the B ring data to the theoretical models of H.H. Aumann and H.H. Kieffer (1973, Astrophys. J.186, 305–311) in order to set constraints on the average particle size and thermal inertia. The rather rapid heating after exit from eclipse points to low-conductivity-particle surfaces, similar to the water frost surfaces of Galilean satellites. If the surface conductivity is indeed low, one cannot determine an upper limit for the particle size through such infrared observations, since only the uppermost millimeters experience a thermal response during eclipse. However, based on these infrared data alone, it is clear that particles of radius equal to a few millimeters or less cannot occupy a significant fraction of the ring surface area, because-regardless of thermal inertia-their thermal response is much faster than observed.     

Modelling the effects of soil type and root distribution on shallow groundwater resources

Vegetated, shallow groundwater environments typically have high environmental and economic value. A sound understanding of the complex interactions and feedbacks between surface vegetation and groundwater resources is crucial to managing and maintaining healthy ecosystems while responding to human needs. A vegetated shallow groundwater environment was modelled using the software HYDRUS 2D to investigate the effects of several combinations of soil type and root distributions on shallow groundwater resources. Three rainfall regimes coupled to both natural and anthropogenically affected groundwater conditions were used to investigate the effect that combinations of four soil types and five root distributions can have on (a) groundwater level drops, (b) groundwater depletion, (c) groundwater recharge and (d) water stress conditions. Vegetation with roots distributed across the whole unsaturated zone and vegetation with dimorphic root systems (i.e. roots having larger concentrations both near the surface and the capillary fringe) behaved differently from vegetation growing roots mainly near the saturated zone. Specifically, vegetation with roots in the unsaturated zone caused water‐table drops and groundwater depletions that were half the amount due to deep‐rooted vegetation. Vegetation with a large portion of roots near the soil surface benefited from rainfall and was less vulnerable to water‐table lowering; as such, the fraction of the total area of roots affected by water stress conditions could be 40% smaller than in the case with deep‐rooted vegetation. However, roots uniformly distributed in the unsaturated zone could halve groundwater recharge rates observed in bare soils. Our analysis provided insights that can enable the formulation of site‐ and purpose‐specific management plans to respond to both human and ecosystem water requirements. Copyright © 2015 John Wiley & Sons, Ltd.