Anthropogenic Impacts on Summer Precipitation in Central Arizona,U.S.A.

This article explores the possibility of urbanization‐ and irrigation‐induced increases in summer precipitation totals in central Arizona. Maximum precipitation impacts are hypothesized to occur downwind of the Phoenix area in the Lower Verde basin. Results from statistical tests indicate that summer precipitation totals in the Lower Verde basin are greater than totals in nearby basins. Precipitation totals in the basin also appear to be equivalent to totals at more monsoon‐impacted stations in eastern Arizona. While this research is preliminary, the results do provide encouraging evidence of the existence of anthropogenically enhanced summer precipitation in central Arizona.     

Drainage in finite-sized unsaturated zones

After the initiation of gravity drainage, water is often assumed to be either (a) draining under unit gradient, or (b) at capillary/gravity equilibrium. Both of these simplifications can be useful, but the regimes of validity of each assumption must be delineated. Water pressures are measured versus time and distance as water drains out of a 1.6 m long sand column to determine the relative effects of capillary and gravitational forces during drainage. For medium sized sands (0.15–0.3 mm in diameter), the capillary pressure is constant in space in a large region of the column for over 12 days, and the water continues to flow under unit gradient for relatively long time scales. Similar results are seen for finer sands, but with a much faster approach to equilibrium. Numerical simulations and analytical estimates are presented and compare favorably to the measurements. Together, the experimental, theoretical and analytical results are used to calculate when capillary/gravity equilibrium is reached as a function of porous media properties and length of the unsaturated zone. The ratio of the length of the unsaturated zone to the bubbling pressure is a key parameter in determining the drainage regime, and that even for relatively short unsaturated zones the equilibrium time scale can be on the order of years.     

Continuous wavelet-like filter for a spherical surface and its application to localized admittance function on Mars

We have developed a 2D isotropic continuous wavelet-like transform for a spherical surface. The transform is simply defined as the surface convolution between the original field and a kernel, based on the zeroth-order Bessel function with a spherical correction. This spherical correction violates the geometric similarity for the various scales of the kernels, which becomes more apparent at longer wavelengths. We found numerically that this transform is practically equivalent to a Gaussian bandpass filter in the spherical harmonic domain. We have applied this wavelet-like transform on the recently acquired Martian gravity and topography fields. Using a ratio constructed locally from these two fields, we have constructed a map describing the lateral variations of the localized admittance function on Mars.     

Organic litter: dominance over stones as a source of interrill flow roughness on low‐gradient desert slopes at Fowlers Gap,arid western NSW,Australia

Thirty‐six runoff plot experiments provide data on flow depths, speeds, and Darcy–Weisbach friction coefficients (f) on bare soil surfaces, and surfaces to which were added sufficient extra plant litter or surface stones to provide projected cover of 5, 10 and 20 per cent. Precision flow depth data were derived with a computer‐controlled gantry and needle gauge for two different discharges for each plot treatment. Taking a fixed flow intensity (Reynolds number, Re = 150) for purposes of comparison shows means of f = 17·7 for bare soil surfaces, f = 11·4 for added stone treatments, and f = 23·8 for added litter treatments. Many individual values of f for stone treatments are lower than for the bare soil surface, but all litter treatments show increases in fcompared to bare soil. The lowering of f in stone treatments relates to the submerged volume that the stones occupied, and the associated concentration of flow onto a smaller part of the plot surface. This leads to locally higher flow intensities and lower frictional drag along threads of flow that the obstacles create. Litter causes higher frictional drag because the particles are smaller, and, for the same cover fraction, are 100 times more numerous and provide 20 times the edge or perimeter length. Along these edges, which in total exceed 2·5 m g^?1 (equivalent to 500 m m^?2 for a loading of 2 t ha^?1), surface tension draws up water from between the litter particles. This reduces flow depth there, and as a consequence of the lower flow intensity, frictional drag rises. Furthermore, no clear passage remains for the establishment of flow threads. These findings apply to shallow interrill flows in which litter is largely immobile. The key new result from these experiments is that under these conditions, a 20 per cent cover of organic litter can generate interrill frictional retardation that exceeds by nearly 41 per cent that of a bare soil surface, and twice that contributed by the same cover fraction of surface stones. Even greater dominance by litter can be anticipated at the many dryland sites where litter covers exceed those tested here. Copyright © 2002 John Wiley & Sons, Ltd.     

Web-Based interrogation of large-scale geophysical data sets from handheld devices

This paper describes the issues related to using handheld devices to interrogate wirelessly large-scale geophysical data over the Web using a client–server configuration. A modification of our current Web-based data interrogation program addresses specifically the limitations of current handheld devices with wireless access, such as processor speed, screen size and resolution, memory, and network speeds. The system uses a combination of Java, Python, and C in order to run remotely from the handheld device, while incorporating the power and memory capacity of a large workstation server.     

Smoothing and Change Point Detection for Gamma Ray Count Data

Gamma ray detectors are used to measure the natural radioactivity of rocks. For a number of boreholes drilled at a site the gamma ray detector is lowered into each borehole and counts of gamma ray emissions at different depths are recorded as the instrument is gradually raised to ground level. The profile of gamma counts can be informative about the geology at each location. The raw count data are highly variable, and in this paper we describe the use of adaptive smoothing techniques and change point models in order to identify changes in the geology based on the gamma logs. We formulate all our models for the data in the framework of the class of generalized linear models, and describe computational methods for Bayesian inference and model selection for generalized linear models that improve on existing techniques. Application is made to gamma ray data from the Castelreagh Waste Management Centre which served as a hazardous waste disposal facility for the Sydney region between March 1974 and August 1998. Understanding the geological structure of this site is important for further modelling the transport of pollutants beneath the waste disposal area.     

Are catch-up reefs an artefact of coring?

Drill cores through modern coral reefs commonly show a time lag in reef initiation followed by a phase of rapid accretion to sea level from submerged foundations – the so-called ‘catch-up response’. But because of the difficulty of drilling in these environments, core distribution is usually restricted to accessible areas that may not fully represent reef history, especially if the reef initiated in patches or developed with a prograde or retrograde geometry. As a consequence, core data have the potential to give a misleading impression of reef development, particularly with respect to the timing of initiation and response to sea-level rise. Here, we use computer models to simulate keep-up reef development and, from them, quantify variations in the timing of reef initiation and accretion rate using mock cores taken through the completed simulations. The results demonstrate that cores consistently underestimate the timing of reef initiation and overestimate the reef accretion rate so that, statistically, a core through a keep-up reef will most likely produce a catch-up pattern – an initiation lag followed by a phase of rapid accretion to sea level. This implies that catch-up signatures may be an artefact of coring and that keep-up reefs are significantly more common than previous core studies claim.     

Papua New Guinea Highlands: palaeomagnetic constraints on terrane tectonics

Oblique convergence since the Early Cenozoic between the northward-moving Australian plate, westward-moving Pacific plate and almost stationary Eurasian plate has created a world-ranking tectonic zone in the eastern Indonesia–New Guinea–Southwest Pacific region (Tonga–Sulawesi megashear) that is notorious for its complex mix of tectonic styles and terrane juxtapositions. Unlike an ancient analog—the Mesozoic–Cenozoic Cordillera of North America—palaeomagnetic constraints on terrane motions in the zone are few. To improve the framework of quantitative control on such motions and therefore our understanding of the development of the zone, results of a palaeomagnetic study in the Highlands region of Papua New Guinea (PNG), in the southern part of the New Guinea Orogen, are reported. The study yields new insights into terrane tectonics along the Australian craton's active northern margin and confirms the complexity of block rotations to be expected at the local scale in tectonically intricate zones. The study is based on more than 500 samples (21 localities) collected from an interior and an exterior zone of New Guinea's central cordillera. The two zones are separated by the Tahin and Stolle–Lagaip–Kaugel Fault zones and collectively represent the para-autochthonous northern margin of the Australian craton. Samples from the interior zone, which in the study area comprises a cratonic spur of uncertain—probably displaced—origin, come from Triassic to Miocene sediments and subordinate volcanics of the Kubor Anticline, Jimi Terrane, and Yaveufa Syncline (16 localities) in the central and eastern Highlands. Samples from the exterior zone, which represent a basement-involved, Pliocene foreland fold-and-thrust belt, come from Middle Eocene to Middle Miocene carbonates and clastics (five localities) in the southern Highlands of the Papuan Fold Belt. Results permit us to constrain the tectonic evolution of the two zones palaeomagnetically. Using mainly thermal demagnetization techniques, three main magnetic components have been identified in the collection: (1) a recent field overprint of both normal and reverse polarity; (2) a pervasive overprint of mainly normal polarity that originated during extensive Middle to Late Miocene intrusive activity in the central cordillera; and (3) a primary component which has been identified in only 7 of the 21 localities (5 of 11 stratigraphic units represented in the collection). All components show patterns of rotation that are consistent within the zones, but differ between them. In the interior zone (central and eastern Highlands), large-scale counterclockwise rotations of between 30°+ and 100°+ have been established throughout the Kubor Anticline and Jimi Terrane, with some clockwise rotation present in the southern part of the Yaveufa Syncline. In contrast, in the Mendi area of the exterior zone (southern Highlands), clockwise rotations of between 30°+ and 50°+ can be recognized. These contrasting rotation patterns across the Tahin and Stolle–Lagaip–Kaugel Fault zones indicate decoupling of the two tectonic zones, probably along basement-involved faults. The clockwise rotations in the southern Highlands of the Papuan Fold Belt are to be expected from its structural grain, and are probably governed by regional basement faults and transverse lineaments. In contrast, the pattern of counterclockwise rotations in the Kubor Anticline–Jimi Terrane cratonic spur of the central and eastern Highlands was unexpected. The pattern is interpreted to result from non-rigid rotation of continental terranes as they were transported westward across the northeastern margin of the Australian craton. This margin became reorganised after the Middle Miocene, when the steadily northward-advancing Australian craton impinged into the westward-moving Pacific plate/buffer-plate system. Transpressional reorganisation under the influence of the sinistral Tonga–Sulawesi megashear became enhanced with Mio-Pliocene docking, and subsequent southward overthrusting, of the Finisterre Terrane onto the northeastern margin of the Australian craton.     

Climatic and volcanic forcing revealed in a 50,000-year diatom record from Lake Massoko, Tanzania

The interactions between climatic and volcanic forcing on diatom communities contained in a 50,000-year sedimentary sequence from Lake Massoko, Tanzania, were examined. At the century scale, 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels, whereas at the millennial scale, diatom-inferred shifts in precipitation–evaporation based on conductivity optima and diatom life-form ratios were broadly similar to lake-level reconstructions from Lake Rukwa, Lake Malawi, and others in the region. Some fluctuations of Lake Massoko are consistent with the precession-driven changes in insolation, but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone. Sea surface temperatures are strongly correlated to multimillennial-scale climate patterns over this region of Africa.