Turbulence in an almond orchard: Spatial variations in spectra and coherence

The spatial variability of turbulence in a fully-leafed almond orchard was studied. Two three-dimensional sonic anemometers were used to measure turbulence spectra and coherence at different vertical and lateral separations inside the canopy. Peak frequencies of the horizontal velocity components, normalized by local horizontal wind speed, are greater in the canopy crown than in the trunkspace. Peak-normalized frequencies for the vertical velocity power spectra are similar in the canopy crown and in the subcanopy trunkspace. Spectral slopes in the inertial subrange are more negative than those predicted with Kolmogorov's -2/3 theory. It is thought that the foliage elements act to short-circuit the eddy cascade. Lateral separation of the instruments in the subcanopy trunkspace has little effect on the shape of the velocity spectra. On the other hand, lateral and vertical velocity coherences between spatially separated sensors are low inside the canopy. These low coherences are due to the Eulerian length scales being of the same order of magnitude as the separation distances of the anemometers. Phase angles between velocity components are about zero for small separation distances. When the two instruments are separated by 9 m and one instrument is positioned in a row while the other is between two rows, vertical velocities are about 180 deg out of phase and the streamwise velocities are about 40 to 60 deg out of phase. These data support the contention that preferred differences occur between within- and between-row wind flow regimes.     

Improvement of ERS-1 orbits using along-track accelerations from DORIS data on SPOT2

In long-arc precise orbit determinations of altimetric satellites such as ERS-1, large errors may occur from mismodelling of aerodynamic drag and solar radiation pressure. Such surface forces for non-spherical satellites require accurate modelling of the effective area and particle-surface interactions, but the dominant source of error is neutral air density as derived from thermospheric models for aerodynamic drag. Several techniques can be employed to alleviate air-drag mismodelling but all require the solution of additional parameters from the tracking data. However, for ERS-1 the sparsity of laser range data limits the application of such empirical techniques. To overcome this, use can be made of the dense DORIS Doppler tracking for SPOT2 which is in a similar orbit to ERS-1. A recent investigation by CNES examined the use of drag scale factors from SPOT2 to constrain the ERS-1 orbit. An improvement to that methodology is to consider along-track mismodelling as observed by timing errors in the Doppler data for each pass of SPOT2. The along-track correction to the acceleration as derived from SPOT2 can then be applied to ERS-1 orbits, solving for a scale factor to absorb systematic errors - particularly that arising from the 50 km altitude difference. Results are presented of the associated improvement in ERS-1 orbits as derived from concurrent SPOT2 arcs. It will be seen that the procedure not only improves the laser range fit, but more importantly, leads to more precise radial positioning as evident in the altimeter and crossover residuals.     

The reserves of extracted resources: Historical data

The available data on known reserves of extracted resources have not previously been tabulated and graphed. In this article we compile the reserves data in one place for the first time. This serves to (1) show the trends in the reserves series, (2) make the series available to others, and (3) place the reserves data in the context of price data for which longer series have been available, and with which they can now be seen to be consistent in both showing increasing availability rather than increasing scarcity.     

Geochronology and sedimentology of the lower Passaic River,New Jersey

In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for ²¹⁰Pb activity at predetermined depths and the rate of sediment accretion (cm yr^?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from ²¹⁰Pb measurements (R_Pb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent ¹³⁷Cs analyses were focused on this depth interval. Sediment accretion rates derived from ¹³⁷Cs measurements (R_Cs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr^?1 to 10.2 cm yr^?1 and averaged 6.8 cm yr^?1. The R_Cs estimates for cores from this area ranged from 3.8 cm yr^?1 to 8.9 cm yr^?1 and averaged 6.6 cm yr^?1. The R_Cs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr^?1 and 0.66 cm yr^?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.     

Ultra-wideband radar studies of steep crested waves with scanning laser measurements of wave slope profiles

We report results of ultra wide-band radar sea spike experiments using steep and weakly breaking non-linear water surface features in a wave tank. To generate these features we used a 1 s paddle wave and wind waves for a sequence of wind speeds. A scanning laser was used to measure synchronously the surface slope profile across 12 cm along the wave propagation direction once per radar pulse. A time domain reflectometer (TDR) radar transmitted short horizontally polarized pulses at X-band, several hundred picoseconds long, to give a range resolution of 10 cm. A radar range of 36 cm was digitally sampled so that surface feature echoes could be tracked through the area continuously with 5 ms temporal resolution with each instrument. We report results considering the wave slope component in the propagation direction and the corresponding curvature component. For the conditions studied, two types of features which produce sea spike radar echoes were generated–a non-linear feature near the crest front of the wind wave, caused by extreme steepening as a result of the passage of the paddle wave, and a steepened blocked wind wave in the trough of the paddle wave, caused by the local orbital current of the 1 s wave being nearly equal to and opposite the phase velocity of the wind wave.     

Simple analytical Green's functions for ray perturbations in layered media

The total Green's function for two-point boundary-value problems can be related to the propagator for initial-value problems. A very simple expression for the Green's function is obtained when the unperturbed medium may be described by material with a constant gradient in quadratic slowness. The derivation requires a correct understanding of assumptions made in the propagator solution. Expressions are also obtained for Green's function in multilayered media.     

Volcanic geology and eruption frequency,São Miguel,Azores

Six volcanic zones comprise São Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoação caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoação caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on São Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150–250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau.     

Modeling basin-scale hydrology in support of physical climate and global biogeochemical studies: An example using the Zambezi River

Human activity is an important agent defining the contemporary hydrologic cycle. We have documented the potential impacts of impoundment, land use change and climate change on the Zambezi River system in southern Africa and found that they can be substantial. A full analysis requires construction and parameterization of a simulation for the entire catchment. This paper develops a strategy for implementing catchment-scale models of the major hydrologic processes operating within the basin. A coherent data set for calibrating the models has also been assembled. The algorithms consist of a Water Balance (WBM) and a Water Transport (WTM) operating at 1/2^o spatial scale and at monthly timesteps. These models transform complex patterns of regional climatology into estimates of soil water, evapotranspiration, runoff, and discharge through rivers of various size. The models are dependent on the characteristics of the terrestrial surface, principally soil texture and land cover. A simulated river network is also required. Additional tabular data sets are essential for model testing and calibration. These include subcatchment areas; observed river discharge at selected points; flooding, storage and loss characteristics of major wetlands; floodwave translation; and, volume, surface area, withdrawal and evaporative losses from impoundments. An important design consideration for the numerous impoundments in the Zambezi requires an understanding of the seasonal variation in discharge, in particular how it might respond to climate and land use change. The research strategy offered here lays a framework for addressing such issues. Although the primary focus of this work is hydrologic, we discuss how the model can be extended to consider constituent transport and biogeochemical cycling issues at the continental scale.