TVZHR profile for the 1991 Perseid shower

Image intensified video detection systems were used to observe the 1991 Perseid meteor shower from two locations in eastern Canada. In 29.6 hours of total observing time a total of 668 meteors were detected, of which 403 were Perseids. We derived a profile of TVZHR (television zenithal hourly rate) values for the 1991 Perseid shower over the solar longitude (epoch 2000) interval 138°51 to 141°01. The apparent limiting stellar magnitudes of the observing systems were +9.4 and +8.8 (corresponding to limiting meteor magnitudes for our geometry ranging from +8.7 to +7.0). Within the observing period, the maximum TVZHR rate was approximately 1600, and occurred at solar longitude 139.9°. This is in good agreement with the second peak observed by visual observers. The data suggest that TVZHR values should be divided by a factor of approximately 5 to compare TVZHR and ZHR values.     

Portable autonomous vertical profiler for estuarine applications

The design and implementation of a portable autonomous vertical profiler are documented and example data sets from a mesotidal estuary and a microtidal, wind-driven estuary are presented. The profiler sampling range dynamically adjusts to changes in water column depth, and a typical vertical sampling resolution of 4 cm is attained. Example data show the onset and vertical and temporal extent of stratification in the water column. The profiler is ideal for water quality monitoring programs in shallow estuaries that require frequent profiles of hydrographic, chemical, and biological parameters that can be measured in situ.     

The influence of floodplain geohydrology on the distribution of Sarcocornia pillansii in the Olifants Estuary on the West Coast, South Africa

The distribution of Sarcocornia pillansii (Moss) A.J. Scott was determined by water-table depth and electrical conductivity (EC) of the groundwater. Where the groundwater was accessible (<1.5 m) and had a low EC (<80 mS cm⁻¹), S. pillansii extended its roots down to the water-table where a suitable water potential gradient was shown to exist between the soil and roots. In areas where the groundwater was too deep and/or hypersaline, the plants grew on hummocks. The unconfined aquifer below the floodplain is linked to the estuary and although diurnal tidal waves were dampened, water-table level fluctuations were recorded between tidal events. The complex geomorphology of the floodplain influences groundwater flow, in turn affecting the distribution of the salt marsh vegetation.     

Transformation of dilative and contractive landslide debris into debris flows—An example from marin County, California

The severe rainstorm of January 3, 4 and 5, 1982, in the San Francisco Bay area, California, produced numerous landslides, many of which transformed into damaging debris flows. The process of transformation was studied in detail at one site where only part of a landslide mobilized into several episodes of debris flow. The focus of our investigation was to learn whether the landslide debris dilated or contracted during the transformation from slide to flow.

The landslide debris consisted of sandy colluvium that was separable into three soil horizons that occupied the axis of a small topographic swale. Failure involved the entire thickness of colluvium; however, over parts of the landslide, the soil A-horizon failed separately from the remainder of the colluvium.

Undisturbed samples were taken for density measurements from outside the landslide, from the failure zone and overlying material from the part of the landslide that did not mobilize into debris flows, and from the debris-flow deposits. The soil A-horizon was contractive and mobilized to flows in a process analogous to liquefaction of loose, granular soils during earthquakes. The soil B- and C-horizons were dilative and underwent 2 to 5% volumetric expansion during landslide movement that permitted mobilization of debris-flow episodes.

Several criteria can be used in the field to differentiate between contractive and dilative behavior including lag time between landsliding and mobilization of flow, episodic mobilization of flows, and partial or complete transformation of the landslide.     

The characterization of axenic culture systems suitable for plant propagation and experimental studies of the submersed aquatic angiospermPotamogeton pectinatus (Sago Pondweed)

Clonal lines of the submersed aquatic angiospermPotamogeton pectinatus were grown in three culture systems. The first, which used sucrose as a carbon source in a liquid medium, supported vigorous vegetative growth and can be used to propagate large numbers of plants in axenic conditions. In this culture system, plants were responsive to increasings photosynthetically active radiation (PAR) photon flux density (PFD) and were photosynthetically competent. However, their growth was heterotrophic and root development was poor. When these plants were transferred to a second nonaxenic culture system, which used 16-1 buckets containing artificial sediments and tap water, growth was autotrophic and plants were morphologically identical to field-harvestedP. pectinatus. The last culture system which consisted of a sand substrate and inorganic nutrient bathing solution aerated with 135 ml min^?1 ambient air enhanced to 3.0% CO₂ was axenic and supported autotrophic growth by plants that were also morphologically normal.     

A reassessment of the economic effects of global climate change on U.S. agriculture

This study uses recent GCM forecasts, improved plant science and water supply data and refined economic modeling capabilities to reassess the economic consequences of long-term climate change on U.S. agriculture. Changes in crop yields, crop water demand and irrigation water arising from climate change result in changes in economic welfare. Economic consequences of the three GCM scenarios are mixed; GISS and GFDL-QFlux result in aggregate economic gains, UKMO implies losses. As in previous studies, the yield enhancing effects of atmospheric CO₂ are an important determinant of potential economic consequences. Inclusion of changes in world food production and associated export changes generally have a positive affect on U.S. agriculture. As with previous studies, the magnitude of economic effects estimated here are a small percentage of U.S. agricultural value.     

The Use of Invertebrates in Ground Water Monitoring: A Rising Research Field

The use of invertebrates as biomonitors of ground water quality is a relatively new approach that has come of age with the development of ground water ecology. The benefits of such an approach are illustrated by four examples of field biomonitoring from several sites in various hydrogeological settings. Contamination of the interstitial zone by heavy metals in some sectors of the Rhóne River (France) was shown by the scarcity of insect species; sewage pollution in the saturated zone of a karstic aquifer was indicated by the low relative abundances of stygobites as compared with those of stygophiles and stygoxenes; and enrichment with organic matter of an underflow was clearly demonstrated by the extremely high density of ground water invertebrates such as oligochaetes, ostracods, and isopods. Examination of the spatial changes in the composition and abundance of invertebrate assemblages was also useful in determining the direction and intensity of water fluxes between a river and its underflow, as well as in delineating the reduced or oxidized zones in a manganese-polluted aquifer. Finally, the selected case studies emphasized the variety of methodological approaches that could be developed in ground water contamination biomonitoring, as well as the complementary and sometimes new information provided by this innovative method in comparison with that obtained by conventional pollution monitoring techniques.