A stochastic distributed model of soil erosion by overland flow

A model has been built to simulate runoff and erosion on slopes of varying form with soil of varying hydraulic conductivity, porosity, storage capacity, and particle size distribution. The model is based on known physical laws and takes into account spatially dependent random variation in the initial conditions of the soil. The rainfall is similarly both random and autocorrelated in space. Detailed results show how runoff is generated as a result of slow conductivity and high water table, how this leads to erosion, the degradation of the land surface, the capture of flow lines, and the development of networks of rills. The last accords well with results from laboratory experiments. Changing the initial conditions produces results that accord with intuition and reality.     

A note on quaternary climate modelling using Boolean delay equations

The Boolean delay equation model of Ghil et al. for the study of Quaternary ice ages has been re-examined and also extended to include a hydrological feedback mechanism that directly influences the thermohaline circulation. For the basic Ghil et al. model (with the original and with corrected time delays), we show that the maximum duration for a high ice volume state depends continuously but not monotonically on the time delays of the problem. In particular, both short spiky glacials and longer glacials can be obtained from identical parameter values by simply choosing different initial conditions. In the extended model, we find that an additional temperature-hydrology-ocean feedback mechanism tends to generate longer glacials, but in neither case do the average time scales of model variability compare favourably with those of the major Quarternary glacials.     

Correlation of the lower devonian rocks of australasia

Alkali basaltic rocks from the Southern Highlands, N.S.W., contain oxide phases of both high and low pressure origin. The two phases are readily distinguished using chemical and textural criteria.

Chemical data for low‐pressure Fe‐Ti oxides indicate that oxygen fugacities of the host lavas range from 10‐^12.8 to 10‐⁸ atm at 950° to 1110°C. In most cases, the oxygen fugacities of the individual lava flows appear to be principally a function of temperature and intrinsic chemical equilibria existing at the time of formation of the basaltic liquid. However, some relatively differentiated flows shows a high degree of oxidation due to volatile enrichment with fractionation. Rare glassy flows show dendritic crystallization of Fe‐Ti oxides. Most flows in which abundant olivine was the first phase to be precipitated also contain Cr‐rich spinels associated, and apparently coeval, with the earliest‐crystallizing olivine.     

Combining Metric Aerial Photography and Near‐Infrared Videography to Define Within‐Field Soil Sampling Frameworks

Abstract

This paper investigates the combination of metric aerial photography and near‐infrared (NIR) videography data to improve the design of field‐survey sampling frameworks. Spatial data collection can contribute up to 80% of the cost of deploying a Geographic Information System (GIS) based Decision Support System (DSS). The use of remotely sensed information, field survey using differential Global Positioning System (dGPS) and geostatistical interpolation methods maximises data quality for a given rate of sampling.

Medium‐format colour aerial photography and NIR videography were orthorectified to the national map base and mosaiced using ERDAS Imagine. The green and red layers of the aerial photography were combined with the NIR videography to form a false‐colour composite image. Two sampling strategies were tested. The first stratified sampling on a per field basis, creating four points per hectare, randomly located within each field. The second strategy used the remotely sensed information to identify within‐field variability classes for each field, using red‐green difference or normalised difference vegetation index (NDVI) models. These variability classes were used as a sub‐stratification framework with each class sampled at the same rate of 4 per hectare. For both strategies the sample points were generated within ESRI ArcView and were located in the field using dGPS. Maps of stone content were created using geostatistical methods and validated against samples collected on a 100 metre grid. It was concluded that combining the two image sources to create a within‐field stratification framework improved the precision of the results obtained from field‐survey.     

Vapor Intrusion Screening at Petroleum UST Sites

Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m³ and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m³ at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m³ for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.     

CHANGING SPATIAL CHARACTERISTICS OF WILLAMETTE VALLEY FARMS

This article reflects on the racial configuration of urban space. Previous research tends to posit racial segregation and diversity as either endpoints on a continuum of racial dominance or mirror images of one another. We argue that segregation and diversity must be jointly understood; they are necessarily related, although not inevitably as binary opposites. Our view is that the neighborhood geographies of U.S. metropolitan areas are simultaneously and increasingly marked by both racial segregation and racial diversity. We offer an approach that classifies neighborhoods based jointly on their compositional diversity and their racial dominance, illustrated by an examination of the neighborhood racial structure of several large metropolitan areas for 1990 and 2000. Compositional diversity increased in all metropolitan areas in ways rendered visible by our approach, including a sharp reduction in the number of highly segregated white neighborhoods, transitioning mostly into moderately diverse yet still white-dominated neighborhoods, and a fourfold increase in the number of highly diverse neighborhoods. Even so, many highly segregated spaces remain, especially for whites and blacks. Latino-dominated spaces show a mix of persistence and emergence. Although compositional diversity is increasing, highly diverse neighborhoods are still rare and are the least persistent of all racial configurations. Our approach clearly demonstrates the “both/and”-ness of segregation and diversity.