Weekend–weekday aerosols and geographic variability in cloud-to-ground lightning for the urban region of Atlanta, Georgia, USA

We characterized the differences in warm-season weekday and weekend aerosol conditions and cloud-to-ground (CG) flashes (1995–2008) for an 80,000 square kilometer region around Atlanta, Georgia, a city of 5.5 million in the humid subtropics of the southeastern United States. An integration of distance-based multivariate techniques (hierarchical agglomerative clustering, multiresponse permutation procedures, fuzzy kappa statistics, and Mantel tests) indicated a greater concentration of CG flash activity within a 100 km radius around Atlanta under weekday aerosol concentrations. On weekends, these effects contracted toward the city. This minimized any weekly anthropogenic cycle over the more densely populated urban center even though this location had a higher flash density, a higher percentage of days with flashes, and stronger peak currents over the course of a week compared to the surrounding region. The sharper contrasts in weekday and weekend lightning regime developed outside the perimeter of the city over nonurban land uses. Here, lightning on weekend and weekdays differed more in its density, frequency, polarity, and peak current. Across the full extent of the study region, weekday peak currents were stronger and flash days more frequent, suggesting that weekly CG lightning signals have a regional component not tied to a single city source. We integrate these findings in a conceptual model that illustrates the dependency of weekly anthropogenic weather signals on spatial and temporal extent.     

Nomadic pastoralists adopt subsistence strategies that maximise long-term household survival

Summary Optimality approaches have been used to investigate the adaptiveness of human subsistence strategies mainly in hunter-gatherer societies. However the static optimality models used are not suitable for investigating the long-term costs and benefits of decisions, especially in societies where wealth is accumulated and hence resources gained in one year have an influence on future years. Here I use dynamic optimality modelling to investigate the adaptiveness of a subsistence strategy in the Gabbra, a nomadic pastoralist group. I show that herders forego short-term gain in favour of long-term household survival. This is done by herders sometimes manipulating the life histories of their sheep. In herds where the breeding rate has been slowed, by restricting ewes access to males, females have greater longevity, but fewer offspring per year, than in unmanipulated herds. Models maximising offtake from the herd predict that herd breeding-rate should never be slowed. Models maximising short-term household survival predict herd-breeding rate should always be slowed. Models of long-term household survival predict that herd-breeding rate should be slowed only by relatively wealthy households. This is the behaviour observed. Poor and wealthy house-holds adopt different behaviours, yet all are following optimal strategies for their own level of wealth, that maximise their long-term survival. This is the behaviour that would be predicted on the basis of evolutionary theory, given that a family takes many years to be raised. Correspondence to: R. Mace     

Computer-aided classification of forest cover types from small scale aerial photography

The US National Park Service must map forest cover types over extensive areas in order to fulfill its goal of maintaining or reconstructing presettlement vegetation within national parks and monuments. Furthermore, such cover type maps must be updated on a regular basis to document vegetation changes. Computer-aided classification of small scale aerial photography is a promising technique for generating forest cover type maps efficiently and inexpensively. In this study, seven cover types were classified with an overall accuracy of 62 percent from a reproduction of a 1120,000 color infrared transparency of a conifer-hardwood forest. The results were encouraging, given the degraded quality of the photograph and the fact that features were not centered, as well as the lack of information on lens vignetting characteristics to make corrections. Suggestions are made for resolving these problems in future research and applications. In addition, it is hypothesized that the overall accuracy is artificially low because the computer-aided classification more accurately portrayed the intermixing of cover types than the hand-drawn maps to which it was compared.     

FACTORS AFFECTING SPRING RUNOFF ON TWO FORESTED WATERSHEDS1

ABSTRACT Spring runoff from two forested watersheds in northern Minnesota is a function of annual snowfall, soil water recharge, and water supply rates. A drainage basin with a clay soil and a hardwood overstory had greater snowmelt and water supply rates than another drainage basin with a sandy soil and conifer overstory. The average soil water recharge rate for the clay soil was 28 percent less than for the sandy soil. The lower recharge rate of the clay soil resulted in spring runoff which averaged 40 percent of water supplied during the three year study while an average of two percent was produced on the sandy soil. Soil frost which affected soil water recharge varied between soil types and was influenced by amount of soil water storage and snow cover.