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Drought termination: Concept and characterisation

Progress in physical geography - Pon., 11/28/2016 - 14:55

There are numerous anecdotal examples of drought terminations documented throughout the historical record on most continents. The end of a drought is the critical time during which water resource managers urgently require information on the replenishment of supplies. Yet this phase has been relatively neglected by the academic community, with much of the existing body of research on drought termination assessing the likelihood of droughts ending rather than its temporal profile. In particular, there has been little effort to characterise drought termination events themselves. This is partly explained by existing definitions of drought termination as a specific point in time when drought is considered to have finished, rather than a more holistic consideration based on approaches developed within biological sciences. There is also a lack of understanding about how drought termination propagates through the hydrological cycle. This paper specifically examines and reviews available research on drought termination, highlighting limitations associated with current definitions and offering suggestions for characterising the temporal stages of drought. An alternative definition of drought termination is proposed: a period between the maximum negative anomaly and a return to above-average conditions. Once this phase has been delineated, the duration, rate and seasonality of drought termination can be derived. The utility of these metrics is illustrated through a case study of the 2010–2012 drought in the UK, and the propagation of drought termination between river flows and groundwater levels.

Effectiveness of debris flow mitigation strategies in mountainous regions

Progress in physical geography - Pon., 11/28/2016 - 14:55

Debris flows represent major hazards in most mountainous regions of the world where they repeatedly result in disasters. In order to protect people and infrastructure against future debris flows, many debris flow catchments have been artificially intervened by employing various mitigation measures, including civil engineering works. However, the commonly adapted engineering measures, such as check dams, are not effective for every debris flow catchment, and the failure of such measures even causes more damage, e.g. the Sanyanyu debris flow catchment in Zhouqu, China, killed 1756 people. In order to research the effectiveness of engineering strategies and explore much more effective mitigation works for debris flows in the mountainous regions, we took the Bailong River catchment of Southern Gansu of China as study area, with special emphasis on Sanyanyu debris flow catchment (with civil engineering works) and Goulinping debris flow catchment (without civil engineering works), and comparatively analysed the two catchments. The comparative results show that both catchments have similar material source, geomorphological/environmental and climatic conditions, however, vegetation cover and rock hardness are poorer in Goulinping than in Sanyanyu, the catchment that underwent larger-scale debris flows, suggesting that the mitigation measures had been applied in Sanyanyu catchment were inappropriate. Subsequently, we simulated the effectiveness of controlling debris flow peak discharge with check dams at the lower part of Sanyanyu and Goulinping catchment using the Kanako simulator, and summarised argument based on the hypothesis and facts from positive and negative aspects. We draw the conclusion that it is not reasonable to build check dams in the two catchments and instead, drainage channels should be primarily considered for reducing debris flow hazards in such densely populated areas. Finally, we undertook detailed field investigations and experiments on the native plants in the region, and found that the ecological mitigation measure with planting Robinia Pseudoacacia on the debris flow deposits is an effective method to alleviate debris flow hazards. It is concluded that channel works combined with ecological measures are the preferable approaches to minimize the debris flow damage in debris flow catchments characterised with high mountains, concentrated rainfalls and active neotectonic movement.

Degradation of frigid swampy meadows on the Qinghai-Tibet Plateau: Current status and future directions of research

Progress in physical geography - Pon., 11/28/2016 - 14:55

The frigid swampy meadows of the Qinghai–Tibet Plateau in western China have suffered widespread degradation. The severe degradation of such meadows can lead to the formation of a landscape commonly known as heitutan. Much research has been undertaken to study its triggers, formation mechanism, grading of severity, and rehabilitation. These studies are comprehensively reviewed in this paper. In particular, it attempts to reconcile two divergent theories on heitutan formation by proposing a new model to elucidate the role of a given factor at each stage of degradation. In this model, climate desiccation is the most important in transforming healthy meadow to dry grassland, while rodent attack is the most important in worsening dry grassland to degraded meadow. Overgrazing is the most important factor during the early stages of degradation. Wind and water erosion is important during the final stage, when a large quantity of loose materials has been accumulated by rodent burrowing. Also covered in this paper is how to rehabilitate heitutan to productive use, including the effectiveness of various measures. Finally, this paper identifies the gap in our understanding of swampy meadow degradation and spells out a future research agenda. It is concluded that future research on heitutan will undergo a transformation from descriptive to quantitative. The expected research outcome will inform herders of the appropriate strategies that can be adopted to prevent heitutan formation. This proactive approach will minimize economic loss from swampy meadow degradation and reduce its adverse impact on the environment.

Beyond the Bioclimatic Law: Geographic adaptation patterns of temperate plant phenology

Progress in physical geography - Pon., 11/28/2016 - 14:55

Almost a century ago, observed geographic patterns of plant phenology (such as leaf-out and flowering) were summarized in Hopkins’ Bioclimatic Law. This law describes phenology as varying along climatic gradients by latitude, longitude, and altitude. Yet phenological patterns are not only affected by contemporary climatic differences across space, but also by underlying geographic variations in plant genetics that arise from long-term climatic adaptation. The latter influence on geographic patterns in phenology has been undervalued to this day, mainly due to the difficulty of quantifying it. This study outlines a methodology for bridging this knowledge gap through delineating geographic adaption patterns using common garden and cloned plant phenology. Through synthesizing existing literature, typical geographic adaptation patterns in both spring and autumn phenology of many temperate tree species are identified. Under uniform environment, spring leaf-out of colder climate-adapted populations of a certain species is either earlier than warmer climate-adapted ones due to lower thermal requirements, or later because of higher chilling (for dormancy release) demands. The former leads to a countergradient pattern as it is opposite to an in situ observation, while the latter leads to a cogradient pattern. Autumn leaf senescence, on the other hand, expresses a consistent cogradient pattern that is related to latitude and constrained by the populations’ varied photoperiod requirements. These geographic adaptation patterns allow a clearer understanding of geographical variations in phenological responses to climate change, and provide a theoretical basis for spatially explicit phenological models. In addition, given that these adaptive patterns reveal genotype-based variabilities, they are potentially useful for more accurately tracking phenology-dependent ecosystem processes (e.g. species distribution) and non-weather-related vegetation changes. As a unique subfield of physical geography with broad environmental implications, this line of research needs to be further developed by furnishing a stronger and more explicit spatial structure into current phenological studies.

Pronival ramparts: A review

Progress in physical geography - Pon., 11/28/2016 - 14:55

Pronival ramparts are debris ridges formed at the downslope margins of perennial or semi-permanent snowbeds beneath bedrock cliffs. These landforms, also previously known as protalus ramparts, are located in periglacial environments, but the apparent simplicity of rampart formation made these landforms far less interesting than other modified forms of talus in cold environments. As a result, limited research, use of supposed relict examples and assumed formative mechanisms led to the misidentification of ramparts, circular arguments regarding genesis and inappropriate palaeo-environmental inferences. Several advances have, however, been made in the past few decades, particularly where actively-forming ramparts have been studied. Thus, this paper provides a review of research on pronival ramparts. In particular, focus is placed on the advances made in our understanding of rampart genesis, identification (diagnostic criteria) and palaeo-environmental significance. Notable advances include the development of a retrogressive model of rampart genesis to supplement the conventional downslope model of development, revised diagnostic criteria for field identification and the use of calibration equations during Schmidt-hammer exposure dating of pronival rampart. The use of pronival ramparts as palaeo-environmental indicators is also examined to determine what relict examples of these landforms may reveal about past climates.

The influence of Weaver and Dales (1978) paper, "Trampling effects of hikers, motorcycles and horses in meadows and forests"

Progress in physical geography - Pon., 11/28/2016 - 14:55

Trampling of vegetation and soils by recreational activities is a recurrent concern in many lands where trail use is common. The degree of impact can vary with the myriad types of trail users. The Weaver and Dale article, "Trampling effects of hikers, motorcycles and horses in meadows and forests", was the first to provide some insights about how different trampling agents might produce varied impacts. Weaver and Dale showed that horses and motorcycles produced greater impacts to soils and vegetation than hikers. This article appeared early in a relatively young field of study. The direct results of their research are still valuable today, but the greater impact of their paper was that it ushered in a phase of experimental trampling research that is ongoing. Because of their research and continual influence, land managers can now make better decisions about what type of recreation activity to discourage or encourage.