Thresholds,mode switching,and emergent equilibrium in geomorphic systems

Landform and landscape evolution may be convergent, whereby initial differences and irregularities are (on average) reduced and smoothed, or divergent, with increasing variation and irregularity. Convergent and divergent evolution are directly related to dynamical (in)stability. Unstable interactions among geomorphic system components tend to dominate in earlier stages of development, while stable limits often become dominant in later stages. This results in mode switching, from unstable, divergent to stable, convergent development. Divergent‐to‐convergent mode switches emerge from a common structure in many geomorphic systems: mutually reinforcing or competitive interrelationships among system components, and negative self‐effects limiting individual components. When the interactions between components are dominant, divergent evolution occurs. As threshold limits to divergent development are approached, self‐limiting effects become more important, triggering a switch to convergence. The mode shift is an emergent phenomenon, arising from basic principles of threshold modulation and gradient selection. As an example, the relationships among flow concentration, erosive force, and channel incision in fluvial systems are examined in the context of mode switching and thresholds. The commonly observed divergence in channel growth and fluvial dissection and network development, eventually transitioning to a stable, convergent configuration, is an emergent outcome of gradient selection and threshold modification, and does not imply any goal functions of balancing mass fluxes or limiting change. Copyright © 2013 John Wiley & Sons, Ltd.     

Some rainfall-related thresholds for erosion and sediment yield in the upper Yangtze River basin

This study examines rainfall thresholds for erosion and sediment yield in the upper Yangtze River basin. Sediment reduction effects of soil conservation measures depend on the magnitudes of rainstorm. When the latter is less than a critical threshold, sediment reduction effects of soil conservation measures are positive; when this magnitude is exceeded, the effect is negative. An analysis based on data from the Jialingjiang River shows that the sediment reduction by soil conservation measures increased with annual precipitation to a peak, and then decreased to a negative value. The annual precipitation at the peak and zero values of sediment reduction are 970 and 1,180 mm, respectively, which can be regarded as two thresholds. Annual precipitation at the zero-value of sediment reduction has a return period of 25 years. In general, the design standard of soil conservation works in China is related with rainstorms with return periods of 10–20 years. When the magnitude of rainstorm exceeds this, the soil conservation works may be partly or totally destroyed by rainstorms, and the previously trapped sediment may be released, resulting in a sharp increase in sediment yield. It was also found in the lower Jinshajiang River that when annual precipitation exceeds 1,050 mm or high-flow season precipitation exceeds 850 mm, the annual sediment yield increased sharply. These can also be regarded as key rainfall thresholds for erosion and sediment yields. When precipitation is less than the two thresholds, dominant erosion types are sheet, rill and gully erosions. When precipitation crosses the two thresholds, debris flows may occur more frequently. As a result, the previously stored loose sediment is released and sediment yield increases sharply.     

The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.     

浙江省四大平原区土壤元素背景值特征

浙江省地球物理地球化学勘查院,浙江 杭州 310005
摘要：20世纪90年代初,浙江省地球物球地球化学勘查院先后在杭嘉湖平原区、宁绍平原区、温黄平原区、金衢盆地等四大块平原区开展生态环境地球化学调查,获得了数以万计的定量分析数据,编绘了各类相关图件及说明书。将数以万计的分析数据,经技术处理,最终取得了全省土壤中元素背景含量。     

Ecological thresholds of phytoplankton community across environmental gradients in the harmful algal blooms-frequently-occurring,subtropical coastal waters,East China Sea

Phytoplankton communities can response immediately and directly to environmental changes,and thus have been applied as reliable biotic indicators in aquatic systems.This study provided insights into the relationships concerning ecological thresholds of phytoplankton communities and individual taxon in response to environmental changes in coastal waters of northern Zhejiang Province,East China Sea.Results demonstrated that there existed seasonal variations of phytoplankton community ecological thresholds of which spring being higher than those in summer.As for individual species,Prorocentrum donghaiense and Noctiluca scintillans were identified as the most tolerant and sensitive indicator species in spring and summer,respectively.They exhibited strong indications in response to environmental changes.These findings highlighted that phytoplankton community structure in this region was stable when environmental gradients were below the thresholds of sensitive species,whereas potential harmful algal blooms may occur when environmental gradients exceeded the thresholds of tolerant species.     

Towards Establishing Climatic Thresholds for Slope Instability: Use of a Physically-based Combined Soil Hydrology-slope Stability Model

— Shallow landslides are often linked to high magnitude rainstorms. Research has attempted to establish threshold rainfall totals that trigger shallow landslides, based mainly on field evidence. Complications arise because not all regolith has the same hydrological behaviour, and research frequently fails to take this into account. This paper uses a combination of field and modelling approaches to explore the triggering rainfall thresholds for shallow failures in deforested hill country of New Zealand. It emphasises the role of variations in regolith hydrology, focussing on unsaturated and saturated zone responses. By using a modelling approach, detailed variations in pore pressure (positive and negative) responses are investigated, developing ideas initially derived from field evidence. This paper defines and develops earlier research that establishes values for maximum and minimum probability thresholds for shallow landslides, and provides a more generalised model that can be applied more widely. Hydrological mechanisms for shallow landslides are investigated in greater detail than previously possible using a Combined Hydrology And Stability Model (CHASM).Received: 20 August 2001     

海洋资源环境承载能力超载阈值确定方法探讨

超载阈值是资源环境承载能力监测预警体系构建中的关键因子,其确定是当前资源环境承载能力评价方法研究中的重点和难点问题。本文首先分析了资源环境承载能力超载阈值的定义和内涵,结合国内外研究进展和国家发改委下发的《资源环境承载能力监测预警技术方法(试行)》,分析目前海洋资源环境承载能力评价采用的超载阈值确定方法及潜在的问题。试行方法主要是从海洋功能区划及环境质量标准的合规性,以及指标多年变化情况确定超载阈值,对资源环境保护与社会经济发展的关系协调考虑较少,评价方法的创新性以及评价结果的管理效能还较弱。本文还从海洋资源环境对于社会经济发展的限制性分析视角,探索构建区域差异性指标体系和更具科学性及管理适用性的超载阈值确定新思路。     

湿地“淹埋深-历时-频率”阈值研究的理论和方法

针对"淹埋深-历时-频率"(S, D, F)阈值研究存在的问题,根据土壤学、植物生态学、系统论、地理系统学说以及反演理论等的相关原理,系统分析了湿地(S, D, F)阈值确定的理论和方法。结果表明:在正常情况下,水饱和至地表的最大地下水埋深、湿地植物主要根系分布层下限和沉水植物分布下界淹水深分别是湿地土壤上界、湿地植被上界和湿地下界的S阈值;D阈值只能通过湿地土壤或湿地植被边界界定在水位过程线上反演,稳定湿地边界的D阈值应取与50%频率值对应的D值或D值的多年平均值,波动湿地边界的D阈值是常数;"非正常情况下"湿地的D阈值应参照湿地水文地貌分类的同一区域子类中"正常情况下"的湿地的D阈值;检验D阈值科学性的标准是利用D阈值确定的湿地水文边界必须与湿地土壤边界或湿地植被边界耦合。     

羟基自由基杀灭压载水中有害赤潮物种的生物有效性研究

本研究采用大气压下强电离放电协同气液混溶技术,高效制备羟基自由基(·OH)杀灭3个门的典型有害赤潮物种,使用荧光染色、测定光合作用潜能等生物学检测方法确定·OH致死阈值。结果表明,5.05×104 cells/mL的赤潮异弯藻(Heterosigma akashiwo)、5.28×104 cells/mL的亚历山大藻(Alexandrium tamarense)和5.02×104 cells/mL的中肋骨条藻(Skeletonema costatum),其致死阈值分别为1.24 mg/L、2.01 mg/L、1.12 mg/L,此时其叶绿素a分解率分别为77%、85%和74%。利用光学显微镜观察,处理前后藻细胞结构有明显的改变。因此,·OH致死方法可有效地杀灭压载水中的有害赤潮藻。     

Development of a thresholds approach for real‐time flash flood prediction in complex geomorphological river basins

Flash floods represent one of the deadliest and costliest natural disasters worldwide. The hydrological analysis of a flash flood event contributes in the understanding of the runoff creation process. This study presents the analysis of some flash flood events that took place in a complex geomorphological Mediterranean River basin. The objective of the present work is to develop the thresholds for a real‐time flash flood forecasting model in a complex geomorphological watershed, based on high‐frequency data from strategically located hydrological and meteorological telemetric stations. These stations provide hourly real‐time data which were used to determine hydrological and meteorological parameters. The main characteristics of various hydrographs specified in this study were the runoff coefficients, lag time, time to peak, and the maximum potential retention. The estimation of these hydrometeorological parameters provides the necessary information in order to successfully manage flash floods events. Especially, the time to peak is the most significant hydrological parameter that affects the response time of an oncoming flash flood event. A study of the above parameters is essential for the specification of thresholds which are related to the geomorphological characteristics of the river basin, the rainfall accumulation of an event, the rainfall intensity, the threshold runoff through karstic area, the season during which the rainfall takes place and the time intervals between the rainstorms that affect the soil moisture conditions. All these factors are combined into a real‐time‐threshold flash flood prediction model. Historical flash flood events at the downstream are also used for the validation of the model. An application of the proposed model is presented for the Koiliaris River basin in Crete, Greece. Copyright © 2011 John Wiley & Sons, Ltd.