Brackish marshes erode twice as fast as saline marshes in the Mississippi Delta region

Marsh soil properties vary drastically across estuarine salinity gradients, which can affect soil strength and, consequently, marsh edge erodibility. Here, we quantify how marsh erosion differs between saline and brackish marshes of the Mississippi Delta. We analyzed long-term (1932–2015) maps of marsh loss and developed an algorithm to distinguish edge erosion from interior loss. We found that the edge erosion rate remains nearly constant at decadal timescales, whereas interior loss varies by more than 100%. On average, roughly half of marsh loss can be attributed to edge erosion, the other half to interior loss. Based on data from 42 cores, brackish marsh soils had a lower bulk density (0.17 vs. 0.27 g/cm³), a higher organic content (43% vs. 26%), a lower shear strength (2.0 vs. 2.5 kPa), and a lower shear strength in the root layer (13.8 vs. 20.7 kPa) than saline marsh soils. We then modified an existing marsh edge erosion model by including a salinity-dependent erodibility. By calibrating the erodibility with the observed retreat rates, we found that the brackish marsh is two to three times more erodible than the saline marshes. Overall, this model advances the ability to simulate estuarine systems as a whole, thus transcending the salinity boundaries often used in compartmentalized marsh models.     

2018年春霜冻对江西省北部茶叶生产影响的定量评估

为探究江西省北部茶区春霜冻发生特点,深入开展茶叶春霜冻灾害定量评估工作,采用实地调查方式,获取江西省北部12个茶场2018年4月7—9日春霜冻的灾情资料和同期气象观测资料,采用加权求和法构建茶叶春霜冻灾害损失定量评估模型,对春霜冻灾害县进行灾害定量评估和检验。结果表明：婺源、浮梁等江西省北部茶叶主产县达到轻—重度春霜冻气象灾害等级,灾害指标等级计算结果与实地灾情调查结果较一致;轻度、中度、重度气象灾害等级的茶场减产率依次为10%—30%、30%—50%和50%以上;根据不同气象灾害等级的受灾面积及其减产率构建江西省北部茶叶春霜冻灾害损失定量评估模型,计算得到此次春霜冻灾害过程造成婺源县春茶直接经济损失为1.23亿元,模型评估结果和实际灾情相一致,模型能较为客观、准确的评估春霜冻灾害对茶叶生产造成的损失。     

降水变化对陕北黄土高原植被覆盖度和高度的影响

植被特性及生长模式是土壤侵蚀研究的重要因子，而降水是植被生长的主要限制因素。通过 3年的观测数据，分析了陕北黄土高原地区降水年际变化和年内变化对 6种植被覆盖度和高度的影响，得到如下主要结论：①年降水量对刺槐林、灌木林、荒草地和农地植被的影响较大，平水年份和干旱年份最大覆盖度相差约一倍。而对于植被高度来说，受影响最大的是休闲地和农地植被。②降水的年内分布，尤其是 6～8月份的降水量是限制植被覆盖度和高度的主要原因。对于荒草地、农耕地覆盖度和高度的影响尤为显著。农作物覆盖度、高度甚至每年播种日期和种类都受到生长季降雨量的制约。研究结果对于估计黄土高原地区植被参数的变化，以及对土壤侵蚀模拟和退耕还林还草都具有参考价值。     

甘肃永靖黑方台地区灌溉诱发型滑坡发育特征 及力学机制

水是诱发黄土滑坡最为积极的因素,以灌溉诱发型滑坡尤为典型.基于野外水工环地质综合调查,对黑方台地区灌溉渗透诱发滑坡的发育特征进行总结.通过分析特定孕灾地质条件下滑坡发育特征的差异,认为黄土滑坡形成的力学机制以滑移-压致拉裂-剪断型和蠕滑-拉裂-滑移型为主,而黄土-基岩滑坡以滑移-压致-拉裂型为主.滑移-压致拉裂-剪断型黄土滑坡为原位溯源后退扩展式,系灌溉水入渗后造成黄土饱和带厚度增大,不仅造成台缘周边地下水溢出带部位渗透压力增大,且抗剪强度显著弱化,从而在坡体自重作用下发生蠕滑剪切.锁固段突然破坏使滑坡快速启动,滑体冲击致早期饱水滑床产生液化,产生高位抛射,借助原始地形滑体加速运移形成高速远程运动.蠕滑-拉裂-滑移型黄土滑坡是黄土底部饱和带厚度较薄或灌溉入渗影响较小的地段,以后缘拉裂为主激发形成.渗灌溉水长期入渗造成下伏基岩顺倾泥岩层面软化泥化,从而在顺向坡段产生黄土-基岩顺层滑坡,该类滑坡以低速短程滑坡为主.     

Influence of spatial correlation of strong ground motion on uncertainty in earthquake loss estimation

In addition to the mean values of possible loss during an earthquake, parameters of the probability distribution function for the loss to a portfolio (e.g. fractiles and standard deviation) are very important. Recent studies have shown that the proper treatment of ground‐motion variability and, particularly, the correlation of ground motion are essential for the estimation of the seismic hazard, damage and loss for distributed portfolios. In this study, we compared the effects of variations in the between‐earthquake correlation and in the site‐to‐site correlation on seismic loss and damage estimations for the extended objects (hypothetical portfolio) and critical elements (e.g. bridges) of a network. A scenario earthquake approach and a portfolio containing a set of hypothetical building and bridges were used for the purpose. We showed that the relative influences of the types of correlation on characteristics of loss distribution and the probability of damage are not equal. In some cases, when the median values of loss distribution or the probability that at least one critical element of a lifeline will be damaged are considered and when the spatial correlation of ground motion is used, the possible variations in the between‐earthquake correlation may be neglected. The shape of the site‐to‐site correlation function (i.e. the rate of decrease of the coefficient of spatial correlation with separation distance) seems also to be important when modelling spatially correlated ground‐motion fields. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of grass soil cover on water runoff and soil detachment under rainfall simulation in a sub‐humid South African degraded rangeland

In most regions of the world overgrazing plays a major role in land degradation and thus creates a major threat to natural ecosystems. Several feedbacks exist between overgrazing, vegetation, soil infiltration by water and soil erosion that need to be better understood. In this study of a sub‐humid overgrazed rangeland in South Africa, the main objective was to evaluate the impact of grass cover on soil infiltration by water and soil detachment. Artificial rains of 30 and 60 mm h^?1 were applied for 30 min on 1 m² micro‐plots showing similar sandy‐loam Acrisols with different proportions of soil surface coverage by grass (Class A: 75–100%; B: 75–50%; C: 50–25%; D: 25–5%; E: 5–0% with an outcropping A horizon; F: 0% with an outcropping B horizon) to evaluate pre‐runoff rainfall (Pr), steady state water infiltration (I), sediment concentration (SC) and soil losses (SL). Whatever the class of vegetal cover and the rainfall intensity, with the exception of two plots probably affected by biological activity, I decreased regularly to a steady rate <2 mm h^?1 after 15 min rain. There was no significant correlation between I and Pr with vegetal cover. The average SC computed from the two rains increased from 0·16 g L^?1 (class A) to 48·5 g L^?1 (class F) while SL was varied between 4 g m^?2 h^?1 for A and 1883 g m^?2 h^?1 for F. SL increased significantly with decreasing vegetal cover with an exponential increase while the removal of the A horizon increased SC and SL by a factor of 4. The results support the belief that soil vegetation cover and overgrazing plays a major role in soil infiltration by water but also suggest that the interrill erosion process is self‐increasing. Abandoned cultivated lands and animal preferred pathways are more vulnerable to erosive processes than simply overgrazed rangelands. Copyright © 2011 John Wiley & Sons, Ltd.     

Using palm‐mat geotextiles on an arable soil for water erosion control in the UK

To date, most studies of the effectiveness of geotextiles on soil erosion rates and processes have been conducted in laboratory experiments for less than 1 h. Hence, at Hilton (52°33′ N, 2°19′ W), UK, the effectiveness of employing palm‐mat geotextiles for soil erosion control under field conditions on arable loamy sands was investigated. Geotextile‐mats constructed from Borassus aethiopum (Borassus palm of West Africa) and Mauritia flexuosa (Buriti palm of South America) leaves are termed Borassus mats and Buriti mats, respectively. Duplicate runoff plots (10 m × 1 m on a 15° slope) had five treatments (bare, permanent grass, Borassus total plot cover, Borassus buffer strip and Buriti buffer strip). Borassus covered plots had about 72% ground cover and to differentiate between this treatment and Borassus buffer strips, the former treatment is termed Borassus completely‐covered. Runoff and eroded soil were collected from each bounded plot in a concrete gutter, leading to a receptacle. Results from 08/01/2007–23/01/2009 (total precipitation = 1776·5 mm; n = 53 time intervals) show that using Borassus buffer strips (area coverage ～10%) on bare soil decreased runoff volume by about 71% (P > 0·05) and soil erosion by 92% (P < 0·001). Bare plots had nearly 29·1 L m^?2 runoff and 2·36 kg m^?2 soil erosion during that period. Borassus buffer strip, Buriti buffer strip and Borassus completely‐covered plots had similar effects in decreasing runoff volume and soil erosion. Runoff volumes largely explain the variability in soil erosion rates. Although buffer strips of Borassus mats were as effective as whole plot cover of the same mats, the longevity of Borassus mats was nearly twice that of Buriti mats. Thus, use of Borassus mats as buffer strips on bare plots is highly effective for soil erosion control. The mechanisms explaining the effectiveness of buffer strips require further studies under varied pedo‐climatic conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of grass and shrub cover on the critical unit stream power in overland flow

Improved knowledge of the effects of grass and shrub cover in overland flow can provide valuable information for soil and water conservation programs.Laboratory simulated rainfall studies were conducted to determine effects of grass and shrub on runoff and soil loss and to ascertain the relationship between the rate of soil loss and the unit stream power of runoff for a 20°slope subjected to rainfall intensities of 45,87,and 127 mm/h.The results indicated that the average runoff rates ranged from 4.2 to 73.1 mm/h for grass plots and from 9.3 to 58.2 mm/h for shrub plots.Runoff rates from shrub plots were less than those from grass plots for all but the 45 mm/h rainfall intensity regime. Average soil loss rates varied from 5.7 to 120.3 g/min.m~2 for grass plots and from 5.6 to 84.4 g/min.m~2 for shrub plots.Soil loss rates from shrub plots were generally lower than those from grass plots.Runoff and soil loss were strongly influenced by soil surface conditions due to the formation of erosion pits and rills.The rate of soil loss increased linearly with the unit stream power of runoff on both grass and shrub plots.Critical unit stream power values were 0.0127 m/s for grass plots and 0.0169 m/s for shrub plots.Shrub plots showed a greater stability to resist soil detachment and transport by surface flow than grass plots.     

地震应急评估与决策指标体系的构建

地震应急的损失评估与决策工作是地震应急救援的基础,需要在基础数据的支持下,对震害损失和决策目标相关信息进行科学的预估.然而,灾情评估中存在基础数据精度不够高、内容不足、没有一个成体系的、完整的工作标准可遵循等问题.本文从应急决策的需求出发,构建了地震应急评估与决策指标体系,试图利用数据指标化的方法解决上述问题.     

春季低温连阴雨灾害对农作物产量影响评估

依据河南省春季低温连阴雨灾害标准,对信阳市1971—2007年春季达到低温连阴雨过程标准的气象资料,分别从3月低温上,4月低温、连阴雨以及低温连阴雨,5月连阴雨等不同的致灾因子持续时间长短、对农作物造成危害程度的大小,将其划分3成个不同的灾害等级。用拉格朗日插值法计算作物的期望产量,用分离法将春季低温连阴雨灾害对作物造成的损失分离出来。结果发现,不同时段的春季低温连阴雨对水稻、小麦、油菜和茶叶造成的危害程度是不同的,以4月低温连阴雨和5月连阴雨危害最大。定量评估春季低温连阴雨灾害损失对防灾减灾和政府决策具有十分重要意义。