Aeolian geomorphology of the Namib Sand Sea

This review is concerned with the aeolian geomorphology of the Namib Sand Sea, part of the Namib Desert in Namibia, south-west Africa, concentrating in particular on studies made over the past 50 years. The paper covers: sand sea organisation and dune patterns; and the dynamics of individual dunes. It demonstrates that studies in the Namib Sand Sea have been important to a much wider understanding of the geomorphological development of global sand seas.     

The nature of moisture at Gobabeb,in the central Namib Desert

This paper reviews the nature of moisture at Gobabeb, Namibia with emphasis on rainfall, and fog. It introduces the observational record produced by the Gobabeb Training and Research Centre and examines nature and cause of the rainfall record from October 1st 1962 to May 30th 2011. Over this period of 17749 days only 381 rainy days produced a total of 1213 mm of rain with an annual average of 25 mm. 2011 has been the wettest year on record also featuring the two most wettest days (March 12th and May the 6th). 1992 has been the driest year with no rain at all. Over the last 3 decades (1979–2009) the number of decadal rain days has decreased from 77 to 56 to 54 days, while total decadal rain amount has increased from 130 mm to 149 mm up to 300 mm. 193 Individual rain events between 1979 and 2009 were linked to synoptic conditions present in the region including the Zaire Air Boundary (ZAB), Tropical Temperate Troughs (TTT), the Angola Low, temperate cold fronts and cut-off lows (850 hgt geopotential height). Cluster analyses in the form of Self Organising Maps (SOMs), suggests that all synoptic states have the potential to produce rain but that the Angolan low dominates with an increase in TTT activity being evident. Fog collection techniques have evolved through time and suggest a range of possible event types, including advected fog, coastal stratus cloud, high stratus cloud, radiation fog and fog drizzle. While each of these has their own meso- and micro-scale synoptic control and may even vary in their bulk and isotopic chemistry, they collectively make a significant moisture contribution to the flora and fauna of the Namib. Additional sources of moisture are gaining appreciation and include the widespread occurrence of hypersaline springs on the Namib gravel plains as well as micro-scale moisture including vapour in desert soils and regolith.     

Psammophily in Namib Desert spiders

A community of psammophilous spiders was studied in the central Namib Desert. Of 20 species, 85% were cursorial. Complex setae, claws or keels facilitate digging, sweeping, carrying, and swimming in or walking on sand, while long spigots enable spiders to bind sand with silk when burrowing down to depths with amenable microclimate (10–120 cm). Sand is excavated either by sweeping it up an incline or carrying it vertically up. Other characteristics of dune spiders were large size, polyphagy, low metabolism, longevity, brood care and seasonal foraging and breeding patterns. The large, dominant heteropodid,Leucorchestris, may determine many characteristics of the Namib arthropod community.     

Variation of Dry Days Since Last Rain (DDSLR) as a measure of dryness along a Mediterranean – Arid transect

Dry periods between rainstorms control the soil moisture conditions, and are the antecedent conditions for various geomorphological, hydrological and ecological processes. An annual distribution of dry condition, based on the Dry Days Since Last Rain (DDSLR) approach is presented, showing the duration and timing of dry periods for various daily rainfall thresholds and probabilities.Daily rainfall data from five stations along a climatic transect in Israel, extending from the Judean Mountains, which have a Mediterranean climate, to the Dead Sea arid region were studied. The results show that DDSLR values increased with increasing aridity. At a 0.1 mm rainfall threshold, the DDSLR values were similar along this transect. However, at higher daily thresholds, differences among different locations along the transect, become more noticeable.     

华北农牧交错带夏季极端气候的趋势分析

利用1956~2001年日降水量及日平均气温资料,分析华北农牧交错带极端气候事件的长期变化趋势。对降水极端事件的分析季节为5~9月,温度为6~8月。结果发现暴雨 (日降水 ≥50 mm) 频数没有显著的线性变化趋势,但1980年前后发生了明显的年代际尺度变化,1980年代到1990年代初,频次显著减少,此期间暴雨发生的时间更加分散。定义连续无雨的天数为干燥事件。发现虽然降水量没有显著的变化,但是严重干燥事件 (连续0降水日 ≥10天) 的频数却呈显著增加趋势 (+8.3%/10a), 超过99%信度水平。最高和最低的10%温度分别定义异常高温和异常低温,则异常高温的频数有显著增加趋势 (+20.9%/10a),特别是1990年代后期以来增加非常突出。异常低温频次有减少的趋势 (-15.1%/10a),超过99%信度水平。严重干燥事件及异常高温事件的强烈增加,可能是造成近来北方干旱频繁发生的重要原因。     

Sub-daily rainfall events in an arid environment with marked climate variability: Variation among wet and dry years at Fowlers Gap,New South Wales,Australia

The nature of rainfall events is explored through six years of below average rainfall, associated with negative Southern Oscillation Index (SOI), and three years of above average rainfall, associated with positive SOI (and strong La Niña conditions), at arid Fowlers Gap, Australia. There is a greater probability of rainfall in wet years, but the events themselves also change significantly. Rainfall depth per event was 116% larger on average in wet years than dry, and average event rainfall rate was 85% higher. However, these results are influenced by a small number of very large events in the wet years, and events of <2 mm occur at about the same rate in dry and wet years. Rainfall event profiles in dry years showed more Huff first quartile events likely to promote partitioning of rain into infiltration. In contrast, larger events in wet years showed a preponderance of Huff third quartile profiles likely to be associated with greater partitioning of rainfall into overland flow. This co-variation in rainfall event profile with annual rainfall, not previously described, is reasoned to increase the amplitude of ecological impacts of the SOI-related rainfall variability at this site.     

In-situ rainwater harvesting and gravel mulch combination for corn production in the dry semi-arid region of China

A field study was conducted to determine the effect of a combination of ridge and furrow method of in-situ rainwater harvesting with gravel mulch on corn production, soil moisture storage, and water-use efficiency in the dry semi-arid region of China. Results showed that plastic-covered ridges had an average runoff efficiency (runoff/rainfall) of 87% as compared to 7% for bare ridges, and could generate runoff at a threshold value of 0·8±0·2 mm rainfall. Bare ridges produced runoff only under high intensity rainfall events, and was ineffective for harvesting rainfall in the study area. The plastic-covered ridge and gravel-mulched furrow method of rainwater harvesting was effective in conserving moisture and increasing yield and water-use efficiency. The grain yield in this treatment was 1·9 times that of the conventional flat soil cultivation (control), and the water-use efficiency was 1·8 times that of the control. The good performance of the film-covered surface ridges and gravel-mulched furrows is attributed to the better utilization of light rains, improvement of infiltration in the root zone, and suppression of evaporation losses.     

Movement of a small slipfaceless dome dune in the Namib Sand Sea, Namibia

The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m³ that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year⁻¹, and is equivalent to an annual sand transport rate of about 1.2 tonnes m⁻¹ year⁻¹. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m⁻¹ year⁻¹, and the resultant potential sand flow is 63 tonnes m⁻¹ year⁻¹. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.     

极端降水事件对科尔沁沙地一年生植被的影响

在科尔沁沙地奈曼旗,大于10 mm的降雨属极端降雨。根据当地多年平均降雨量,设置雨量×降雨次数的双因素模拟试验(T₁,共288 mm、分18次;T₂,共288 mm、分9次;T₃,共576 mm、分36次;T₄共576 mm、分18次;CK,接收当年的自然降雨),考察极端降雨对科尔沁沙地一年生植被的密度、多样性指数、生物量及根冠比的影响。结果表明:(1)在萌发期,植株密度在各处理下均维持在2 000株·m^-2;而在生长发育后期,T₃、T₄维持在400株·m^-2,而T_I、T₂维持在1 600株·m^-2,CK下为1 200株·m^-2,说明降雨总量决定了能完成生活史的沙地一年生植被的数量。(2)T₁、T₂、T₃3种极端降雨模式均显著提高沙地一年生植被的生物多样性指数,说明科尔沁沙地一年生植被的生物多样性不是由年降雨量单一因素决定的,而是由每次降雨量与降雨次数的分布共同决定的。(3)T₃、T₄显著降低了沙地一年生植被的水分利用效率。     

黑河流域日降水格局及其时间变化

利用10个气象站1950—2000年日降水资料,对黑河流域的降水及其时空格局进行了研究。结果表明:①降水事件以≤5 mm的降水为主,占全年降水事件的82%,0～10 d间隔期占全年无降水期的40.7％,＞10 d间隔期占59.3％;②年降水日数与降水量之间存在显著的正相关性(r=0.78,P<0.01),＞10 mm降水总量与年降水量呈显著正相关,而≤5 mm降水总量与年降水量的相关性差;③＞10 mm和≤5 mm的降水总量变化范围分别为:0～262.6 mm,20.3～172.7 mm ,变异系数分别为0.25～0.92,0.17～0.25;④近50 d来,黑河流域近50 d降水量的变化总体上呈平稳上升趋势,≤5 mm的降水日呈现小幅下降的趋势,≥10 mm的降水日呈小幅上升的趋势。0～10 d的降水间隔期出现的频率总体保持不变,＞10 d间隔期出现的频率出现明显的下降趋势,年内总降水日数保持不变。     

Holocene environmental changes in the Gebel Umm Hammad, Eastern Desert, Egypt

Gebel Umm Hammad in the Red Sea Mountains east of Quseir, Egypt, today enjoys small but irregular amounts of winter rain, enabling the widening of joint controlled openings in the Thebes Limestone. Cavities are especially affected by flaking, while rock fragmentation is more active on the outside. The sedimentological and botanical study of fan deposits at the outlet of a karstic shaft in the Tree Shelter showed the local Holocene environmental evolution. Three periods of different degree of aridity can be considered: (i) Before 8120±45 BP (UtC-5389), bedload aggradation points to rare but occasionally heavy rains, lasting for several hours, attaining intensities of more than 76 mm/h and covering some 20 km². Wadi flash floods occasionally attained bankfull stage. (ii) Since 8120±45 BP (UtC-5389), such heavy rains have not occurred in the Egyptian Red Sea Mountains. Instead, a more moderate but maybe wetter precipitation regime was established. The karstic shafts were active, and there was water and life in the desert. Two humid pulses can be distinguished within this period. The first occurs at ±8000 BP, the second between 6630±45 (GrN-22560) and 6770±60 BP (GrN-22562). (iii) After the last wet culmination, there was a gradual shift to drier conditions. Shortly after ±5000 BP, modern climatic conditions are believed to have been attained. Today, the occasional rain storms are less heavy than before ±8000 BP. Bankfull stage river floods do not occur. Instead, secondary channels are eroded in the wadi beds. The general arid character during the whole period and the inherent local and temporal variations in precipitation patterns might explain apparent aberrations between the palaeoenvironmental evolution of the Tree Shelter site and other remote study areas in Egypt and Sudan.     

500 Years of rainfall variability and extreme hydrological events in southeastern Spain drylands

The semiarid SE fringe of the Iberian Peninsula is considered one of the most sensitive to extreme floods and droughts in the western Mediterranean area. The controlling climatic mechanisms are nevertheless difficult to predict and model. A combined documentary-sedimentary-instrumental 500 years comprehensive register of climatic data (rainfall and flooding) was collated for analysing the decadal to centennial scale hydrological response. Wet years are closely linked to the presence of autumn (SON) positive anomalies (e.g. early 18th century). However, continuous, decadal wet periods seem to correspond in time to both autumn and spring (MAM) positive rainfall anomaly years (e.g. 1570/90, 1830/40, 1870/1900). High frequencies of large floods were registered during the late Medieval Warm Period (AD 950–1200), and during some decades of the Little Ice Age with an average of 0.22 floods/year (1440–1490, 1520–1570, 1600–1740, 1770–1800, 1820–1840, 1870–1900), but flood frequency decreased in the 20th Century (1945–1973; 0.14 floods/year). During wet phases (e.g. late 19th century), large floods occurred during all seasons, whereas a predominantly autumn extreme flooding (>70%) is linked to a rainfall patterns with higher inter-annual variability (e.g. 1945–1973). The recurrence of dry phases is higher since early 17th century, and the frequency of continuous wetter phases lower than the ones with marked annual variability. This results in a trend with less frequent high magnitude catastrophic floods. This study confirms a shift from autumn rainfall maxima towards winter since the early 1990's. The tendency towards longer dry periods and increased inter-annual variability (with 1–3 years maximum wet spells) and a changing seasonal rainfall distribution are thought to be key in modelling projections for this specific arid Mediterranean region.     

Daily precipitation changes in the semi-arid region over northern China

The semi-arid area in northern China is an agro-pastoral transition zone. The environment there is sensitive to climate variation, particularly to precipitation change. Daily precipitation records of 30-gauge stations from May to September during 1956–2000 are analysed in this study. The precipitation amounts show only slightly decreasing trends; however, some aspects of the rainfall characteristics display significant changes. Compared to the 1950s, rainy days have been reduced by about 8 days in the 1990s. Significant change is observed in the number of days when there is light rain (<10 mm day⁻¹). The days with light rain also show a significant tendency toward a stronger intensity, whereas those days with moderate and above moderate rains do not. At the same time, the long duration precipitation (3days) event is found to decrease remarkably with a linear trend of −5.6% per 10 years. In contrast, the frequency of long dry spells (10 consecutive days without rainfall) is becoming more frequent at a rate of 7.2% per 10 years. Taking 30 stations as a whole the maximum daily rainfall displays no evident trend. But the median of the maximums shows a step-like drop beginning in 1979 and the low values are notable until the early 1990s. This is coincident with the large-scale climate regime shift over East Asia, probably suggesting a response of regional daily precipitation extremes due to the change of the East Asian summer monsoon system.     

毛乌素沙地流动沙丘土壤水分对降雨的响应

为探讨半干旱区流动沙丘土壤水分对降雨的响应,采用AV-3665R型雨量传感器、ECH₂O-5土壤水分传感器同步监测毛乌素沙地东北缘流动沙丘2013年降雨及0～200 cm深度土壤体积含水量动态,分析土壤水分含量变化特征、降雨入渗特征及降雨对土壤水的补给作用。结果表明:5-11月累积降雨399.4 mm,显著(p<0.01)影响0～200 cm深度土壤水分含量; 且0～200 cm深度土壤体积含水量较低(6.49%±1.12%)时53.8 mm降雨、较高(10.22%±1.96%)时24.2 mm降雨湿润锋能够到达200 cm; 试验期间399.4 mm累积降雨对土壤水有一定补给作用,其中45.7±14.1 mm降雨蓄存在0～200 cm深度土壤中,占同期降雨的(11.4%±3.5%,且随着时间的延长,蓄存水大部分将渗漏到200 cm以下补给深层土壤水。     

Throughfall and its spatial variability in a temperate rainforest of simple structure

There are few throughfall data from southern hemisphere closed-forest, and none from Tasmanian callidendrous cool-temperate rainforest, which has a simpler structure than most primary rainforests. We determined throughfall, measured its local spatial variation, and tested its relationships with rainfall, rainfall intensity, wind speed, canopy dryness, canopy cover, and other structural variables in a cool-temperate callidendrous rainforest in Tasmania. Eighty-two percent of the precipitation was measured as throughfall, which occurred after 2.3 mm of rain fell on dry canopies. The cumulative rainfall in 25 randomly located funnel rain gauges on the forest floor varied from 160 to 567 mm. Canopy cover and other structural variables did not predict the spatial pattern of throughfall. While throughfall in rainfall events was related to rainfall amount and intensity, wind speed did not affect throughfall as a percentage of rainfall. Percentages of throughfall to rainfall over 100 for many low rainfall events may indicate a contribution of fog drip to precipitation on the forest floor. The high local spatial variability in throughfall indicates the mean moisture conditions on the forest floor may not be a good indicator of the potential for localised fire damage.     

Productivity, management and development problems in the arid rangelands of the central Mendoza plains (Argentina)

The central Mendoza plains (56,000 km²), located in the eastern foothills of the Andes, belong to the vast phytogeographic province of ‘Monte’ (c.500,000 km²) in Argentina. This area is a typical warm arid zone and vegetation is typically a shrubland. Perennial grasses are predominantly C₄species. Land-use is for livestock grazing; the average stocking rate is 28 ha per large stock unit (LSU), year-long. Ecological and range research have been actively pursued in the region by IADIZA over the past 25 years. Two experiment stations were established 25 and 10 years ago, respectively: The Ñacuñán Biosphere Reserve and the El Divisadero Cattle and Range Station. Primary production and its relationship to climatic and edaphic parameters were analysed and compared in the two stations. Rain-use efficiency (RUE, kg DM ha⁻¹year⁻¹mm⁻¹) and useful rains efficiency (URE, P −0·1 PETp) were similar in both cases (RUE = 1·7 and 2·0; URE = 3·0 and 2·9, respectively), when herbs are concerned. However, the variability of annual production was much higher on the silty soils of Ñacuñán than on the coarse sands of El Divisadero; PRVR (Production to Rain Variability Ratio) was 2·5 and 0·53, respectively. Utilization of such experimental figures allows for the prediction of primary production and carrying capacity for a given year, based on rainfall probabilities and, therefore, to determine stocking rates in a rational manner. Basing stocking rates on annual dependable rains (f 0·8) and on useful rains probability (P − 0·1 PETp) proved to be a sound and realistic assessment, validated by long-term experiments and experience. The results of the studies on brush control, cattle diet composition, animal productivity and feed value of forage species were analysed. Current and future research programmes in IADIZA for the Mendoza plains are mentioned. Although research on resource inventory and basic ecological processes in the Mendoza plains should carry on, the emphasis and therefore the resources should henceforth be devoted to the consequences of resource management alternatives on overall arid land productivity, i.e. research for development. A better integration of applied ecology for management and investigations on basic arid land ecological processes should appear in IADIZA programmes.     

The effect of rainfall and habitat on abundance and diversity of birds in a fenced protected area in the central Saudi Arabian desert

Factors influencing avian abundance and diversity in central Saudi Arabia were investigated over a 28-month period using walked belt transects in five local habitat types in a 2200 km²fenced desert protected area established in 1989. Enclosure of the protected area has resulted in the development of a mosaic of densely vegetated grassland and savanna areas amongst the sand and gravel plains which dominate the surrounding landscape. Overall bird diversity showed biannual peaks corresponding to spring and autumn passage migration, when 30–40 species were recorded; troughs occurred in summer or winter (9–12 species) following poor spring or autumn rains. Bird numbers responded dramatically to heavy rainfall (> 20 mm in a month) occurring two months earlier, with major peaks recorded in autumn 1992 and May–June 1994. Numbers of granivorous species, principally larks, fluctuated mostly in relation to rainfall, whereas largely insectivorous species tended to remain more stable. A desert locust plague in spring/summer 1993, combined with poor spring rains, resulted in the lowest numbers and species diversity recorded during the study.     

1960—2017年太湖流域不同等级降水时空特征

基于太湖流域1960—2017年逐日降水数据,运用Mann-Kendall非参数检验法、R/S分析等方法,分析太湖流域不同等级降水的时空变化特征,并探讨了不同等级降水对年降水的影响。结果表明：1）近60年来,流域小雨发生率最高,为73.55%;年总降水量中,中雨量所占比例最大,为32.05%。小雨发生率呈显著减少趋势,暴雨贡献率呈显著增加趋势。2）太湖流域大雨、暴雨的降水量和降水日数都呈显著增加,小雨日数显著减少,小雨强度、年总降水强度显著增强。3）不同等级降水变化趋势的空间分布存在明显差异。小雨日数与年总降水日数,以及小雨强度与年总降水强度的变化趋势空间格局相一致。中雨日数、大雨日数、暴雨日数变化趋势的空间分布与其对应的降水量变化趋势的空间格局相似。4）R/S分析结果显示,小雨、暴雨、年总降水相关指标（小雨量除外）都表现出较强的持续性,未来变化趋势与过去相一致。5）近60年来,太湖流域年总降水量、降水日数、年总降水强度的变化,分别受中雨量、小雨日数、暴雨量的影响较大。在旱年流域年降水量偏少受大雨量减少的影响较大,而涝年年降水量偏多受暴雨增加的影响较大。     

季风与地形对台湾降水的影响

本文以季风环流的季节变化及地形两大因子对台湾降水的影响进行了研究分析，结果显示：东北季风和地形共同影响，造成冬半年台湾东北部多雨和西南部干旱；夏半年由于受西南季风的影响，出现了岛西部降水略多于岛东部的现象，且由于西南季风的影响，缓解了西南部干旱．台湾地形对降水的影响居全国之冠，海拔高度每上升100m，年降水量递增值都大于100mm，海拔500mm左右的低层递增率更是高达266．5mm／100m．