黄河下游河槽横断面调整规律及治理方式探讨

通过对影响河槽挟沙能力诸因素的分析,作者认为目前黄河下游只有河槽形态是一个人为可调节的控制河道淤积的要素。分析黄河下游河道横剖面形态的特征及其调整规律,考虑来沙组成变化、河槽综合阻力变化,计算得到黄河下游典型断面的实际挟沙能力及平均水沙条件下河槽平衡输沙横剖面,经与实际断面形态对比,得出必须以多级河槽方式缩窄7000m³/s流量以下即中、小水期的河槽,方能显着减少黄河下游河槽中的淤积的结论。这样才能真正实现潘季驯的“束水攻沙”的治黄方略。     

长江口水沙入海通量的观测与分析

在长江口用声学多普勒流速剖面仪（ADCP）进行走航式断面观测,结果表明,采集水样获取的悬沙浓度与ADCP记录的声学信号（后向散射强度）之间存在显著相关关系,因此可据ADCP声学数据获得沿观测断面的高时空分辨率的悬沙浓度剖面。分析结果表明．在徐六泾附近断面以往复流占优势．大潮期间单位时间最大水、悬沙通量值出现于涨潮期．落潮历时较长．落潮期水、悬沙通量分别大于涨潮期水、悬沙通量,净通量向海。长江口南槽拦门沙外侧主要为旋转流．涨潮期间内的水、悬沙通量均分别大于落潮期间的水、悬沙通量。2003年11月12日的观测结果表明．潮周期内通过徐六泾断面向海输运的水与悬沙通量分别为10^9m^3和10^8kg量级：径流量与悬沙输运率分别为10^4m^3／s和10^3kg／s量级。1998年以来徐六泾ADCP流量观测数据与同时间的大通径流量之间存在着显著的相关关系．故可由大通径流量估算徐六泾径流量。作为长江人海径流量．以作为历史资料整编的一种手段。结果表明,长江人海年径流总量略大于大通年径流总量：而在枯水期间人海径流量小于大通径流量。     

Remote sensing of stream depths with hydraulically assisted bathymetry (HAB) models

This article introduces a technique for using a combination of remote sensing imagery and open-channel flow principles to estimate depths for each pixel in an imaged river. This technique, which we term hydraulically assisted bathymetry (HAB), uses a combination of local stream gage information on discharge, image brightness data, and Manning-based estimates of stream resistance to calculate water depth. The HAB technique does not require ground-truth depth information at the time of flight. HAB can be accomplished with multispectral or hyperspectral data, and therefore can be applied over entire watersheds using standard high spatial resolution satellite or aerial images. HAB also has the potential to be applied retroactively to historic imagery, allowing researchers to map temporal changes in depth.We present two versions of the technique, HAB-1 and HAB-2. HAB-1 is based primarily on the geometry, discharge and velocity relationships of river channels. Manning's equation (assuming average depth approximates the hydraulic radius), the discharge equation, and the assumption that the frequency distribution of depths within a cross-section approximates that of a triangle are combined with discharge data from a local station, width measurements from imagery, and slope measurements from maps to estimate minimum, average and maximum depths at a multiple cross-sections. These depths are assigned to pixels of maximum, average, and minimum brightness within the cross-sections to develop a brightness–depth relation to estimate depths throughout the remainder of the river.HAB-2 is similar to HAB-1 in operation, but the assumption that the distribution of depths approximates that of a triangle is replaced by an optical Beer–Lambert law of light absorbance. In this case, the flow equations and the optical equations are used to iteratively scale the river pixel values until their depths produce a discharge that matches that of a nearby gage.R² values for measured depths versus depths estimated by HAB-1 and HAB-2 are 0.51 and 0.77, respectively, in the relatively simple Brazos River, Texas. R² values for HAB-1 and HAB-2 are 0.46 and 0.26, respectively, in the Lamar River, a complex mountain river system in Yellowstone National Park. Although the R² values are moderate, depth maps and cross-sections derived from the HAB techniques are consistent with typical stream geomorphology patterns and provide far greater spatial coverage and detail than could be achieved with ground-based survey techniques. Improved depth estimates can be achieved by stratifying the river into different habitat types that normalize for differences in turbulence and substrate.     

亚洲降水中δ18O沿不同水汽输送路径的变化

利用IAEA/WMO全球监测网和青藏高原的监测站,建立了由赤道地区经我国西南水汽通道至长江中下游的南方水汽输送路径、沿西风带自我国西部经华北至日本的北方水汽输送路径以及自南亚穿喜马拉雅山到我国青藏高原的水汽输送路径的取样剖面,比较了三条水汽路径在不同季节降水中啄18O的变化及其与温度、降水量的关系。沿南方水汽路径,低纬度地区取样站降水中平均啄18O的季节差异较小。沿北方水汽路径,郑州以西取样站平均啄18O的季节差均大于郑州以东的取样站。随着经度的增加,降水中平均啄18O的季节差减小。沿高原水汽路径,印度次大陆南部降水中的啄18O相对较高,随着纬度的增加,降水中啄18O逐渐减小。在翻越喜马拉雅山后,由于强烈的洗涤作用,降水中啄18O急剧下降。     

Effects of peat on the shapes of alluvial channels: examples from the Cumberland Marshes, Saskatchewan, Canada

An avulsion of the lower Saskatchewan River in the 1870s inundated a large segment of peat-covered floodplain that subsequently has become aggraded with a broad (500 km²) belt of alluvium deposited by the redirected flow. Routing of water and sediment discharge through the avulsion-affected area has been accomplished mainly by networks of sandy bedded anastomosed channels that have formed, evolved, and abandoned as the alluvial belt prograded down the floodplain slope. These processes continue today, though at a much-reduced rate. New channels, formed by crevassing and basinward extension of distributaries, are initially small and shallow, with bottom elevations situated within the avulsive alluvium but above the pre-avulsion peat (floodplain) surface. Subsequent enlargement and downcutting of many of these channels eventually uncovers the underlying peat layer whose resistance to erosion exerts significant influence on cross-sectional shape and further channel development. Peat-floored channels tend to have rectangular cross-sections, high ratios of average to maximum depth (D/D_max), and a large range of width-to-depth ratios. If the channel continues to enlarge, the peat layer eventually becomes breached, commonly leading to temporarily irregular cross-sections caused by localized scouring at the breach sites. Eventually, the peat layer is completely eroded from the channel floor by undercutting and slumping, after which channel shape becomes governed mainly by other perimeter characteristics. Channels unaffected by peat, either before the peat layer is encountered during early channel development or after it is entirely removed, tend to have low width/depth ratios and a large range of D/D_max values.     

Recent sedimentary histories of shallow lakes in the guatemalan savannas

Shallow basins in the savannas of Peten, Guatemala filled with water after 305±55 BP (calibrated age+1430–1660 AD). Aguadas Chimaj and Chilonche possess dilute waters and iron-rich, clayey sediments that are poor in Ca and Mg, reflecting the highly weathered nature of riparian soils. Low ²¹⁰Pb flux rates to Chimaj (0.085 pCi cm^-2 yr^-1) and Chilonche (0.134 pCi cm^-2 yr^-1) are attributed to low ²²²Rn emission rates from the nearby Caribbean Sea. Mean sediment accumulation rates in Chimaj and Chilonche for the past 150 years are 0.015 g cm^-2 yr^-1 and 0.047 g cm^-2 yr^-1 respectively. Forest expansion after 305 BP is documented in pollen profiles from the small aguadas and larger Lake Oquevix. Regional reforestation postdates the 9th century Classic Maya collapse and coincides with indigenous depopulation that was a consequence of European intrusion that began in the early 1500s. The timing of forest regrowth indicates the importance of historical anthropogenic factors in controlling Peten's vegetation. Nevertheless, other sedimentological lines of evidence (e.g. lithology, algal remains and charcoal particles) suggest that changing climate and/or local hydrology may have played a role in the reforestation process.     

Georadar Investigations of the Peretolchin Glacier (Southern Siberia)

Presented are the results obtained from radar profiling of the Peretolchin glacier in the Munku-Sardyk mountain range in the south of Siberia (June 2014 and May 2016) using georadar OKO-2 with the ABDL Triton antenna unit operating at the radiation frequency of 50 and 100 MHz. The ice thickness was determined from the profiles and the ice volumes from the cross-sectional areas and distances between profiles. The ice volume was calculated for the body of the Peretolchin glacier to be 0.007 ± 0.0019 km³. An analytical method was also used to determine the ice volume according to the type and area of glacier. It is established that the correlation between the glacier volume and area is expressed by a power function V = kS ^p . The mean ice volume, calculated with due regard for different coefficients, was 0.0061 km³. The GlabTop model that assumes the cross-sectional form of the glacier was used to infer the ice volume: the mean volume for two cross-sections (parabolic and elliptic) was 0.0073 km³. It is found that over 110 years since the start of the investigation into the morphology of the Peretolchin glacier, it has increased twice in its length and in area, its volume has decreased by a factor of 3.7, and the lower boundary of the open part of the glacier has risen 184 meters during that period.     

Recent morphological channel changes in a deltaı̈c environment. The case of the Rhône River,France

Channel morphological changes were measured at two sites within the Rhône delta along the main channel of the Rhône River over one century. Results show an average erosion rate of 2.8 mm year⁻¹ during the 20th century and decreasing to <1.5 mm year⁻¹ since the 1960s. Erosion affected site 2 more than site 1 but cross-sections can alternately be in erosion or accretion depending on bed topography and/or floor lithology. Riverbed adjustments are partly explained by the boundary shear stress and specific stream power but local factors also influence the Rhône River evolution. Results compared with those obtained upstream or in other European rivers suggest that the lower Rhône River has developed an original response to external stresses: the first part of the 20th century is characterised by an important degradation followed by a decrease in the incision rate since the 1960s. The initiation of the erosion phase is assumed to be linked to the end of the Little Ice Age, to the reforestation of the alpine hillsides, to the earlier management and to the reduction of the sedimentary yield (according to the literature). The second phase possibly corresponds to the relaxation period. In this perspective, the effects of dams are probably less significant than we would suppose up to now. It implies that such a large river records morphological responses which may vary all along the fluvial system in relation to the time and space scales involved.     

阿克苏河流域的面雨量序列及其与径流关系

以数字高程模型 (DEM) 的1km×1km网格数据为基础,对阿克苏河流域14个气象站和水文站的1961~2000年的年降水资料进行了自然正交分解 (EOF),通过回归分析,建立主要特征向量与地理因子的插值模型,给出了一个面雨量序列的计算方法,为建立气候要素的区域平均序列提供了一个有效的解决方案,并由此推算出年阿克苏流域平均年降水量的空间分布以及面雨量序列。径流量与面雨量之比 (R/P) 平均为0.43,最高为0.69 (1997年),最低为0.30 (1963年)。计算出的阿克苏河流域面雨量序列与阿克苏河实测径流量序列的趋势变化率分别为5.79×10⁸ m³/10a和4.29×10⁸ m³/10a,两者均表现出增加趋势,但面雨量的增加速率要比径流量大一些,年际变化幅度也要大,面雨量和径流量的变差系数Cv值分别为0.17和0.13。阿克苏河年径流量的变化与夏季0^oC层高度、年面雨量有着十分密切的关系,表明20世纪90年代以来新疆气候的变化是阿克苏河流域径流稳定增加的一个非常重要的因素。     

近50年中国光合有效辐射的时空变化(英文)

Based on long-term measurement data of weather/ecological stations over China,this paper calculated and produced annually-and seasonally-averaged Photosynthetically Active Radiation(PAR) spatial data from 1961 to 2007,using climatological calculations and spatialization techniques.The spatio-temporal variation characteristics of annually-and seasonally-averaged PAR spatial data over China in recent 50 years were analyzed with Mann-Kendall trend analysis method and GIS spatial analysis techniques.The results show that:(1) As a whole,the spatial distribution of PAR is complex and inhomogeneous across China,with lower PAR in the eastern and southern parts of China and higher PAR in the western part.Mean annual PAR over China ranges from 17.7 mol m-2 d-1 to 39.5 mol m-2 d-1.(2) Annually-and seasonally-averaged PAR of each pixel over China are averaged as a whole and the mean values decline visibly with fluctuant processes,and the changing rate of annually-averaged PAR is-0.138 mol m-2 d-1/10a.The changing amplitudes among four seasons are different,with maximum dropping in summer,and the descending speed of PAR is faster before the 1990s,after which the speed slows down.(3) The analysis by each pixel shows that PAR declines significantly(α=0.05) in most parts of China.Summer and winter play more important roles in the interannual variability of PAR.North China is always a decreasing zone in four seasons,while the northwest of Qinghai-Tibet Plateau turns to be an increasing zone in four seasons.(4) The spatial distributions of the interannual variability of PAR vary among different periods.The interannual variabilities of PAR in a certain region are different not only among four seasons,but also among different periods.     

Influences of environmental changes on water storage variations in Central Asia

The spatio-temporal pattern of the global water resource has significantly changed with climate change and intensified human activities. The regional economy and ecological environment are highly affected by terrestrial water storage (TWS), especially in arid areas. To investigate the variation of TWS and its influencing factors under changing environments, the response relationships between TWS and changing environments (climate change and human activities) in Central Asia have been analyzed based on the Gravity Recovery and Climate Experiment (GRACE) data, Climatic Research Unit (CRU) climate data and Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing data products (MOD16A2, MOD13A3 and MCD12Q1) from 2003 to 2013. The slope and Pearson correlation analysis methods were used. Results indicate that: (1) TWS in about 77 % of the study area has decreased from 2003 to 2013. The total change volume of TWS is about 2915.6 × 10⁸ m³. The areas of decreased TWS are mainly distributed in the middle of Central Asia, while the areas of increased TWS are concentrated in the middle-altitude regions of the Kazakhstan hills and Tarim Basin. (2) TWS in about 5.91% of areas, mainly distributed in the mountain and piedmont zones, is significantly positively correlated with precipitation, while only 3.78% of areas show significant correlation between TWS and temperature. If the response time was delayed by three months, there would be a very good correlation between temperature and TWS. (3) There is a significantly positive relationship between TWS and Normalized Difference Vegetation Index (NDVI) in 13.35% of the study area. (4) The area of significantly positive correlation between TWS and evapotranspiration is about 31.87%, mainly situated in mountainous areas and northwestern Kazakhstan. The reduction of regional TWS is related to precipitation more than evaporation. Increasing farmland area may explain why some areas show increasing precipitation and decreasing evapotranspiration. (5) The influences of land use on TWS are still not very clear. This study could provide scientific data useful for the estimation of changes in TWS with climate change and human activities.     

Fractures and drainage in the granite mountainous area: A study from Sierra Nevada,USA

This paper provides an analysis of relationships between drainage patterns and fractures in the part of Sierra Nevada, California, north of the Yosemite Valley. Bedrock is Cretaceous granite and cut by numerous lineaments of various orientation, length and geomorphic expression. We have mapped fractures and drainage lines from aerial photographs, 1:40 000 scale, in four test areas ranging in size from 32.5 to 266 km². Azimuths are shown on rose diagrams for fractures and drainage lines and then visually and statistically compared. The coincidence of drainage and fracture patterns is strong, which implies causal relationships. In plan, the majority of valleys follow fractures even if this locally means a different orientation in respect to the regional slope arising from tectonic tilt of the range. Main streams occupy deeply incised troughs coincident with ‘master fractures’ of regional extent. Among two principal fracture directions, SSW–NNE to SW–NE and WSW–ENE, the former exerts more control on the drainage lines. The presence of a central zone of structural weakness within the major valleys provided significant constraints for the course of glacial erosion and may explain why multiple Pleistocene glaciers did not succeed in transforming valley cross-sections into expected U-shapes.     

Influence of vegetation change on watershed hydrology: implications for paleoclimatic interpretation of lacustrine δ18O records

Stratigraphic shifts in the oxygen isotopic (¹⁸O) and trace element (Mg and Sr) composition of biogenic carbonate from tropical lake sediment cores are often interpreted as a proxy record of the changing relation between evaporation and precipitation (E/P). Holocene ¹⁸O and Mg and Sr records from Lakes Salpetén and Petén Itzá, Guatemala were apparently affected by drainage basin vegetation changes that influenced watershed hydrology, thereby confounding paleoclimatic interpretations. Oxygen isotope values and trace element concentrations in the two lowland lakes were greatest between ~ 9000 and 6800 ¹⁴C-yr BP, suggesting relatively high E/P, but pollen data indicate moist conditions and extensive forest cover in the early Holocene. The discrepancy between pollen- and geochemically-inferred climate conditions may be reconciled if the high early Holocene ¹⁸O and trace element values were controlled principally by low surface runoff and groundwater flow to the lake, rather than high E/P. Dense forest cover in the early Holocene would have increased evapotranspiration and soil moisture storage, thereby reducing delivery of meteoric water to the lakes. Carbonate ¹⁸O and Mg and Sr decreased between 7200 and 3500 ¹⁴C-yr BP in Lake Salpetén and between 6800 and 5000 ¹⁴C-yr BP in Lake Petén Itzá. This decline coincided with palynologically documented forest loss that may have led to increased surface and groundwater flow to the lakes. In Lake Salpetén, minimum ¹⁸O values (i.e., high lake levels) occurred between 3500 and 1800 ¹⁴C-yr BP. Relatively high lake levels were confirmed by ¹⁴C-dated aquatic gastropods from subaerial soil profiles ~ 1.0–7.5 m above present lake stage. High lake levels were a consequence of lower E/P and/or greater surface runoff and groundwater inflow caused by human-induced deforestation.     

Steady, balanced rates of uplift and erosion of the Santa Monica Mountains, California

Topographic change in regions of active deformation is a function of rates of uplift and denudation. The rate of topographic development and change of an actively uplifting mountain range, the Santa Monica Mountains, southern California, was assessed using landscape attributes of the present topography, uplift rates and denudation rates. Landscape features were characterized through analysis of a digital elevation model (DEM). Uplift rates at time scales ranging from 10⁴ to 10⁶ years were constrained with geological cross-sections and published estimates. Denudation rate was determined from sediment yield data from debris basins in southern California and from the relief of rivers set into geomorphic surfaces of known age. First-order morphology of the Santa Monica Mountains is set by large-scale along-strike variations in structural geometry. Drainage spacing, drainage geometry and to a lesser extent relief are controlled by bedrock strength. Dissection of the range flanks and position of the principal drainage divide are modulated by structural asymmetry and differences in structural relief across the range. Topographic and catchment-scale relief are ≈300–900 m. Mean denudation rate derived from the sediment yield data and river incision is 0.5±0.3 mm yr^?1. Uplift rate across the south flank of the range is ≈0.5±0.4 mm yr^?1 and across the north flank is 0.24±0.12 mm yr^?1. At least 1.6–2.7 Myr is required to create either the present topographic or the catchment-scale relief based on either the mean rates of denudation or uplift. Although the landscape has had sufficient time to achieve a steady-state form, comparison of the time-scale of uplift and denudation rate variation with probable landscape response times implies the present topography does not represent the steady-state form.     

The ratio between chrysophycean cysts and diatoms in temperate,mountain lakes: some recommendations for its use in paleolimnology

The ratio between chrysophycean cysts and diatom valves (CD ratio) in lake sediments has been suggested as a useful indicator of changing trophic state conditions in oligotrophic lakes. Other environmental factors, however, may influence the CD ratio because chrysophycean cysts usually reflect conditions in the planktonic environment and diatoms reflect benthic conditions. We investigated the CD ratio in 76 mountain lakes in the Pyrenees to determine the environmental drivers that influence the ratio and assess its value for paleoenvironmental inference. The lakes surveyed included a broad range with respect to bedrock type, altitude and surface area, characteristics that cover much of the variability that can be found in cold, oligotrophic mountain lakes. Lake depth and Ca²⁺ concentration explain most of the variation in the CD ratio. Trophic state factors (e.g. total phosphorus, TP) play a secondary role. As a predictor, CD ratio performs primarily as a lake depth indicator. The predictive models can be improved if trophic state (i.e. TP) and chemical conditions (Ca²⁺) are known or can be estimated independently. Use of the CD ratio for inferring Ca²⁺ oscillations only makes sense in lakes with Ca²⁺ <200 µeq/L or in those that oscillate below and above this threshold through time. Other interpretations of the CD ratio (e.g. lake trophic state changes, ice-cover duration) make sense if complementary paleolimnological evidence indicates that neither water depth nor Ca²⁺ concentration changed significantly. Indeed, paleolimnological interpretation of the CD ratio requires considering the particular characteristics of the lake and may vary depending on the temporal scale considered. This study provides some guidelines for evaluating critically the use of the CD ratio.     

Response of diatom community in Lugu Lake (Yunnan–Guizhou Plateau,China) to climate change over the past century

A mean annual temperature increase has been recorded on the Yunnan–Guizhou Plateau of China during the last century. This temperature increase has been significantly greater since the 1950s. Thus, paleolimnological analyses may be utilized to better understand ecological responses to recent changing climate over decadal to centennial timescales, especially in regions with sparse lake monitoring data. Here, we present paleolimnological results from a ²¹⁰Pb/¹³⁷Cs-dated sediment core spanning approximately the last ~250 years from a remote, alpine, semi-closed oligotrophic lake (Lugu Lake) on the northwestern Yunnan–Guizhou Plateau. Sediment profiles of diatoms, geochemical variables (LOI₅₅₀, TOC and C/N) and median grain size were analyzed and compared with the climate data (1951 AD–2010 AD) from the Lijiang weather station. Endogenous productivity of Lugu Lake has increased gradually over the last 30 years. The majority of diatom taxa encountered in the core are typical of alkaline oligotrophic lakes. Diatom assemblages were dominated by Cyclostephanos dubius, Cyclotella taxa, and fragilarioid taxa. Diatom species composition has changed significantly with three assemblage shifts at different scales over the ~250-year period. Diatom species diversity reveals a distinct increase before ~1970 AD, followed by a decline. In addition, a decreasing trend in diatom cell-size was consistent with recent warming trends. Redundancy analysis (RDA) shows that regional air temperature trends (annual, spring, summer, and winter) have played a significant role (p < 0.05) in determining diatom compositional changes over the past six decades. Results of this study suggest that regional warming is the main driving force behind recent changes in diatom composition at Lugu Lake, while nutrients may also have impact on the diatom change in recent 10 years.     

Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (δ¹⁵N) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δ¹⁵N of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN),  %N in the sediments and lake morphometric variables were the best predictors of sedimentary δ¹⁵N, explaining 66 % of its variation. Significant relationships were also found between sediment δ¹⁵N and human-derived N load ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.23, p < 0.001), the latter being a strong predictor of TN ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δ¹⁵N. Alberta lake sedimentary δ¹⁵N signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δ¹⁵N was best explained by  %N in the sediments and lake volume. Overall, our findings support the use of δ¹⁵N in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δ¹⁵N are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.     

太湖流域土地利用变化对流域产水量的影响

根据土地详查和土地利用总体规划资料，分析太湖流域近２０年土地利用的现状特点和将来变化趋势。并以 １９９１年降雨类型分别计算了 １９８６、１９９６和 ２０１０年土地利用状况下流域的产水量。结果表明，太湖流域土地利用变化的主要特点是耕地面积的减少和建设用地面积的增加。同样降雨类型，１９９６年流域下垫面状况下的产水量比 １９８６年的多１０．１８ × １０８ ｍ３，２０１０年产水量将比 １９８６年多 １２ × １０８ｍ３。太湖流域土地利用变化对流域产水量有较大影响，这与２０世纪９０年代以来太湖持续高水位的现象相吻合。     

Erosion rates in badland areas recorded by collectors,erosion pins and profilometer techniques (Ebro Basin,NE-Spain)

In badland areas of the Ebro Basin, in a semiarid climate, two erosion plots (257 m²; 5° slope and 128 m²; 23° slope) on exposed Tertiary clays were monitored over two years (Nov. 1991–Nov. 1993). This material is characterized by high sodium absorption ratios which lead to high soil dispersivity. The dominant erosion processes in both plots are rilling and sheet erosion. Rainfall intensity was recorded at a weather station, connected to a data-logger, sediment production for single events was collected in tanks, and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques. Significant runoff was produced only by rainfall events above 5 mm. Another threshold at 20 mm rain was noted. For rainfalls higher than 20 mm, the 23° slope plot shows a greater runoff response than the 5° one. Rainfall events exceeding this threshold showed a higher sediment production for the steeper slope. In the relationship between precipitation and sediment concentration, an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity. Runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations. In both plots, the erosion pins show that erosion rates in rill areas are 25–50% higher than in the interrill areas. Sediment yield recorded by collector devices was higher than the rates measured by erosion pins. The erosion rates based on rill cross-sections by profilometers were higher than the ones recorded by collectors.     

Stream Channel Enlargement Response to Urban Land Cover in Small Coastal Plain Watersheds,North Carolina

Stream channel response to urban land use has not been well documented for southeastern Coastal Plain streams. In this study, urban channel response was evaluated in small Inner Coastal Plain watersheds (<5 km²) in eastern North Carolina. Reaches were selected across a range of watershed total impervious area (0-67% TIA). Channel dimensions and sediment grain size data were collected along 20 urban (>10% TIA) and 20 rural reaches (<10% TIA), and at 10 stormwater outfall sites (180 cross-sections). Urban cross-sectional area, channel incision ratio, and channel grain size (gravel%, D₅₀, and D₈₄) were greater, relative to rural channels. Bankfull cross-sectional areas were approximately 1.78 times greater for urban watersheds than for rural watersheds. Channels in urban watersheds were incised and had median full-channel capacities approximately 3.4 times greater than channels draining rural watersheds. Watershed TIA explained 65-72% of channel capacity enlargement. Urban expansion in the region began in the 1960s, with major urbanization occurring over the last 25 years. Channels draining urban watersheds are still responding to this land use change by downcutting and widening. Urban channel incision has frequently cut off streams from their floodplains, reducing floodplain sediment retention and water quality functions.