Influence of groundwater and topography on stream drying in semi-arid headwater streams

Non-perennial streams comprise over half of the global stream network and impact downstream water quality. Although aridity is a primary driver of stream drying globally, surface flow permanence varies spatially and temporally within many headwater streams, suggesting that these complex drying patterns may be driven by topographic and subsurface factors. Indeed, these factors affect shallow groundwater flows in perennial systems, but there has been only limited characterisation of shallow groundwater residence times and groundwater contributions to intermittent streams. Here, we asked how groundwater residence times, shallow groundwater contributions to streamflow, and topography interact to control stream drying in headwater streams. We evaluated this overarching question in eight semi-arid headwater catchments based on surface flow observations during the low-flow period, coupled with tracer-based groundwater residence times. For one headwater catchment, we analysed stream drying during the seasonal flow recession and rewetting period using a sensor network that was interspersed between groundwater monitoring locations, and linked drying patterns to groundwater inputs and topography. We found a poor relationship between groundwater residence times and flowing network extent (R² < 0.24). Although groundwater residence times indicated that old groundwater was present in all headwater streams, surface drying also occurred in each of them, suggesting old, deep flowpaths are insufficient to sustain surface flows. Indeed, the timing of stream drying at any given point typically coincided with a decrease in the contribution from near-surface sources and an increased relative contribution of groundwater to streamflow at that location, whereas the spatial pattern of drying within the stream network typically correlated with locations where groundwater inputs were most seasonally variable. Topographic metrics only explained ~30% of the variability in seasonal flow permanence, and surprisingly, we found no correlation with seasonal drying and down-valley subsurface storage area. Because we found complex spatial patterns, future studies should pair dense spatial observations of subsurface properties, such as hydraulic conductivity and transmissivity, to observations of seasonal flow permanence.     

Streamflow regime of a lake-stream system based on long-term data from a high-density hydrometric network

Northern landscapes are dominated by a mosaic of lakes and streams, yet only a limited number of studies have explored how these lake-stream networks influence streamflow regimes. In order to gain further insight into the hydrologic behaviour of lake-stream systems, we conducted a study using long-term streamflow data to investigate the annual-, seasonal- and event-scale streamflow regimes of a lake-stream network at the Turkey Lakes Watershed (TLW) in central Ontario, Canada. Streamflow metrics were compared for seven lake and 12 no-lake catchments within the TLW, in addition to 14 no-lake catchments from other forested landscapes. It was difficult to attribute patterns in annual streamflow regimes to the influence of lakes due to the confounding influence of catchment size; however, streamflow regimes appeared to be less flashy at locations with more lake influence. In addition, lake catchments showed high similarity in streamflow regimes across seasons, whereas no-lake catchments showed more similarity to lake catchments during wet seasons but less similarity during dry seasons. Event-scale streamflow regimes further downstream from lake outlets were associated with greater increases in peakflow response and hydrograph rise rate following rain events than locations closer to lake outlets. Antecedent conditions were also important for both the peakflow response and rise rate, but less so than the amount of rainfall during the event. Variability in streamflow across lake-stream networks appears to be driven by interactions between delayed contributions from lakes and relatively rapid runoff contributions from hillslopes and tributaries without lakes. In addition, streamflow regimes are influenced by temporal changes in lake storage deficits, which are a function of lake and catchment properties, as well as hydrometeorological conditions. Our results highlight that a network-scale perspective that incorporates lakes and streams is needed to understand how these landscapes will hydrologically respond to environmental change.     

Riparian canopy openings on mountain streams: Landscape controls upon temperature increases within openings and cooling downstream

How much stream temperatures increase within riparian canopy openings and whether stream temperatures cool downstream of these openings both have important policy implications. Past studies of stream cooling downstream of riparian openings have found mixed results including rapid, slow, and no cooling. We collected longitudinal profiles of stream temperatures above, within, and below riparian forest openings along stream segments within otherwise forested riparian conditions to evaluate how sensitivity of stream temperatures to riparian conditions varied across landscape factors. We conducted these temperature surveys across openings in 12 wadeable streams within and near the Upper Little Tennessee River Basin in western North Carolina and northeastern Georgia. Basin areas ranged from 74 to 6,913 ha, and bankfull channel widths varied from 3.4 to 16.4 m. Stream temperatures were collected every 15 min using HOBO® data loggers for 2 weeks in each stream, repeated later in summer in some streams. Reference temperatures were highest in stream reaches at low elevations and with large drainage areas. Stream temperature increases in the middle of riparian gaps were highest when streams drained small high-elevation watersheds, and increases at the end of openings were highest when the opening length was large relative to watershed size. Downstream from openings, cooling rates were greatest in small, high-elevation headwater streams and also increased with larger increases in canopy cover. Stream segments that warmed the most within openings also featured higher cooling rates downstream. The data show that stream temperature sensitivity to canopy change is highly dependent on network position and watershed size. A better understanding of stream temperature responses to riparian vegetation may be useful to land managers and landowners prioritizing riparian forest restoration.     

Stream geochemical response to reductions in acid deposition in headwater streams: Chronic versus episodic acidification recovery

In the decades since dramatic reductions in acid emissions and subsequent deposition in North America and Europe, the vast majority of research on aquatic chemical recovery has focused on trends in acid–base concentrations during baseflow conditions. Missing from such assessments is consideration of higher flow periods, when flow paths and chemical concentrations change and episodic acidification may occur. Stream samples collected weekly and bi‐hourly during high‐flow storm events from 1992 to 2015 at three watersheds representing a gradient in response to acid deposition in the south‐eastern United States were used to evaluate temporal trends in acid anions, base cations, acid neutralizing capacity (ANC), and pH for each of five distinct flow exceedance regimes, as well as trends in concentration–discharge (C–Q) relationships. For the most well‐buffered watershed (mean ANC = 220 μeq L^?1), ANC and pH increased at a similar rate for baseflow and storm flow conditions. Decreases in sulfate controlled ANC trends at higher flows, whereas smaller sulfate decreases in combination with base cation increases resulted in similar ANC improvements for low‐flow periods. For the most acid sensitive watershed (mean ANC = 10 μeq L^?1), no trends in ANC or pH were observed for the lowest flows (>90% flow exceedance), whereas reductions in sulfate resulted in significant increases in ANC during higher flow conditions. At all sites, greater rates of sulfate decline during high‐flow, as compared with low‐flow, conditions are likely a result of a reduced capacity of near surface soils to adsorb and retain sulfur in these non‐glaciated watersheds. Overall, consistent increases in pH (~0.01–0.02 pH units year^?1) during higher flow conditions (<10% flow exceedance) in contrast to the variable trends observed during lower flows (>50% flow exceedance) illustrate that episodic acidification is recovering at an equal or greater rate than chronic acidification in these watersheds.     

Meteoroid streams at Mars: possibilities and implications

In order to assess the possibility of meteoroid streams detectable from the surface of Mars as meteor showers we have derived minimum distances and associated velocities for a large sample of small body orbits relative to the orbits of Mars and the Earth. The population ratio for objects approaching to within 0.2 AU of these two planets is found to be approximately 2:1. The smaller relative velocities in the case of Mars appears to be the main impediment to the detection of meteors in the upper atmosphere of that planet. We identify five bodies, including the unusual object (5335) Damocles and periodic comet 1P/Halley, with relative orbital parameters most suitable to produce prominent meteor showers. We identify specific epochs at which showers related to these bodies are expected to occur. An overview of possible detection methods taking into account the unique characteristics of the Martian environment is presented. We pay particular attention on the effects of such streams on the dust rings believed to be present around Mars.     

Doppler tomography of Algols

The technique of Doppler tomography has been influential in the study of mass transfer in Algol‐type interacting binaries. The Algols contain a hot blue dwarf star with a magnetically‐active late‐type companion. In the close Algols, the gas stream flows directly into the photosphere of the blue mass‐gaining star because it does not have enough room to avoid impact with that star. Doppler tomograms of the Algols have been produced from over 2500 time‐resolved spectra at wavelengths corresponding to Hα, Hβ, He I (6678 Å), Si II (6371 Å) and Si IV (1394 ° A). These tomograms display images of accretion structures that include a gas stream, accretion annulus, accretion disk, stream‐star impact region, and occasionally a source of chromospheric emission associated with the cool, mass‐losing companion. Some Algol systems alternate between streamlike and disk‐like states, and provide direct evidence of active mass transfer within the Algols. This work produced the very first images of the gas stream for the entire class of interacting binaries, and demonstrated that the Algols are far more active than formerly believed, with variability on time scales of weeks to months. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

多次散射对向上漫射散射系数及漫射吸收系数影响的数值研究

在假定水体表面为镜面、水体中无内光源及非弹性散射的情况下、利用多次散射理论, 研究了在固定体散射函数的情况下, 不同的入射天顶角、单次散射反照率和散射系数对向上漫射散射系数, 上、下行漫射吸收系数的影响。结果表明: 在紧贴水表的下层, 向上漫射散射系数随入射辐射天顶角的增大而增大, 但不随单次散射反照率和散射系数的改变而改变; 当入射辐射为垂直入射时, 向上漫射散射系数的深度廓线随深度呈现单调增的趋势, 并逐步接近一个恒定状态; 随着入射天顶角的增大, 该单调现象发生了变化, 出现先增大后减小, 再逼进恒定状态, 且这个恒定值随单次反照率增大而增大。下行漫射吸收系数随着入射天顶角的增大, 其廓线由单调增加过渡到先增大后减小; 在散射系数和单次散射反照率相同的条件下, 不同入射天顶角的下行漫射吸收系数逐渐收敛于同一稳定的状态。且单次散射率越小, 这种收敛的速度越快, 达到恒定状态的深度也越浅。上行漫射吸收系数的主要特征基本与下行漫射吸收系数类似, 而且两个恒定值均大于属于固有光学参数的吸收系数, 其中向上漫射吸收系数最大。     

2011年梅雨期长江中下游地区两次暴雨过程数值模拟对比分析

基于WRF数值预报模式,对2011年梅雨期6月9—10日和14—15日长江中下游地区两次暴雨过程（分别简称“6·10”过程和“6·14”过程）进行数值模拟,重点对比分析了暴雨期间西南涡的活动与高低空急流耦合配置之间的关系。结果表明：1） 西南涡的活动和高低空急流耦合配置与暴雨活动关系密切,是造成两次暴雨过程范围和强度差异的重要因素。2）“6·10”过程中,一个浅薄的西南涡系统受青藏高原浅槽东移北缩减弱影响,向东北方向移动,同时西南低空急流位置偏北,导致暴雨区位置偏北;“6·14”过程中,一个深厚的西南涡系统受高空浅槽东移发展加深影响,沿长江缓慢东移,伴随西南低空急流位置偏南,降水缓慢向东移动,导致暴雨区位置偏南。3） 两次过程的强降水中心均位于高低空急流耦合区,“6·10”过程中,在长江中下游地区形成的高低空急流耦合区范围偏小且强度偏弱,因此辐合上升运动偏弱,不利于形成大范围的强降水;“6·14”过程中,在长江下游地区形成大范围高低空急流耦合的环流形势,强烈的辐合上升运动配合充足的水汽供应,最终形成大范围强降水。     

Spatial variation of the vegetative roughness in Mediterranean torrential streams affected by check dams

This study focuses on the spatial variations in vegetative roughness associated with morphological channel adjustments due to the presence of check dams in Mediterranean torrential streams. Manning’s n values were estimated using methods established by previous studies of submerged and non-submerged vegetation in laboratory flume experiments and field work. The results showed a linear decrease in shrub density and rate of variation of the roughness coefficient versus degree of submergence with increasing distance upstream from the check dam, while downstream, the filling of the check dam and the bed incision had the most influence. A regression analysis was applied by fitting the data to different models: relationships between Manning’s n and the flow velocity were found to be of the power type for shrubs in all upstream sections, while relationships of flow velocity versus hydraulic radius in the sections closest to check dams showed a good fit to second-order polynomial equations.     

Variable hillslope-channel coupling and channel characteristics of forested mountain streams in glaciated landscapes

Channel morphology of forested, mountain streams in glaciated landscapes is regulated by a complex suite of processes, and remains difficult to predict. Here, we analyze models of channel geometry against a comprehensive field dataset collected in two previously glaciated basins in Haida Gwaii, B.C., to explore the influence of variable hillslope–channel coupling imposed by the glacial legacy on channel form. Our objective is to better understand the relation between hillslope–channel coupling and stream character within glaciated basins. We find that the glacial legacy on landscape structure is characterized by relatively large spatial variation in hillslope–channel coupling. Spatial differences in coupling influence the frequency and magnitude of coarse sediment and woody material delivery to the channel network. Analyses using a model for channel gradient and multiple models for width and depth show that hillslope–channel coupling and high wood loading induce deviations from standard downstream predictions for all three variables in the study basins. Examination of model residuals using Boosted Regression Trees and nine additional channel variables indicates that ~10 to ~40% of residual variance can be explained by logjam variables, ~15–40% by the degree of hillslope–channel coupling, and 10–20% by proximity to slope failures. These results indicate that channel classification systems incorporating hillslope–channel coupling, and, indirectly, the catchment glacial legacy, may present a more complete understanding of mountain channels. From these results, we propose a conceptual framework which describes the linkages between landscape history, hillslope–channel coupling, and channel form. © 2018 John Wiley & Sons, Ltd.