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Dissolved organic carbon (DOC) concentrations vary among headwaters, with variation typically decreasing with watershed area. We hypothesized that streamflow intermittence could be an important source of variation in DOC concentrations across a small watershed, through (a) temporal legacies of drying on organic matter accumulation and biotic communities and (b) spatial patterns of connectivity with DOC sources. To test these hypotheses, we conducted three synoptic water chemistry sampling campaigns across a 25.5‐km2 watershed in south‐eastern Idaho during early spring, late summer, and late fall. Using changepoint analysis, we found that DOC variability collapsed at a consistent location (watershed areas ~1.3 to ~1.8 km2) across seasons, which coincided with the watershed area where variability in streamflow intermittence collapsed (~1.5 km2). To test hypothesized mechanisms through which intermittence may affect DOC, we developed temporal, spatial, and spatio‐temporal metrics of streamflow intermittence and related these to DOC concentrations. Streamflow intermittence was a strong predictor of DOC across seasons, but different metrics predicted DOC depending on season. Seasonal changes in the effects of intermittence on DOC reflected seasonal changes from instream to flowpath controls. A metric that captured spatial connectivity to sources significantly predicted DOC during high flows, when DOC is typically controlled by transport. In contrast, a reach‐scale temporal metric of intermittence predicted DOC during the late growing season, when DOC is typically controlled by instream processes and when legacy effects of drying (e.g., diminished biological communities) would likely affect DOC. The effects of intermittence on DOC extend beyond temporal legacies at a point. Our results suggest that legacy effects of intermittence do not propagate downstream in this system. Instead, snapshots of spatial patterns of intermittence upstream of a reach are critical for understanding spatial patterns of DOC through connectivity to DOC sources, and these processes drive patterns of DOC even in perennial reaches. 相似文献
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As saline streams are geographically widespread in arid and semiarid regions, flow intermittence frequently occurs as another stressor factor apart from salinity. Rambla Salada is a temporary naturally saline stream with an intermittent reach upstream. This stream is an ideal scenario to study the effects of intermittent stream flow on macroinvertebrate community composition and biological traits in a naturally saline Mediterranean stream. This study analysed three sites with different hydrological regimes (one intermittent and two perennials). Flow intermittence exerted low pressure on the macroinvertebrate composition and biological traits which led to the loss of drought-intolerant species and taxa rather than acting as a selective force to promote desiccation-resistant taxa. Macroinvertebrate community at the intermittent site was a subset of the community found in perennial sites, and the presence of flier taxa at this site helped avoid flow cessation. These minor changes have consequences to assess the ecological quality of these saline temporary streams in the context of the Water Framework Directive, given the major differences revealed by some indices between the intermittent and perennial sites as the former obtained lower values due to the presence of few desiccation-intolerant species, which significantly increased the value of those biological indices. 相似文献
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Flow perturbation due to the orographic forcing at the top of a steep ridge has been investigated. Spectral and wavelet analyses of longitudinal and vertical wind velocity components highlight the presence of low-frequency perturbations produced by topography. Wavelet kurtosis reveals the intermittent character of these perturbations. The application of a multi-resolution data filter allows the detection of the topographically forced structures and the consequent statistical characterisation. The detected topographically forced structures remain active only for a 15% of the time on average. Nevertheless, they appear very efficient in the momentum transport, accounting for about 50% of the downward momentum flux produced in the low frequency range. Quadrant analysis highlights how topographical forcing produces the weakening of ejections and the strengthening of sweeps at the top of the ridge. 相似文献
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夜间近地面稳定边界层湍流间歇与增温 总被引：1，自引：1，他引：1
在夜间晴空条件下，近地面大气湍流表现出很强的间歇性，这种间歇现象导致夜间气温短时的急剧下降，随后大幅度增温。近地面大幅度增温表明此时存在着很大的湍流热通量散度，常通量层的概念这时不存在。从各高度层温度和风速变化的曲线上分析，我们发现湍流大多在距离地面较高一点的高度上发生发展，然后向下层传递，尽管上层的湍流可能是由于下层的某一触发机制向上传递而引起的。湍流偶尔也出现自下向上传递的过程，但这一过程较少发生。湍流的这种上下传递说明稳定边界层大气经常处于非平衡状态，在运用相似理论研究稳定边界层大气结构时要特别注意这一情形。 相似文献
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本文将辩证思维方法运用于西安地裂缝研究中,阐述了西安地裂缝的时空观和间隙性,指出地裂缝的活动和地震没有必然的联系,同时也不足以抑制一次大震的发生。 相似文献
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