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81.
Stream temperature and riparian microclimate were characterized for a 1·5 km wildfire‐disturbed reach of Fishtrap Creek, located north of Kamloops, British Columbia. A deterministic net radiation model was developed using hemispherical canopy images coupled with on‐site microclimate measurements. Modelled net radiation agreed reasonably with measured net radiation. Air temperature and humidity measured at two locations above the stream, separated by 900 m, were generally similar, whereas wind speed was poorly correlated between the two sites. Modelled net radiation varied considerably along the reach, and measurements at a single location did not provide a reliable estimate of the modelled reach average. During summer, net radiation dominated the surface heat exchanges, particularly because the sensible and latent heat fluxes were normally of opposite sign and thus tended to cancel each other. All surface heat fluxes shifted to negative values in autumn and were of similar magnitude through winter. In March, net radiation became positive, but heat gains were cancelled by sensible and latent heat fluxes, which remained negative. A modelling exercise using three canopy cover scenarios (current, simulated pre‐wildfire and simulated complete vegetation removal) showed that net radiation under the standing dead trees was double that modelled for the pre‐fire canopy cover. However, post‐disturbance standing dead trees reduce daytime net radiation reaching the stream surface by one‐third compared with complete vegetation removal. The results of this study have highlighted the need to account for reach‐scale spatial variability of energy exchange processes, especially net radiation, when modelling stream energy budgets. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
82.
Michael J. Paterson 《Journal of Paleolimnology》1994,11(2):189-200
Remains of Cladocera were examined in short sediment cores from three Adirondack lakes with mean pHs below 5 and a fourth with a mean pH of 6.5. These cores were collected as part of the Paleoecological Investigation of Recent Lake Acidification (PIRLA I) project. Historical and paleolimnological evidence suggests that pH has decreased in each of the acid lakes in recent decades. In all of the study cores, the greatest changes in net accumulation rates, assemblage composition, and species richness occurred in recently deposited sediments. The similar timing of events in all lakes suggests that a regional disturbance was responsible. In the three acid lakes, there was a strong association of changes in cladoceran assemblages and diatom, chrysophyte, and geochemical evidence of acidification. The occurrence of recent changes in non-acid Windfall Pond indicates that other factors may also have affected Cladocera in the study lakes.This is the fifteenth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series. 相似文献
83.
84.
本文利用常规探测资料,分析了1983年3月25日发生在珠江三角洲地区的一次强对流天气过程背景流场的几个重要特征,如高空急流的结构与变化、辐散风场及与之相联系的背景次级环流、风场垂直切变和水平切变等. 相似文献
85.
D. M. Vaughn 《地球表面变化过程与地形》1990,15(6):525-537
Brush Creek drains a 76·1 km2 watershed within urban Kansas City, Missouri and eastern Kansas. A concrete-lined reach trending 6·1 km through the Plaza District of Kansas City, Missouri has been the focus for several major floods over the past ten years. Channel geometry, slope, and floodwater elevations were determined in the field for segments of the concrete-lined section of Brush Creek for a flood event that occurred on September 18, 1986. Discharge was computed by indirect methods and compared to a value determined from a rating curve established by the Water Resources Division of the U.S.G.S. Boundary shear stress, unit stream power, and average velocity were also computed in order to establish a quantitative relationship between sediment distribution, volume, and size fractions; and flow dynamics operating throughout the channel during this event. Boundary shear stress ranged from 91-96 Nm?2, stream power was 528-557 Wm?2, while average velocity was 5-8 ms?1. These values were sufficient to displace concrete slabs as large as 5 m long by 4·6 m wide by 0·23 m thick weighing an estimated 12 245 kg. As the channel was sediment free and unsecured prior to the flood, the distribution of deposits and subsequent channel scour provide valuable evidence for potentially hazardous sections of this urban stream. 相似文献
86.
Nikolaos P. Nikolaidis Vicki S. Nikolaidis Jerald L. Schnoor 《Aquatic Sciences - Research Across Boundaries》1991,53(4):330-345
Monte-Carlo simulations were used to assess the extent of shortterm alkalinity depressions occuring in Sierra Nevada lakes due to acidic deposition events. The Episodic Event Model (EEM) was used to simulate spring snowmelt events. Snow course data, precipitation data and lake acidification surveys were used to derive values for the EEM parameters. Spring snowmelt events were shown to have great impacts on the water quality of Sierran lakes. Lakes are likely to be most affected by the early-spring snowmelt event because the epilimnion depth is at a minimum, which indicates minimum dilution. Under annual average loading conditions, no Sierran lake has been reported as acidic although 29% of the lakes have alkalinities less than 40 µeq/L indicating a sensitivity to acidification. In simulations of early-spring snowmelt events, using present-day acidic loading conditions, it was estimated 79% ± 9% of the lakes would experience shortterm lake alkalinity depressions to levels less than 40 µeq/L. The results provided by the model simulations are valuable in establishing upper and lower limits on the extent of possible episodic acidification to lake-resources-at-risk. The most critical parameters controlling the magnitude of lake alkalinity depressions during snowmelt episodic events are a) the lake area to watershed area ratio — a measure of input loading, and b) the epilimnion volume — a measure of dilution and mixing. 相似文献
87.
88.
Ian C. Grieve 《地球表面变化过程与地形》1984,9(1):35-41
Dissolved organic matter (DOM) and iron in a moorland stream were determined at 8-hour intervals over a 6-month period to examine relationships with streamflow. Regression of both solutes on discharge were positive and explained 50–70 per cent of the variance in the solute data, but better predictions were obtained in both cases when a covariate reflecting temporal variation in the relationships was introduced (explained variance 80–90 per cent). Variations in the regression of Fe on DOM were also identified, indicating differences in the complexing power of DOM for Fe and possible variations in the chemical composition of the DOM. 相似文献
89.
Rock salt is approximately 1000 times more soluble than limestone and thus displays high rates of geomorphic evolution. Cave stream channel profiles and downcutting rates were studied in the Mount Sedom salt diapir, Dead Sea rift valley, Israel. Although the area is very arid (mean annual rainfall ≈ 50 mm), the diapir contains extensive karst systems of Holocene age. In the standard cave profile a vertical shaft at the upstream end diverts water from a surface channel in anhydrite or clastic cap rocks into the subsurface route in the salt. Mass balance calculations in a sample cave passage yielded downcutting rates of 0–2 mm s?1 during peak flood conditions, or about eight orders of magnitude higher than reported rates in any limestone cave streams. However, in the arid climate of Mount Sedom floods have a low recurrence interval with the consequence that long-term mean downcutting rates are lower: an average rate of 8·8 mm a?1 was measured for the period 1986–1991 in the same sample passage. Quite independently, long-term mean rates of 6·2mm a?1 are deduced from 14C ages of driftwood found in upper levels of 12 cave passages. These are at least three orders of magnitude higher than rates established for limestone caves. Salt cave passages develop in two main stages: (1) an early stage characterized by high downcutting rates into the rock salt bed, and steep passage gradients; (2) a mature stage characterized by lower downcutting rates, with establishment of a subhorizontal stream bed armoured with alluvial detritus. In this mature stage downcutting rates are controlled by the uplift rate of the Mount Sedom diapir and changes of the level of the Dead Sea. Passages may also aggrade. These fast-developing salt stream channels may serve as full-scale models for slower developing systems such as limestone canyons. 相似文献
90.
塔里木河在1958年前是一条淡水河,河水的矿化度在各段及不同季节均未超过1.0g/L,现全河段各月平均矿化度除8月洪水季节<1.0g/L,其余各月都在1.0g/L以上。上游阿拉尔水文站平均每年有5个月为1~3g/L,4个月为3~5g/L,2个月超过5g/L,最高5月份达到6.3g/L.阿拉尔的淡水所占比例不足年平均径流量的35%.下游卡拉全部为微咸水。造成塔里木河水矿化度升高的原因,除气候干旱、蒸发强烈、土壤含盐量高等自然因素外,主要是灌溉、排水不合理,上游三条主要支流灌溉引水增加,使补给干流的淡水资源量由1960年前的50.0亿m3减少到1990年后的42.0亿m3;与此同时大量农田排水泄入塔里木河,每年带入盐量达467.4万t.要改善塔里木河水质,必须减少农田排水泄入,实行咸、淡分流,退耕还林;并在源流区适度开发地下水,使三源流向塔里木河输水量每年不少于46.0亿m3。 相似文献