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  • 1
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    Stream nitrate uptake and transient storage over a gradient of geomorphic complexity, north-central Colorado, USA (2011)
    Wiley
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    Publication Date: 2011-11-05
    Description: The understanding of nutrient uptake in streams is impeded by limited understanding of how geomorphic setting and flow regime interact with biogeochemical processing. This study investigates these interactions as they relate to transient storage and nitrate uptake in small agricultural and urban streams. Sites were selected across a gradient of channel conditions and management modifications and included three 180-m long geomorphically distinct reaches on each of two streams in north-central Colorado. The agricultural stream has been subject to historically variable cattle-grazing practices and the urban stream exhibits various levels of stabilization and planform alteration. Reach-scale geomorphic complexity was characterized using highly detailed surveys of channel morphology, substrate, hydraulics, and habitat units. Breakthrough-curve modeling of conservative bromide (Br - ) and non-conservative nitrate (NO 3 - ) tracer injections characterized transient storage and nitrate uptake along each reach. Longitudinal roughness and flow depth were positively associated with transient storage, which was related to nitrate uptake, thus underscoring the importance of geomorphic influences on stream biogeochemical processes. Additionally, changes in geomorphic characteristics due to temporal discharge variation led to complex responses in nitrate uptake. Copyright © 2011 John Wiley & Sons, Ltd.
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  • 2
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    Influences of sudden changes in discharge and physical stream characteristics on transient storage and nitrate uptake in an urban stream (2014)
    Wiley
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    Publication Date: 2014-06-25
    Description: Changes in the physical structure of urban streams can occur abruptly due to flashy high flow events and subsequently alter stream processes, including transient storage and nitrate uptake. We examined temporal variability in transient storage and nitrate uptake by exploring the effects of altered physical characteristics resulting from a single high flow event in three reaches of Spring Creek, an urban stream in Fort Collins, Colorado, USA. Study reaches of varying geomorphic and hydraulic characteristics were chosen to represent distinct geomorphic settings in terms of substrate size, sinuosity, bed slope, and degree of rehabilitation and structural controls. We performed detailed physical characterizations and multiple nutrient injections of Br − and NO 3 − to estimate transient storage and nitrate uptake in each reach. A comparison of pre- and post-flood data indicate that transient storage and nitrate uptake are highly context-specific and mediated by interactions between geomorphic setting and flood discharge. In the two reaches that showed significant post-flood increases in transient storage (250% to 350% increases in F med 200 ), the pool-riffle reach exhibited a significant increase in uptake velocity, while the channelized reach did not. In contrast, transient storage decreased post-flood in the third reach containing hydraulic structures. These complex responses likely reflect reach-specific differences in hyporheic vs. in-channel storage. This study shows that repeat injections are necessary to describe nutrient dynamics because transient storage and nitrate uptake can be highly variable over time (showing changes on the order of 100%) due to variation in discharge and geomorphically influential flow events. This article is protected by copyright. All rights reserved.
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  • 3
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    Satellite perspectives on the spatial patterns of new snowfall in the Southern Appalachian Mountains (2014)
    Wiley
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    Publication Date: 2014-03-18
    Description: ABSTRACT The Southern Appalachian Mountains (SAM) are a heavily forested mid-latitude mountain region which provide an ideal location for assessing the suitability of satellite-derived snow maps and explicitly linking atmospheric circulation to the spatial patterns of new snowfall. Although a variety of synoptic-scale circulation regimes contribute to mean annual snowfall, which ranges from roughly 25 cm in the lowest valleys to over 250 cm at the highest elevations, atmospheric circulation processes have largely been absent from efforts seeking to quantify the spatial patterns of new snowfall. In this paper, we examine the suitability of fractional snow cover (FSC) maps from the Moderate Resolution Imaging Spectroradiometer (MODIS) and determine the spatial extent of snowfall according to synoptic-scale circulation. FSC maps are analyzed after 122 snow events from 2006-12 to provide a suitability analysis of MODIS products for use in the SAM. For each event, we calculate FSC distribution, total snow covered area, and compare with available in-situ observations. Results indicate that the SAM presents unique meteorological, physical, and spectral characteristics that are ideal for evaluating the suitability of MODIS for measuring snow cover. Out of 122 observed snow events, 63 are considered suitable for analysis with the FSC maps. The highest FSC values are observed after Gulf/Atlantic Lows and southeastward tracking clipper systems. We conclude that MODIS data can be successfully used to link broader atmospheric circulation processes of snowfall with the spatial patterns of snow cover. This article is protected by copyright. All rights reserved.
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  • 4
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    Comparison of the Effects of Unidirectional and Sign-Alternating Temperature Gradients on the Sintering of Ice Spheres (2016)
    Wiley
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    Publication Date: 2016-10-28
    Description: To simplify the complex snow structures that occur in nature, polycrystalline ice spheres were produced and arranged vertically to model the sintering process. By controlling the temperatures on both the top and bottom of the ice sphere array, the effect of upward and downward vapor transfers was examined. The evolution of the neck areas between ice spheres was observed using X-ray Computed Micro-Tomography. As frequently observed under the basal part of a snow layer and previous experiments of snow temperature gradient metamorphism, depth hoar structures were formed along neck areas and their formation was found to be directly related to the vapor transfer direction. To model the TG inversion that can be induced in nature by daily cycles of radiative heating and cooling, we also performed sign-alternating temperature gradient experiments on the ice sphere arrays. The morphological evolution of the neck and the associated vapor transfer were examined through image analysis and 2D modeling. The final microstructures of the neck area turned out to be a symmetrical distribution of ice protrusions bridging neighboring ice spheres.
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  • 5
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    The high hydraulic conductivity of three wooded tropical peat swamps in northeast Peru: measurements and implications for hydrological function (2013)
    Wiley
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    Publication Date: 2013-05-09
    Description: The form and functioning of peatlands depend strongly on their hydrological status, but there are few data available on the hydraulic properties of tropical peatlands. In particular, the saturated hydraulic conductivity ( K ) has not previously been measured in Neotropical peatlands. Piezometer slug tests were used to measure K at two depths (50 and 90 cm) in three contrasting forested peatlands in the Peruvian Amazon; Quistococha, San Jorge, and Buena Vista. Measured K at 50 cm depth varies between 0.00032 and 0.11 cm s -1 , and at 90 cm it varies between 0.00027 and 0.057 cm s -1 . Measurements of K taken from different areas of Quistococha showed that spatial heterogeneity accounts for c. 20 % of the within-site variance, and that depth is a good predictor of K . However, K did not vary significantly with depth at Buena Vista and San Jorge. Statistical analysis showed that c. 18 % of the variance in the K -data can be explained by between-site differences. Simulations using a simple hydrological model suggest the relatively high K values could lead to lowering of the water table by 〉 10 cm within c. 48 m of the peatland edge for domed peatlands, if subjected to a drought lasting 30 days. However, under current climatic conditions, even with high K , peatlands would be unable to shed the large amount of water entering the system via rainfall through subsurface flow alone. We conclude that most of the water leaves these peatlands via overland flow and/or evapotranspiration. This article is protected by copyright. All rights reserved.
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  • 6
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    Spectrophotometric discrimination of river dissolved organic matter (2002)
    Wiley
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    Publication Date: 2002-01-01
    Description: There is a need to be able to differentiate the dissolved organic matter (DOM) fraction in river waters. Research in the 1970s and 1980s has attempted to utilize both absorbance and flourescence to distinguish between DOM fractions in river waters, but both were limited by the available technology. Total organic carbon content has, therefore, been widely used as a standard method of measuring DOM concentration, although it has little power to differentiate DOM fractions. Recent advances in flourescence spectrophotometry enable rapid and optically precise analysis of DOM. Here, we show how a combination of both flourescence and absorbance can be used to discriminate statistically between spatial variations of DOM in tributaries in a small catchment of the Ouseburn, NE England. The results of the discriminant analysis suggest that about 70% of the samples can be correctly classified to its tributary. Discriminant function 1 explains 60·8% of the variance in the data and the fulvic-like fluorescence intensity has the largest absolute correlation within this function; discriminant function 2 explains a further 21·5% of the variance and the fulvic-like fluorescence emission wavelength has the largest absolute correlation within this function. The discriminant analysis does not correctly classify all tributaries every time, and successfully discriminates between the different tributaries 70% of the time. Occasions when the tributary waters are less well discriminated are due to either episodic pollution events (at two sites) or due to tributaries that have strong seasonal trends in spectrophotometric parameters, which allows the sites to be misclassified. Results suggest that spectrophotometric techniques have considerable potential in the discrimination of DOM in rivers. © 2002 John Wiley and Sons, Ltd.
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  • 7
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    Land use and water quality (2003)
    Wiley
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    Publication Date: 2003-01-01
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  • 8
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    Spectrophotometric properties of surface water dissolved organic matter in an afforested upland peat catchment (2008)
    Wiley
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    Publication Date: 2008-01-01
    Description: Many upland catchments in the UK have undergone afforestation; their characteristic waterlogged soils require extensive pre-plantation ground drainage to allow tree establishment. In peatland areas this can result in very highly coloured runoff and enhanced dissolved organic matter (DOM) export in rivers of naturally high concentrations. In 1966, the Coalburn Experimental Catchment, northern England, was established to investigate the impact of afforestation on an upland peat catchment. Here we report the variations in DOM spectrophotometric properties of streamflow in the catchment at canopy closure, especially with respect to potential carbon sources within the artificial drainage ditches. Drainage ditches are characterized by water that has higher absorption coefficients and which is more highly coloured than in the catchment tributaries. Ditched, afforested areas produce more highly-coloured runoff waters that are more fluorescent and absorbent normalized to carbon concentration compared to ditches in open moorland. Ditches that had been experimentally re-excavated have organic matter of different spectrophotometric character, with higher dissolved organic carbon concentration and less aromatic or lower molecular weight material. It is hypothesized that this is due to the exposure of bare peat faces within and adjacent to the ditches that are more susceptible to drying compared to vegetated areas. The large extent of this drainage network acts as both a rapid transport network increasing hydrological connectivity and a pool for the storage of DOM, which is of different spectrophotometric character under low flow conditions, depending on management conditions. Copyright © 2007 John Wiley & Sons, Ltd.
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  • 9
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    Structural evolution during ice-sphere sintering (2010)
    Wiley
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    Publication Date: 2010-06-10
    Description: Two-dimensional ice-sphere arrays were produced in a cold room and studied as a geometrically simplified model of dry snow aggregates. The structural evolution of the ice-sphere arrays under either a quasi-isothermal condition or a temperature gradient was examined temporally. Two observational techniques were combined for the structural characterization. Optical microscopy was used to perform periodic observations, whereas scanning electron microscopy was used to both acquire high-resolution images and confirm the hexagonal crystal structure. Different thermal conditions led to the development of different morphologies of necks between two ice spheres by affecting the water vapour transfer in the pore spaces and the sublimation-condensation of water molecules. The microstructure of necks could also play a significant role in the structure development at later times by inducing or enhancing the local temperature gradient. This feature was discussed on the basis of steady-state heat transfer modelling of a two-sphere structure. Copyright © 2010 John Wiley & Sons, Ltd.
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  • 10
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    Observations of the morphology and sublimation-induced changes in uncoated snow using scanning electron microscopy (2010)
    Wiley
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    Publication Date: 2010-01-01
    Description: The morphology of a fresh-snow specimen and the sublimation-induced structural changes in an individual snow crystal were examined using a high-vacuum scanning electron microscope (SEM) equipped with a cryo system. The snow specimen was collected during a snowfall in Hanover, NH, USA, and was initially maintained at liquid nitrogen temperature in order to prevent any structural changes. Without coating, snow crystals can sublimate in the SEM chamber at a rate that is mainly determined by the temperature of the specimen. After the initial morphology was examined at -180 oC, sublimation was initiated by slowly increasing the stage temperature, and changes in morphology were recorded by taking images of an individual snow crystal intermittently and recording the temperatures and times. Severe morphological destructions due to sublimation occurred at around -150 oC. This approach allowed us to closely observe both the initial morphology and the structural changes in snow crystals as they underwent sublimation, a significant process during snow metamorphism. These observations also serve as a reference for temperature selection in the study of uncoated snow with a similar morphology using a high-vacuum SEM. Copyright © 2010 John Wiley & Sons, Ltd.
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