Intra-annual variability of urban effects on streamflow

While considerable research has established the impacts of urbanization on streamflow, there has been little emphasis on how intra-annual variations in streamflow can deepen the understanding of hydrological processes in urban watersheds. This study fills this critical research gap by examining, at the monthly scale, correlations between land-cover and streamflow, differences in streamflow metrics between urban and rural watersheds, and the potential for the inflow and infiltration (I&I) of extraneous water into sewers to reduce streamflow. We use data from 90 watersheds in the Atlanta, GA region over the 2013–2019 period to accomplish our objectives. Similar to other urban areas in temperate climates, Atlanta has a soil-water surplus in winter and a soil-water deficit in summer. Our results show urban watersheds have less streamflow seasonality than do rural watersheds. Compared to rural watersheds, urban watersheds have a much larger frequency of high-flow days during July–October. This is caused by increased impervious cover decreasing the importance of antecedent soil moisture in producing runoff. Urban watersheds have lower baseflows than rural watersheds during December–April but have baseflows equal to or larger than baseflows in rural watersheds during July–October. Intra-annual variations in effluent data from wastewater treatment plants provide evidence that I&I is a major cause of the relatively low baseflows during December–April. The relatively high baseflows in urban watersheds during July–October are likely caused by reduced evapotranspiration and the inflow of municipal water. The above seasonal aspects of urban effects on streamflow should be applicable to most urban watersheds with temperate climates.     

A new approach for measuring surface hydrological connectivity

The development of surface hydrological connectivity is a key determinant of flood magnitude in drylands. Thresholds in runoff response may be reached when isolated runoff-generating areas connect with each other to form continuous links to river channels, enabling these areas to contribute to flood hydrographs. Such threshold behaviour explains observed nonlinearities and scale dependencies of dryland rainfall–runoff relationships and complicates attempts at flood prediction. However, field methods for measuring the propensity of a surface to transmit water downslope are lacking, and conventional techniques of infiltration measurement are often inappropriate for use on non-agricultural drylands. Here, we argue for a reconceptualization of the dryland surface runoff process, suggesting that the downslope transfer of water should be considered alongside surface infiltration; that is, there is a need for the “aggregated” measurement of infiltration and overland flow hydraulics. Surface application of a set volume of water at a standardized rate generates runoff that travels downslope; the distance it travels downslope is determined by infiltration along the flow, integration of flow paths, and flow resistance. We demonstrate the potential of such a combined measurement system coupled with structure-from-motion photogrammetry to identify surface controls on runoff generation and transfer on dryland hillslopes, with vegetation, slope, surface stone cover, and surface roughness all having a significant effect. The measurement system has been used on slopes up to 37° compared with the flat surface typically required for infiltration methods. On average, the field workflow takes ~10–15 min, considerably quicker than rainfall simulation. A wider variety of surfaces can be sampled with relative ease, as the method is not restricted to stone and vegetation-free land. We argue that this aggregated measurement represents surface connectivity and dryland runoff response better than standard hydrological approaches and can be applied on a much greater variety of dryland surfaces.     

Widespread fluid infiltration during metamorphism of the Witwatersrand goldfields: generation of chloritoid and pyrophyllite

Abstract Chloritoid and pyrophyllite occur together in all major goldfields of the Witwatersrand Basin and are widespread in virtually all rock types of the upper Witwatersrand Supergroup, including metaconglomeratic reefs and altered mafic rocks. Both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages, but they are also developed locally in the regionally persistent metapelites that have basin-wide extent. Pyrophyllite is particularly common in foliated zones, adjacent to quartz veins, and near unconformably overlying auriferous conglomerates. The wide distribution of chloritoid and pyrophyllite in metapelites of the Witwatersrand Basin is attributed to alteration of chlorite-rich shales, rather than to unusual premetamorphic starting materials. This alteration event involved the redistribution of many elements, with up to 40% volume loss, mainly due to removal of silica. Removal of most of the Mg and some Fe accounts for the stabilization of chloritoid and pyrophyllite. Relatively immobile elements included Al, Ti, Nb, Cr, V, P, La and Ce, whereas Si, Fe, Mn, Zn, Co, Ni, Cu, Mg and Ca were lost, and K, Rb and Ba were introduced by an infiltrating fluid. The alteration event is inferred to have been within the chloritoid and pyrophyllite stability field (and thus syn-metamorphic) as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite, rather than to lower grade minerals such as kaolinite. Muscovite–chlorite–chloritoid and muscovite–chloritoid–pyrophyllite assemblages are attributed to fluid buffering along appropriate curves, as their production by metamorphism of lower grade mineral mixes is considered unlikely, based on the present bulk rock compositional data. A metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration. The transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration. Widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite, quartz veining and textures. Fluid:rock ratios calculated from a silica budget in one metapelitic horizon exceed 100:1 over many square kilometres. These values need not imply multi-pass fluid flow, as much of the silica migration may be redistribution on a scale of a few metres, from source rocks into veins. Although infiltration during metamorphism may have affected much of the upper Witwatersrand succession, channelized fluid flow within reef packages, along faults and unconformities and in certain metaconglomerates and metapelites is inferred.     

西北干旱区间歇性河流与含水层水量交换研究进展与展望

本文依托国家自然科学基金青年项目“河水温度对干旱区宽浅型河床渗透系数影响的定量研究”,并梳理了其他相关研究,总结了中国西北干旱区间歇性河流与含水层水量交换研究所面临的基础科学问题及当前所取得的研究进展。主要结论为：以宽浅型沙质河床为基本特征的西北干旱区内陆河流域下游河流,其河床每年经历数次干湿交替与冻融过程。受河水温度与河流水动力条件的影响,河床作为河流与含水层相互作用的重要界面,其渗透性能具有高度时空变异性,已成为河水与地下水水量交换研究的难点与热点。分析指出,在环境变化和当前交叉学科迅猛发展的背景下,以河流与含水层相互作用为核心的潜流带水文学发展面临新的机遇与挑战。     

From orthogneiss to migmatite: Geochemical assessment of the melt infiltration model in the Gföhl Unit (Moldanubian Zone, Bohemian Massif)

The Gföhl Unit is the largest migmatite terrain of the Variscan orogenic root domain in Europe. Its genesis has been until now attributed to variable degrees of in situ partial melting. In the Rokytná Complex (Gföhl Unit, Czech Republic) there is a well-preserved sequence documenting the entire migmatitization process on both outcrop and regional scales. The sequence starts with (i) banded orthogneiss with distinctly separated monomineralic layers, continuing through (ii) migmatitic mylonitic gneiss, (iii) schlieren migmatite characterised by disappearance of monomineralic layering and finally to (iv) felsic nebulitic migmatite with no relics of the original banding.

While each type of migmatite shows a distinct whole-rock geochemical and Sr–Nd isotopic fingerprint, the whole sequence evolves along regular, more or less smooth trends for most of the elements. Possible mechanisms which could account for such a variation are that the individual migmatite types (i) are genetically unrelated, (ii) originated by equilibrium melting of a single protolith, (iii) formed by disequilibrium melting (with or without a small-scale melt movement) or (iv) were generated by melt infiltration from external source. The first scenario is not in agreement with the field observations and chemistry of the orthogneisses/migmatites. Neither of the remaining hypotheses can be ruled out convincingly solely on whole-rock geochemical grounds. However in light of previously obtained structural, petrologic and microstructural data, this sequence can be interpreted as a result of a process in which the banded orthogneiss was pervasively, along grain boundaries, penetrated by felsic melt derived from an external source.

In terms of this melt infiltration model the individual migmatites can be explained by different degrees of equilibration between the bulk rock and the passing melt. The melt infiltration can be modelled as an open-system process, characterised by changes of the total mass/volume and accompanied by gains/losses in many of the major- and trace elements. The modelling of the mass balance resulted in identification of a component added by a heterogeneous nucleation of feldspars, quartz and apatite from the passing melt. This is in line with the observed presence of new albitic plagioclase, K-feldspar and quartz coatings as well as resorption of relict feldspars. At the most advanced stages (schlieren and nebulitic migmatites) the whole-rock trace-element geochemical variations document an increasing role for fractional crystallization of the K-feldspar and minor plagioclase, with accessory amounts of monazite, zircon and apatite.

The penetrating melt was probably (leuco-) granitic, poor in mafic components, Rb rich, with low Sr, Ba, LREE, Zr, U and Th contents. It probably originated by partial melting of micaceous quartzo-feldspathic rocks.

If true and the studied migmatites indeed originated by a progressive melt infiltration into a single protolith resembling the banded orthogneiss, this until now underappreciated process would have profound implications regarding rheology and chemical development of anatectic regions in collisional orogens.     

Micropiping processes and biancana evolution in southeast Tuscany, Italy

Biancane badlands consisting of small domes dissected by rills and micropipes, with rough disordered microrehef, can be found along the Apennines in Italy. The dominant processes forming biancane differ from those of badlands formed on smectite-rich mudrocks, as micropipes associated with pseudokarstic enlargement of pores and cracks predominate and form the main routes for evacuation of eroded material.Biancana evolution is controlled by water infiltration into intact bedrock, producing an erodible weathering ‘rind’ which is more porous than intact rock. This rind is easily removed by rill or micropipe flow, and erosion is therefore ‘weathering-controlled’, depending on rind production by infiltrating water. Infiltration is initially slow and stepped, due to slow water movement through very small capillary pores in intact rock alternating with rapid filling of macropores and cracks. This occurs due to rapid matrix pore enlargement by dispersion and/or dissolution. The infiltration pattern is accurately reproduced by a model built on progressive development of weathering layers by moisture penetration. Model results are consistent with weathering rind depths and erosion observed in the field, and show that a pipe network can be generated on newly exposed rock by the rainfall of one year.Propagation of the pipe network diverts a progressively larger proportion of runoff into micropipes, expanding weathering rind production within the biancana as well as on the surface. Internal weathering and flow progressively dominate with few unweathered corestones, and the biancana gradually collapses into a penultimate “soufflé-like” form.     

三轴应力状态下单裂隙灾水机理的实验研究

介绍了单裂隙试样在不同三轴应力状态下，其导水性能的变化规律，充填物对裂隙导水性的影响，并得出了突水判据。     

Porosity and size gradation of saturated gravel with percolated fines

Fine particles may infiltrate through coarse alluvial beds and eventually saturate the subsurface pore space. It is essential to understand the conditions that lead to bed saturation, and to forecast the packing characteristics of saturated beds to assess the effect of excess fine sediment supply on a number of processes that occur in the stream–sediment boundary. To address this problem, in this study, a new method is introduced to predict the grain‐size distribution for the saturated condition, and the resulting porosity decrease, given the characteristics of the bed and the supplied sediments. The new method consists of the numerical aggregation of infilling fines in a finite bed volume, during which the bed properties change to affect further infilling. An existing semi‐empirical, particle packing model is implemented to identify these properties. It is shown that these types of models are adequate to describe regimes of natural sediment fabric quantitatively, and are thus useful tools in the analysis of sediment infiltration processes. Unlike previous developments to quantify saturated bed conditions, which assume that the supplied material is uniform and finer than the bed pore openings, the method developed herein considers poorly sorted fines, and can identify size fractions that are able to ingress into the bed due to being smaller than the particles that form the bed structure. Application of the new method to published experimental data showed that the final content of infiltrated fines is strongly sensitive to the initial bed packing density, highlighting the need to measure and understand open‐work gravel deposits. In addition, the new method was shown to be suitable for assessing the degree of bed saturation, when it was applied to a published data set of field samples.     

Effects of Coal Gangue Content on Water Movement and Solute Transport in a China Loess Plateau Soil

The mining industry has grown strongly in China in recent decades, resulting in large amounts of coal gangues, which cause water and soil pollution, soil erosion, and various other environmental problems. They are often used in reclamation projects in attempts to restore land damaged by mining, hence they are frequently present (in widely varying proportions) in the topsoil in areas around mines. Their presence can strongly affect key soil variables, including its bulk density, structure, water retention, water movement, and solute transport rates. In the study presented here, the effects of gangue contents on infiltration, saturated hydraulic conductivity, and solute transport parameters of a Chinese Loess plateau soil were examined. The results show that infiltration rates and saturated hydraulic conductivity decreased with increasing gangue content. The Peck–Watson equation modeled these relationships well, but Bouwer–Rice equations provided poorer matches with the acquired data. Cumulative infiltration over time was described well by both the Philip equation and Kostiakov equation. Both the simplified convection–dispersion equation and a two‐region model described the solute transport processes well. In addition, the dispersion increased, while both the Peclet number and mobile water fraction decreased, with increases in gangue contents.