DOC budgets of drained peat catchments: implications for DOC production in peat soils

The blocking of drainage ditches in peat has been proposed as a possible mitigation strategy for the widely observed increases in dissolved organic carbon (DOC) concentrations from northern peatlands. This study tested the hypothesis that drain‐blocking could lead to lower DOC concentrations by measuring the DOC export from a series of small peat‐covered catchments over a period of 2 years. Six catchments were chosen: two were pristine that had never been drained; three where drains had been blocked (one in 1995, and two in 2003); and a control peat drain catchment where the drain was left unblocked throughout the study. In the case where drains were blocked as part of thus study, the drains were observed for 2 months before blocking and 2 years after blocking. The results show that: (i) high concentrations of DOC can come from water ponded in the drain; (ii) the DOC export (flux of DOC per area of catchment) from the six study catchments shows a high degree of positive correlation with both catchment size and water yield; (iii) distinctly lower DOC export with water yield was observed for the catchments containing higher‐order channels (>27 500 m²) as opposed to single drain catchments (>7500 m²); (iv) drain‐blocking resulted in a statistically significant decrease in DOC export (average was 39%) but the effect upon DOC concentration explained only 1% of the variance in the data. The results suggest that drain blocking works by decreasing the flow from the drain, not by changing the production of DOC in the peat. The change in export with catchment size implies a considerable removal of DOC from large catchments. Copyright © 2009 John Wiley & Sons, Ltd.     

Seasonal controls on DOC dynamics in nested upland catchments in NE Scotland

Spatial and temporal variability of hydrological responses affecting surface water dissolved organic carbon (DOC) concentrations are important for determining upscaling patterns of DOC export within larger catchments. Annual and intra‐annual variations in DOC concentrations and fluxes were assessed over 2 years at 12 sites (3·40–1837 km²) within the River Dee basin in NE Scotland. Mean annual DOC fluxes, primarily correlated with catchment soil coverage, ranged from 3·41 to 9·48 g m^?2 yr^?1. Periods of seasonal (summer–autumn and winter–spring) DOC concentrations (production) were delineated and related to discharge. Although antecedent temperature mainly determined the timing of switchover between periods of high DOC in the summer‐autumn and low DOC in winter‐spring, inter‐annual variability of export within the same season was largely dependent on its associated water flux. DOC fluxes ranged from 1·39 to 4·80 g m^?2 season^?1 during summer–autumn and 1·43 to 4·15 g m^?2 season^?1 in winter–spring.Relationships between DOC areal fluxes and catchment scale indicated that mainstem fluxes reflect the averaging of highly heterogeneous inputs from contrasting headwater catchments, leading to convergent DOC fluxes at catchment sizes of ca 100 km². However, during summer–autumn periods, in contrast to winter–spring, longitudinal mainstem DOC fluxes continue to decrease, most likely because of increasing biological processes. This highlights the importance of considering seasonal as well as annual changes in DOC fluxes with catchment scale. This study increases our understanding of the temporal variability of DOC upscaling patterns reflecting cumulative changes across different catchment scales and aids modelling of carbon budgets at different stages of riverine systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Release of dissolved organic carbon from upland peat

This study examines the release of dissolved organic carbon (DOC) from upland peat during the period of the autumn flushing. Hydroclimatic conditions were monitored in conjunction with measurements of absorbance and the E4/E6 ratio of the stream draining an 11·4 km² upland peat catchment in northern England. During two months of monitoring the effects of 67 separate rainfall events were examined showing that:

• The peat behaves hydrologically as if it were a two end‐member system consisting of old, interevent, and new, event, water. Runoff is initiated by percolation excess of new water at the acrotelm–catotelm interface.
• The discharge of dissolved organic matter behaves like a three end‐member system with the between‐event water being low in DOC and storm events being characterized by two types of water. Initial runoff being characterized by new water rich in DOC that gives way to new water depleted in DOC. This transition can be ascribed to the runoff progressing from throughflow within the acrotelm progressing to saturation‐excess overland flow.
• Depletion of DOC during storm events is accompanied by a change in the character of the DOC as the E4/E6 ratio changes. This suggests that the decrease in DOC during events is the result of exhaustion of reserves rather than changes in the flowpaths being utilized by runoff.
• The amount of carbon released in any event is critically dependent upon the time between events during which oxidation processes generate a reservoir of available carbon. Production of available carbon in the catchment is as high as 4·5 g C per day per m³ of peat, suggesting a turnover rate of peat of the order of 42 years. Copyright © 2002 John Wiley & Sons, Ltd.

     

Fluvial carbon flux from headwater peatland streams: significance of particulate carbon flux

The extensive blanket peatlands of the UK uplands account for almost half of total national terrestrial carbon storage. However, much of the blanket peat is severely eroded so that the contemporary role of the peatland system in carbon sequestration is compromised by losses of organic carbon in dissolved (DOC) and particulate (POC) form in the fluvial system. This paper presents the first detailed assessment of dissolved and organic carbon losses from a severely eroded headwater peatland (River Ashop, South Pennines, UK). Total annual fluvial organic carbon losses range from 29–106 Mg C km,^‐2 decreasing from the headwaters to the main catchment outlet. In contrast to less eroded systems fluvial organic carbon flux is dominated by POC. POC:DOC ratios decrease from values of 4 in the headwaters to close to unity at the catchment outlet. These results demonstrate the importance of eroding headwater sites as sources of POC to the fluvial system. Comparison with a range of catchment characteristics reveals that drainage density is the best predictor of POC:DOC but there is scatter in the relation in the headwaters. Steep declines in specific POC yield from headwater catchments are consistent with storage of POC within the fluvial system. Key to the significance of fluvial carbon flux in greenhouse gas budgets is understanding the fate of fluvial carbon. Further work on the fate of POC and the role of floodplains in fluvial carbon cycling is urgently required. Copyright © 2012 John Wiley & Sons, Ltd.     

Long‐term records of dissolved organic carbon flux from peat‐covered catchments: evidence for a drought effect?

This study considers three long records of dissolved organic carbon (DOC) flux from two catchments with peat‐covered headwaters. The catchments vary in size from 11 to 818 km² and the records are at least 12 years old, with one record going back to 1965. The study compares both annual and monthly DOC flux records with a range of hydroclimatic indicators in order to test which component of droughts may contribute to increasing DOC flux. The study found that: (1) there was no significant correlation between any of the proposed drought variables and DOC flux in any of the study catchments over periods of up to 34 years; (2) the most important variable for explaining the DOC flux was the runoff from the catchments overlying a seasonal cycle and an underlying upward trend was present in some records; (3) the residual time‐series, after removal of the best‐fit models, showed no evidence of increased production after times of severe drought. The lack of any evidence for any additional biogeochemical reactions associated with drought supports evidence that DOC loss from peat is limited by its solubility and that its production is fast on the time‐scale of runoff events. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of Ultrafiltration for Characterization of the DOC of Elbe River Water

An ultrafiltration methodology and the results of its application on Elbe river water are presented. Compared with other methods, Ultrafiltration allows fractionation of the molecular-weight fractions of DOC for higher volumes of solution in a shorter time. To characterize the molecular-weight fractions, the sum parameter DOC (β(DOC)) and the absorption coefficient at 254 nm a₂₅₄ were measured, together with determination of a₂₅₄/ β(DOC) for further characterization of the DOC fractions and to provide information about changes in parts of the UV-active substances of the total dissolved organic material. In the period May 1992 to November 1994, the DOC molecular-weight composition of 65 samples of Elbe river water were determined. At two sampling locations (Torgau-East and Meissen), the DOC content β(DOC) was approximately 6 mg/L. The high-molecular weight fraction (> 10000 g/mol) gave a mean of 13% of the total DOC content β₀(DOC). The measurements showed that the molecular composition of DOC in the Elbe does not greatly differ. There was also no clear seasonal influence on the molecular-weight composition of the DOC content. During the period of investigation, the percentage part of β(DOC) and a₂₅₄/β(DOC) of the high-molecular weight fraction of the total dissolved organic material increased, while the a₂₅₄/β(DOC) values of the middle molecular-weight fraction remained fairly constant. The dependence of β(DOC) on discharge of the Elbe river is low. Changes in molecularweight composition of DOC were only observed during the steepest increase in river discharge during a flood event, with another spectrum of substances responsible for the increase in β(DOC) when compared with the β(DOC) for low or middle water level.     

干旱区高山泥炭磁学特性研究

对干旱区高山—新疆阿尔泰山中段连续的泥炭沉积序列进行详细系统的磁学分析,获得泥炭沉积物中磁性矿物的类型、含量以及粒径大小等磁学特性,探讨了在富含大量有机质的氧化还原条件下磁性矿物的保存与变化机理.岩石磁学结果表明沉积物中亚铁磁性矿物的富集程度低,磁性较弱.主要含有磁铁矿、赤铁矿、顺磁性矿物以及大量的抗磁性矿物组分,并且证实泥炭沉积物中不可能含有生物成因的趋磁细菌.沉积物的磁性颗粒主要以细颗粒为主,但同时还存在粗颗粒成分.研究结果指示在泥炭表层酸性的亚氧环境中,亚铁磁性矿物在较短的时间内伴随着部分溶解和改造,导致沉积物磁性浓度的降低和粒径的减小,快速的沉积和埋藏之后,长期处于缺氧的碱性还原环境下,磁铁矿发生的变化很小或基本不会再次被改造.     

Fluvial carbon export and CO2 efflux in representative nested headwater catchments of the eastern La Plata River Basin

This study involved a baseline evaluation of fluvial carbon export and degas rates in three nested rural catchments (1 to 80 km²) in Taboão, a representative experimental catchment of the Upper Uruguay River Basin. Analyses of the carbon content in stream waters and the catchment carbon yield were based on 4‐year monthly in situ data and statistical modeling using the United States Geological Survey load estimator model. We also estimated p CO₂ and degas fluxes using carbonate equilibrium and gas‐exchange formulas. Our results indicated that the water was consistently p CO₂ saturated (~90% of the cases) and that the steep terrain favors high gas evasion rates. The mean calculated fluvial export was 5.4 tC·km^?2·year^?1 with inorganic carbon dominating (dissolved inorganic carbon:dissolved organic carbon ratio >4), and degas rates (~40 tC km^?2·year^?1) were nearly sevenfold higher than the downstream export. The homogeneous land use in this nested catchment system results in similar water‐quality characteristics, and therefore, export rates are expected to be closely related to the rainfall–runoff relationships at each scale. Although the sampling campaigns did not fully reproduce storm‐event conditions and related effects such as flushing or dilution of in‐stream carbon, our results indicated a potential link between dissolved inorganic carbon and slower hydrological pathways related to subsurface water storage and movement.     

气候变化对长江流域汉江和赣江径流的影响

近几年来,国家气候中心己经建立了中国主要四大流域气候对水资源影响评估的模式框架.本文拟进一步证明其中之一的两参数分布式月水量平衡水文模式对长江之上汉江和赣江两子流域径流的模拟能力,结果表明该水文模式对目前气候条件下径流模拟效果较好,运行稳定,可用于实时业务运行.在此基础上,利用ECHAM4和HadCM2两GCM(General Circulation Model)未来气候情景模拟结果及目前实测气候情况,对汉江和赣江两子流域的径流对未来气候变化的敏感性进行评估.经检验,两GCM对未来气候,特别是降水情景模拟存在一定差异,因此,造成径流对气候变化的响应不同,这充分反映了全球模式模拟结果不确定性在气候变化影响研究中的重要性.     

A comparative overview of weathering intensity and HCO3 flux in the world's major rivers with emphasis on the Changjiang,Huanghe, Zhujiang (Pearl) and Mississippi Rivers

In this paper, general relationships of riverine bicarbonate concentrations and fluxes as a function of drainage basin mineral content and runoff are examined using a database of the 25 largest rivers in the world. Specific HCO₃⁻ flux normalized to unit basin area, which peaks in the mid latitudes, was found to be strongly correlated with the carbonate mineral content of river basins, while river HCO₃⁻ concentration was related to the balance of precipitation and evaporation. Within this global context, the weathering patterns of CO₂ in a few large rivers (Changjiang, Huanghe, Pearl, and Mississippi rivers) were examined in further detail. The Zhujiang (Pearl River), especially its largest branch (Xijiang), was characterized by the highest specific weathering rate among all the world's large rivers due to an exceptionally high carbonate mineral content (over 80%) in its drainage basin and its warm and wet environment. It has a moderate level of HCO₃⁻ concentration, however, due to dilution by relatively high precipitation in the watershed. In stark contrast, the Huanghe (Yellow River) has one of the lowest specific weathering rates because of low carbonate mineral content and a dry climate. However, it has a high HCO₃⁻ concentration due largely to the concentrating effects of high evaporative water loss, as a result of arid weather and the agricultural use of water through irrigation systems, as well as carbonate-containing surficial deposits (i.e., loess). The strong correlation between specific HCO₃⁻ fluxes and discharge in all four rivers with different discharge seasonality suggests that higher precipitation in drainage basins promotes higher weathering rates.     

Flow prediction in high altitude ungauged catchments: A case study in the Italian Alps (Pantano Basin,Adamello Group)

We develop a simple model to evaluate the daily flow discharges in the ablation season for the 11 km² Pantano basin in the Retiche Italian Alps, based upon the data gathered during a three years field campaign. The Pantano basin embeds the Venerocolo debris covered and the Avio debris free glaciers, covering 2.14 km² in the Adamello Group, where the widest Italian glacier Adamello is located. First, degree-day models based upon air temperature are tuned to calculate snow and ice melt at daily scale. Glaciers’ meteorological data are collected from an automatic weather station (AWS), operating on the glacier during summer 2007. The melt factors in the debris covered areas of the glacier are estimated against debris thickness, using a data driven parameterization. The flow discharge from the catchment is estimated using semi distributed flow routing for the ablation seasons of four years, from 2006 to 2009. The predicted discharges are compared to those derived from inverse reservoir's routing at the Benedetto lake, catching the basin outflow. The proposed approach is valuable as a tool to investigate the hydrology of poorly gauged glacierized areas, including those with debris covered ice, widely diffused and yet poorly understood. Pending accurate parameterization the approach is usable for water resources evaluation and for long term assessment of the climate change impact on the glacierized areas within the Alps.     

Predicting average annual groundwater levels from climatic variables: an empirical model

On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment.     

乌江磷浓度与通量变化及来源解析

磷(P)是长江流域备受关注的污染物。乌江是长江八大支流之一,位于三峡水库近库尾江段。武隆断面是乌江入长江控制断面。对1998—2019时期武隆断面径流量、悬浮泥沙浓度(SS)与输沙量、磷浓度与通量(包括总磷(TP)、溶解态磷(DP)和颗粒态磷(PP))年际变化及季节特征进行研究,并基于河流基流分割原理对磷的来源进行了解析。结果表明,(1)1998—2019年,乌江武隆断面径流量在一定幅度内上下波动,而悬浮泥沙浓度和输沙量下降剧烈。(2)22年来,乌江TP和DP浓度与通量总体上呈先升高后下降的趋势,2009—2013年为磷污染峰值期,TP和DP浓度与通量远高于其它时期。(3)2007年是一个重要的时间节点,该节点前,TP的赋存形态以颗粒态为主,颗粒态磷在总磷中的占比均值为65%;该节点后,TP的赋存形态转变为以溶解态为主,颗粒态磷占比均值下降为16%,相应地,溶解态磷占比由35%上升为84%。水沙条件改变是磷形态发生显著变化的主要驱动力,磷污染程度亦是磷形态变化的重要影响因素。(4)磷通量在年内的季节分布发生了显著变化,丰水期磷通量减少,枯水期磷通量增加。(5)1998—2012、2009—2013和2014—2019年3个时期点源负荷占比分别为23.5%、36.8%和62.1%,呈增加趋势。(6)建议制定适宜的总磷控制目标,结合目前所存在的磷污染风险点,进一步强化监管,侧重源头治理。     

Surface soil moisture retrievals from remote sensing: Current status,products & future trends

Advances in Earth Observation (EO) technology, particularly over the last two decades, have shown that soil moisture content (SMC) can be measured to some degree or other by all regions of the electromagnetic spectrum, and a variety of techniques have been proposed to facilitate this purpose.In this review we provide a synthesis of the efforts made during the last 20 years or so towards the estimation of surface SMC exploiting EO imagery, with a particular emphasis on retrievals from microwave sensors. Rather than replicating previous overview works, we provide a comprehensive and critical exploration of all the major approaches employed for retrieving SMC in a range of different global ecosystems. In this framework, we consider the newest techniques developed within optical and thermal infrared remote sensing, active and passive microwave domains, as well as assimilation or synergistic approaches. Future trends and prospects of EO for the accurate determination of SMC from space are subject to key challenges, some of which are identified and discussed within.It is evident from this review that there is potential for more accurate estimation of SMC exploiting EO technology, particularly so, by exploring the use of synergistic approaches between a variety of EO instruments. Given the importance of SMC in Earth’s land surface interactions and to a large range of applications, one can appreciate that its accurate estimation is critical in addressing key scientific and practical challenges in today’s world such as food security, sustainable planning and management of water resources. The launch of new, more sophisticated satellites strengthens the development of innovative research approaches and scientific inventions that will result in a range of pioneering and ground-breaking advancements in the retrievals of soil moisture from space.     

中国8大湖泊冬季水-气界面甲烷通量初步研究

采用静态箱-气相色谱法对中国不同地区的8个湖泊(洞庭湖、鄱阳湖、巢湖、南四湖、洪泽湖、抚仙湖、洱海、滇池)冬季水体水-气界面甲烷(CH4)通量进行了24 h连续观测,对中国湖泊冬季CH4的总释放量进行了估算.结果表明:鄱阳湖、巢湖、南四湖、洪泽湖和滇池24 h内各时段均为大气CH4的源,其通量分别为0.818、0.021、0.034、0.019、0.163mg/(m2·h);洞庭湖、抚仙湖和洱海部分时段为大气CH4的汇,但从24 h平均通量来看,仍为大气CH4的源,通量分别为0.199、0.012、0.044mg/(m2·h).冬季湖泊水体CH4通量空间差异较小,其大小主要受风速的影响,与水温、箱内温度和DOC没有明显的相关关系.中国湖泊冬季(90 d)CH4总释放量大约为3.22±2.75×107 kg,约为1990年中国稻田CH4总释放量的2.8‰.     

A 3-d model of quaternary terrace development,simulations of terrace stratigraphy and valley asymmetry: A case study for the allier terraces (limagne,France)

The combined effects of climate and tectonism on general terrace stratigraphy and valley asymmetry during the last half million years in the Allier system (France) are simulated by a 3-D conceptual model (LIMTER). This model allows the formulation and evaluation of long term terrace formation scenarios for the Allier system. Simulation results suggest that terrace stratigraphy in the study area is mainly the result of internal dynamics and climatic change. Local tectonism contributed to the development of unpaired terraces while the general regional uplift played a dominant role in determining terrace formation and preservation in general.     

Runoff reduction due to environmental changes in the Sanchuanhe river basin

Recently, runoff in many river basins in China has been decreasing. Therefore, the role that climate change and human activities are playing in this decrease is currently of interest. In this study, we evaluated an assessment method that was designed to quantitatively separate the effects of climate change and human activities on runoff in river basins. Specifically, we calibrated the SIMHYD rainfall runoff model using naturally recorded hydro-meteorologic data pertaining to the Sanchuanhe River basin and then determined the effects of climate change and human activities on runoff by comparing the estimated natural runoff that occurred during the period in which humans disturbed the basin to the runoff that occurred during the period prior to disturbance by humans. The results of this study revealed that the S1MHYD rainfall runoff model performs well for estimating monthly discharge. In addition, we found that absolute runoff reductions have increased in response to human activities and climate change, with average reductions of 70.1% and 29.9% in total runoff being caused by human activities and climate change, respectively. Taken together, the results of this study indicate that human activities are the primary cause of runoff reduction in the Sanchuanhe River basin.     

Quantitative assessment of the impact of climate variability and human activities on runoff changes: a case study in four catchments of the Haihe River basin,China

Quantitative evaluation of the effect of climate variability and human activities on runoff is of great importance for water resources planning and management in terms of maintaining the ecosystem integrity and sustaining the society development. In this paper, hydro‐climatic data from four catchments (i.e. Luanhe River catchment, Chaohe River catchment, Hutuo River catchment and Zhanghe River catchment) in the Haihe River basin from 1957 to 2000 were used to quantitatively attribute the hydrological response (i.e. runoff) to climate change and human activities separately. To separate the attributes, the temporal trends of annual precipitation, potential evapotranspiration (PET) and runoff during 1957–2000 were first explored by the Mann–Kendall test. Despite that only Hutuo River catchment was dominated by a significant negative trend in annual precipitation, all four catchments presented significant negative trend in annual runoff varying from ?0.859 (Chaohe River) to ?1.996 mm a^?1 (Zhanghe River). Change points in 1977 and 1979 are detected by precipitation–runoff double cumulative curves method and Pettitt's test for Zhanghe River and the other three rivers, respectively, and are adopted to divide data set into two study periods as the pre‐change period and post‐change period. Three methods including hydrological model method, hydrological sensitivity analysis method and climate elasticity method were calibrated with the hydro‐climatic data during the pre‐change period. Then, hydrological runoff response to climate variability and human activities was quantitatively evaluated with the help of the three methods and based on the assumption that climate and human activities are the only drivers for streamflow and are independent of each other. Similar estimates of anthropogenic and climatic effects on runoff for catchments considered can be obtained from the three methods. We found that human activities were the main driving factors for the decline in annual runoff in Luanhe River catchment, Chaohe River catchment and Zhanghe River catchment, accounting for over 50% of runoff reduction. However, climate variability should be responsible for the decrease in annual runoff in the Hutuo River catchment. Copyright © 2012 John Wiley & Sons, Ltd.     

极端气候事件综合危险性等级指标构建及近60年来长江流域极端气候综合分析

本文利用经过均一化订正的长江流域共669个气象站近60年(1961—2020年)逐日观测资料,采用相对阈值和绝对阈值相结合的极值分析方法,对长江流域近60年极端高温事件、极端低温事件、极端干旱事件和极端降水事件进行识别,分析了年发生频率和线性变化趋势.在此基础上,考虑到全国极端气候事件发生情况,构建了多个极端气候事件综合危险性等级指标,比较客观地给出了长江流域极端气候事件综合危险性等级.研究结果表明,相对于全国其他地区,长江流域大部分地区极端气候综合危险性等级较高,虽然自1961年以来综合年发生频率呈现弱的线性减少趋势,但自20世纪90年代以来,长江流域极端气候事件发生的危险性相对于全国其他地区明显偏高.通过对不同极端气候事件危险性和变化规律研究,结果表明：长江流域近60年极端干旱事件年发生频率呈现线性减少趋势,与全国他其区域相比较,长江流域大部分地区极端干旱发生的危险性等级都在中级以上,说明长江流域容易发生极端干旱事件;长江流域近60年极端降水事件年发生频率呈现弱的增加趋势,危险性等级指数分析表明,高危险区主要位于长江中下游地区,湖南西部、江西大部、湖北南部等地发生极端降水事件的危险性很高;近60年长江流域大部分地区极端高温事件显著增加,尤其进入21世纪以来发生更加频繁,但相对于全国其他地区,危险性等级较低;近60年长江流域极端低温事件显著减少,但相对于全国其他地区,极端低温事件发生的危险性增加明显.进入21世纪以来,长江流域极端气候事件的综合危险性不断增加,极端高温和极端干旱相伴而生的高温干旱复合型事件频繁发生,极端降水事件和极端低温事件在全国的占比不断升高,造成的社会经济影响越来越严重,说明长江流域加强极端气候事件风险防范的重要性和紧迫性.