Sediment budgets for a sediment‐laden river: the lower Wei River in the period 1960–1990

Fluvial sediment delivery is the main form of sediment transfer from the land to the sea, but this process is currently undergoing significant variations due to the alteration of catchment and base level controls related to climate change and human activities, especially the widespread construction of dams. Using the lower Wei River as an example and an integrated approach, this study investigates the variation of fluvial sediment delivery, as well as the connectivity under the effects of both controls. Based on hydrological records and channel cross‐section surveys, sediment budgets were constructed for two periods (1960–1970, 1970–1990) after the dam was closed in 1960. In the period 1960–1969, due to the elevated base level (327.2 ± 1.62 m) caused by the dam, the aggradation rate was 0.451 × 10⁸ t yr^‐1 in the channel and 0.716 × 10⁸ t yr^‐1 on the floodplain, indicating that the positive lateral connectivity between these locations was enhanced. As a consequence, serious sediment storage resulted in a sediment delivery ratio (SDR) that was smaller than that occurring before 1960. In the period 1970–1990, sweeping soil and water conservation (SWC) measures were implemented, resulting in a reduction of the connectivity between the trunk and tributaries, and a decrease of ~31% in the mean sediment input. In addition, together with the base level fluctuation in the range of 327.47 ± 0.49 m, the annual variation in sediment storage was primarily dependent on the water–sediment regime affected by the SWC. The negative lateral connectivity was enhanced between the channel and floodplain via bank erosion. Consequently, the aggradation rate was reduced by 89% on the floodplain and by 96% in the channel. Sediment output continued to decrease primarily due to the SWC practices and climate changes in this period, whereas the SDR increased due to the enhanced longitudinal connectivity between the upstream and downstream. Copyright © 2014 John Wiley & Sons, Ltd.     

Inter‐annual variability in the effects of riparian woodland on micro‐climate,energy exchanges and water temperature of an upland Scottish stream

The influence of riparian woodland on stream temperature, micro‐climate and energy exchange was investigated over seven calendar years. Continuous data were collected from two reaches of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland) with contrasting land use characteristics: (1) semi‐natural riparian forest and (2) open moorland. In the moorland reach, wind speed and energy fluxes (especially net radiation, latent heat and sensible heat) varied considerably between years because of variable riparian micro‐climate coupled strongly to prevailing meteorological conditions. In the forested reach, riparian vegetation sheltered the stream from meteorological conditions that produced a moderated micro‐climate and thus energy exchange conditions, which were relatively stable between years. Net energy gains (losses) in spring and summer (autumn and winter) were typically greater in the moorland than the forest. However, when particularly high latent heat loss or low net radiation gain occurred in the moorland, net energy gain (loss) was less than that in the forest during the spring and summer (autumn and winter) months. Spring and summer water temperature was typically cooler in the forest and characterised by less inter‐annual variability due to reduced, more inter‐annually stable energy gain in the forested reach. The effect of riparian vegetation on autumn and winter water temperature dynamics was less clear because of the confounding effects of reach‐scale inflows of thermally stable groundwater in the moorland reach, which strongly influenced the local heat budget. These findings provide new insights as to the hydrometeorological conditions under which semi‐natural riparian forest may be effective in mitigating river thermal variability, notably peaks, under present and future climates. © 2014 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.     

Carbon emissions induced by cropland expansion in Northeast China during the past 300 years

Assessments of the impacts of land use and land cover changes(LUCC) on the terrestrial carbon budget, atmospheric CO2 concentration, and CO2-related climatic change are important to understand the environmental effects of LUCC and provide information about the effects of historical carbon emissions. Using regional land cover reconstructions from historical records, with a bookkeeping model, we estimated the carbon sink changes caused by historical cropland expansion in Northeast China during the past 300 years. The conclusions are as follows:(1) There was a dramatic land reclamation of cropland during the past 300 years in Northeast China. Approximately 26% of the natural land was cultivated, and 38% of the grassland and 20% of the forest and shrubland were converted to cropland.(2) The carbon emission induced by cropland expansion between 1683 and 1980 was 1.06–2.55 Pg C, and the estimation from the moderate scenario was 1.45 Pg C. The carbon emissions of the soil carbon pool was larger than that from the vegetation carbon pool and comprised more than 2/3 of the total carbon emissions.(3) The carbon emissions of the three provinces in Northeast China were different. Heilongjiang Province had the largest carbon emissions, and Jilin Province had the second largest emissions.(4) The primary source of carbon emissions was forest reclamation(taking 60% of the total emissions in the moderate scenario), the secondary source was grassland cultivation(taking 27%), and the tertiary sources were shrubland and wetland reclamation(taking 13%). Examination on the data accuracy revealed that the high-resolution regional land cover data allowed the carbon budget to be evaluated at the county level and improved the precision of the results. The carbon emission estimation in this study was lower than those in previous studies because of the improved land use data quality and various types of land use change considered.     

Energetics characteristics of the super magnetic storm on November 20,2003 based on 3D global MHD simulation

The three-dimensional global magnetohydrodynamic model(PPM-LR MHD)is employed to investigate the energy budget in the solar wind-magnetosphere system during the super magnetic storm on November 20,2003,one of the biggest storms during the last decade with Dst~-500 n T.During this event,about 23%solar wind kinetic energy is transferred into the magnetosphere.The total energy input is estimated to be about 9.50×1017 J,about 14 times of a moderate storm.The energy dissipation via the inner magnetosphere is less than the energy input with the coupling efficiency of~63.3%.The energy dissipated via ring current injection is less than that via high-latitude ionosphere at the initial stage of the super storm.Furthermore,both the simulation results and the empirical results indicate that the ratio of ring current injection to the total energy output increases with the enhancement of the magnetospheric activity level.These are consistent with the statistical results we have got before.The empirical equations underestimate the solar wind kinetic energy,the energy input,and the energy dissipation via high-latitude ionosphere compared with the simulation results;however,the coupling efficiency of the high-latitude ionosphere(23.4%)is close to the simulation result(23.1%)during the entire storm time period.     

摄食水平对卵形鲳鲹幼鱼的生长和能量收支的影响

水温28±1℃条件下,投喂人工配合饲料,研究摄食水平1%、2%、3%、4%、5%(每日投喂量占初始实验鱼湿重的质量百分比)及饱食对卵形鲳鲹幼鱼(初始体重5.77±0.45g)生长及能量收支的影响。结果表明,卵形鲳鲹幼鱼特定生长率、转化效率、摄食率随着摄食水平的提高而增长,摄食水平达4%时干重特定生长率与饱食组差异无显著性(P>0.05),摄食水平达5%时,湿重、蛋白质和能量生长率与饱食组差异无显著性(P>0.05);摄食水平达4%时能值转化效率与饱食组差异无显著性(P>0.05),摄食水平达5%时,干重、蛋白质转化效率与饱食组差异无显著性(P>0.05);摄食水平达2%时干重、蛋白质表观效率与饱食组差异无显著性(P>0.05),摄食水平达4%时,能量表观效率与饱食组差异无显著性(P>0.05)。生长能分配率随摄食水平升高而显著增加(P<0.05),代谢能、排粪能及排泄能分配率的变化则相反。建立了6种摄食水平条件下卵形鲳鲹幼鱼的能量收支方程。     

Quantification of nitrate storage in the vadose (unsaturated) zone: a missing component of terrestrial N budgets

National terrestrial nitrogen budgets for many developed countries have been calculated as part of the management of impacts of N on the environment, but these rarely represent the subsurface explicitly. Using estimates of vadose zone travel time and agricultural nitrate loading, we quantify, for the first time, the total mass of nitrate contained in the vadose zone of aquifers in England and Wales. This mass peaked in 2008 at 1400 kt N (800 to >1700 kt N from sensitivity analyses), which is approximately 2.5 to 6 times greater than saturated zone estimates for this period and indicates that the subsurface is an important store of reactive nitrogen. About 70% of the nitrate mass is estimated to be in the Chalk, with the remainder split between the Permo‐Triassic sandstones, the Jurassic Oolitic limestones and minor aquifers. Current controls on fertilizer application mean that the vadose zone is now a nitrate source, and in 2015 we estimate the net flux from the unsaturated zone to groundwater to be 72 kt N/a. The mass of nitrate in the vadose zone should be included in future terrestrial nitrogen budgets at national and global scales to improve ecosystem management. British Geological Survey © NERC 2015. Hydrological Processes © 2015 John Wiley & Sons, Ltd.     

Morphological dynamics of upland headwater streams in the southern North Island of New Zealand

Short-term channel dynamics of mountain stream reaches in the southern North Island of New Zealand were assessed over two successive 3-month periods using morphological budgeting. Response to floods varies between reaches, even when the catchments were located close to each other and had similar characteristics. The reaches on the Central Volcanic Plateau experienced least morphological change, while streams with steep catchments and migrating planform in the Tararua and Ruahine Ranges showed frequent channel adjustments. Channel response is conditioned by intrinsic variables rendering reaches responsive or robust to the effects of floods, and this is likely to reflect the degree of connectivity between slopes and channels, and reaches.     

Channel and floodplain sediment dynamics in a reach of the tropical meandering Rio Beni (Bolivian Amazonia)

The study investigates interactions, water and sediment exchanges, between a rapidly migrating meander and its associated floodplain at fine temporal and spatial scales. The Beni River, an Amazonian free meandering river, makes the transition between Andean ranges and Amazonian lowlands. For the period 2002–2006, an assemblage of tools and methods (water and sediment discharges, topometric and bathymetric surveys, sedimentation rate estimations from unsupported ²¹⁰Pb and sediment trapping system) was used to jointly analyse the influence on the sediment budget of external factors (mainly water and sediment discharge) and the inherent behaviour of the system. The main issue addressed is the investigation of the complex relationship between ‘morphological conditioning’ of fluvial landform and process. The first part of the study was undertaken with the aim of linking erosion–deposition in an active meander with water and sediment fluxes. The three inter‐annual evolutions are characterized by very unequal sediment budgets; the first two intervals underwent predominant erosion, and the latter slight accumulation. Digital elevation models, evaluated for the active meander, demonstrate that sedimentation on the point bar depends more on external factors than erosion of the concave bank, which fluctuates slightly. The second part of the study, focusing on water and sediment exchanges between active bend and floodplain, examines the respective parts played by overbank flow and by an abandoned channel on the diffusion and sequestration of sediment. The association of short‐ and long‐term estimation of sedimentation rates suggests that floodplain construction is associated with two different processes and rhythms of sediment transportation. Finally, a sediment budget is proposed for the Beni River in the upper part of the Amazonian lowlands. Copyright © 2010 John Wiley & Sons, Ltd.     

Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic—A model study,Part II: Carbon budget and fluxes

The 3-d coupled physical–biogeochemical model ECOHAM (version 3) was applied to the Northwest-European Shelf (47°41′–63°53′N, 15°5′W–13°55′E) for the years 1993–1996. Carbon fluxes were calculated for the years 1995 and 1996 for the inner shelf region, the North Sea (511,725 km²). This period was chosen because it corresponds to a shift from a very high winter-time North Atlantic Oscillation Index (NAOI) in 1994/1995, to an extremely low one in 1995/1996, with consequences for the North Sea physics and biogeochemistry. During the first half of 1996, the observed mean SST was about 1 °C lower than in 1995; in the southern part of the North Sea the difference was even larger (up to 3 °C). Due to a different wind regime, the normally prevailing anti-clockwise circulation, as found in winter 1995, was replaced by more complicated circulation patterns in winter 1996. Decreased precipitation over the drainage area of the continental rivers led to a reduction in the total (inorganic and organic) riverine carbon load to the North Sea from 476 Gmol C yr⁻¹ in 1995 to 340 Gmol C yr⁻¹ in 1996. In addition, the North Sea took up 503 Gmol C yr⁻¹ of CO₂ from the atmosphere. According to our calculations, the North Sea was a sink for atmospheric CO₂, at a rate of 0.98 mol C m⁻² yr⁻¹, for both years. The North Sea is divided into two sub-systems: the shallow southern North Sea (SNS; 190,765 km²) and the deeper northern North Sea (NNS; 320,960 km²). According to our findings the SNS is a net-autotrophic system (net ecosystem production NEP>0) but released CO₂ to the atmosphere: 159 Gmol C yr⁻¹ in 1995 and 59 Gmol C yr⁻¹ in 1996. There, the temperature-driven release of CO₂ outcompetes the biological CO₂ drawdown. In the NNS, where respiratory processes prevail (NEP<0), 662 and 562 Gmol C yr⁻¹ were taken up from the atmosphere in 1995 and 1996, respectively. Stratification separates the productive, upper layer from the deeper layers of the water column where respiration/remineralization takes place. Duration and stability of the stratification are determined by the meteorological conditions, in relation to the NAO. Our results suggest that this mechanism controlling the nutrient supply to the upper layer in the northern and central North Sea has a larger impact on the carbon fluxes than changes in lateral transport due to NAOI variations. The North Sea as a whole imports organic carbon and exports inorganic carbon across the outer boundaries, and was found to be net-heterotrophic, more markedly in 1996 than in 1995.     

Determination of the freshwater budget of tidal flats from measurements near a tidal inlet

The freshwater budget of a tidal flat area is evaluated from long-term hydrographic time series from an observation pole positioned in a tidal channel in the Hörnum Basin (Germany). For each tidal cycle, the freshwater budget is calculated from the total imported and exported water volumes and the corresponding mean densities. The variability of the budget on a tidal scale is characterised by a period of twice the tidal period, exhibiting a minimum when the tidal flats are dry around daylight hours during the foregoing low tide, and a maximum when low tide occurs at night; enhanced evaporation on the flats at daylight hours is identified as the driving process. On the average over one year, while winter observations are missing, the freshwater budget is negative for the years 2002–2005 and positive only for 2006. The interannual mean is negative and amounts to a freshwater loss of about 2 mm day⁻¹, although the large-scale climate in this region is humid. The results demonstrate that the bulk parametrisations for the latent and sensible heat flux between the ocean and the atmosphere must not be applied for the tidelands.