Morphology of riffle–pool sequences in the River Severn,England

Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero‐crossing’, the ‘spectral analysis’ and the ‘control‐point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ _p) and channel width ( W ), λ _p = 3 W , applies to the River Severn. Reach‐average riffle height ( H ) is a constant proportion of bankfull depth ( h ); typically H ≅ 0·16 h . Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed‐level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L _r) to water depth, i.e. L _r = α2π h, wherein α ≅ 1 for steep near‐equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent‐flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self‐similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd.     

Phase‐shifts in shear stress as an explanation for the maintenance of pool–riffle sequences

The stability of the pool–rif?e sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of pool–rif?e sequence bars during high ?ows, offsetting gradual scour of rif?es and deposition in pools during low ?ows. Despite several attempts, reversal has rarely been recorded in ?eld measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase‐shifted with respect to the pool–rif?e sequence bedform pro?le, so that maximum shear stress occurs upstream of rif?e crests at high ?ow, and downstream at low ?ow. Such phase‐shifts produce gradients of shear stress that explain rif?e deposition, and pool scour, at high ?ow, in accord with sediment continuity. The proposal is supported by results of a one‐dimensional hydraulic model applied to the surveyed bathymetry of a pool–rif?e sequence in a straight reach of a gravel‐bed river. In the sequence studied, the upstream phase‐shift in shear stress at high ?ow was associated with variations in channel width, with width minima occurring upstream of rif?e crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of rif?e crests. Assuming that the width variation is itself the result of ?ow de?ection by rif?e crests at low ?ow, and associated bank‐toe scour downstream, low and high ?ow can be seen to have complementary roles in maintaining alluvial pool–rif?e sequences. Copyright © 2004 John Wiley & Sons, Ltd.     

Field assessment of surface water–groundwater connectivity in a semi‐arid river basin (Murray–Darling,Australia)

In semi‐arid and arid river basins, understanding the connectivity between rivers and alluvial aquifers is one of the key challenges for the management of groundwater resources. The type of connection present (gaining, losing‐connected, transitional and losing‐disconnected) was assessed at 12 sites along six Murray–Darling Basin river reaches. The assessments were made by measuring the hydraulic head in the riparian zone near the rivers to evaluate if the water tables intersected the riverbeds and by measuring fluid pressure (ψ) in the riverbeds. The rationale for the latter was that ψ will always be greater than or equal to zero under connected conditions (either losing or gaining) and always lesser than or equal to zero under losing‐disconnected conditions. A mixture of losing‐disconnected, losing‐connected and gaining conditions was found among the 12 sites. The losing‐disconnected sites all had a riverbed with a lower hydraulic conductivity than the underlying aquifer, usually in the form of a silty clay or clay unit 0.5–2 m in thickness. The riparian water tables were 6 to 25 m below riverbed level at the losing‐disconnected sites but never lower than 1 m below riverbed level at the losing‐connected ones. The contrast in water table depth between connected and disconnected sites was attributed to the conditions at the time of the study, when a severe regional drought had generated a widespread decline in regional water tables. This decline was apparently compensated near losing‐connected rivers by increased infiltration rates, while the decline could not be compensated at the losing‐disconnected rivers because the infiltration rates were already maximal there. Copyright © 2012 John Wiley & Sons, Ltd.     

Lithium isotope variations in Tonga–Kermadec arc–Lau back‐arc lavas and Deep Sea Drilling Project (DSDP) Site 204 sediments

Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reservoirs such as subducting sediments and altered oceanic crust. The Tonga–Kermadec arc–Lau back‐arc provides an end‐member of subduction zones with the coldest thermal structure on Earth. Reported here are Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. The arc and back‐arc lavas range from basalts to dacites in composition with SiO₂ = 48.3–65.3 wt% over which Li concentrations increase from 2 ppm to 16 ppm. Li/Y ratios range from 0.08 to 0.77 and from 0.24 to 0.65 in the arc and back‐arc lavas, respectively. The majority of the lavas have δ⁷Li that ranges from 2.5 ‰ to 5.0 ‰ with an average of (3.6 ±0.7) ‰, similar to that reported from other arcs and there is no distinction between the arc front and back‐arc lavas. The pelagic sediments have variable Li concentrations (33–133 ppm) and δ⁷Li that ranges from 1.2 ‰ to 10.2 ‰ while the volcaniclastic sediments have an even greater range of Li concentrations (3.6–165 ppm) and generally higher δ⁷Li values (8–14 ‰). However, δ⁷Li in the lavas does not correlate with commonly used trace element ratio or isotope signatures indicative of slab‐derived fluids or the sediments. This is probably because the range of δ⁷Li in the lavas and sediments overlap. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust.     

An experimental test of whether bar instability contributes to the formation,periodicity and maintenance of pool–riffle sequences

Pool–riffle sequences (PRSs) are periodic river‐bed morphologies with wavelengths several times the channel width. Causes of PRS formation and maintenance are not clearly understood, which may limit the effectiveness of protection and rehabilitation measures. Some confusion has existed about whether the PRS morphology is the same as or distinct from alternate bars. In this paper we investigate whether the bar instability forming alternate bars also contributes to PRS formation, periodicity and maintenance. This was unclear because bar instability occurs only when the ratio of channel width/depth exceeds a critical value, generally understood to be approximately 10, which is larger than the width‐to‐depth ratio of many PRSs. A mobile‐bed physical model is used to test whether bar instability occurs in channels characteristic of PRS morphology, with low width‐to‐depth ratio, and high relative roughness. The physical model was scaled from a prototype PRS in a gravel and cobble bed river. Alternate bars formed in the model at channel width‐to‐depth ratios as low as 3·8. The wavelength of the alternate bars formed was generally 2·2–5 times channel width, which was similar to the prototype PRS. Therefore, bar instability can occur in virtually all PRSs, and it contributes to the widespread formation of periodic PRS morphology. The model showed that maintenance of the bar height in the prototype PRS also depends on variations in channel width. It is concluded that periodic PRSs are formed and maintained by the interaction between bar instability, and flow deflections associated with variations in channel geometry such as width variation. Resonance between bar instability and three‐dimensional bed forms such as alternate bars and variations in channel geometry. Variations in channel geometry are also important in determining the location and dimensions of individual pools and bars. Copyright © 2008 John Wiley & Sons, Ltd.     

Recent increase of river–floodplain suspended sediment exchange in a reach of the lower Amazon River

We analyzed variation of channel–floodplain suspended sediment exchange along a 140 km reach of the lower Amazon River for two decades (1995–2014). Daily sediment fluxes were determined by combining measured and estimated surface sediment concentrations with river–floodplain water exchanges computed with a two‐dimensional hydraulic model. The average annual inflow to the floodplain was 4088 ± 2017 Gg yr^?1 and the outflow was 2251 ± 471 Gg yr^?1, respectively. Prediction of average sediment accretion rate was twice the estimate from a previous study of this same reach and more than an order of magnitude lower than an estimate from an earlier regional scale study. The amount of water routed through the floodplain, which is sensitive to levee topography and increases exponentially with river discharge, was the main factor controlling the variation in total annual sediment inflow. Besides floodplain routing, the total annual sediment export depended on the increase in sediment concentration in lakes during floodplain drainage. The recent increasing amplitude of the Amazon River annual flood over two decades has caused a substantial shift in water and sediment river–floodplain exchanges. In the second decade (2005–2014), as the frequency of extreme floods increased, annual sediment inflow increased by 81% and net storage increased by 317% in relation to the previous decade (1995–2004). Copyright © 2017 John Wiley & Sons, Ltd.     

Flow reversal over a natural pool–riffle sequence: a computational study

A computational study is presented on the hydraulics of a natural pool–rif?e sequence composed of mixed cobbles, pebbles and sand in the River Lune, northern England. A depth‐averaged two‐dimensional numerical model is employed, calibrated with observed data at the ?eld site. From the computational outputs, the occurrence of longitudinally double peak zones of bed shear stress and velocity is found. In particular, at low discharge there exists a primary peak zone of bed shear stress and velocity at the rif?e tail in line with the local maximum energy slope, in addition to a secondary peak at the pool head. As discharge increases, the primary peak at the rif?e tail at low ?ow moves toward the upstream side of the rif?e along with the maximum energy slope, showing progressive equalization to the surrounding hydraulic pro?les. Concurrently, the secondary peak, due to channel constriction, appears to stand at the pool head, with its value increasing with discharge and approaching or exceeding the primary peak over the rif?e. The existence of ?ow reversal is demonstrated for this speci?c case, which is attributable to channel constriction at the pool head. A dynamic equilibrium model is presented to reconstruct the pool–rif?e morphology. A series of numerical modelling exercises demonstrates that the pool–rif?e morphology is more likely produced by shallow ?ows concentrated with coarse sediments than deep ?ows laden with low concentrations of ?ne sediments. It is concluded that channel constriction can, but may not necessarily, lead to competence reversal, depending on channel geometry, ?ow discharge and sediment properties. Copyright © 2003 John Wiley & Sons, Ltd.     

Hydrochemical variations during flood pulses in the south‐west China peak cluster karst: impacts of CaCO3–H2O–CO2 interactions

High‐resolution measurements of rainfall, water level, pH, conductivity, temperature and carbonate chemistry parameters of groundwater at two adjacent locations within the peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China, were made with different types of multiparameter sonde. The data were stored using data loggers recording with 2 min or 15 min resolution. Waters from a large, perennial spring represent the exit for the aquifer's conduit flow, and a nearby well measures water in the conduit‐adjacent, fractured media. During flood pulses, the pH of the conduit flow water rises as the conductivity falls. In contrast, and at the same time, the pH of groundwater in the fractures drops, as conductivity rises. As Ca²⁺ and HCO₃^? were the dominant (>90%) ions, we developed linear relationships (both r² > 0·91) between conductivity and those ions, respectively, and in turn calculated variations in the calcite saturation index (SI_C) and CO₂ partial pressure (P) of water during flood pulses. Results indicate that the P of fracture water during flood periods is higher than that at lower flows, and its SI_C is lower. Simultaneously, P of conduit water during the flood period is lower than that at lower flows, and its SI_C also is lower. From these results we conclude that at least two key processes are controlling hydrochemical variations during flood periods: (i) dilution by precipitation and (ii) water–rock–gas interactions. To explain hydrochemical variations in the fracture water, the water–rock–gas interactions may be more important. For example, during flood periods, soil gas with high CO₂ concentrations dissolves in water and enters the fracture system, the water, which in turn has become more highly undersaturated, dissolves more limestone, and the conductivity increases. Dilution of rainfall is more important in controlling hydrochemical variations of conduit water, because rainfall with higher pH (in this area apparently owing to interaction with limestone dust in the lower atmosphere) and low conductivity travels through the conduit system rapidly. These results illustrate that to understand the hydrochemical variations in karst systems, considering only water–rock interactions is not sufficient, and the variable effects of CO₂ on the system should be evaluated. Consideration of water–rock–gas interactions is thus a must in understanding variations in karst hydrochemistry. Copyright © 2004 John Wiley & Sons, Ltd.     

Precise Re–Os mineral isochron and Pb–Nd–Os isotope systematics of a mafic–ultramafic sill in the 2.0 Ga Onega plateau (Baltic Shield)

We present Re–Os, Sm–Nd and Pb–Pb isotope and trace element data for the Konchozero sill, a layered mafic–ultramafic intrusion in the Early Proterozoic Onega plateau, one of the oldest continental flood basalt provinces on Earth. The Sm–Nd and Pb–Pb combined mineral and whole-rock isochron ages of 1988±34 and 1985±57 Ma for the sill coincide with the age of ferropicrites from Pechenga (the Kola Peninsula). The lithostratigraphic, chemical and isotope evidence suggest the derivation of Pechenga lavas and the Onega plateau volcanics from a single mantle plume. Peridotite and gabbro whole-rock samples, and primary ulvospinel and ilmenite mineral separates from the sill yield a Re–Os isochron with a slope corresponding to an age of 1969±18 Ma, γ_Os(T) =−0.61±5.9. This age is consistent with the other isotope data, and indicates the closed-system behavior of Re and Os in the rocks. The peridotites and ulvospinel have high Os concentrations (2.5–14 ppb) and low ¹⁸⁷Re/¹⁸⁸Os ratios (0.35–1.1), thus allowing a more accurate determination of the weighted average initial ¹⁸⁷Os/¹⁸⁸Os of 0.1144±0.0019 (2σ_pop), γ_Os(T) =+0.77±1.7. This value is lower than that determined by Walker et al. (Geochim. Cosmochim. Acta 61 (1997) 3145–3160) for the Pechenga lavas (γ_Os(T) =+6.1±0.7), and implies a substantial Os-isotope heterogeneity in this ancient plume. Compared to the Onega plateau primary basalt magma, Pechenga ferropicrites are relatively enriched in iron and Ni, have lower (Nb/Th)_N ratios (2.1 vs 1.1) and less radiogenic Nd-isotope compositions (_Nd(T) = +3.1 and +1.4, respectively), but share similar low-radiogenic Pb-isotope characteristics (μ₁=8.57 and 8.60). Incorporation of small amounts (1.5%) of outer core material into the hotter central part of the plume and subsequent contamination of the Pechenga ferropicritic magmas with the 2.9 Ga Belomorian gneisses can explain the observed chemical and isotope variations in the two provinces provided that the core had <0.25 ppm of Pb.     

Water and air dynamics within a deep vadose zone of a karst massif: Observations from the Lukina jama–Trojama cave system (−1,431 m) in Dinaric karst (Croatia)

Extreme heterogeneity of karst systems makes them very challenging to study. Various processes within the system affect its global response, usually measured at karst springs. Research conducted in caves provides a unique opportunity for in situ analysis of separate processes in karst underground. The aim of the present study was to research the water and air dynamics within a deep karst system. Air and water basic physical parameters across the Lukina jama–Trojama cave system (?1,431 m) were continuously monitored during a 1‐year period. Recorded hydrograph of the siphon lake at the bottom of the cave was used to interpret the characteristics of an unexplored phreatic/epiphreatic conduit network. Water origin in the siphon was determined based on temperature and electrical conductivity. Air temperature and humidity monitoring revealed a strong inflow of air of sub‐zero temperature into the upper portion of the cave during winter. Cave passage morphology was interpreted as the main determinant of air dynamics, which caused ice to accumulate extensively in the upper portions of the cave and caused the temperature on the top of the homothermic zone to be significantly below the mean outside temperature. Air dynamics also lowered the temperature of water flowing through the cave vadose zone and feeding the phreatic zone of the massif. The pronounced temperature difference between the phreatic zone and the top of the homothermic zone probably contributed to the thermal gradient observed in the cave, which is steeper than in ice‐free caves in the area. Our results enabled the development of a conceptual model that describes coupling between air and water dynamics in the cave system and its surroundings.     

Tidal effects on variations of fresh–saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island (Jeju Island, Korea)

We conducted various field studies at the seawater intrusion monitoring wells located in the eastern part of Jeju Island, Korea, to observe the tidal effect on groundwater–seawater flow in the coastal aquifer. Studies included monitoring the fluctuations of groundwater and tide levels, electrical and temperature logging, and 2-D heat-pulse flowmeter tests. According to time-series analysis, tidal effects on groundwater level reached up to 3 km inland from the coastline. Water-level variation was more sensitive to tidal fluctuations near the coast, and more related to rainfall toward inland areas. Temporal and spatial variations in the shape and location of the freshwater–saltwater interface were analyzed using data from nine monitoring wells. The results indicated that the interface toe is located at a distance of 6–8 km from the coastline and its location was related to geological layers present. Long-term seasonal variations revealed no major changes in the interface; minor variations were due to moving boundary conditions induced by tidal fluctuations. Using the two-dimensional heat-pulse flowmeter, groundwater flow directions and velocities at four tidal stages were measured on three monitoring wells drilled into the multilayered aquifers. This direct measurement enabled us to relate the differences of flow velocities and directions with geology and tidal fluctuations. Combining the results of EC logging and flowmeter tests, we found a zone where freshwater and saltwater moved alternately in opposite directions, as influenced by the tidal fluctuations. Integrating various physical logging and flowmeter data with water-level fluctuations improved our understanding of the behavior of fresh and seawater flow in the coastal aquifers.     

Delineation of saltwater intrusion zones using the time domain electromagnetic method: the Nabeul–Hammamet coastal aquifer case study (NE Tunisia)

The time domain electromagnetic method (TDEM) is applied to monitor, to delineate and to map the saltwater intrusion zones in the Mediterranean Plio‐Quaternary aquifer. Forty‐two TDEM soundings were carried out in the coastal plain of Nabeul–Hammamet region (NE Tunisia). TDEM resistivity data were correlated with the existing borehole logging data to assign them to a particular lithology and to provide information about the position of the freshwater–seawater transition zone. The geoelectric sections showing the vertical configuration of seawater intrusion, with the brackish‐salty‐saturated zones, have a resistivity ranging from ~0.1 to 5 Ω?m and are detected at a depth lower than 1.5 m. The salinized zones are located at Nabeul (Sidi Moussa, Sidi El Mahrsi, Al Gasba and Mrazgua) and at Hammamet (Touristic zone of Hammamet north and south, Baraket Essahel) and reached a distance of 4 km from the coastline, indicating a severe state for the aquifer in these zones. These TDEM results are confirmed by the increase of chloride concentration content in the analysed water samples of monitoring wells. Moreover, in the northeastern part, the presence of a saltwater front located far from the coast and along the NW–SE major surface fault can be explained by two hypothesis: (i) this fault seems to provide a conduit for seawater to move readily towards the water wells and (ii) the clay and gypsum infiltration of marine Messinian deposits through the fault plane leads to low resistivities. Finally, it comes out from this study that TDEM survey has successfully depicted salinized zones of this coastal aquifer. Copyright © 2012 John Wiley & Sons, Ltd.     

Rainfall–runoff modelling of ephemeral streams in the Valencia region (eastern Spain)

This paper presents preliminary results from the application of a transfer‐function rainfall–runoff model to ephemeral streams in Mediterranean Spain. Flow simulations have been conducted for two small catchments (Carraixet and Poyo basins), located in close proximity to one another yet with significantly different geological characteristics. Analysis of flow simulations for a number of high‐flow events has revealed the dominant influence of the rainfall on the catchment response, particularly for high‐rainfall events. Particular success has been attained modelling the highest magnitude events in both catchments and for all events in the faster responding (Poyo) catchment. In order to investigate the viability of the model for forecasting floods in ungauged catchments, additional investigations have been conducted by calibrating the model for one catchment (donor catchment) and then applying it to another (receptor catchment). The results indicate that this can be successful when either the donor catchment is a fast response catchment or when the model is calibrated using a high‐magnitude event in the donor catchment, providing that the modelled receptor catchment event is of a lower magnitude. Copyright © 2002 John Wiley & Sons, Ltd.     

Moisture transport and seasonal variations in the stable isotopic composition of rainfall in Central American and Andean Páramo during El Niño conditions (2015–2016)

High‐elevation tropical grassland systems, called Páramo, provide essential ecosystem services such as water storage and supply for surrounding and lowland areas. Páramo systems are threatened by climate and land use changes. Rainfall generation processes and moisture transport pathways influencing precipitation in the Páramo are poorly understood but needed to estimate the impact of these changes, particularly during El Niño conditions, which largely affect hydrometeorological conditions in tropical regions. To fill this knowledge gap, we present a stable isotope analysis of rainfall samples collected on a daily to weekly basis between January 2015 and May 2016 during the strongest El Niño event recorded in history (2014–2016) in two Páramo regions of Central America (Chirripó, Costa Rica) and the northern Andes (Cajas, south Ecuador). Isotopic compositions were used to identify how rainfall generation processes (convective and orographic) change seasonally at each study site. Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) air mass back trajectory analysis was used to identify preferential moisture transport pathways to each Páramo site. Our results show the strong influence of north‐east trade winds to transport moisture from the Caribbean Sea to Chirripó and the South American low‐level jet to transport moisture from the Amazon forest to Cajas. These moisture contributions were also related to the formation of convective rainfall associated with the passage of the Intertropical Convergence Zone over Costa Rica and Ecuador during the wetter seasons and to orographic precipitation during the transition and drier seasons. Our findings provide essential baseline information for further research applications of water stable isotopes as tracers of rainfall generation processes and transport in the Páramo and other montane ecosystems in the tropics.     

Comparison analysis of the summer monsoon precipitation between northern and southern slopes of Tanggula Mountains,Qinghai–Xizang (Tibetan) Plateau: a case study in summer 1998

Based on the precipitation data obtained through GEWEX Asian Monsoon Experiment–Tibet fieldwork from May to September 1998, this study investigated the features of the summer monsoon precipitation on the northern and southern slopes of the huge Tanggula Mountains in the Qinghai–Xizang (Tibetan) Plateau. The results show that the precipitation on the southern slope is about 50% higher than on the northern slope, whereas the frequency and diurnal pattern of the precipitation are very similar. The mean precipitation intensity on the southern slope is larger than on the northern slope. In most cases, the daily precipitation showed similar variation on both slopes, demonstrating that the precipitation processes might be similar. In the summer monsoon period, the local convective precipitation contributed to the total precipitation on both slopes and such a contribution on the southern slope is larger. Copyright © 2006 John Wiley & Sons, Ltd.     

Temporal variation of stable isotopes in a precipitation–groundwater system: implications for determining the mechanism of groundwater recharge in high mountain–hills of the Loess Plateau,China

The groundwater in shallow loess aquifers in high mountain–hills in the western Loess Plateau in China is almost the sole water resource for local residents. However, the question about how the loess groundwater naturally circulates in these high mountain–hills, characterized by low precipitation and high potential evaporation, remains unclear. The objectives of this study are to evaluate the application of hydrogen and oxygen isotopes to (1) examine temporal variations of the isotopic composition of precipitation and shallow groundwater and (2) uncover the mechanism of groundwater recharge in high mountain–hills. Results from 2 years of monitoring data show a difference in the stable isotopes for groundwater and local precipitation between the winter and summer periods. Similar to precipitation, stable isotopes in groundwater are observed to be depleted in winter and enriched in summer, particularly in oxygen isotope. A prominent characteristic is that H and O isotopes of groundwater show a very clear response to strong precipitation in the rainy season in 2013. The results highlight that local precipitation is the likely recharge source for groundwater in shallow loess aquifers. Annual recharge from local precipitation maintains the groundwater resource in the shallower loess aquifer. The mechanisms governing shallow loess groundwater recharge in high mountain–hills were evaluated. In addition to possible vertical slow percolation of soil water through the unsaturated zone, rapid groundwater recharge mechanisms have been identified as temporal preferential infiltration through sinkholes, slip surface or landslide surface and through the interface of loess layer and palaeo‐soils. Most groundwater can be recharged after a heavy rainy season. Copyright © 2015 John Wiley & Sons, Ltd.     

Experimental study of the zooplankton impact on the trophic structure of phytoplankton and the microbial assemblages in a temperate wetland (Argentina)

An experimental study using mesocosms was conducted in the main shallow lake of a temperate wetland (Otamendi Natural Reserve, Argentina) to analyse the impact of zooplankton on phytoplankton and the microbial assemblages. The lake is characterised by the presence of a fluctuating cover of floating macrophytes, whose shading effects shape the phytoplakton community and the ecosystem functioning, which was absent during the study period. The experiment was run in situ using polyethylene bags, comparing treatments with and without zooplankton. The cascade effect of zooplankton on phytoplankton and the lower levels of the microbial food web (ciliates, heterotrophic nanoflagellates (HNF) and picoplankton) were analysed.A significant zooplankton grazing on the nano-phytoplankton fraction (3–30 μm) was observed. Conversely, large algae (filamentous cyanobacteria, colonial chlorophytes and large diatoms) increased in all mesocosms until day 10, suggesting that they were not actively grazed by zooplankton during this period. However, from day 10 until day 17 this fraction decreased in the enclosures with mesozooplankton, probably due to an increase in the abundance of large herbivores.The results of the experiment would also indicate a trophic cascade effect on the lower levels of the microbial community. In the treatment where zooplankton was removed, the abundance of ciliates followed the same increasing pattern as the abundance of HNF, but with a time lag in its response. In the enclosures without zooplankton, HNF remained relatively constant throughout the experiment, whereas ciliates strongly decrease during the last week. Total picoplankton abundance increased in the enclosures with mesozooplankton, thus supporting the existence of a four-link trophic cascade (copepods–microzooplankton–HNF–picoplankton). Zooplankton composition changed significantly from the beginning until the end of the experiment; cyclopoid nauplii and rotifers were notoriously dominant at t₀, whereas 10 days later the community showed a more equitable proportion of cyclopoids, calanoids, nauplii, cladocerans and rotifers.     

Spatiotemporal analysis of climate variability (1971–2010) in spring and summer on the Loess Plateau,China

By using the Variable Infiltration Capacity model with Palmer Drought Severity Index (VIC‐PDSI) model and Standardized Precipitation Index (SPI), spatiotemporal trends of climate variation during the main growing seasons for plants of Loess Plateau between 1971 and 2010 were detected and characterized. The VIC‐PDSI model is established by combining the VIC model with PDSI. The simulation results and the grids system of VIC were applied to substitute for the two‐layer bucket‐type model to do the hydrological accounting, which could improve the physical mechanism of PDSI and expand its application range. Our results suggest that the climate of the study area has experienced a drying and warming trend during the past four decades. Apart from some individual years and regions, there was a perpetuation of water deficit over the Plateau both in spring and summer. The drought frequency increased from southeast to northwest in spring, while the drought frequency decreased from southeast to northwest in summer. The climate in the southern part of the Loess Plateau, accounting for 23.3% of the study region, showed a significant drying and warming trend in spring over the past four decades. The climate variability detected by VIC‐PDSI model shows good agreement with that monitored by SPI. Since a large part of the study region frequently suffered from water shortage during the main growing seasons for plants, people living in such drought‐prone areas should take measures to prevent the negative effects on agricultural production, reforestation, and regional food security caused by drought. Copyright © 2013 John Wiley & Sons, Ltd.     

Contributions of slab fluid and sediment melt components to magmatism in the Mariana Arc–Trough system: Evidence from geochemical compositions and Sr,Nd, and noble gas isotope systematics

This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, ⁸⁴Kr/⁴He and ¹³²Xe/⁴He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of ³He/⁴He and ⁴⁰Ar/³⁶Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in ²⁰Ne/²²Ne and ²¹Ne/²²Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess ¹²⁹Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.