Ephemeral Research Articles

Multi-User Location Proximity Prediction in Offline Ephemeral Social Networks

Multi-User Location Proximity Prediction in Offline Ephemeral Social Networks

Widespread dieback of riparian trees on a dammed ephemeral river and evidence of local mitigation by tributary flows.

Ephemeral rivers act as linear oases in drylands providing key resources to people and wildlife. However, not much is known about these rivers’ sensitivities to human activities. We investigated the landscape-level determinants of riparian tree dieback along the Swakop River, a dammed ephemeral river in Namibia, focusing on the native ana tree (Faidherbia albida) and the invasive mesquite (Prosopis spp.). We surveyed over 1,900 individual trees distributed across 24 sites along a 250 km stretch of the river. General linear mixed models were used to test five hypotheses relating to three anthropogenic threats: river flow disruption from damming, human settlement and invasive species. We found widespread dieback in both tree populations: 51% mortality in ana tree, with surviving trees exhibiting 18% canopy death (median); and 26% mortality in mesquite, with surviving trees exhibiting 10% canopy death. Dieback in the ana tree was most severe where trees grew on drier stretches of the river, where tributary flow was absent and where mesquite grew more abundantly. Dieback in the mesquite, a more drought-tolerant taxon, did not show any such patterns. Our findings suggest that dieback in the ana tree is primarily driven by changes in river flow resulting from upstream dam creation and that tributary flows provide a local buffer against this loss of main channel flow. The hypothesis that the invasive mesquite may contribute to ana tree dieback was also supported. Our findings suggest that large dams along the main channels of ephemeral rivers have the ability to cause widespread mortality in downstream riparian trees. To mitigate such impacts, management might focus on the maintenance of natural tributary flows to buffer local tree populations from the disruption to main channel flow.

Hydraulic flood routing in an ephemeral channel: Wadi Mekerra, Algeria

Wadis are a vital source of water in most arid and semi-arid countries. Catastrophic flash floods occurring in wadis are a threat to many societies. The Wadi Mekerra basin in the North-West of Algeria represents one of these semi-arid regions the most affected by floods and has been always one of the main concerns of managers in Sidi Bel Abbes province. In this work, we present the main climatic and hydromorphometric features of Wadi Mekerra watershed. We discuss the main factors that cause flooding in the studied catchment. A hydraulic model based on the Runge-Kutta Discontinuous Galerkin (RKDG) finite element scheme and a limited (existing) field data was applied and compared with measured flow data. The objective was to assess whether a reliable hydraulic flood routing model of a Wadi in a semi-arid region could be developed, with limited cross section data supplemented by topographic map information. A comparison between the measured and simulated results shows that the model used is appropriate for determining the flood hydrograph at the outlet of the watercourse.

Effect of Protection Level in the Hydroperiod of Water Bodies on Doñana’s Aeolian Sands

Mediterranean temporary ponds on Doñana’s aeolian sands form an extensive system of small dynamic water bodies, dependent on precipitation and groundwater, of considerable importance for biodiversity conservation. Different areas of the aeolian sands have received different levels of environmental protection since 1969, and this has influenced the degree of conservation and the flooding dynamic of these temporary surface waters. We use the Landsat series of satellite images from 1985 to 2014 to study the temporal dynamic of small temporary water bodies on the aeolian sands in relation to the protection level and to distance to water abstraction pressures from agriculture and residential areas. The results show that even with small and ephemeral water bodies optical remote sensing time-series are an effective way to study their flooding temporal dynamics. The protected areas of the aeolian sands hold a better preserved system of temporary ponds, with a flooding dynamic that fluctuates with precipitation. The unprotected area shows an increase in mean hydroperiod duration, and surface flooded, and a decline in hydroperiod variability. This seems to be due to the creation of irrigation ponds and the artificialization of the flooding regime of the natural temporary ponds, that either receive excess irrigation water or dry-up due to the lowering of the groundwater table level. Although a decline in hydroperiod duration of temporary ponds is seen as negative to the system, an increase in hydroperiod of surface waters due to artificialization, or a decline in variability cannot be considered as positive compensatory effects.

The recharge process in alluvial strip aquifers in arid Namibia and implication for artificial recharge

Alluvial strip aquifers associated with ephemeral rivers are important groundwater supply sources that sustain numerous settlements and ecological systems in arid Namibia. More than 70 % of the population in the nation’s western and southern regions depend on alluvial aquifers associated with ephemeral rivers. Under natural conditions, recharge occurs through infiltration during flood events. Due to the characteristic spatial and temporal variability of rainfall in arid regions, recharge is irregular making the aquifers challenging to manage sustainably and they are often overexploited. This condition is likely to become more acute with increasing water demand and climate change, and artificial recharge has been projected as the apparent means of increasing reliability of supply. The article explores, through a case study and numerical simulation, the processes controlling infiltration, significance of surface water and groundwater losses, and possible artificial recharge options. It is concluded that recharge processes in arid alluvial aquifers differ significantly from those processes in subhumid systems and viability of artificial recharge requires assessment through an understanding of the natural recharge process and losses from the aquifer. It is also established that in arid-region catchments, infiltration through the streambed occurs at rates dependent on factors such as antecedent conditions, flow rate, flow duration, channel morphology, and sediment texture and composition. The study provides an important reference for sustainable management of alluvial aquifer systems in similar regions.

Habitat drives dispersal and survival of translocated juvenile desert tortoises

Summary In spite of growing reliance on translocations in wildlife conservation, translocation efficacy remains inconsistent. One factor that can contribute to failed translocations is releasing animals into poor‐quality or otherwise inadequate habitat. Here, we used a targeted approach to test the relationship of habitat features to post‐translocation dispersal and survival of juvenile Mojave desert tortoises Gopherus agassizii. We selected three habitat characteristics – rodent burrows, substrate texture (prevalence and size of rocks) and washes (ephemeral river beds) – that are tied to desert tortoise ecology. At the point of release, we documented rodent burrow abundance, substrate texture and wash presence and analysed their relationship to the maximum dispersal. We also documented the relative use by each individual for each habitat characteristic and analysed their relationships with survival and fatal encounters with a predator in the first year after release. In general, the presence of refugia or other areas that enabled animals to avoid detection, such as burrows and substrate, decreased the overall mortality as well as predator‐mediated mortality. The presence of washes and substrate that enhanced the tortoises’ ability to avoid detection also associated with the reduced dispersal away from the release site. These results indicate an important role for all three measured habitat characteristics in driving dispersal, survival or fatal encounters with a predator in the first year after translocation. Synthesis and applications. Resource managers using translocations as a conservation tool should prioritize acquiring data linking habitat to fitness. In particular, for species that depend on avoiding detection, refuges such as burrows and habitat that improved concealment had notable ability to improve the survival and dispersal. Our study on juvenile Mojave desert tortoises showed that refuge availability or the distributions of habitat appropriate for concealment are important considerations for identifying translocation sites for species highly dependent on crypsis, camouflage or other forms of habitat matching.

Geomorphic change and sediment transport during a small artificial flood in a transformed post-dam delta: The Colorado River delta, United States and Mexico

The Colorado River delta is a dramatically transformed landscape. Major changes to river hydrology and morpho-dynamics began following completion of Hoover Dam in 1936. Today, the Colorado River has an intermittent and/or ephemeral channel in much of its former delta. Initial incision of the river channel in the upstream ∼50km of the delta occurred in the early 1940s in response to spillway releases from Hoover Dam under conditions of drastically reduced sediment supply. A period of relative quiescence followed, until the filling of upstream reservoirs precipitated a resurgence of flows to the delta in the 1980s and 1990s. Flow releases during extreme upper basin snowmelt in the 1980s, flood flows from the Gila River basin in 1993, and a series of ever-decreasing peak flows in the late 1990s and early 2000s further incised the upstream channel and caused considerable channel migration throughout the river corridor. These variable magnitude post-dam floods shaped the modern river geomorphology. In 2014, an experimental pulse-flow release aimed at rejuvenating the riparian ecosystem and understanding hydrologic dynamics flowed more than 100km through the length of the delta’s river corridor. This small artificial flood caused localized meter-scale scour and fill of the streambed, but did not cause further incision or significant bank erosion because of its small magnitude. Suspended-sand-transport rates were initially relatively high immediately downstream from the Morelos Dam release point, but decreasing discharge from infiltration losses combined with channel widening downstream caused a rapid downstream reduction in suspended-sand-transport rates. A zone of enhanced transport occurred downstream from the southern U.S.-Mexico border where gradient increased, but effectively no geomorphic change occurred beyond a point 65km downstream from Morelos Dam. Thus, while the pulse flow connected with the modern estuary, deltaic sedimentary processes were not restored, and relatively few new open surfaces were created for establishment of native riparian vegetation. Because water in the Colorado River basin is completely allocated, exceptional floods from the Gila River basin are the most likely mechanism for major changes to delta geomorphology for the foreseeable future.

Analysis of subsurface temperature data to quantify groundwater recharge rates in a closed Altiplano basin, northern Chile

Quantifying groundwater recharge is a fundamental part of groundwater resource assessment and management, and is requisite to determining the safe yield of an aquifer. Natural groundwater recharge in arid and semi-arid regions comprises several mechanisms: in-place, mountain-front, and mountain-block recharge. A field study was undertaken in a high-plain basin in the Altiplano region of northern Chile to quantify the magnitude of in-place and mountain-front recharge. Water fluxes corresponding to both recharge mechanisms were calculated using heat as a natural tracer. To quantify in-place recharge, time-series temperature data in cased boreholes were collected, and the annual fluctuation at multiple depths analyzed to infer the water flux through the unsaturated zone. To quantify mountain-front recharge, time-series temperature data were collected in perennial and ephemeral stream channels. Streambed thermographs were analyzed to determine the onset and duration of flow in ephemeral channels, and the vertical water fluxes into both perennial and ephemeral channels. The point flux estimates in streambeds and the unsaturated zone were upscaled to channel and basin-floor areas to provide comparative estimates of the range of volumetric recharge rates corresponding to each recharge mechanism. The results of this study show that mountain-front recharge is substantially more important than in-place recharge in this basin. The results further demonstrate the worth of time-series subsurface temperature data to characterize both in-place and mountain-front recharge processes.

Local scour calculation around bridge pier during flood event

In the present study, local scour around bridge pier during flood event especially in flashy hydrographs is investigated, experimentally. The duration of tested hydrographs were deduced from different ephemeral rivers in nature using physical modeling concepts. Final scour depth as well as scour hole width and time development of scour depth were recorded by highresolution cameras. The effects of flood hydrograph rising as well as falling limb and peak discharge on local scour were studied. Results showed that, for hydrographs with similar duration and peak discharge, the influence of time for hydrograph rising limb (Tr) on final scour depth is not of great importance. On the other hand, the scour depth slightly decreased after the hydrograph peak due to the slide of the scour hole slopes and deposition of some of suspended sediments. The maximum decrease was about 5% of the pier diameter. In addition, it is shown that the effect of time for hydrograph falling limb (Tf) on scour depth and width is closely related to parameter Tf/Tr. For example, final scour depth for a typical hydrograph with Tf/Tr = 9.25 is about 42% larger than similar hydrograph with only a rising limb. Overall, experimental results revealed that hydrograph peak discharge plays a much more important role than other parameters in affecting the development of scour hole. Based on the present as well as the previous experimental results, a new modification factor is added to HEC-18 empirical equation to calculate final scour depth during a hydrograph event. Finally, by using stepped-hydrograph model, time development of scour depth was calculated for the present tested flashy hydrographs which showed acceptable accuracy of this method.

Post-Pleistocene Archaeology and Geomorphological Processes on the Namib Desert Coast of South Western Africa

ABSTRACTThe Namib Desert coast is rich in potential food resources, but its extreme aridity imposed severe limits on human settlement in the past. It appears that intensive use of the coast began in the Middle Holocene, although there is evidence of intermittent occupation from the Middle Pleistocene. Coastal sites were closely linked to the desert interior and movement between the two was mainly via the linear oases of ephemeral river systems. Contact was at first sporadic but regular trade with merchant vessels was established in the mid-eighteenth century AD. Coastal geomorphological processes determined the location and accessibility of marine resources used by hunter-gatherer and nomadic pastoral communities. At the same time, marine, fluvial, and aeolian processes have influenced the visibility and survival of archaeological evidence on the coast.

Waterbugs (Heteroptera: Nepomorpha and Gerromorpha) as sources of essential n‐3 polyunsaturated fatty acids in Central Siberian ecoregions

Summary Aquatic systems are considered to be a main source of essential long‐chain n‐3 polyunsaturated fatty acids (PUFA), which are preferentially synthesized by microalgae and transferred along food chains to terrestrial consumers. Emerging aquatic insects comprise a significant part of this transfer of the essential PUFA from water to land. Quantitative data on PUFA content and composition are available mainly for rheophilic insects while taxa that are characteristic of wetlands and stagnant water bodies, such as aquatic Heteroptera, remain relatively unstudied. We investigated the role of various waterbug taxa (Heteroptera: Nepomorpha and Gerromorpha) inhabiting different ecoregions in temperate Central Siberia (Russia) as potential sources of PUFA. The ecoregions were steppe, forest‐steppe, hemiboreal forest and montane coniferous forest. Although these waterbugs insects are aquatic in both larval and adult stages, they can disperse through the landscape and transfer essential PUFAs from water to land so making them potentially available to terrestrial consumers. Species of Naucoridae, Notonectidae and Corixidae were generally dominant in all ecoregions, attaining maximum biomass in the steppe. We showed that habitat or ecoregion played a major role in the variability of fatty acid composition of Notonectidae and Gerridae but not Corixidae. In contrast, the biochemical composition of the only naucoridae, Ilyocoris cimicoides, was largely affected by life stage. Both the dominant families and species within them differed with respect to their mass‐specific contents of essential long‐chain PUFA of the n‐3 family, namely eicosapentaenoic and docosahexaenoic acids. Corixid species had the highest content of these two essential PUFAs among the waterbug studies, and relative to literature reports for other aquatic insects. Corixids thus appear to be a potentially important vector for transfer of the essential biochemical compounds from water to land, especially in steppe ecoregions with numerous ephemeral water bodies.

Ephemeral stream dynamics, land cover changes and climate variability in the marginal grabens of the northern Ethiopian Rift Valley

This study focused on ephemeral streams morphodynamics and their relation with hydro-climatic and bio-physical characteristics in their headwater and graben floors in the marginal grabens along the northern Ethiopian Rift Valley. The Raya graben was taken as a case study area, which is the largest marginal graben in northern Ethiopia. The link between rainfall variability and land cover changes shows that there was a negative correlation between precipitation and deforestation which implies that an increase in precipitation decreases deforestation deterring the peasants from tree cutting. Regarding the relationship between upper catchment characteristics and river morphology, results show that upper catchment area is the most important controlling factor of the length of and the area occupied by the rivers in the graben bottom. A simple hydraulic analysis based on the change of specific discharge as the river ow approaches the bridges demonstrates that the abrupt change in channel width as the river approaches the bridge is the main cause of the thick deposition and, consequently, of the increased frequency of overbank oods upstream of the road bridges in the study area. The results related to channel bank erosion capacity of ash oods show that all the peak discharges were equally important in triggering channel bank erosion. Concomitantly, vegetated channels are much more resist- ant to channel bank erosion than non-vegetated banks. Land changes in graben bottoms dictate that the changes are not simply related to a distributary river system but that human interven- tion (e.g., the conversion of bushland and forests into cropland, settlement on alluvium, and avoidance of losing farmland to other land units) and natural vegetation regeneration (e.g., alluvial deposits into shrubland) are also important. Overall, the study suggests that to control the morphodynamics of the ephemeral rivers and their impact on farming activities in the gra- ben bottom, catchment rehabilitation activities need to focus both on the upper catchment and the river channels in the graben bottom. Key words: ephemeral streams, land change, graben, Rift Valley, land cover, climate variability

Excess River Sedimentation at Bridges in the Raya Graben (Northern Ethiopia)

AbstractThis study investigates the temporal variability and effects of bio‐climatic factors and bridges on local hydro‐geomorphic conditions that cause excess sedimentation and flood hazard in ephemeral rivers of the Raya graben (northern Ethiopia). Normalized difference vegetation index (NDVI) was analyzed using Landsat imageries of 1972, 1986, 2000, 2005, 2010, and 2014. Daily rainfall data of three meteorological stations were considered to analyze the temporal trends and return time intervals of rainfall intensity in 24 h. Stream‐bed gradient and bed material grain size were measured in 20 river reaches. Results show that the maximum NDVI values were recorded in the time interval from 2000 to 2014. Nevertheless, during this decade, the study bridges experienced the largest excess sedimentation, in some bridges up to 1·6 m. Different factors to account for it are discussed as no evidence of rainfall pattern, accountable for an increase in sediment delivery from the headwaters, is found. The field data show that channel narrowing at bridges is the main cause of thick upstream sedimentation and of increasing frequency of overbank flows during the last 15 years. A simple hydraulic approach is presented to explain the variability of sediment deposition at bridges and change in bed material grain size between the main stem and the reach right upstream of the bridge. Copyright © 2016 John Wiley & Sons, Ltd.

Pace of Landscape Change and Pediment Development in the Northeastern Sonoran Desert, United States

Pediments of the Sonoran Desert in the United States have intrigued physical geographers and geomorphologists for nearly a century. These gently sloping bedrock landforms are a staple of the desert landscape that millions visit each year. Despite the long-lived scientific curiosity, an understanding of the processes operating on the pediment has remained elusive. In this study we revisit the extensive history of pediment research. We then apply geospatial, field, and laboratory cosmogenic 10Be nuclide dating and back-scattered electron microscopy methods to assess the pace and processes of landscape change on pediment systems abutting the Salt River in Arizona. Our study focuses on the Usery pediments linked to base-level fluctuations (river terraces) of the Salt River. Relict pediment surfaces were reconstructed with dGPS data and kriging methodologies utilized in ArcGIS—based on preserved evidence of ancient pediment surfaces. 10Be ages of Salt River terraces established a chronology of incision events, where calculating the volume between the reconstructed relict pediment and modern surface topography established minimum erosion rates (∼41 mm/ka to ∼415 mm/ka). Pediment area and length appear to have a positive correlation to erosion rate and development of planar pediment surfaces. Field and laboratory observations reveal that pediment systems adjust and stabilize at each Salt River terrace. Relief reduction across the pediment begins with pediment channel incision via headward erosion. Next, tributary drainage capture begins and collapses interfluves. Lateral stream erosion promotes planation where the porosity of decayed granite along channel banks exceeds the bedrock underneath ephemeral channels.

Flood routing in an ephemeral channel with compound cross-section

Natural phenomenon of surface and subsurface flow interaction is an intrinsic component of the hydrological processes in any watershed. It is a highly sensitive process, especially in arid and semi-arid regions, and should be considered while dealing with any water management activity in these regions. This paper describes a novel approach for flood routing in an ephemeral channel with compound cross-sections. The proposed mathematical model couples the numerical solution for complete Saint-Venant equations for surface flow with the numerical solution for one-dimensional Richards equation for sub-surface flow through an iterative procedure. Recently developed interactive divided channel (IDC) method is incorporated for simulating the main channel and flood plain flow interactions. In the one-dimensional surface and pseudo two-dimensional sub-surface (1DSP2DSS) model presented here, the effect of lateral variation in infiltration rate at a cross section arising due to (i) lateral variation in flow depth and (ii) lateral variation in soil characteristics is incorporated by considering infiltration into different soil columns for main channel and flood plains. The proposed model is verified by comparing the model results with those available in literature for benchmark problems. Simulations are presented to demonstrate the capability of the model for flood routing in ephemeral channels with flood plains and the effect of lateral variation in infiltration rate on transmission losses.

A comparison of temperature regimes in dry-season waterholes in the Flinders and Gilbert catchments in northern Australia

Dry-season waterholes in ephemeral rivers provide vital habitat for aquatic biota, whose survival is dependent on the waterholes lasting throughout the dry season with temperatures that are not lethal. To examine this in the Flinders and Gilbert Rivers, 20-min temperature measurements were taken during the 2012–2013 dry season in 10 waterholes in each catchment. These data were used to derive thermal-frequency curves that quantify how often waterhole temperature exceeds thresholds for (1) the optimum growth of tropical fish and (2) their lethal temperature. Waterholes that remained deeper than ~0.5m throughout the dry season provided thermally suitable fish refugia, especially if they were turbid, because this decreased the risk of exposure to undesirable temperatures at the bottom of the waterhole. However, surface temperatures in these waterholes often exceeded optimal and even potentially lethal temperatures, so fish may have had to move to cooler water at the bottom of these waterholes. The risk to aquatic species in waterhole refugia is, therefore, primarily determined by depth, with shallow waterholes presenting the greatest risk because they become thermally unsuitable well before (1–2 months) they fully dry out.

Validating a mass balance accounting approach to using 7Be measurements to estimate event‐based erosion rates over an extended period at the catchment scale

AbstractUse of the fallout radionuclides cesium‐137 and excess lead‐210 offers important advantages over traditional methods of quantifying erosion and soil redistribution rates. However, both radionuclides provide information on longer‐term (i.e., 50–100 years) average rates of soil redistribution. Beryllium‐7, with its half‐life of 53 days, can provide a basis for documenting short‐term soil redistribution and it has been successfully employed in several studies. However, the approach commonly used introduces several important constraints related to the timing and duration of the study period. A new approach proposed by the authors that overcomes these constraints has been successfully validated using an erosion plot experiment undertaken in southern Italy. Here, a further validation exercise undertaken in a small (1.38 ha) catchment is reported. The catchment was instrumented to measure event sediment yields and beryllium‐7 measurements were employed to document the net soil loss for a series of 13 events that occurred between November 2013 and June 2015. In the absence of significant sediment storage within the catchment's ephemeral channel system and of a significant contribution from channel erosion to the measured sediment yield, the estimates of net soil loss for the individual events could be directly compared with the measured sediment yields to validate the former. The close agreement of the two sets of values is seen as successfully validating the use of beryllium‐7 measurements and the new approach to obtain estimates of net soil loss for a sequence of individual events occurring over an extended period at the scale of a small catchment.

Russia: TVEL/Siberian Chemical Combine – TiO2

Although it is commonly assumed that the trace-fossil record of eolian dunes and associated environments is invariably poor, a systematic review of the available information indicates that this is not necessarily the case. A model involving five main phases of colonization of desert environments through the Phanerozoic is proposed in this paper. The first phase (Cambrian–Silurian) involved animal incursions into coastal dune fields directly from the sea, although it is unlikely that these animals would have remained for long periods of time in coastal deserts. The second phase (Devonian) reflects the activities of dune pioneers that left their fluvial habitat to enter temporary or permanently into inland deserts. The third phase (Carboniferous–Permian) involved the colonization of deserts by tetrapods. The fourth phase (Triassic–Cretaceous) involved a major exploitation of the infaunal ecospace as reflected by the appearance of more varied behavioral patterns in sub-superficial structures. The fifth phase (Paleogene–Recent) reflects the appearance of the ecological dynamic that characterizes modern desert communities. The invertebrate ichnofacies for eolian dunes is re-named herein as the “Octopodichnus–Entradichnus Ichnofacies” honoring the seminal work of previous workers who simultaneously tackled the issue of eolian dune ichnofacies. The Chelichnus ichnofacies is retained for vertebrate trace-fossil assemblages in eolian settings. Both ichnofacies occur in mobile and temporary stabilized sandy substrates, subject to frequent erosion and deposition, and to strong seasonality. Desert settings consist of complex mosaics of habitats or physical units associated with organism activity. Trace-fossil distribution can be understood as reflecting the partitioning of desert settings in a mosaic of landscape units, characterized by water content and its temporal fluctuations, nutrient availability, nature of the substrate, and the dominant organisms present. In turn, desert systems are dynamic entities that change as a response to regional climate. Landscape units, such as eolian sand seas, salt flat and playa lake systems, ephemeral rivers and alluvial fans, interact in response to regional-scale climate variations in hyper-arid, arid, and semiarid climatic settings. Ancient deserts completely developed under hyper-arid climatic conditions rarely preserve trace fossils due to the absence of moisture near the surface. However, the alternation of wet periods may represent windows for life development and thus, preservation of biogenic structures. Arid deserts display complex patterns of dunes combined with dry, wet, and flooded interdunes. Dry desert elements (e.g. dunes, interdunes, sand sheets) typically record the Entradichnus–Octopodichnus and Chelichnus Ichnofacies. Slight rises in regional precipitation produce elevation of the water table and increase of fluvial discharges that provide water and sediment to the system. These processes may result in the local concentration of trace fossils in wet interdunes and ephemeral fluvial systems, illustrating the Scoyenia and Chelichnus Ichnofacies. In semiarid systems playa lakes expand by the addition of freshwater, evolving into freshwater lakes, and fluvial systems may become more common; lake margins and fluvial overbanks typically contain trace-fossil assemblages that may be ascribed to the Scoyenia Ichnofacies.

Geomorphological impacts of an extreme flood in SE Spain

Long-term field studies in semiarid ephemeral streams are rare. These geomorphic data are essential for understanding the nature of the processes in order to develop modelling for risk assessments and management. An extreme flood event on 28 September 2012 affected the Murcia region of SE Spain, including long-tem monitoring sites on two fluvial systems in the Guadalentín basin, the Nogalte and Torrealvilla. Detailed morphological data were collected before and immediately after the event; and the amount of morphological change, erosion, and deposition have been related to peak flow conditions at the sites.On the Nogalte channel, peak flow reached 2500m3s−1 at the downstream end of the catchment in less than 1h. The event had a recurrence interval of >50years based on rainfall records and damage to old irrigation structures. The major effect in the braided, gravel channel of the Nogalte was net aggradation, with massive deposition in large flat bars. The measured changes in bankfull capacity were highly correlated with most hydraulic variables. Net changes in cut-and-fill in cross sections on the Nogalte were highly related to peak discharge and stream power but much less so to measures of hydraulic force (velocity, shear stress, unit stream power). Relationships of amount of erosion to hydraulic variables were much weaker than for amount of deposition, which was largely scaled to channel size and flow energy. Changes on the Torrealvilla were much less than on the Nogalte, and net erosion occurred at all sites. Sites on the Nogalte channel in schist exhibited higher deposition than those of the Torrealvilla sites on marl for the same hydraulic values.Overall, less morphological change took place in the extreme event on the Nogalte than predicted from some published hydraulic relations, probably reflecting the high sediment supply and the hydrological characteristics of the event. The results demonstrate the high degree of adjustment of these channels to the occasional, high magnitude, flash flood events and that such events need to be allowed for in management. The detailed quantitative evidence produced by these long-term monitoring sites provide valuable, rare data for modelling morphological response to flood events in ephemeral channels.

Rain and channel flow supplements to subsurface water beneath hyper-arid ephemeral stream channels

Summary In hyper-arid regions, ephemeral stream channels are important sources of subsurface recharge and water supply for riparian vegetation, but few studies have documented the subsurface water content dynamics of these systems. This study examines ephemeral channels in the hyper-arid western Sonoran Desert, USA to determine how frequently water recharges the alluvial fill and identify variables that affect the depth and persistence of recharge. Precipitation, stream stage, and subsurface water content measurements were collected over a three-year study at six channels with varying contributing areas and thicknesses of alluvial fill. All channels contain coarse alluvium composed primarily of sands and gravels, and some locations also have localized layers of fine sediment at 2–3 m depth. Rain alone contributed 300–400 mm of water input to these channels over three years, but water content responses were only detected for 36% of the rain events at 10 cm depth, indicating that much of the rain water was either quickly evaporated or taken up by plants. Pulses of water from rain events were detected only in the top meter of alluvium. The sites each experienced ⩽5 brief flow events, which caused transient saturation that usually lasted only a few hours longer than flow. These events were the only apparent source of water to depths >1 m, and water from flow events quickly percolated past the deepest measurement depths (0.5–3 m). Sustained saturation in the shallow subsurface only developed where there was a near-surface layer of finer consolidated sediments that impeded deep percolation.