Microbially-mediated transformations are important for removing wastewater chemicals in rivers. In-stream degradation rates are likely to be affected by channel morphology and size because these factors determine the extent of chemical contact between the water column and the microbial community in fixed biofilms, where most microbially-mediated processes are expected to take place. We hypothesize that transformation rate constants are inversely proportional to hydraulic radius (R: the ratio of channel cross-sectional area to wetted perimeter). Microbial transformations are also predicted to be controlled systematically by bed sediment characteristics. This will be a function of sediment surface area per unit volume (for biofilm colonisation) and hyporheic exchange. Here, we test the above hypotheses in controlled 14-day laboratory experiments, by monitoring nitrification and biodegradation in stirred tanks with three different water depths and three different sediment sizes (sand, gravel and cobbles). Transformation rate constants were inversely proportional to depth, supporting the notion that channel morphology represents a significant system-specific control. Rate constants for nitrification were highest in the gravel treatments and lowest in sand. This suggests that intermediate grain sizes simultaneously allow good biofilm colonisation and solute exchange between sediment and the water column. There was, however, little difference in biodegradation rate constants between different sediment sizes. This study highlights the need to consider geomorphology in models of chemical exposure.

Single- and multi-thread gravel-bed ephemeral channels in semi-arid and arid regions have a characteristic repeating, channel-wide pattern of low angle, fine-grained 'flats' alternating with steeper, coarse-grained 'bars'. The genesis of these macroforms, while a topic of ongoing discussion, has not been fully elucidated. We have documented the formation of these macroforms after both artificially homogenising the bed material of a reach of the Nahal Yatir in the northern Negev, Israel, and ensuring its surface was planar. Here, we integrate the empirical data gained from these field experiments with a novel mathematical model. The model we propose, within a one-dimensional framework, is based on the analysis of flow over an initially planar, erodible bed consisting of a bimodal grain size mixture of sediment. We apply linear stability analysis to derive a solution that provides insight into the formational dynamics of these macroforms. Our results indicate that bar-flat patterns may arise from a free-instability mechanism driven by sediment size heterogeneity, provided that the standard deviation of the sediment grain size distribution (GSD) is sufficiently large. Application of the model to a selection of ephemeral channels of the northern Negev suggests that repeating bar-flat patterns are likely to develop during the recession of flash flood hydrographs, specifically when flow conditions approach the critical threshold for bedload transport. Besides providing a possible explanation for the formation of these macroforms, this study also contributes to a broader understanding of geomorphic processes in dryland river systems.

Repeating fluvial macroforms are ubiquitous. The streamwise alternation of steeper, coarse‐grained, cobble/pebble bars and near‐horizontal, fine‐grained, sand/granule flats is characteristic of upland, single‐thread, dryland channels. We have monitored the rate of formation of a bar‐flat sequence by flash floods, noting the disposition and extent of each macroform as they evolve. To accomplish this, the bed material of a straight reach of the Nahal Yatir in the northern Negev Desert, Israel, was thoroughly mixed to a depth of 0.5 m with the aid of a mini excavator, obliterating a well‐formed sequence of bars and flats. The ten flow events of two succeeding rain seasons were recorded, as were the changing topography and textural roughness of the bed following each complete post‐flood dewatering. Embryonic flats, texturally like those present before disturbance, formed under the first flow event, occupying a fifth of the total length of the flats that had existed in the reach before experimental disturbance. Their length increased and their inclination decreased with each flow event, returning to the natural, pre‐disturbed, aggregated length within two rain seasons. Restoration of the pre‐disturbance bar‐flat sequence was almost complete, the location of bars differing only marginally in places. Resemblance with the natural original was high, reflecting the sedimentary dynamism of desert flash floods. A mechanism that models and explains the formation and stability of these macroform sequences is developed in a companion paper.

Microbially mediated transformations, such as nitrification and biodegradation, play a crucial role in removing pollutants from rivers. Although in-stream removal rate coefficients are often assumed to be spatially and temporally constant, they are likely affected by the channel shape and size because these factors control contact between the water column and fixed biofilms. Here, we test the hypothesis that transformation rate constants are inversely proportional to the hydraulic radius (R: ratio of the channel cross-sectional area to wetted perimeter) in dye tracing experiments conducted in two U.K. rivers with contrasting morphologies: (1) the River Maun (shallow: mean bankfull R = 1.25 m) and (2) the River Calder (deep: mean bankfull R = 3 m). In each case, a slug of rhodamine WT was injected upstream of a wastewater outfall, and samples were collected downstream, staggered by the rhodamine travel time. Rate constants were derived for sucralose, ammonium, caffeine, and linear alkylbenzenesulfonate. Sucralose (persistent, hydrophilic, and exclusively of wastewater origin) was used as a conservative tracer to adjust model fits for dilution. Higher rate coefficients were observed for all biotransformed pollutants in the Maun compared to the Calder, supporting the hypothesis and highlighting the need to consider geomorphology in models of chemical behavior.

Gravel bed channels of single-thread ephemeral streams in drylands are typified by alternations of steep, coarse-grained bars and less steep, fine-grained 'flats'. The origin of both macroforms has been discussed but not fully explained. We studied the generation of this macroform sequence under field conditions. Relevantly, we provide a possible explanation regarding their formation, using a simple mathematical model fed by data obtained from field experiments carried out on the Yatir channel in the northern Negev, Israel. The mathematical model is developed considering the one-dimensional model of flow over an erodible bed composed of a bimodal mixture of sediments. The solution is obtained via the application of linear stability analysis. Results suggest that the bar-flat pattern occurs when the standard deviation of the sediment mixture is relatively large, as in the case of the Yatir channel, and the flow approaches critical conditions for bedload transport.

The aeolian landforms of the Mojave Desert in the SW USA have been studied in detail over the last three decades, particularly in terms of their relationship to the region's topography and Pleistocene climate / sediment supply histories, as well as wider developments, such as the aeolian "sediment state" concept. In this context, the evolution of the Mojave River and its associated palaeolakes is thought to have been a key control on long-term sediment supply to aeolian systems, and luminescence dating chronologies for a range of (resulting) aeolian landforms have been related to these hydrological changes. Here we argue that at least some of these aeolian chronologies need to be re-assessed. We focus on luminescence chronologies for aeolian landforms within and marginal to the Cady Mountains, a mountain block adjacent to the Mojave River and palaeolake Manix, east of Barstow, California. We demonstrate that quartz in this locale exhibits several malign luminescence properties, and that low temperature K-feldspar infrared stimulated luminescence (IRSL) consistently exhibits high anomalous fading rates. Both contribute to age underestimation. We address these issues via post-infrared IRSL (pIRIR) and post-isothermal post-IR (pIt-IR) analyses of K-feldspars. The resulting ages span the last 120 ka and imply phases of aeolian activity of a substantially greater antiquity than previously inferred. Notably, at one well-studied site - the Soldier Mountain sand ramp - the new ages suggest a landform dating not to Marine Isotope Stage (MIS) 1 or 2, as previously suggested, but more likely to MIS 5. The Cady Mountain record indicates that the only period of the last glacial cycle lacking evidence for aeolian sedimentation is 40-9 ka, broadly consistent with expectations of increased regional humidity. These results also suggest that site topographic context may influence the age structure of aeolian deposits. In this instance, sand ramps consistently represent the oldest type of deposit (range 40-120 ka), while early-mid Holocene dune accumulation is associated with sandsheets and valley-fill sands. Based on these findings, we argue that there is a need to critically re-assess the existing regional luminescence age database, and that there is potential to significantly revise our understanding of the region's aeolian system responses, and associated paleoenvironmental interpretations.

Plastic accumulation in the marine environment is a major concern given the harmful effects and longevity of plastics at sea. Although rivers are likely to significantly contribute to the flux of plastic to marine systems, the behaviour of plastic debris in fluvial systems remains poorly understood and estimates of riverine plastic flux derived from field measurements and modelling efforts are highly uncertain. This paper presents a new probabilistic model of plastic transport in rivers which describes the main processes controlling plastic displacement and which predicts the statistical distribution of travel distances for individual items of buoyant macroplastic debris. Macroplastic transport is controlled by retention in temporary stores (or traps) created by vegetation, bank roughness elements and other obstacles. The behaviour of these traps is represented in the model via a series of Bernoulli trials conducted in a Monte Carlo simulation framework. The model was applied to a tracer experiment in a small 1.1 km river reach. Three replicates were used for calibration and three for validation. For each replicate, 90 closed air-filled polyethylene terephthalate (PET) bottles were introduced at the upstream end of the reach and the location of each bottle was recorded after 24 h. Bottles were chosen as “model” macroplastic litter items given their high usage and littering rate. Travel distances were low. The average and maximum distances travelled over 24 h were 231 m and 1.1 km, respectively. They were also variable. The coefficient of variation of travel distances was 0.94. Spatial patterns were controlled by the location and characteristics of discrete traps. The model was able to describe the observed travel distance distributions reasonably well, suggesting that modelling plastic behaviour in longer reaches and even whole catchments using a stochastic travel distance approach is feasible. The approach has the potential to improve estimates of river plastic flux, although significant knowledge gaps remain (e.g., the rate and location of plastic supply to river systems, the transport behaviours of different types of plastic debris and trap effectiveness in different types of river system, season, and discharge).

Topography fundamentally influences the distribution and morphology of aeolian landforms via the modification of surface wind flow and the creation of space for sediment deposition. This has been observed at both landform (individual topographic dune forms) and macro-landscape (sand sea) scales. Although previous studies have considered several aspects of the impact of topography on aeolian landforms, the patterns of landscape-scale aeolian sediment accumulation that emerge at the meso-scale, within topographically complex environments have received less consideration. To address this, we present an approach that combines information on the presence of surficial sand (via remote sensing) with the morphometric feature classification method, LandSerf. Using the Cady Mountains in the Mojave Desert as a case study, we explore the relationships between sand cover and topographic indices over length scales of 10(2)-10(3) m. Field observations are then used to refine our understanding of these patterns. Aeolian deposits across the Cady Mountains are strongly controlled by the topography. Although sand cover is often continuous and highly variable in depth, four archetypal "accommodation space types" are identified from the morphometric analysis: Slopes, Plains, Valley-Fills, and Slope-Valley composite. Specific aeolian land -forms within these accommodation spaces may manifest as sand ramps and climbing -falling dunes, particularly on mountain front Slopes, and as sand sheets on downwind Plains within the mountain block. In areas of high sediment supply these may also coalescence, as exemplified by the extensive and compositionally complex Slope-Valley composites in the northern Cady Mountains. In conjunction with field observations, we argue that topography, moderated by proximity to sediment supply, strongly influences the character of the aeolian sedimentary record. However, even within the relatively complex landscape studied here, 90% of the mapped sand accumulation is associated with the four identified accommodation space types. The implication is that areas of such complex topography are amenable to analysis within the scheme outlined and that this can potentially be used to support interpretations of accompanying dune chronologies. (C) 2021 The Author(s). Published by Elsevier B.V.

The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near‐bed flow dynamics, and thus on fluid advection at the sediment–water interface. This study provides the first quantification of this effect for water‐worked gravel beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double‐averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three‐fold. First, local pressure gradients, and therefore rates of material transport, across the sediment–water interface are likely to differ between impermeable and permeable beds. Second, near‐bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse‐grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

The Mojave Desert presents an array of Pleistocene lacustrine deposits and aeolian landforms to which, at times, it has proved challenging to apply luminescence methods. We tested the suitability of K-feldspar post-IR IRSL methods using two sites with independent radiocarbon dating shorelines at Harper Lake and Silver Lake - considering: 1) overall performance of the post-IR IRSL 225 degrees C (PIRIR225) protocol; 2) effect of test dose size on pIRIR(225) D-e; 3) anomalous fading correction of pIRIR(225) ages; 4) preliminary single grain pIRIR(225) results. We observe consistently good performance of the single aliquot pIRIR(225) protocol, with good dose recovery, acceptable recycling ratios, low recuperation and low inter-aliquot scatter. The pIRIR(225) ages for Silver Lake (8.8 +/- 0.4 and 11.3 +/- 0.5 ka) and Harper Lake (both 25.4 +/- 1.4 ka) are in substantially better agreement with the independent dating than low temperature (50 degrees C) IRSL and quartz OSL ages. pIRIR(225) fading rates are reduced to similar to 2.0-2.5% per decade, but there remains a tendency for under-estimation when using uncorrected ages. A need for fading correction is further implied at Harper Lake via comparison with multi-elevated temperature (MET)-PIR age plateaus and pIRIR(290) measurements, although at the younger Silver lake site these methods produce ages nearly identical to the uncorrected pIRIR(225) ages. Preliminary single grain pIRIR(225) measurements suggest a similar to 25-30% usable grain yield. At Silver Lake the single grain and single aliquot ages agree well despite over-dispersion of the single grain equivalent dose distribution. At Harper Lake the single grain and single aliquot pIRIR(225) ages also agree well, although a population of insensitive, lower D-e grains is observed. These grains are not associated with significantly higher fading rates.

Differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume. The temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size, microtopography, and bed organization at both grain and mesoscales. Measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds. Structured mobile armors formed quickly in each experiment. At the grain scale (1-45mm; 9D(s50)17mm where D-s50 is the median surface particle size), surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor. Particles comprising the armor also became flow aligned and developed imbrication. At a larger scale (100-200mm), the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations. Metrics of mobile armor structure showed remarkable consistency over prolonged periods of near-constant transport, demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure. Bed structuring was least developed in the experiments conducted at the lowest flow strength. However, little difference was observed in the structural metrics of the mobile armors generated at higher flows. Although the range of transport rates studied was limited, the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low ((*)approximate to 0.03 where (*) is the dimensionless shear stress).

Resistance to flow determines several important hydraulic parameters in streams and rivers and must be properly represented in models for estimating water discharge and sediment transport. This paper reviews our understanding of how flow resistance is generated in open channel flows and evaluates the different approaches used to model the flow resistance that originates at the bed in coarse-grained alluvial rivers. The Manning equation is routinely used as a flow predictor in hydraulic models of open channel flow. However, defining the energy gradient and roughness coefficient can be problematic and as a result, estimates of velocity and discharge are subject to considerably uncertainty and error. Attempts to develop more physically based models of flow resistance exploit well-established principles of engineering fluid mechanics. Forces responsible for generating resistance at the channel bed are of two kinds—shearing forces and pressure forces. The former are generated by transfers of fluid momentum and give rise to skin friction. The latter are generated by pressure gradients around roughness elements and give rise to form drag. The relative importance of skin friction and form drag varies with the relative submergence—the ratio of the flow depth (Y) (or hydraulic radius (R)) to the size of the bed material (D). Skin friction dominates at high relative submergence (Y≫D) and models for such conditions are based on boundary layer theory and the results of its application to engineering studies of pipe flow. Models assume a logarithmic velocity profile through the depth and have the form 1/√ff∝log10 (R/ks) where ff is the Darcy–Weisbach roughness coefficient and ks is a bed roughness parameter scaled on bed material size (ks≈3.5D84 where D84 is the surface size for which 84% is finer than). Such models can be approximated by a power equation of the form 1/√ff∝(R/D)b with an exponent of 1/6. The assumption of a logarithmic velocity profile is invalid for low relative submergence conditions (Y≈D). For these flow conditions, flow resistance models are based on either hydraulic geometry or roughness layer theory. The equations that result from the two approaches have been shown to be equivalent and, moreover, to be approximated by 1/√ff∝(R/D). Since this, in turn, is equivalent to a power law with an exponent of 1, a variable power relation which is asymptotic to the logarithmic flow law and roughness layer laws at high and low relative submergence respectively provides a single equation that can be used in both deep and shallow flows as it explicitly accounts for the changing sources of flow resistance as relative submergence changes. A similar approach using dimensionless hydraulic geometry yields an equation with different parameter sets for deep and shallow flows. Assessments of the predictive performance of a variety of flow resistance equations suggest that the dimensionless hydraulic geometry equation for low-high relative submergence performs best, closely followed by the logarithmic flow law and the variable power flow law. Predictive errors, however, can be significant and further research is needed to better understand the physics of flow over rough boundaries and to incorporate this understanding in improved models of flow resistance.

ABSTRACTSingle‐thread, gravel‐bed streams of moderate slope in the northern Negev are characterized by three channel units: bars exhibit steeper than average slopes and poorly sorted mixtures of small–medium cobbles and coarse–very coarse pebbles; flats are associated with more gentle slopes and well‐sorted medium–fine pebbles and granules; and transitional units have intermediate slopes and grain size. In general, all three units are planar, span the full channel width and have well‐defined boundaries. Bars and flats are more common than the transitional units and alternate downstream for distances of several hundred metres, forming sequences that are reminiscent of the riffle–pool structure commonly observed in humid‐temperate gravel‐bed rivers. A notable contrast is the absence of significant bed relief: bars lack crests and flats lack depressions. The relative lack of bed relief in bar–flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport. This reduction of bed relief lowers channel roughness, which in turn increases flow velocity and, therefore, the ability of the channel to transmit the large sediment loads it receives. Although our analyses pertain to a semi‐arid river system, the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments. Copyright © 2012 John Wiley & Sons, Ltd.

Earth Surface Processes and LandformsVolume 35, Issue 11 p. 1349-1356 Exchanges Standing proud: a response to 'Soil-erosion models: where do we really stand?' by Smith et al. John Wainwright, Corresponding Author John Wainwright [email protected] Sheffield Centre for International Drylands Research, Department of Geography, University of Sheffield, Sheffield, UK Institut für Geoökologie, Universität Potsdam, Potsdam, GermanySheffield Centre for International Drylands Research, Department of Geography, University of Sheffield, Winter Street, Sheffield, S10 2TN, UKSearch for more papers by this authorAnthony J. Parsons, Anthony J. Parsons Sheffield Centre for International Drylands Research, Department of Geography, University of Sheffield, Sheffield, UKSearch for more papers by this authorEva N. Müller, Eva N. Müller Institut für Geoökologie, Universität Potsdam, Potsdam, GermanySearch for more papers by this authorRichard E. Brazier, Richard E. Brazier Department of Geography, University of Exeter, Exeter, UKSearch for more papers by this authorD. Mark Powell, D. Mark Powell Department of Geography, University of Leicester, Leicester, UKSearch for more papers by this author John Wainwright, Corresponding Author John Wainwright [email protected] Sheffield Centre for International Drylands Research, Department of Geography, University of Sheffield, Sheffield, UK Institut für Geoökologie, Universität Potsdam, Potsdam, GermanySheffield Centre for International Drylands Research, Department of Geography, University of Sheffield, Winter Street, Sheffield, S10 2TN, UKSearch for more papers by this authorAnthony J. Parsons, Anthony J. Parsons Sheffield Centre for International Drylands Research, Department of Geography, University of Sheffield, Sheffield, UKSearch for more papers by this authorEva N. Müller, Eva N. Müller Institut für Geoökologie, Universität Potsdam, Potsdam, GermanySearch for more papers by this authorRichard E. Brazier, Richard E. Brazier Department of Geography, University of Exeter, Exeter, UKSearch for more papers by this authorD. Mark Powell, D. Mark Powell Department of Geography, University of Leicester, Leicester, UKSearch for more papers by this author First published: 26 August 2010 https://doi.org/10.1002/esp.2047Citations: 8Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat REFERENCES Abrahams AD. 2003. A bedload transport equation for sheet flow. Journal of Hydraulic Engineering 129: 159–163. 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Assessing the impact of erosion on semi arid archæological sites. In Past and Present Soil Erosion, M Bell, J Boardman (eds). Oxbow Books: Oxford; 228–241. Wainwright J, Parsons AJ, Abrahams AD. 1995. Simulation of raindrop erosion and the development of desert pavements. Earth Surface Processes and Landforms 20: 277–291. Wainwright J, Parsons AJ, Abrahams AD. 1999. Field and computer simulation experiments on the formation of desert pavement. Earth Surface Processes and Landforms 24: 1025–1037. Wainwright J, Parsons AJ. 2002. The effect of temporal variations in rainfall on scale dependency in runoff coefficients. Water Resources Research 38(12): 1271. DOI. 10.1029/2000WR000188 Wainwright J, Mulligan M. 2003. Introduction. In Environmental Modelling: Finding Simplicity in Complexity, J Wainwright, M Mulligan (eds). John Wiley & Sons: Chichester; 1–4. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM, Fenti B. 2008a. A transport-distance approach to scaling erosion rates: 1. Background and model development. Earth Surface Processes and Landforms 33: 813–826. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM, Fenti B. 2008b. A transport-distance approach to scaling erosion rates: 2. Sensitivity and evaluation of MAHLERAN. Earth Surface Processes and Landforms 33: 962–984. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM, Fenti B. 2008c. A transport-distance approach to scaling erosion rates: 3. Evaluating scaling characteristics of MAHLERAN. Earth Surface Processes and Landforms 33: 1113–1128. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM. 2009. Response to Hairsine's and Sander's 'Comment on "A transport-distance based approach to scaling erosion rates:" Parts 1, 2 and 3 by Wainwright et al.'. Earth Surface Processes and Landforms 34: 886–890. Yalin MS. 1972 Mechanics of Sediment Transport. Pergamon Press: Oxford. Zhang X, Drake NA, Wainwright J, Mulligan M. 1999. Comparison of slope estimates from low resolution DEMs: scaling issues and a fractal method for their solution. Earth Surface Processes and Landforms 24: 763–779. Zhang X, Drake NA, Wainwright J. 2002. Scaling land-surface parameters for global scale soil-erosion estimation. Water Resources Research 38(10): 1180. DOI. 10.1029/2001WR000356 Citing Literature Volume35, Issue1115 September 2010Pages 1349-1356 ReferencesRelatedInformation

Dryland alluvial rivers vary considerably in character. In terms of processes, high energy, sediment-laden flash floods in upland rivers contrast dramatically with the low sediment loads and languid flows of their lowland counterparts while from a form perspective, the unstable wide, shallow and sandy braid plains of piedmont rivers are quite different from the relatively stable, narrow, deep and muddy channels of anastomosing systems (Nanson et al. 2002). It is also apparent that few, if any, morphological features are unique to dryland rivers. The variety of dryland river forms and the absence of a set of defining dryland river characteristics makes it difficult to generalise about dryland rivers and raises questions about whether it is necessary (or even desirable) to consider dryland river systems separately from those in other climatic zones. Indeed, as noted in the introduction to this volume, the recent shift away from the study of morphogenesis within specific climatic regimes (e.g. Tricart and Cailleux 1972) towards the study of geomorphological processes per se (e.g. Bates et al. 2005) has largely undermined the distinctiveness of desert geomorphology. This is not to say rivers draining different climatic regions do not differ in aspects of their behaviour. They clearly do, as exemplified in several reviews of tropical (Gupta 1995), periglacial (McEwen and Matthews 1998) and dryland (Graf 1988; Knighton and Nanson 1997; Reid and Frostick 1997; Tooth 2000a) fluvial geomorphology. However, given the diversity of dryland river mor-

Earth Surface Processes and LandformsVolume 34, Issue 6 p. 882-885 Letters to ESEX Comment on ‘A transport-distance based approach to scaling erosion rates’: Parts 1, 2 and 3 by Wainwright et al. P. B. Hairsine, Corresponding Author P. B. Hairsine [email protected] CSIRO Land and Water PO Box 1666 Canberra 2601 AustraliaCSIRO Land and Water PO Box 1666 Canberra 2601 Australia.Search for more papers by this authorG. C. Sander, G. C. Sander Department of Civil and Building Engineering, Loughborough University, Loughborough, LE11 3TU, UKSearch for more papers by this author P. B. Hairsine, Corresponding Author P. B. Hairsine [email protected] CSIRO Land and Water PO Box 1666 Canberra 2601 AustraliaCSIRO Land and Water PO Box 1666 Canberra 2601 Australia.Search for more papers by this authorG. C. Sander, G. C. Sander Department of Civil and Building Engineering, Loughborough University, Loughborough, LE11 3TU, UKSearch for more papers by this author First published: 13 March 2009 https://doi.org/10.1002/esp.1782Citations: 5AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat References Beuselinck L, Govers G, Steegen A, Hairsine PB, Poesen J. 1999. Evaluation of the simple settling theory for predicting sediment deposition by overland flow conditions. Earth Surface Processes and Landforms 24: 993–1007. Beuselinck L, Hairsine PB, Govers G, Poesen J. 2002a. Evaluating a single-class net deposition equations across a range of conditions. Water Resources Research 38(7): 14/1–14/11. Beuselinck L, Hairsine PB, Sander GC, Govers G. 2002b. Evaluating a multi-class net deposition equation across a range of conditions. Water Resources Research 38. DOI: 10.1029/2001WR000250 Beven K. 1989. Changing ideas in hydrology – the case of physically-based models. Journal of Hydrology 105(1–2): 157–172. Ellis TW, Hairsine PB, Seaton S. 2006. Evaluation of a multi-class net deposition model using field measurements in contour channels. Transactions of the American Society of Agricultural and Biological Engineers 49(5): 1341–1350. Einstein H. 1937. Der Geschiebebetrieb als Wahrscheinlichkeitsproblem. Verlag Rascher: Zurich. [English translation in H Shen (ed.). 1972. Sedimentation Symposium to Honour H.A. Einstein. Colorado State University: Fort Collins, CO. ] Foster GR, Meyer LD. 1972. A closed form soil-erosion equation for upland areas. In Sedimentation, HW Shen (ed.). Colorado State University: Fort Collins, CO; 12.1–12.19. Grayson R, Bloschl G. 2000. Spatial processes, organisation and patterns. In Spatial Patterns in Catchment Hydrology: Observations and Modelling, R Grayson, G Bloschl (eds). Cambridge University Press: Cambridge. Hairsine PB, Rose CW. 1992a. Modelling water erosion due to overland flow using physical principles: I. Uniform flow. Water Resources Research 28(1): 237–243. Hairsine PB, Rose CW. 1992b. Modelling water erosion due to overland flow using physical principles: II. Rill flow. Water Resources Research 28(1): 245–250. Hairsine PB, Sander GC, Rose CW, Parlange JY, Hogarth WL, Lisle I, Rouhipour M. 1999. Unsteady soil erosion due to rainfall impact: a model of sediment sorting on the hillslope. Journal of Hydrology 220(3–4): 115–128. Hairsine PB, Beuselinck L, Sander GC. 2002. Sediment transport through an area of net deposition. Water Resources Research 38(6): 1086. DOI: 10./1029/2001WR000265 Heilig A, De Bruyn D, Walter MT, Rose CW, Parlange JY, Steenhuis TS, Sander GC, Hairsine PB, Hogarth WL, Walker LP. 2001. Testing a mechanistic soil erosion model with a simple experiment. Journal of Hydrology 244(1–2): 9–16. Hussein J, Yu BF, Ghadiri H, Rose CW. 2007. Prediction of surface flow hydrology and sediment retention upslope of a vetiver buffer strip. Journal of Hydrology 338(3–4): 261–272. Lisle IG, Rose CW, Hogarth WL, Hairsine PB, Sander GC, Parlange JY. 1998. Stochastic sediment transport in soil erosion. Journal of Hydrology 204(1–4): 217–230. Orszaghova J. 2007. Water-induced Soil Erosion Modelling, MSc Dissertation, University of Oxford. Polyakov VO, Nearing MA. 2003. Sediment transport in rill flow under deposition and detachment conditions. Catena 51(1): 33–43. Rose CW, Yu B, Hogarth WL, Okom AEA, Ghadiri H. 2003. Sediment deposition from flow at low gradients into a buffer strip – a critical test of re-entrainment theory. Journal of Hydrology 280(1–4): 33–51. DOI: 10.1016/S0022-1694(03)00184-7 Sander GC, Hairsine PB, Beuselinck L, Govers G. 2002. Steady state sediment transport through an area of net deposition: multi-size class solutions. Water Resources Research 38(6): 1087. DOI: 10.1029/2001WR000323 Sander GC, Parlange J-Y, Barry DA, Parlange MB, Hogarth WL. 2007. Limitation of the transport capacity concept in sediment transport modelling. Water Resources Research 43(2): W02403. DOI: 10.1029/2006WR005177 Silberstein RP. 2006. Hydrological models are so good, do we still need data? Environmental Modelling and Software 21(9): 1340–1352. DOI: 10.1016/j.envsoft.2005.04.019 Van Oost K, Beuselinck L, Hairsine PB, Govers G. 2004. Spatial evaluation of a multi-class sediment transport and deposition model. Earth Surface Processes and Landforms 29(8): 1027–1044. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM, Fenti B. 2008a. A transport-distance approach to scaling erosion rates: 1. Background and model development. Earth Surface Processes and Landforms 33(5): 813–826. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM, Fenti B. 2008b. A transport-distance approach to scaling erosion rates: 2. Sensitivity and valuation of Mahleran. Earth Surface Processes and Landforms 33(6): 962–984. Wainwright J, Parsons AJ, Müller EN, Brazier RE, Powell DM, Fenti B. 2008c. A transport-distance approach to scaling erosion rates: 3. Evaluating scaling characteristics of Mahleran. Earth Surface Processes and Landforms 33(7): 1113–1128. Citing Literature Volume34, Issue6May 2009Pages 882-885 ReferencesRelatedInformation

The process basis of existing soil‐erosion models is shown to be ill‐founded. The existing literature builds directly or indirectly on Bennett's (1974) paper, which provided a blueprint for integrated catchment‐scale erosion modelling. Whereas Bennett recognized the inherent assumptions of the approach suggested, subsequent readings of the paper have led to a less critical approach. Most notably, the assumption that sediment movement could be approximated by a continuity equation that related to transport in suspension has produced a series of submodels that assume that all movement occurs in suspension. For commonly occurring conditions on hillslopes, this case is demonstrably untrue both on theoretical grounds and from empirical observations. Elsewhere in the catchment system, it is only partially true, and the extent to which the assumption is reasonable varies both spatially and temporally. A second ground‐breaking paper – that of Foster and Meyer (1972) – was responsible for subsequent uncritical application of a first‐order approximation to deposition based on steady‐state analysis and again a weak empirical basis. We describe in this paper an alternative model ( Mahleran – Model for Assessing Hillslope‐Landscape Erosion, Runoff And Nutrients) based upon particle‐travel distance that overcomes existing limitations by incorporating parameterizations of the different detachment and transport mechanisms that occur in water erosion in hillslopes and small catchments. In the second paper in the series, we consider the sensitivity and general behaviour of Mahleran , and test it in relation to data from a large rainfall‐simulation experiment. The third paper of the sequence evaluates the model using data from plots of different sizes in monitored rainfall events. From this evaluation, we consider the scaling characteristics of the current form of Mahleran and suggest that integrated modelling, laboratory and field approaches are required in order to advance the state of the art in soil‐erosion modelling. Copyright © 2008 John Wiley & Sons, Ltd.

Eight runoff plots, located within a small catchment within the Walnut Gulch Experimental Watershed, southern Arizona, were constructed to test the argument that sediment yield (kg m(-2)) decreases as plot length increases. The plots ranged in length from 2 m to 27(.)78 m. I Runoff and sediment loss from these plots were obtained for ten natural storm events. The pattern of sediment yield from these plots conforms to the case in which sediment yield first increases as plot length increases, but then subsequently decreases. Data from the present experiment indicate that maximum sediment yield would occur from a plot 7 m long. Analysis of both runoff and sediment yield from the plots indicates that the relationship of sediment yield to plot length derives both from the limited travel distance of individual entrained particles and from a decline in runoff coefficient as plot length increases. Particle-size analysis of eroded sediment confirms the role of travel distance in controlling sediment yield. Whether in response to the finite travel distance of entrained particles or the relationship of runoff coefficient to plot length, the experiment clearly demonstrates that the erosion rates for hillslopes and catchments cannot be simply extrapolated from plot measurements, and that alternative methods for estimating large-area erosion rates are required. Copyright (c) 2006 John Wiley & Sons, Ltd.