Modelling catchment‐scale shallow landslide occurrence and sediment yield as a function of rainfall return period

A model‐based method is proposed for improving upon existing threshold relationships which define the rainfall conditions for triggering shallow landslides but do not allow the magnitude of landsliding (i.e. the number of landslides) to be determined. The SHETRAN catchment‐scale shallow landslide model is used to quantify the magnitude of landsliding as a function of rainfall return period, for focus sites of 180 and 45 km² in the Italian Southern Alps and the central Spanish Pyrenees. Rainfall events with intensities of different return period are generated for a range of durations (1‐day to 5‐day) and applied to the model to give the number of landslides triggered and the resulting sediment yield for each event. For a given event duration, simulated numbers of landslides become progressively less sensitive to return period as return period increases. Similarly, for an event of given return period, landslide magnitude becomes less sensitive to event duration as duration increases. The temporal distribution of rainfall within an event is shown to have a significant impact on the number of landslides and the timing of their occurrence. The contribution of shallow landsliding to catchment sediment yield is similarly quantified as a function of the rainfall characteristics. Rainfall intensity–duration curves are presented which define different levels of landsliding magnitude and which advance our predictive capability beyond, but are generally consistent with, published threshold curves. The magnitude curves are relevant to the development of guidelines for landslide hazard assessment and forecasting. Copyright © 2011 John Wiley & Sons, Ltd.     

Assessing the performance of a physically based hydrological model using a proxy‐catchment approach in an agricultural environment

Physically based models are useful frameworks for testing intervention strategies designed to reduce elevated sediment loads in agricultural catchments. Evaluating the success of these strategies depends on model accuracy, generally established by a calibration and evaluation process. In this contribution, the physically based SHETRAN model was assessed in two similar U.K. agricultural catchments. The model was calibrated on the Blackwater catchment (18 km²) and evaluated in the adjacent Kit Brook catchment (22 km²) using 4 years of 15 min discharge and suspended sediment flux data. Model sensitivity to changes in single and multiple combinations of parameters and sensitivity to changes in digital elevation model resolution were assessed. Model flow performance was reasonably accurate with a Nash–Sutcliffe efficiency coefficient of 0.78 in Blackwater and 0.60 in Kit Brook. In terms of event prediction, the mean of the absolute percentage of difference (μAbs_diff) between measured and simulated flow volume (Qv), peak discharge (Qp), sediment yield (Sy), and peak sediment flux (Sp) showed larger values in Kit Brook (48% [Qv], 66% [Qp], 298% [Sy], and 438% [Sp]) compared with the Blackwater catchment (30% [Qv], 41% [Qp], 106% [Sy], and 86% [Sp]). Results indicate that SHETRAN can produce reasonable flow prediction but performs less well in estimation of sediment flux, despite reasonably similar hydrosedimentary behaviour between catchments. The sensitivity index showed flow volume sensitive to saturated hydraulic conductivity and peak discharge to the Strickler coefficient; sediment yield was sensitive to the overland flow erodibility coefficient and peak sediment flux to raindrop/leaf soil erodibility coefficient. The multiparameter sensitivity analysis showed that different combinations of parameters produced similar model responses. Model sensitivity to grid resolution presented similar flow volumes for different digital elevation model resolutions, whereas event peak and duration (for both flow and sediment flux) were highly sensitive to changes in grid size.     

Application of the SHETRAN basin‐scale,landslide sediment yield model to the Llobregat basin,Spanish Pyrenees

The first application of the SHETRAN basin‐scale, landslide erosion and sediment yield model is carried out for a major landsliding event in the upper 505 km² of the Llobregat basin, in the eastern Spanish Pyrenees, in November 1982. The model simulates the spatial distribution of shallow landslides and their sediment yield. Acknowledging uncertainty in the model parameter evaluation, the aim of the application was not to reproduce the observed occurrence of landslides as accurately as possible with one simulation, but to bracket the observed pattern with several simulations representing uncertainty in the key input conditions. Bounds on the landslide simulations were thus determined as a function of uncertainty in the vegetation root cohesion (used in the model factor of safety calculations). The resulting upper bound considerably overestimates the observed pattern (17 000 landslides compared with an observation of around 700), but it reproduces several of the principal clusters in the observed pattern. The lower bound contains around 500 landslides. The sediment yield estimates (2670–14 630 t km^?2) are comparable to measurements elsewhere in the Pyrenees for extreme events. The results demonstrate an ability to simulate the basin‐scale landslide response to a rainfall event and the resulting sediment yield. They also highlight the need for further research in setting the uncertainty bounds and in avoiding large overestimates of landslide occurrence arising in part from a current inability to model small‐scale controls for a basin of the given size. Copyright © 2006 John Wiley & Sons, Ltd.     

Short‐term discharge and suspended sediment fluctuations in the proglacial Skeldal River,north‐east Greenland

The summer discharge pattern of the Skeldal River, which drains a 560 km² partly glacierized catchment in north‐east Greenland, is dominated by diurnal oscillations reflecting variations in the melt rate of snow and ice in the basin. Superimposed on this diurnal pattern are numerous short‐lived discharge fluctuations of irregular periodicity and magnitude. The larger fluctuations are described and attributed to both rainfall events and periodic collapse of the glacier margin damming flow from beneath the Skelbrae glacier. Other minor fluctuations are less readily explained but are associated with changes in the channelized and distributed reservoirs and possibly temporary blockage of subglacial conduits caused by ice melt with subsequent damming. Fluctuations in suspended sediment concentration (SSC) are normally associated with discharge fluctuations, although examples of ‘transient flushes’ were observed where marked increases in SSC occurred in the absence of corresponding discharge variations. A strong relationship between the event discharge increase and event SSC increase for rainfall‐induced events was established, but no such relationship existed for non‐rainfall‐induced events. There is some evidence for an exhaustion effect in the SSC patterns both at the event time‐scale and as the month proceeds. A mean suspended sediment load of 1765 ± 0·26 t day^?1 was estimated for the study period, which would be equivalent to a suspended sediment yield of 732 ± 4 t km^?2 year^?1. Copyright © 2001 John Wiley & Sons, Ltd.     

An analysis of the impact of spatial variability in rainfall on runoff and sediment predictions from a distributed model

This paper investigates the effect of introducing spatially varying rainfall fields to a hydrological model simulating runoff and erosion. Pairs of model simulations were run using either spatially uniform (i.e. spatially averaged) or spatially varying rainfall fields on a 500‐m grid. The hydrological model used was a simplified version of Thales which enabled runoff generation processes to be isolated from hillslope averaging processes. Both saturation excess and infiltration excess generation mechanisms were considered, as simplifications of actual hillslope processes. A 5‐year average recurrence interval synthetic rainfall event typical of temperate climates (Melbourne, Australia) was used. The erosion model was based on the WEPP interrill equation, modified to allow nonlinear terms relating the erosion rate to rainfall or runoff‐squared. The model results were extracted at different scales to investigate whether the effects of spatially varying rainfall were scale dependent. A series of statistical metrics were developed to assess the variability due to introducing the spatially varying rainfall field. At the catchment (approximately 150 km²) scale, it was found that particularly for saturation excess runoff, model predictions of runoff were insensitive to the spatial resolution of the rainfall data. Generally, erosion processes at smaller sub‐catchment scales, particularly when the sediment generation equation had non linearity, were more sensitive to spatial rainfall variability. Introducing runon infiltration reduced the total runoff and sediment yield at all scales, and this process was also most sensitive to the rainfall resolution. Copyright © 2011 John Wiley & Sons, Ltd.     

Flood‐based analysis of high‐magnitude sediment transport using a non‐parametric method

Upland erosion and the resulting reservoir siltation is a serious issue in the Isábena catchment (445 km² Central Spanish Pyrenees). During a three‐month period, water and sediment fluxes have been monitored at the catchment outlet (Capella), two adjacent subcatchments (Villacarli, 41 km²; Cabecera, 145 km²) and the elementary badland catchment Torrelaribera (8 ha). This paper presents the results of the monitoring, a method for the calculation of a sedigraph from intermittent measurements and the derived sediment yields at the monitored locations. The observed suspended sediment concentrations (SSCs) demonstrate the role of badlands as sediment sources: SSCs of up to 280 g l^?1 were encountered for Villacarli, which includes large badland areas. SSCs at the Cabecera catchment, with great areas of woodland, barely exceeded 30 g l^?1. SSCs directly at the sediment source (Torrelaribera) were comparable to those at Villacarli, suggesting a close connection within this subcatchment. At Capella, SSCs of up to 99 g l^?1 were observed. For all sites, SSC displayed only a loose correlation with discharge, inhibiting the application of a simple sediment rating curve. Instead, ancillary variables acting as driving forces or proxies for the processes (rainfall energy, cumulative discharge, rising/falling limb data) were included in a quantile regression forest model to explain the variability in SSC. The variables with most predictive power vary between the sites, suggesting the predominance of different processes. The subsequent flood‐based calculation of sediment yields attests high specific sediment yields for Torrelaribera and Villacarli (6277 and 1971 t km^?2) and medium to high yields for Cabecera and Capella (139 and 410 t km^?2) during the observation period. In all catchments, most of the sediment was exported during intense storms of late summer. Later flood events yield successively less sediment. Relating upland sediment production to yield at the outlet suggests considerable effects of sediment storage within the river channel. Copyright © 2008 John Wiley & Sons, Ltd.     

The 1996 Biescas campsite disaster in the Central Spanish Pyrenees,and its temporal and spatial context

On the evening of the 7 August 1996 an intense storm occurred over the Arás catchment near Biescas in the central Pyrenees. Eighty-seven people were killed as a result of the subsequent flood, which hit a campsite located on the alluvial fan at the outlet of the 18·8 km² catchment. This paper presents the main results of a hydromorphological study of the event. The Betés subcatchment received the most intense rainfall, estimated at somewhat in excess of 250 mm, which resulted in a peak flow from this tributary of 300 m³ s⁻¹. Just downstream from the Betés river junction, flow in the main channel reached 400 m³ s⁻¹, increasing to around 500 m³ s⁻¹ further downstream. Rainfall in the larger Aso tributary was less intense, and in the head reaches flow remained within-bank, representing a one in two-year return period event. Flow from this tributary did not exceed 100 m³ s⁻¹, indicating that the Betés subcatchment supplied some 75% of the flow from just 28·7% of the catchment area. The extreme flows caused the collapse of a series of sediment trap dams in the Arás channel downstream of the Betés junction. This resulted in the addition of 68 000 m³ of sediment to an already disastrous event. Data from other rain gauges in the area showed both the extremely local nature of the event, and the problems of return period analysis for such storms, whose peaks are rarely observed at gauges. Together with the high geomorphological risks of the zone, this leads to the conclusion that a new method of spatial and temporal risk analysis is required for infrastructure planning. © 1997 John Wiley & Sons, Ltd.     

Estimation of suspended sediment loads and delivery in an incised upland headwater catchment,south‐eastern Australia

Historically upland headwater catchments in south‐eastern Australia have undergone extensive gully erosion that has removed large amounts of sediment to lowlands. Recent research suggests these upland areas may continue to dominate fine sediment loads in lowland rivers. Improved understanding of sediment transfer through upland headwater catchments may have implications for interpreting downstream sediment supply. In this study a nested catchment design was utilized to examine suspended sediment yields and delivery from a small tributary sub‐catchment (1·64 km²) to the study catchment outlet (53·5 km²). Monitoring of suspended sediment concentration and discharge was undertaken for a period of nearly two years and used to estimate suspended sediment loads. Estimated total suspended sediment exports over the period of monitoring were 24·16 t from the sub‐catchment and 550·3 t from the catchment, which are generally less than previous reported small catchment yields in south‐eastern Australia. The extent of sediment delivery was examined using between‐site ratios of specific sediment yield per unit area and incised channel length. Sediment delivery was high under average rainfall conditions, but seasonally dependent. Both suspended sediment yields and the extent of delivery peaked over spring months, supplemented by remobilization of sediment stored during summer months in the main catchment channel. The findings of this study suggest much of the suspended sediment exported from small incised upland sub‐catchments (1–2 km²) may be delivered to downstream reaches under average rainfall conditions, which, in conjunction with the findings of previous research supports the potential importance of contributions from these areas to suspended sediment loads in lowland rivers during high flow periods. Copyright © 2007 John Wiley & Sons, Ltd.     

Physically‐based modelling of double‐peak discharge responses at Slapton Wood catchment

Heavy winter rainfall produces double‐peak hydrographs at the Slapton Wood catchment, Devon, UK. The first peak is saturation‐excess overland flow in the hillslope hollows and the second (i.e. the delayed peak) is subsurface stormflow. The physically‐based spatially‐distributed model SHETRAN is used to try to improve the understanding of the processes that cause the double peaks. A three‐stage (multi‐scale) approach to calibration is used: (1) water balance validation for vertical one‐dimensional flow at arable, grassland and woodland plots; (2) two‐dimensional flow for cross‐sections cutting across the stream valley; and (3) three‐dimensional flow in the full catchment. The main data are for rainfall, stream discharge, evaporation, soil water potential and phreatic surface level. At each scale there was successful comparison with measured responses, using as far as possible parameter values from measurements. There was some calibration but all calibrated values at one scale were used at a larger scale. A large proportion of the subsurface runoff enters the stream from three dry valleys (hillslope hollows), and previous studies have suggested convergence of the water in the three large hollows as being the major mechanism for the production of the delayed peaks. The SHETRAN modelling suggests that the hillslopes that drain directly into the stream are also involved in producing the delayed discharges. The model shows how in the summer most of the catchment is hydraulically disconnected from the stream. In the autumn the catchment eventually ‘wets up’ and shallow subsurface flows are produced, with water deflected laterally along the soil‐bedrock interface producing the delayed peak in the stream hydrograph. Copyright © 2007 John Wiley & Sons, Ltd.     

Physically based modelling of sediment generation and transport under a large rainfall simulator

A series of large rainfall simulator experiments was conducted in 2002 and 2003 on a small plot located in an experimental catchment in the North Island of New Zealand. These experiments measured both runoff and sediment transport under carefully controlled conditions. A physically based hydrological modelling system (SHETRAN) was then applied to reproduce the observed hydrographs and sedigraphs. SHETRAN uses physically based equations to represent flow and sediment transport, and two erodibility coefficients to model detachment of soil particles by raindrop erosion and overland flow erosion. The rate of raindrop erosion also depended on the amount of bare ground under the simulator; this was estimated before each experiment. These erodibility coefficients were calibrated systematically for summer and winter experiments separately, and lower values were obtained for the summer experiments. Earlier studies using small rainfall simulators in the vicinity of the plot also found the soil to be less erodible in summer and autumn. Limited validation of model parameters was carried out using results from a series of autumn experiments. The modelled suspended sediment load was also sensitive to parameters controlling the generation of runoff from the rainfall simulator plot; therefore, we found that accurate runoff predictions were important for the sediment predictions, especially from the experiments where the pasture cover was good and overland flow erosion was the dominant mechanism. The rainfall simulator experiments showed that the mass of suspended sediment increased post‐grazing, and according to the model this was due to raindrop detachment. The results indicated that grazing cattle or sheep on steeply sloping hill‐country paddocks should be carefully managed, especially in winter, to limit the transport of suspended sediment into watercourses. Copyright © 2006 John Wiley & Sons, Ltd.     

Using high‐resolution isotope data and alternative calibration strategies for a tracer‐aided runoff model in a nested catchment

Testing hydrological models over different spatio‐temporal scales is important for both evaluating diagnostics and aiding process understanding. High‐frequency (6‐hr) stable isotope sampling of rainfall and runoff was undertaken during 3‐week periods in summer and winter within 12 months of daily sampling in a 3.2‐km² catchment in the Scottish Highlands. This was used to calibrate and test a tracer‐aided model to assess the (a) information content of high‐resolution data, (b) effect of different calibration strategies on simulations and inferred processes, and (c) model transferability to <1‐km² subcatchment. The 6‐hourly data were successfully incorporated without loss of model performance, improving the temporal resolution of the modelling, and making it more relevant to the time dynamics of the isotope and hydrometric response. However, this added little new information due to old‐water dominance and riparian mixing in this peatland catchment. Time variant results, from differential split sample testing, highlighted the importance of calibrating to a wide range of hydrological conditions. This also provided insights into the nonstationarity of catchment mixing processes, in relation to storage and water ages, which varied markedly depending on the calibration period. Application to the nested subcatchment produced equivalent parameterization and performance, highlighting similarity in dominant processes. The study highlighted the utility of high‐resolution data in combination with tracer‐aided models, applied at multiple spatial scales, as learning tools to enhance process understanding and evaluation of model behaviour across nonstationary conditions. This helps reveal more fully the catchment response in terms of the different mechanistic controls on both wave celerites and particle velocities.     

Interpreting nested storm event suspended sediment-discharge hysteresis relationships at large catchment scales

Reducing soil erosion and sediment delivery into rivers is a major aim for land management in New Zealand. Therefore, it is important to identify areas of sediment generation and their relationship to in-stream suspended sediment concentrations and water quality attributes. It is possible to infer and assess sediment sources and dynamics using storm event suspended sediment concentration-discharge hysteresis shape and loop direction. Research in small catchments has achieved some success; however, research in larger (>10³ km²) catchments has shown the inherent difficulty of interpreting hysteresis patterns at larger scales. In this paper, we use a nested, long-term suspended sediment monitoring program across a large catchment (3,903 km²: Manawatū in New Zealand) to address these challenges. We evaluate the hysteresis patterns of five major tributaries (subcatchment areas 329–1,298 km²) of the Manawatū River together with the hysteresis patterns at the gauged catchment outlet. Hysteresis patterns of the Manawatū subcatchments can be characterized as predominantly clockwise, that is, high hysteresis index (HI) value. Larger storms (discharge >2 × 10⁷ m³) increase the likelihood of clockwise hysteresis directions, whereas smaller storms (discharge <2 × 10⁷ m³) are more likely to be anticlockwise. The link between suspended sediment concentration-discharge hysteresis and subcatchment sediment sources becomes increasingly attenuated within the larger subcatchments. High antecedent discharge negatively correlates to HI values, suggesting conditions immediately before the storm have an influence on whether the catchment is “primed” or “exhausted” with available sediment. The different storm categories indicate that within this catchment, whereas hysteresis patterns vary due to the spatial origin of discharge and sediment to some extent, storm magnitude has a stronger impact on hysteresis dynamics than spatial origin.     

Modelling long‐term contaminant migration in a catchment at fine spatial and temporal scales using the UP system

A method has been developed to simulate the long‐term migration of radionuclides in the near‐surface of a river catchment, following their release from a deep underground repository for radioactive waste. Previous (30‐year) simulations, conducted using the SHETRAN physically based modelling system, showed that long‐term (many decades) simulations are required to allow the system to reach steady state. Physically based, distributed models, such as SHETRAN, tend to be too computationally expensive for this task. Traditional lumped catchment‐scale models, on the other hand, do not give sufficiently detailed spatially distributed results. An intermediate approach to modelling has therefore been developed which allows flow and transport processes to be simulated with the spatial resolution normally associated with distributed models, whilst being computationally efficient.The approach involves constructing a lumped model in which the catchment is represented by a number of conceptual water storage compartments. The flow rates to and from these compartments are prescribed by functions that summarize the results from physically based distributed models run for a range of characteristic flow regimes. The physically based models used were, SHETRAN for the subsurface compartments, a particle tracking model for overland flow and an analytical model for channel routing. One important advantage of the method used in constructing the lumped model is that it makes down scaling possible, in the sense that fine‐scale information on the distributed hydrological regime, as simulated by the physically based distributed models, can be inferred from the variables in the lumped model that describe the hydrology at the catchment scale. A 250‐year flow simulation has been run and the down scaling process used to infer a 250‐year time‐series of three‐dimensional velocity fields for the subsurface of the catchment. This series was then used to drive a particle tracking simulation of contaminant migration. The concentration and spatial distribution of contaminants simulated by this model for the first 30 years were in close agreement with SHETRAN results. The remaining 220 years highlighted the fact that some of the most important transport pathways to the surface carry contaminants only very slowly so both the magnitude and spatial distribution of concentration in surface soils are not apparent over the shorter SHETRAN simulations. Copyright © 1999 John Wiley & Sons, Ltd.     

Water and salt balance modelling to predict the effects of land-use changes in forested catchments. 3. The large catchment model

This paper presents an application of a long-term, large catchment-scale, water balance model developed to predict the effects of forest clearing in the south-west of Western Australia. The conceptual model simulates the basic daily water balance fluxes in forested catchments before and after clearing. The large catchment is divided into a number of sub-catchments (1–5 km² in area), which are taken as the fundamental building blocks of the large catchment model. The responses of the individual subcatchments to rainfall and pan evaporation are conceptualized in terms of three inter-dependent subsurface stores A, B and F, which are considered to represent the moisture states of the subcatchments. Details of the subcatchment-scale water balance model have been presented earlier in Part 1 of this series of papers. The response of any subcatchment is a function of its local moisture state, as measured by the local values of the stores. The variations of the initial values of the stores among the subcatchments are described in the large catchment model through simple, linear equations involving a number of similarity indices representing topography, mean annual rainfall and level of forest clearing. The model is applied to the Conjurunup catchment, a medium-sized (39·6 km²) catchment in the south-west of Western Australia. The catchment has been heterogeneously (in space and time) cleared for bauxite mining and subsequently rehabilitated. For this application, the catchment is divided into 11 subcatchments. The model parameters are estimated by calibration, by comparing observed and predicted runoff values, over a 18 year period, for the large catchment and two of the subcatchments. Excellent fits are obtained.     

Effects of the catchment runoff coefficient on the performance of TOPMODEL in rainfall–runoff modelling

Effects of the catchment runoff coefficient on the performance of TOPMODEL in simulating catchment rainfall–runoff relationships are investigated in this paper, with an aim to improve TOPMODEL's simulation efficiency in catchments with a low runoff coefficient. Application of TOPMODEL in the semi‐arid Yihe catchment, with an area of 2623 km² in the Yellow River basin of China, produced a Nash–Sutcliffe model efficiency of about 80%. To investigate how the catchment runoff coefficient affects the performance of TOPMODEL, the whole observed discharge series of the Yihe catchment is multiplied with a larger‐than‐unity scale factor to obtain an amplified discharge series. Then TOPMODEL is used to simulate the amplified discharge series given the original rainfall and evaporation data. For a set of different scale factors, TOPMODEL efficiency is plotted against the corresponding catchment runoff coefficient and it is found that the efficiency of TOPMODEL increases with the increasing catchment runoff coefficient before reaching a peak (e.g. about 90%); after the peak, however, the efficiency of TOPMODEL decreases with the increasing catchment runoff coefficient. Based on this finding, an approach called the discharge amplification method is proposed to enhance the simulation efficiency of TOPMODEL in rainfall–runoff modelling in catchments with a low runoff coefficient. Copyright © 2004 John Wiley & Sons, Ltd.     

Improving bank erosion modelling at catchment scale by incorporating temporal and spatial variability

Bank erosion can contribute a significant portion of the sediment budget within temperate catchments, yet few catchment scale models include an explicit representation of bank erosion processes. Furthermore, representation is often simplistic resulting in an inability to capture realistic spatial and temporal variability in simulated bank erosion. In this study, the sediment component of the catchment scale model SHETRAN is developed to incorporate key factors influencing the spatio‐temporal rate of bank erosion, due to the effects of channel sinuosity and channel bank vegetation. The model is applied to the Eden catchment, north‐west England, and validated using data derived from a GIS methodology. The developed model simulates magnitudes of total catchment annual bank erosion (617–4063 t y^‐1) within the range of observed values (211–4426 t yr^‐1). In addition, the model provides both greater inter‐annual and spatial variability of bank eroded sediment generation when compared with the basic model, and indicates a potential 61% increase of bank eroded sediment as a result of temporal flood clustering. The approach developed within this study can be used within a number of distributed hydrologic models and has general applicability to temperate catchments, yet further development of model representation of bank erosion processes is required. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Sensitivity of runoff and soil erosion to climate change in two Mediterranean watersheds. Part I: model parameterization and evaluation

Climate change is expected to effect storm runoff and erosion processes in Mediterranean watersheds at multiple spatial scales. Models are typically applied to estimate these impacts; however, the scarcity of spatially distributed data for parameterization, calibration and validation often prevents application of these models, particularly for larger catchments. This report, the first part of a two‐part article, presents an application and evaluation of the MEFIDIS model for two Mediterranean meso‐scale watersheds (115 and 290 km²) in a data‐scarce environment. A multi‐scale assessment method was used that combines quantitative validation and qualitative evaluation, consisting of three steps: (1) calibration at the small (field) scale using results from rainfall simulation experiments; (2) calibration and validation for catchment‐scale results while changing catchment‐scale parameters only (channel roughness and a parameter controlling the distribution of saturated areas); and (3) qualitative evaluation of within‐watershed erosion processes using empirical estimates of sediment delivery ratio and gully location. The results indicate that calibrating MEFIDIS at the field scale can provide reasonable results for catchment runoff and sediment export and for within‐watershed erosion processes. Copyright © 2009 John Wiley & Sons, Ltd.     

Isotopic investigation of runoff generation in a glacierized catchment in northern Sweden

In this study, summer rainfall contributions to streamflow were quantified in the sub‐arctic, 30% glacierized Tarfala (21.7 km²) catchment in northern Sweden for two non‐consecutive summer sampling seasons (2004 and 2011). We used two‐component hydrograph separation along with isotope ratios (δ¹⁸O and δD) of rainwater and daily streamwater samplings to estimate relative fraction and uncertainties (because of laboratory instrumentation, temporal variability and spatial gradients) of source water contributions. We hypothesized that the glacier influence on how rainfall becomes runoff is temporally variable and largely dependent on a combination of the timing of decreasing snow cover on glaciers and the relative moisture storage condition within the catchment. The results indicate that the majority of storm runoff was dominated by pre‐event water. However, the average event water contribution during storm events differed slightly between both years with 11% reached in 2004 and 22% in 2011. Event water contributions to runoff generally increased over 2011 the sampling season in both the main stream of Tarfala catchment and in the two pro‐glacial streams that drain Storglaciären (the largest glacier in Tarfala catchment covering 2.9 km²). We credit both the inter‐annual and intra‐annual differences in event water contributions to large rainfall events late in the summer melt season, low glacier snow cover and elevated soil moisture due to large antecedent precipitation. Together amplification of these two mechanisms under a warming climate might influence the timing and magnitude of floods, the sediment budget and nutrient cycling in glacierized catchments. Copyright © 2012 John Wiley & Sons, Ltd.     

The assumption of uniform specific discharge: unsafe at any time?

Nearby catchments in the same landscape are often assumed to have similar specific discharge (runoff per unit catchment area). Five years of streamflow from 14 nested catchments in a 68 km² landscape was used to test this assumption, with the hypothesis that the spatial variability in specific discharge is smaller than the uncertainties in the measurement. The median spatial variability of specific discharge, defined as subcatchment deviation from the catchment outlet, was 33% at the daily scale. This declined to 24% at a monthly scale and 19% at an annual scale. These specific discharge differences are on the same order of magnitude as predicted for major land‐use conversions or a century of climate change. Spatial variability remained when considering uncertainties in specific discharge, and systematic seasonal patterns in specific discharge variation further provide confidence that these differences are more than just errors in the analysis of catchment area, rainfall variability or gauging. Assuming similar specific discharge in nearby catchments can thus lead to spurious conclusions about the effects of disturbance on hydrological and biogeochemical processes. Copyright © 2016 John Wiley & Sons, Ltd.     

TOPCAT‐NP: a minimum information requirement model for simulation of flow and nutrient transport from agricultural systems

Future catchment planning requires a good understanding of the impacts of land use and management, especially with regard to nutrient pollution. A range of readily usable tools, including models, can play a critical role in underpinning robust decision‐making. Modelling tools must articulate our process understanding, make links to a range of catchment characteristics and scales and have the capability to reflect future land‐use management changes. Hence, the model application can play an important part in giving confidence to policy makers that positive outcomes will arise from any proposed land‐use changes. Here, a minimum information requirement (MIR) modelling approach is presented that creates simple, parsimonious models based on more complex physically based models, which makes the model more appropriate to catchment‐scale applications. This paper shows three separate MIR models that represent flow, nitrate losses and phosphorus losses. These models are integrated into a single catchment model (TOPCAT‐NP), which has the advantage that certain model components (such as soil type and flow paths) are shared by all three MIR models. The integrated model can simulate a number of land‐use activities that relate to typical land‐use management practices. The modelling process also gives insight into the seasonal and event nature of nutrient losses exhibited at a range of catchment scales. Three case studies are presented to reflect the range of applicability of the model. The three studies show how different runoff and nutrient loss regimes in different soil/geological and global locations can be simulated using the same model. The first case study models intense agricultural land uses in Denmark (Gjern, 114 km²), the second is an intense agricultural area dominated by high superphosphate applications in Australia (Ellen Brook, 66 km²) and the third is a small research‐scale catchment in the UK (Bollington Hall, 2 km²). Copyright © 2007 John Wiley & Sons, Ltd.