The Sediment Budget of a Highly Dynamic Mesoscale Catchment: the River Isábena

GEOMOR-03723; No of Pages 14 Geomorphology xxx (2011) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Geomorphology

journal homepage: www.elsevier.com/locate/geomorph

The sediment budget of a highly dynamic mesoscale catchment: The River Isábena

J.A. López-Tarazón a,⁎, R.J. Batalla a,b,c, D. Vericat a,b,d, T. Francke e a Department of Environment and Soil Sciences, University of Lleida, E-25198 Lleida, Catalonia, Spain b Forest Science Centre of Catalonia, E-25280 Solsona, Catalonia, Spain c Catalan Institute for Water Research, H2O Building, E-17003, Girona, Catalonia, Spain d Institute of Geography and Earth Sciences, Aberystwyth University, Wales, Ceredigion SY23 3DB, UK e Institute of Earth and Environmental Sciences, University of Potsdam, 14476 Potsdam, Germany article info abstract

Article history: The paper presents the sediment budget of the Isábena basin, a highly dynamic 445-km2 catchment located Received 13 January 2011 in the Central Pyrenees that is patched by highly erodible areas (i.e., badlands). The budget for the period Received in revised form 13 August 2011 2007–2009 is constructed following a methodology that allows the interpolation of intermittent measure- Accepted 18 August 2011 ments of suspended sediment concentrations and enables a subsequent calculation of sediment loads. Data Available online xxxx allow specification of the contribution of each subbasin to the water and sediment yield in the catchment outlet. Mean annual sediment load was 235,000 t y−1. Specific sediment yield reached 2000 t km−2 y−1,a Keywords: Sediment budget value that indicates very high sedimentary activity, especially in the case of Villacarli and Lascuarre subcatch- Sediment transport ments, were most badlands are located. The specific sediment yield obtained for the entire Isábena is − − Random forests 527 t km 2 y 1, a high value for such a mesoscale basin. Results show that a small part of the area (i.e., Quantile regression forests 1%) controls most of the catchment's gross sediment contribution. Sediment delivery ratio (ratio between River Isábena sediment input from primary sources and basin export) has been estimated at around 90%, while in-channel Ebro basin storage represents the 5% of the annual load on average. The high connectivity between sediment sources (i.e., badlands) and transfer paths (i.e., streamcourses) exacerbates the influence of the local sediment production on the catchment's sediment yield, a quite unusual fact for a basin of this scale. © 2011 Elsevier B.V. All rights reserved.

1. Sediment budgets: the role of highly erodible areas Loughran, 1989; Phillips, 1991; Collins and Walling, 2004; Brown et al., 2009), as well as by numerous operational problems and the A sediment budget is a quantitative assessment of the sediment gen- costs of assembling representative data sets (Slaymaker, 2003). Nev- eration and movement through a landscape unit. This assessment in- ertheless, some works on in-channel sediment storage (i.e., a factor cludes the identification of erosion and storage zones (i.e., sediment that limits the sediment transport efficiency disconnecting the river sources and sinks), the sediment delivery and transfer processes (includ- system) can be found, with these estimating that the sediment stored ing connectivity and fluvial sediment transport), and the linkages among in the channel represents between 2% and 10% of the total suspended them (e.g., Dietrich and Dunne, 1978; Swanson and Fredriksen, 1982; sediment load (Duijsings, 1986; Lambert and Walling, 1988; Walling Golosov et al., 1992; Nelson and Booth, 2002). Establishing a sediment and Quine, 1993; Walling et al., 1998; Navratil et al., 2010). budget thus provides a means of clarifying the link between upstream Other valuable information can also be derived from the sediment erosion and downstream sediment yield and the role of sediment stor- budget calculations, most notably sediment delivery ratios (i.e., SDR, age across different temporal scales (Walling, 1983, 1999; Dunne, the relation between sediment inputs or gross erosion and the sediment 1994; Trimble, 1995; Trimble and Crosson, 2000; Slaymaker, 2003; exported out of the basin); despite the limitations of the SDR as predic- Walling et al., 2006). Despite the obvious scientific and applied value of tor of sediment transport, it can help to quantify the efficiency of sedi- developing sediment budgets, assembling the necessary information ment conveyance (Walling, 1983), at a catchment scale. The SDR for anything other than small (e.g., b10 km2) drainage basins (Walling shows a scale effect, usually with an inverse relation with drainage et al., 2006)remainsdifficult. area, ranging from a delivery ratio of ca. 100% at microscale basins Traditional techniques available to investigate sediment mobilisa- (i.e., b0.1 km2; Porto et al., 2011) to valuesb10% typically found in me- tion by erosion and sediment storage are hampered by significant soscale basins (i.e., 100–1000 km2; Roehl, 1962; Williams and Berndt, spatial and temporal sampling constraints (Peart and Walling, 1988; 1972; Porto et al., 2011). The scale effect is often explained by the fact that, during transport through a river basin, progressively more sediment is trapped in footslopes, concavities, alluvial plains, the channel it- ⁎ Corresponding author. Tel.: +34 973702895; fax: +34 973702613. self and other sinks, while erosion rates do not increase or even E-mail address: [email protected] (J.A. López-Tarazón). decrease due to decreasing hillslope gradients downstream (Walling,

1983; de Vente et al., 2007). This way, a negative relation is often appar- the period 2007–2009. For this purpose, we have applied a method ent, emphasising the uncertainties with respect to temporal and spatial that allows the interpolation of intermittent measurements of sus- lumping of data on sediment transport, sediment yield and explanatory pended sediment concentrations. To construct the budget, we have factors such as climate, land use and lithology (Walling, 1983). Besides estimated the suspended sediment yield of all the subbasins, as well that, Lu et al. (2005) and Parsons et al. (2006) even questioned the va- as that at the basin outlet upstream of the Barasona Reservoir, togeth- lidity of a spatial and temporal uniform sediment delivery concept and er with the amount of sediment that is stored in the main channel. stressed the fact that the SDR depends on the timescale over which ero- Data are used to determine the contribution of each subbasin to the sion rates and sediment yields are measured. total water and sediment load of the Isábena catchment. The ﬁndings Research on sediment transport in catchments draining highly erod- provide a rationale with which to link sediment production from the ible materials (e.g., soft marls forming badlands) has become of interest badlands with sedimentation in the reservoir, as well as key insights because of the possibility of setting maximum thresholds and magni- into the temporal dynamics, the in-channel sediment storage, the tudes of sediment transport and for allowing model calibration and val- magnitude of sediment transport and its driving forces. The present idation in highly active geomorphic environments (e.g., Gallart et al., paper follows and completes a series started by Francke et al. 2002; Mathys et al., 2005; Mamede et al., 2006; López-Tarazón et al., (2008a,b) and López-Tarazón et al. (2009; 2010; 2011). 2009). Badlands are considered to be characteristic of arid regions, but they also occur in wetter climates (such as in the Mediterranean) that 2. The study area have high intensity storm events (Gallart et al., 2002; Nadal-Romero et al., 2007). The humid badlands are found in mountainous areas, The Isábena is a mesoscale mountainous catchment located in the such as the Southern Alps (e.g., Mathys et al., 2005) and the Pyrenees Central Pyrenees, NE Iberian Peninsula (Fig. 1A). The River Isábena, to- (e.g., Clotet et al., 1988; Nadal-Romero et al., 2006). There, mean annual gether with the Ésera, are the main tributaries of the Cinca, in turn the precipitation is around 700 mm or higher. Rainfall mostly occurs in the second largest tributary of the Ebro (Fig. 1B). The Isábena basin is char- form of high intensity storm events. Vegetation growth is no longer lim- acterised by heterogeneous relief, vegetation, and soil characteristics. El- ited by water availability or slope but by the high erosion rates caused evation varies from 450 m asl at the outlet to 2720 m asl in the northern by freeze–thaw cycles on exposed north-facing slopes (e.g., Regüés headwater part of the basin. The catchment is composed of ﬁve main et al., 2000b). Consequently, very high sediment loads can be delivered subbasins: Cabecera (146 km2, representing 33% of the total catchment from the badlands. area of the Isábena), Villacarli (42 km2,9%),Carrasquero(25km2,6%), This study aims to calculate the sediment budget of the main sys- Ceguera (28 km2,6%),andLascuarre(45km2, 10%) (Fig. 1C; Table 1). tem channel of the Isábena River, a 445-km² catchment located in the The climate is typical of Mediterranean mountainous areas (i.e., it has southern Central Pyrenees draining extensive areas of badlands, for a Continental Mediterranean climate), with mean annual precipitation

Fig. 1. (A) Location of the Ebro and the Isábena basins in the Iberian Peninsula. (B) Location of the Cinca, Ésera, and Isábena basins in the Ebro basin. (C) General map of the Isábena catchment, showing locations of the main badland areas, the Barasona Reservoir, and the sampling sites.

Table 1 Hydrosedimentary parameters and sampling data of the main subbasins and the entire Isábena catchment.

2 a b c d e f g h Subbasin Area (km ) Area (%) Badlands (%) Slope (%) Tc (h) Discharge record Total gaugings Manual SS samples Automatic SS samples Cabecera 146 32.8 0.01 4.4 3.2 14/09/2006–30/09/2009i 13 119 63 Villacarli 42 9.4 5.57 9.2 1.2 14/09/2006–30/09/2009j 12 145 23 Carrasquero 25 5.6 1.95 7.6 0.7 14/06/2007–30/09/2009 2 42 9 Ceguera 28 6.3 0.93 3.2 1.7 22/09/2007–30/09/2009k 332 17 Lascuarre 45 10.1 0.36 2.8 1.2 13/06/2007–30/09/2009 3 46 80 Isábena basinl 445 100.0 0.83 0.4 8.6 01/01/1945–today 5 280 1177

a Percentage of the subbasin area in relation to the total catchment area. b Percentage of the total area covered by badlands. c Mean longitudinal slope. d Concentration time. e Period of discharge sampling. f Number of discharge measurements done at the monitoring sections by means of an electromagnetic current meter (Valeport). g Number of suspended sediment samples taken manually by means of a depth-integrated sampler US DH59. h Number of suspended sediment samples taken by the water stage samplers at the subbasins or by an ISCO automatic sampler in the case of Capella (i.e., EA047). i No data recorded during the period 28/01/2007–13/06/2007. j No data recorded during the periods 28/01/2007–02/03/2007, 08/06/2007–16/11/2007, and 26/04/2008–30/09/2009. k No data recorded during the periods 16/11/2007–18/12/2007 and 19/06/2008–30/10/2008. l All data referred to sampling at the Capella gauging station (i.e., EA047). of 767 mm (ranging from 450 at the lower part and 1600 mm at high al- 1990s to release sediment through the dam bottom outlets resulted titudes) and an average potential evapotranspiration rate of 550 to in around 9 hm3 of sediment being sluiced through the dam (Palau, 750 mm y−1, both showing a strong south–north gradient because of 1998; Avendaño et al., 2000). Nowadays, the reservoir capacity equals topography (Verdú et al., 2007a). Mean temperature varies from 11 °C again that of 1990 (i.e., 76 hm3)(Mamede, 2008). to 14 °C in the southern part and between 9 °C and 11 °C in the northern zone, again reflecting a strong south–north gradient (Verdú et al., 3. Establishing the sediment budget of the main channel 2007b). Vegetation is mainly composed of deciduous woodland, agriculture, pasture, and bushes in the valley bottoms with evergreen oaks, The sediment budget of the main channel of the River Isábena has pines, and bushes on higher ground. been quantified including the sediment that is introduced into the fluvial The northern parts of the catchment are composed of Paleogene and system (i.e., gross erosion from badland areas, input from tributaries), Cretaceous sediments and the southern lowlands are mainly dominated the sediment that moves out of the basin and the temporary storage of by Eocene continental sediments. These areas consist of easily erodible sediments in the main channel. Fig. 2 shows a schematic representation materials (marls, sandstones), leading to the formation of badlands of the model followed for this work. that have proven to be the major source of sediment within the catchment (Fargas et al., 1997; Francke et al., 2008b; Alatorre and Beguería, 3.1. Quantifying sediment mobilisation from badlands 2009; Alatorre et al., 2011). Badlands can mainly be found in the Villacarli and Carrasquero subbasins (6% and 2% of their basin area, respectively), Sediment input (i.e., gross erosion) was calculated from the sedi- and to a lesser degree in the Ceguera and Lascuarre subbasins (1% and ment contribution that was monitored by Francke et al. (2008b) in 0.4% of their area, respectively); they are almost absent from the largest a typical badland formation. Although all details are described in subbasin (Cabecera) where they represent b0.01% of its area (Table 1; Francke (2009) here we present a short summary of the methods. see the extension of badlands in Fig. 1C). Erosion pins were used for this purpose, with their exposed height The hydrology of the basin is characterised by a rain–snow fed re- measured to estimate erosion rates. A total of 65 metal pins were gime. Floods typically occur in spring (mainly as a result of large frontal installed in a selected representative badland unit; 36 of them were precipitation events and, to a lesser degree, snowmelt) and, especially installed homogeneously distributed in the deposition zone, while in late summer and autumn, as a consequence of localised thunder- 29 were installed directly in the bare slope. Differences in the exposed storms. Minimum flows (~0.20 m3 s−1 at the outlet) typically occur length of the pins were measured monthly and after each major thun- in summer, but the river is perennial. Absolute instantaneous maximum derstorm during 4 months. Erosion rates were estimated by extrapo- flows normally occur in autumn. The largest peak recorded at the basin lating length readings and from topographical surveys of the targeted outlet (i.e., Capella gauging station, EA047, see Fig. 1C) took place in badland. Uncertainties related to precision of measurements and rep- summer (August 1963), reaching 370 m3 s−1, a discharge to have a re- resentativeness of the pins' location and distribution within monitor- turn period of 86 years estimated from the series of annual maximum ing plots are well known (Wolman, 1959; Hooke, 1979; Thorne, discharges (1951–2008) using a Gumbel distribution. The mean annual 1981; Lawler, 1986, 1993, 1999). Nevertheless, we considered that discharge at the basin outlet for the entire period of record (1945– erosion pins were the best feasible option to get an initial estimation 3 −1 3 −1 3 −1 2009) is 4.1 m s (Q10=2.14m s and Q90 =8.21m s , where of erosion rates. Qi is the i percentile of the observations). The mean annual water 3 3 3 yield is 177 hm (P10 =68hm and P90=259hm ), a value that repre- 3.2. Monitoring and analysis of suspended sediment fluxes sents ~1.5% of the Ebro basin's total runoff. The River Isábena flows into the Barasona Reservoir (Fig. 1C) at its Suspended sediment transport has been monitored in the Isábena confluence with the River Ésera. The Joaquín Costa Dam, which closes from 2005 to 2009. The instrumentation of the basin was implemented the reservoir, was constructed in the early 1930s. Its original capacity in two phases. Initially, the Capella gauging station (i.e., EA047) was was 71 hm3, but this was enlarged in 1972 providing a new impound- monitored in order to estimate the sediment load at the basin outlet ment capacity of 92 hm3. The reservoir supplies water to the Aragón (Fig. 1C). This gauging station is operated by the Ebro Water Authorities and Catalunya Channel, irrigating more than 100,000 ha of lowland (hereafter CHE). The data for the current paper came from continuous agriculture. Since its construction, the reservoir has been silting up records of discharge (from water stage records calibrated using gaugings at a rate of between 0.3 and 0.5 hm3 of deposited sediment per year obtained during low and flood conditions) and suspended sediment (Francke, 2009; Alatorre et al., 2010). Engineering works during the concentration (from turbidity records calibrated using samples obtained

Fig. 2. A) Schematic representation of the model followed by this work to construct the sediment budget of the Isábena; B) ﬂux diagram containing the different components of the sediment budget. Arrows sizes are not intended to be scaled.

during low flows and flood events). Complementary results (including at 5–15 min intervals and later converted to Q by means of the de- description of probe calibrations) have been reported by López-Tarazón rived water stage–discharge rating curves of each location. Rating et al. (2009, 2010). In 2006 and 2007, additional equipment was curves were obtained by combination of the stage–mean velocity installed and calibrated, which increased the spatial and temporal reso- and stage–area methods; this combination yields data sufficiently ro- lution and precision of the measurements. The five main subbasins were bust to permit extrapolation (Mosley and McKerchar, 1993). To calibrate monitored for discharge and sediment transport, with additional rain them, repeated discharge measurements were made at each monitoring gaugesinstalledineach(specific details of the sampling strategy are section (Table 1) using an electromagnetic flow meter (Valeport 801) provided below). The location of all the instrumentation can be seen in and complemented with cross section surveys (Geodimeter total sta- Fig. 1C, while Fig. 3 shows details of two of the monitoring sections. tion). At the catchment outlet (i.e., Capella), water depth was recorded at a 15-min time interval and then transformed into Q by the calibrated 3.2.1. Discharge and rainfall stage–discharge rating curve developed by López-Tarazón et al. (2010). Discharge (hereafter Q) was measured by means of capacitive Precipitation (hereafter P) was measured by CHE by means of two water stage sensors/loggers (TruTrack WT-HR) installed at suitable tipping-bucket rain gauges located in Les Paules (Cabecera subbasin; cross sections in the subcatchments (in river constrictions below Fig. 1C) and Capella (i.e., EA047, Fig. 1C). To improve P coverage across bridges where available, except for Capella, which is operated by the basin, two Campbell ARG100 tipping-bucket rain gauges were CHE as described before) (Fig. 3). Sensor bias (b10%) was estimated installed in the villages of Villacarli (Villacarli subbasin; Fig. 1C) and by comparing real measurements and sensor readings obtained dur- Roda de Isábena (located downstream of the Carrasquero and Isábena ing field visits that were performed weekly for instrument mainte- confluence; Fig. 1C). Both were connected to a Campbell CR-200 data- nance and episodically for sampling during flood events. The Q logger, set to record at 1-min intervals. To better the performance of measurements were used to calibrate the stage records under differ- the statistical models for sedigraph reconstruction (full details below), ent flow and sediment transport conditions. Flow stage was recorded four additional tipping-bucket rain gauges located within b10 km

Fig. 3. Examples of the instrumentation installed at the monitored subbasins: (A) Cabecera, (B) Lascuarre. Note that the capacitive water stage sensors (TruTrack WT-HR) are installed inside the grey PVC tubes. from the Isábena watershed (see López-Tarazón et al., 2010)werealso and rainfall data). The QRF (Meinshausen, 2006) is a nonparametric incorporated into the analysis. All rain gauges operated by CHE regis- multivariate regression technique based on RF regression tree ensem- tered accumulated rainfall values every 15 min. bles (Breiman et al., 1984). Regression trees (i.e., CARTs, Breiman et al., 1984) are built by recursive data partitioning, which can include both 3.2.2. Sediment transport categorical and continuous data from ancillary data sets. Instead of Suspended sediment transport at the main catchment outlet was using single trees, RF and QRF employ a set of trees, which are grown recorded continuously as turbidity using a high range backscattering from random subsets of the training data. In RF, model estimates are Endress+Hauser Turbimax W CUS41 turbidimeter (measuring based on the mean of all tree predictions, whereas QRF employs the range up to 300 g l−1). A Campbell CR-510 data logger recorded the whole distribution of tree predictions and, hence, offers the possibility turbidity values every 15 min from averages of 5-s instrument read- to assess the accuracy and precision of model estimates (Meinshausen, ings. Turbidity records have been calibrated by means of suspended 2006; Table 2). Both can efficiently handle non-linearities, do not de- sediment concentrations (hereafter SSC) obtained from water sam- pend on assumptions on the distribution of the data and are capable of ples (for more details see López-Tarazón et al., 2009). Mixing can be capturing non-additive behaviour, which makes them a powerful tool assumed complete, as indicated by various tests done with depth in- for SSC modelling (Francke et al., 2008a,b). tegrating samplers. At each of the monitored subbasins, suspended Suspended sediment transport in the monitored Isábena subba- sediment was intermittently sampled using water stage samplers sins was analysed using results from this modelling approach. The (WSS; Fig. 3). The WSS were designed and built following the initial reconstructed continuous sedigraphs have a 15-min resolution, model developed by Schick (1967). Sampler height ranged between which was determined by the maximum resolution, which of the 1 and 2 m, ensuring that they were never fully submerged. The spac- ing between bottle intakes was between 6 and 10 cm (i.e., 1 water sample per each 6–10 cm stage increment). The manual samples Table 2 were collected in 1-litre bottles during individual flood events and Performance of SSC prediction using RF/QRF in comparison with traditional sediment rating curves. routinely (weekly or fortnightly) during other periods. Because of the highly turbulent flow conditions, mixing was also assumed com- Sampling site Model nR2 V(%)a plete and none spatial or depth variability correction factor was ap- Cabecera SSC−Q 161 0.04 – plied. Overall, data collection resulted in a total of 1089 suspended log(SSC)−log(Q) 0.06 – sediment samples across the basin (Table 1). Samples were vacuum RF – 41 – filtered (Millipore, 0.045 mm pore size) or decanted when concentra- QRF 26 − – −1 Villacarli SSC Q 138 0.11 tions were above 2 g l , oven-dried, and weighed to determine the log(SSC)−log(Q) 0.09 – suspended sediment concentration. RF – 52 QRF – 23 3.2.3. Interpolation of SSCs, sedigraph prediction and estimation of sedi- Carrasquero SSC−Q 45 0.08 – log(SSC)−log(Q) 0.16 – ment loads RF – 57 fl The use of traditional ow duration curve methods (Walling, 1984) QRF – 28 to estimate sediment yields was not possible because of the poor statis- Ceguera SSC−Q 33 0.16 – tical relations between Q and SSC at all monitoring sections (Table 2). In log(SSC)−log(Q) 0.16 – – the Isábena, SSCs for a given discharge can vary up to five orders of mag- RF 29 QRF – 18 nitude (see López-Tarazón et al., 2009, for an example at Capella sta- Lascuarre SSC−Q 118 0.13 – tion). Instead of flow duration methods, continuous sedigraphs were log(SSC)−log(Q) 0.15 – derived for all the subcatchments using random forest and quantile re- RF – 60 gression forest models (hereafter RF and QRF, respectively). This QRF – 50 allowed estimation of sediment yields from ancillary data (i.e., discharge a Explained variability.

Please cite this article as: López-Tarazón, J.A., et al., The sediment budget of a highly dynamic mesoscale catchment: The River Isábena, Geo- morphology (2011), doi:10.1016/j.geomorph.2011.08.020 6 J.A. López-Tarazón et al. / Geomorphology xxx (2011) xxx–xxx data; further, ancillary predictors from P and Q data were used to run flows typically present in the other creeks, it was assumed that most the model (Table 3), derived from their primary predictors (e.g., P and of the sediment is generated and supplied from Villacarli. This assump- Q) by using increasing temporal shifts and window sizes, keeping this tion facilitates interpretation of the Isábena sediment budget and per- way the correlation between the derived predictors low (as proposed mits discussion of the role of badland areas on the total sediment by Zimmermann et al., in press). The ancillary data sets were selected load. Despite this limitation, modelling in Villacarli was still applied according to the perceived capability of representing (i.e., drive) rel- for some weeks when the station stayed in operation (i.e., 23 weeks, evant processes as, for example, sediment production on slopes or from 16 November 2007 to 1 April 2008). These weeks form part of in the riverbed, exhaustion of sediment supply on slopes or within the study period presented in this paper; in addition, water and sedi- the riverbed, and dilution (e.g., Schnabel and Maneta, 2005). Julian ment load at this site had measured previously (i.e., 28 weeks, from 1 day of year was used as an additional predictor to capture the pro- October 2006 to the 28 January 2007 and from 2 March 2007 to the nounced seasonality and the change in discharge as a useful indicator 8June2007).TheSSCs for these periods were estimated by applying for intraevent dynamics. Variable importance, VI, a permutation-based the modelling approach described above. Results could then be com- measure (Liaw and Wiener, 2002) was used to rank the influence of pared with those obtained in the other monitored sections for the the predictors on the model; low VI values characterize predictors same period of time and, in addition, were used to relate patterns ob- with a relative low influence on model performance (Fig. 4). The ex- served in Villacarli with those observed in the neighbouring subcatch- planatory power of the VI is limited due that the importance of a var- ments, in the mainstem channel, and in the whole basin. iable can be affected by complex interactions with other predictors (Liaw and Wiener, 2002). Consequently, assessing the influence of 3.3. Estimating fine sediment storage in the mainstem channel the predictors in greater detail remains difficult. Nevertheless, variables with very low VI were omitted from the model for parsimony. In-channel fine sediment storage has been identified as a key com- Model construction and statistical analyses were implemented using ponent controlling sediment dynamics and load during flood events in the randomForest (Liaw and Wiener, 2002) and quantregForest the Isábena (López-Tarazón et al., 2011). The amount of fine sediment (Meinshausen, 2007) packages within the R software (R-Team Devel- stored in the channel bed of the Isábena was determined using the opment Core, 2010). method developed by Lambert and Walling (1988). Field data provided Despite QRF models allowing for the assessment of model uncer- information on the amount of sediment stored on the bed of the chan- tainty, RF models were used for sedigraph reconstruction because of nel at specific sites that can be subsequently extrapolated to hydrauli- their better performance (Table 2). By applying the models to the cally and morphologically equivalent areas. Sampling of sediment data of the complete monitoring period (July 2007–July 2009) SSC storage was done at four different cross-sections located in the lower data for each time step and each site were derived. Thus, SSC values part of the basin, downstream from the main tributaries in order to in- for each time step and subbasin were calculated. Finally, suspended clude the total discharge and sediment transport from the basin. Sam- sediment loads (hereafter SSL) were obtained by multiplying the pling was performed in locations morphologically representative of SSC with the associated discharge value. the characteristics of the lower Isábena mainstem channel (i.e., riffle- In the case of the Villacarli subbasin and because of the poor cover- pool system in typical low gradient gravel-bed river). age of the hydrological data set (i.e., Q), modelling the continuous 15- Although full details are provided in López-Tarazón et al. (2011),here min sedigraph for the whole period was not possible. Data acquisition we present a summary of the sampling strategy. The methodology con- was interrupted mainly because of vandalism and technical problems sists in a metal cylinder (diameter of 0.25 m, surface area 0.20 m2,and (i.e., at-a-site severe sedimentation). Owing to such problems, the ca- height 0.6 m) that is carefully lowered into the water until it rests on pacitive sensor had to be replaced five times during the monitoring pe- the channel bed (avoiding disturbance of the fine sediment) and then riod (the exact period of sampling of each subbasin can be seen in slowly rotated to create a seal with the gravels, thereby establishing the Table 1). Villacarli's sediment loads therefore had to be estimated as exact bed area to be sampled. Next, the channel bed is manually disturbed the difference between the sum of the modelled sediment load of the (i.e., using a rod), in order to re-suspend the fine sediment and, thus, es- other subcatchments and the sediment load calculated at the outlet of timate the sediment storage. Although the surface of the bed is vigorously the basin (Capella). The sediment load resulting from this subtraction agitated during this procedure, special attention is given to avoid the ag- corresponds to the sum of Villacarli's load and the rest of the ungauged itation of the subsurface material. 1-litre replicated samples of water and areas of the basin (altogether 201 km2, 45% of the entire catchment suspended sediment are taken for each of the two steps. Besides, the area). However, based on field observations and (i) the comparably cylinder is positioned at different points (i.e., varying from 3 to 5) in higher abundance of badlands in the Villacarli subcatchment (i.e., up every section to capture the transversal sediment storage variability that to 6% of its total area) in relation to other parts of the basin, (ii) the dif- was observed in the field. Samples are later taken to the laboratory to ferences in precipitation (i.e., the highest rainfall intensities and total determine their suspended sediment concentration by filtering or precipitation amounts were always registered at the Villacarli's rain decanting the water samples (depending on the amount of sediment) gauge), (iii) the relief gradient (i.e., with a longitudinal slopeN9%, the using the procedures described by López-Tarazón et al. (2009).For largest in the entire Isábena basin; Table 1), and (iv) the ephemeral more information about the calculation of the amount of sediment released from the bed and the later extrapolation to the whole catchment see López-Tarazón et al. (2011).

4. Results and discussion Table 3 Summary of the ancillary predictors used to model the sedigraphs and their abbreviation. 4.1. Sediment mobilisation from badlands

General predictor Example Meaning A conservative approach is taken to estimate sediment mobilisation Limbi Limb1 Change in discharge for an i time step from badlands. This prevention is the consequence of the uncertainties re- Sin Sin Day of the year (following the Julian calendar) in a sinusoidal form lated to the precision of the measurements, and the representativeness of

Px − i PCapella-135 Cumulated rainfall at a determinate location the pins' location and distribution within monitoring plots. According to and temporal shift it, minimum mean erosion value of the pin series is taken at −1 Qx − i PCapella-135 Cumulated discharge at a determinate location 2.1 cm y . From this ﬁgure, sediment contribution from all the badlands and temporal shift − in the catchment (i.e., 4.75 km2) reached the order of 260,000 t y 1

Fig. 4. Variable importance of SSC predictions for each subbasin normalised to 100%. Abbreviations are shown in Table 3. Note that in the case of Villacarli, values correspond just to the periods in which modelling was possible.

during the 2-year study period (for measured a bedrock density of 4.2. Water and sediment fluxes and yields in the main channel network 2.61 t m−3). This value would roughly mean an average erosion of − 580 t km2 y 1 for the whole catchment equating, as it will be reported 4.2.1. Modelling SSCs: defining predictors in the next sections, the specific sediment yield obtained for the entire Analysis of VI (Fig. 4) indicates that, with the exception of Cabecera, catchment from sediment transport measurements, and anticipating the predictors containing hydrological information from a timespan of b1h high connectivity between sediment sources and channel networks and, prior the predicted SSC are the most influential. This may reflect the consequently, the outstanding role of the badlands on the Isábena basin's rapid hydrological response to P in most of the study basins. In the sediment load. case of Cabecera, most influential predictors are those related to

Fig. 5. Close-up-view of runoff and suspended sediment concentration (SSC) measured and predicted using RF for a given ﬂood at each of the measuring sections: 20 November 2007 in Cabecera; 31 October 2008 in Carrasquero; 8 April 2008 in Ceguera; 5 October 2007 in Lascuarre.

2 from the values obtained at the basin outlet, a certain overestimation is − likely to occur. Water yield during 2008–2009 followed a similar pattern: 3

SSYs (t km Cabecera yielded 97.8 hm ; while Villacarli, Carrasquero, Ceguera, and Lascuarre yielded 9.5, 7.3, 15.6, and 9.7 hm3, respectively. Moreover,

(t) maximum Q increases with increasing catchment size (Table 5). The smallest subcatchments (Carrasquero and Ceguera, with 25 and SSL 28 km2, respectively) showed the smallest Q peaks (2.02 and

) 3 −1 3 3.25 m s , respectively) during the study period. Cabecera, as the larg-

WY (hm est subbasin draining the headwaters of the Isábena, experienced the highest Q, with a maximum recorded value of 63 m3 s−1.TheQ at Capella ) 2 is mainly controlled by the input from Cabecera that, on average, yielded − 68% of the runoff of the entire catchment (very similar to values reported

SSYs (t km by Verdú et al., 2007b). The irregular distribution of rainfall and the different runoff responses were reﬂected in the occurrence of ﬂoods at each of

(t) the subcatchments (21 ﬂood events occurred in Cabecera, 24 occurred in

SSL Carrasquero, 31 in Ceguera, and 33 floods in Lascuarre). Farther downstream, 30 floods were recorded at the Capella gauging station. As de- ) 3 scribed above, determining the exact number of floods that occurred in

WY (hm Villacarli was not possible because of the data gaps in the Q record. Nev- ertheless, based on the periods of available Q data at Villacarli and the rest ) 2

− of the gauging sections, apparently the total number of ﬂoods at each subbasin was very similar (Table 6). SSYs (t km The SSC follows a different behaviour: the smallest subbasins (Carrasquero and Ceguera) showed maximum values at around 107 (t) ) at the whole basin and the subbasins during the study period. and 141 g l−1, respectively; while in Villacarli and Lascuarre maximum

SSL −1 SSYs concentrations reached 278 and 396 g l , respectively. In all four cases,

) SSC ranged over 5 orders of magnitude. The return of SSC to pre-ﬂood 3 levels is usually quicker than the recession of the hydrograph (regard- WY (hm less of the rainfall intensity), a fact that may suggest a degree of exhaus-

) ﬂ

2 tion of sediment; the decline of SSC during ood recession can be −

b interrupted by rainbursts (e.g., the ﬂoodregisteredatLascuarreon11 (t km SSYs April 2009, in which a second peak of the sedigraph was delayed 7 h from the hydrograph, during the ﬂood recession). In the case of Cabecera

c suspended sediment yield ( (largest subbasin) and Capella (entire catchment), SSCs reached 46 and (t) ﬁ a 89 g l−1, respectively, covering in these two cases 4 orders of magni- SSL tude. Nevertheless, notably higher instantaneous SSC (i.e., N300 g l−1)

) had been observed at the Capella gauging station during a ﬂood event 3 a ) and speci recorded in 2006 (López-Tarazón et al., 2009), a year out of the scope SSL (hm WY of the current paper. Sediment dynamics appeared to be mainly inﬂu- )

2 enced by local rainbursts that often produce SSC peaks long after maxi- − mum discharge is reached. For example, a floodeventsampledin Capella on 4 September 2008 had a first instantaneous sediment peak SSYs (t km above 90 g l−1 more than 3 h after the hydrograph peak and a second sediment peak of ca. 70 g l−1 almost 10 h later. This behaviour of the (t) outlet (i.e., counterclockwise hysteresis loops) confirms the patterns SSL found by López-Tarazón et al. (2009), in which 60% of the 73 analysed ), suspended sediment load ( )

3 ﬂoods followed that loop shape. This pattern may be partly driven by WY

WY (hm the sediment availability in the mainstem channel near the outlet of ) 2 − Table 5

), water yield ( ﬁ SSYs (t km Summary of measured discharge (Q), speci c discharge (Qs) and suspended sediment P concentration (SSC) data at the six monitored sections during the study period.

Sections Q (m3 s− 1) Q (l s− 1 km− 2) SSC (g l− 1)

(t) s

SSL Min Mean Max Min Mean Max Min Median Max

Cabecera 0.73 2.94 63.06 5.00 20.14 431.92 0.001 0.18 45.87 ) c suspended sediment yield has been done using its basin size, assuming the role of the ungauged catchment's areas over the sediment load as negligible. 3 ﬁ (n=182)

WY (hm Villacarli N/AN/AN/AN/AN/AN/A 0.001 0.77 277.85 (n=168) Carrasquero 0.04 0.15 2.02 1.60 6.00 80.80 0.001 0.03 106.54 (n=51) P (mm) Ceguera 0.00 0.50 3.25 0.00 17.96 116.07 0.002 0.31 140.63 (n=49) Lascuarre 0.00 0.20 21.25 0.00 4.40 472.22 0.001 31.05 396.22 20082009 745 730 130.8 139.9 225,822 243,524 507 547 85.7 97.8 25,005 47,942 171 328 33.4 9.5 138,382 3,295 65,461 1,559 2.1 7.3 444 3,141 18 126 7.0 15.6 29,377 8,815 1,049 315 9.7 2.6 97,603 53,175 2,169 1,182

– – (n=126) The calculation of the speci

Water and sediment load corresponding to the Villacarli subbasin and the ungauged catchment's area. Capella 0.21 4.02 101.55 0.47 9.03 228.21 0.002 1.35 89.18 Summer 07Autumn 07Winter 99 08Spring 86 08Summer 109 08Autumn 8.5 08 451 106Winter 8.9 09 13.4 235Spring 19,429 09 100.02007 130 10.5 10,044 11,032 44 185,317 259 26.9 23 11,612 416 25 44.7 67,922 57.8 26 6.3 35,577 153 128,413 6.9 63.5 9.1 80 289 116 8.4 23,888 18.8 696 164 305 28.4 1 29,275 42.2 569 5 201 2 15,861 2,237 29.2 4 109 1.6 15 108,352 1.3 2.1 1.2 2,580 11,507 1.5 1.9 12,395 8,430 10,094 4.0 274 0.5 295 201 6,095 240 29,628 17,342 0.4 145 308 705 413 1.8 0.6 0.7 12 29 0.4 2.4 1,417 2.6 33 74 1 1,216 57 4.5 34 474 1 3 49 8,304 19 1 0.0 3.3 297 0.1 2.4 6.0 6,561 6.3 0.0 0 2.3 15,519 234 175 336 7,297 554 44,465 0 12 0.8 6 0 261 988 5.3 0.2 18,275 0.1 0.1 0 3.4 66,189 406 7,776 1,471 0.2 223 8,227 711 173 183 4,913 16 5 109 2008 Average 738 135.4 234,673 527 91.8 36,474 250 21.5 101,922 2,427 4.7 1,793 72 11.3 19,096 682 6.2 75,389 1,675 Period Basin Capella Cabecera Villacarli Carrasquero Ceguera Lascuarre a b (n=544) Table 4 Summary of the seasonal and total precipitation (

Table 6 while Cabecera, Ceguera, and Carrasquero accounted for 25,000, 8815 Villacarli's water yield (WY), modelled sediment load (SSL), number of flood events and 444 t, respectively. Spring 2008 (the wettest season of the study and maximum peak discharge (Q ) registered at the periods where data was avail- max period) was also the season with the largest SSL at the Villacarli monitor- able; comparison with the results obtained at the other subbasins. ing station: 108,350 t were estimated, representing 78% of Villacarli's an- Period nual sediment load and 48% of Isábena basin's sediment load of that year. Subbasin 09/2006–01/ 03/2007–06/ 11/2007–04/ Lascuarre lead the sediment yield for the year 2008–2009, with a total of 2007 2007 2008 97,600 t; 65,500 t were estimated in Villacarli. Cabecera registered Capella WY (hm3) 60.4 60.0 10.4 47,950 t, and Ceguera and Carrasquero accounted for 29,400 and SSL (t) 174,128 35,883 18,094 3150 t, respectively. In addition to having most of the badlands in the Floods (n) 8 6 6 whole catchment, the contribution of Villacarli and Lascuarre to the Q max 88.6 70.8 78.6 − total sediment yield in the basin was also probably driven by the distri- (m3 s 1) Cabecera WY (hm3) 34.5 N/A 12.9 bution of the rain (total amount and intensity) across the basin. During SSL (t) 22,026 N/A 520 the first year, P was rather uniformly distributed along the whole Isábena Floods (n) 8 N/A 4 basin (i.e., b15% of difference in the total annual precipitation between Q max 43.4 N/A 39.8 3 − 1 all the rain gauges); while during the second year the most intense (m s ) fl Villacarli WY (hm3) 6.5 3.5 2.4 events (i.e., those exceeding the limit to generate overland ow; Selby, SSL (t) 74,392 25,244 6,908 1982) occurred mainly in the lower Isábena, where Lascuarre is located. Floods (n) 8 5 6 This feature was also observed in Ceguera in the lower Isábena and in Q max 21.2 1.8 24.0 Carrasquero in the middle Isábena, whose runoff during the second (m3 s− 1) year doubled that observed during the first year. Results in Table 4 con- Carrasquero WY (hm3) N/AN/A 1.0 fi SSL (t) N/AN/A 102 rm the sedimentary behaviour reported by López-Tarazón et al. (2010), Floods (n) N/AN/A 4 i.e., that the gross sediment load in the Isábena basin is mainly controlled Q max N/AN/A 0.6 by P, with the highest sediment yield always registered during the wet- 3 − 1 (m s ) test seasons. Ceguera WY (hm3) N/AN/A 2.4 SSL (t) N/AN/A 168 Suspended sediment load at each subcatchment was related to Floods (n) N/AN/A 5 their area to calculate specific sediment yields (hereafter SSY). Aver- age SSYs for the entire monitoring period ranged over two orders of magnitude between subcatchments (Table 7), varying from the basin (see López-Tarazón et al., 2011, for a complete discussion of 72 t km−2 in Carrasquero, 250 t km− 2 in Cabecera, 682 t km− 2 in the role of in-channel fine sediment storage on sediment transport pat- Ceguera, 1675 t km− 2 in Lascuarre, and 2427 t km−2 in Villacarli. terns). Flood peaks and related sediment concentrations during the For Villacarli, calculations were based on basin area (42 km2) instead study period decreased in relation to rainfall magnitude, as previously of using the size of the whole ungauged area; for this it was assumed reported by López-Tarazón et al. (2010). that almost all sediment is exported directly from Villacarli's basin Water and sediment yields in Villacarli could be modelled for only and the contribution of the rest of the ungauged area is minimum few periods (i.e., when the discharge record was available); compar- (see sediment export from Carrasquero as surrogate of the other ison with the other subcatchments was therefore possible only for small subcatchments). The notable difference in SSY between sub- those periods when concurrent data existed (Table 6). Modelled catchments underlines the predominant role of badlands as the prima- values represent a first approximation of the sediment transport pat- ry sediment source, as well as the different hydrological response. terns in the Isábena catchment. The sediment load in Villacarli was Values from Villacarli and Lascuarre are in the range of those obtained systematically much higher than in the other subcatchments, consti- in similar geomorphic landscapes (i.e., those with highly erodible tuting the most important fraction of the load exported in Capella; areas), such as Vallcebre in the western Pyrenees (2800 t km−2 y−1, water yield was, however, low in relation to Cabecera and Capella 5 years of monitoring; Regüés et al., 2000a), despite being more than but in the same order of magnitude as that in the other subbasins. 40 times larger in area. The SSY obtained for the entire Isábena catch- Together with Villacarli, Lascuarre recorded the highest SSL values ment (Capella data) was 527 t km−2. This value plots above the at both seasonal and annual scales (Table 4; Fig. 6). The SSL for the 350 t km−2 y−1 for the entire Ésera catchment (1600 km2)reported year 2007–2008 was 138,400 t in Villacarli and 53,200 t in Lascuarre; by Sanz-Montero et al. (1996) and ranks high in relation to data for

Fig. 6. The suspended sediment yield of the whole basin for the study period 2007–2009. Plots show seasonally and annually distributed results for each of the subcatchments and for the whole Isábena basin.

Table 7 Water and sediment budget of the Isábena basin; the contribution of each subbasin is represented as its percentage on the total water and sediment load.

Period Basin Cabecera Villacarlia Carrasquero Ceguera Lascuarre

P (mm) WY (%) SSL (%) WY (%) SSL (%) WY (%) SSL (%) WY (%) SSL (%) WY (%) SSL (%)

Summer 07 99 74.0 0.6 19.3 59.2 5.0 0.2 0.0 0.0 1.7 40.0 Autumn 07 86 77.1 6.9 14.9 83.9 6.4 0.3 0.9 1.7 0.7 7.1 Winter 08 109 67.9 2.8 9.2 91.5 5.0 0.7 17.5 3.1 0.4 2.0 Spring 08 451 63.5 12.9 29.2 58.5 0.5 0.2 4.5 4.5 2.3 24.0 Summer 08 106 80.1 4.9 14.1 52.5 4.3 0.3 0.0 0.0 1.6 42.3 Autumn 08 235 69.9 43.1 8.0 18.2 6.7 2.1 12.4 9.7 3.1 26.9 Winter 09 130 63.6 6.3 8.9 48.7 5.8 1.3 14.1 20.5 7.7 23.1 Spring 09 259 73.0 12.4 3.3 23.1 4.2 0.9 10.4 12.1 9.1 51.5 2007–2008 745 65.5 11.1 25.6 61.3 1.6 0.2 5.3 3.9 2.0 23.5 2008–2009 730 69.9 19.7 6.8 26.9 5.2 1.3 11.2 12.1 6.9 40.1 Total 1475 67.8 15.5 15.9 43.4 3.5 0.8 8.3 8.1 4.5 32.1

a Water and sediment yield correspondent to the Villacarli subbasin and the ungauged catchment's area.

44 Mediterranean catchments given by de Vente et al. (2006),both extrapolated to the whole channel length (i.e., 45 km), showed that being long-term estimates derived from reservoir siltation. Fig. 7 and the total storage would equate almost 10,000 t, while from 1260 t in Table 7 present the spatial distribution of water and sediment contribu- spring and to 5900 t in winter. In-channel storage was found variable tion of each subcatchment to the whole basin. in space and time and two interesting trends were pointed out: i) a clear tendency for the stored sediment to increase in the downstream 4.3. In-channel sediment storage direction, probably due to the increase in both the width of the channel and the likelihood of sediment deposition; and ii) a continuous Dynamics of fine sediment stored in the channel bed of the lower year-round sediment accumulation mostly during low flow periods Isábena from field measurements, together with an extrapolation to (which lack sufficient competence to entrain fines). However, a gen- the whole catchment, were already reported by the authors (see eral pattern was difficult to establish due to rainfall irregularity López-Tarazón et al., 2011); the magnitude of in-channel storage (which precludes any long-term extrapolation) and also because of and the seasonal variability were also assessed previously. They the limited length of the study period. reported that, for a 1-year study period (2007–2008), cumulative an- Additional calculations to those done by López-Tarazón et al. (2011) nual storage in the channel bed of the lower Isábena (3.125-km chan- have been made for the period for which all monitoring stations were nel length) was 679 t, varying from 88 t in spring to 408 t in winter. fully operational (November 2007–April 2008; Table 6). In this period, Average seasonal values obtained in the monitored channel reach, the difference between the sediment transported in Capella and the

Fig. 7. Water and sediment subcatchment contributions to the Isábena basin for the study period (A) 2007–2008, (B) 2008–2009, and (C) the whole study period. Water and sediment contributions (in %) of each subbasin and at each period are shown inside the arrows.

4.4. The sediment budget of the main system channel

Data on i) gross erosion (i.e., sediment mobilisation from badlands), ii) seasonal and annual suspended sediment yield for each subcatch- Fig. 8. Relation between sediment delivery ratio and catchment area for data reported ment (i.e., sediment input from the tributaries to the system) and for in catchments of the United States by Roehl (1962) and Williams and Berndt (1972). The SDR of the Isábena catchment is shown. the whole basin (i.e., sediment output of the river system), and iii) sediment stored within the main channel (i.e., factor that modulates the transfer of sediment from sources to outlet) were used to estimate the of 235,000 t y−1. Denudation rate in the badlands, extrapolated from ob- sediment budget of the Isábena basin for the period 2007–2009. This servations during the study period, would attain 2 m×103 y−1.Basin way, the 2-year sediment budget of the Isábena establishes a quantita- denudation rate plots, for instance, remarkably higher in relation to stud- tive relation between sediment input and output and the temporary ies by Dietrich and Dunne (1978) in a basin in Oregon storage of sediment in the main channel. Sediment input (i.e. sediment (0.03 m×103 y−1)andbyBatalla et al. (1995) in the Mediterranean generation), calculated as the contribution from a typical badland for- basin of Arbúcies (0.02 m×103 y−1), both under low-intensity geomor- mation, would reach an average of 260,000 t y−1; the storage in the ac- phic conditions; in contrast, the current study suggests that the Isábena tive bed of the main channel, taken from López-Tarazón et al. (2011), is controlled by high sediment production from the badlands and high can reach up to 5% of the total load transported at the catchment outlet, connectivity between sources and the fluvial network, thus showing in- i.e., 12,000 t y−1; finally, sediment output was estimated as the mean tense geomorphic and sediment transfer activity. suspended sediment yield measured at the Capella gauging station (e.g., 235,000 t y−1) for the study period. 5. Summary and conclusions This sediment budget is limited to a 2-year study period. In absolute terms a balance is present between the sediment input (production from This paper has reported on the sediment budget of the Isábena badlands) and the sediment output (yield at the basin outlet). Clearly, at River basin, a mesoscale mountainous catchment located in the Cen- the temporal scale of the study, the sediment output is the most impor- tral Pyrenees that generates unusually high sediment loads. The work tant component of the budget. This indicates that a large part of the sed- has been focused on the description and quantification of the hydro- iment mobilised from the catchment slopes (at least during the sedimentological dynamics of the five main subbasins (where most monitoring period) easily reached the catchment outlet, providing a sed- active sediment sources, i.e., badlands, are located), as well as of the iment delivery ratio (SDR) of 90%. The study period was relatively dry entire catchment. The study period (2007–2009) is considered mod- with no events inundating the floodplain; thus most sediment supplied erately dry in comparison to the long-term water yield (1945–2009). from the badlands reached the outlet, with the channel acting as inter- The suspended sediment transport series for the main subbasins have mittent storage influencing sediment load dynamics primarily at the been obtained from field data acquisition and modelling. Modelling seasonal scale (as per López-Tarazón et al., 2011). Overall, the in- was performed using a nonparametric multivariate regression tech- channel sediment storage represents a small part of the sediment budget nique (random forest); this procedure allowed the estimation of sed- (ca. 5% of the sediment input), but constitutes an important factor con- iment yields at each subbasin. Overall, the reconstructed sedigraphs trolling both the seasonal variability of the sediment transport (i.e., rang- show moderate agreement with the observed data, despite the limit- ing from 5 to 55% of the seasonal sediment yield; López-Tarazón et al., ed number of samples. Our results illustrate the predictive power of 2011) and the high loads typically transported during baseflows this nonparametric statistical technique and its value in producing (López-Tarazón et al., 2009). The remaining 5% of the budget can easily continuous sediment transport time-series. Results have been com- be attributed to measurement and modelling uncertainties and errors pared to those obtained at the basin outlet and to the estimates of involved in data collection. sediment storage in the main channel in order to establish the sedi- Fig. 8 plots the relation between estimated SDR and catchment areas ment budget. The key findings of the work are: for the present and other studies (Roehl, 1962; Williams and Berndt, 1972). Results follow the same pattern: the inverse relationship be- i. The annual sediment load was 226,000 and 243,500 t for tween the two variables is clear, largely reflecting the increasing oppor- 2007–2008 and 2008–2009, respectively. Despite their rela- tunity for sediment deposition and storage as catchment area increases. tively small area, Villacarli and Lascuarre subcatchments sup- The Isábena departs from the general trend and, despite its size, and ply most of the sediment (102,000 and 75,000 t y−1; 43 and matches with SDRs calculated for smaller basins (b10 km2), illustrating 32% of the total load of the Isábena, respectively). the strong sedimentary dynamics in the basin, best expressed by the ii. Specific sediment yields reach almost 2500 t km− 2 in Villacarli high connectivity between sediment sources and the fluvial network. and 1675 t km− 2 in Lascuarre, illustrating the high erosional Finally, a significant geomorphological implication that can be de- activity (i.e., sediment production and supply) that takes rived from the current and previous investigations is the denudation place in the badland areas. Yields decrease to below rate of the basin. Assuming a uniform contribution over the whole Isá- 600 t km− 2 in the other subbasins, owing to the limited avail- bena catchment area of the sediment exported from the basin, the ability of erodible areas. The specific sediment yield obtained long-term denudation rate can be estimated to be 0.2 m×103 y−1, for the entire Isábena catchment is 530 t km− 2, a value that using a bedrock density of 2.61 t m−3 and a mean sediment contribution is considered high for a basin of its size. A denudation rate for

the entire catchment of 0.2 m×103 y− 1 corroborates the in- Breiman, L.M., Friedman, J., Olshen, R., Stone, C., 1984. Classification and Regression Trees. Wadsworth, Belmont, CA. tensity of the sediment production, delivery, and transfer pro- Brown, A.G., Carey, C., Erkens, G., Fuchs, M., Hoffmann, T., Macaire, J.J., Moldenhauer, cess occurring in the Isábena basin. K.M., Walling, D.E., 2009. From sedimentary records to sediment budgets: multiple iii. A large part of the sediment mobilised from the catchment approaches to catchment sediment flux. Geomorphology 108 (1–2), 35–47. Clotet, N., Gallart, F., Balasch, C., 1988. Medium term erosion rates in a small scarcely slopes reaches the basin outlet, resulting in a delivery ratio of vegetated catchment in the Pyrenees. Catena Supplement 13, 37–47. 90%. This emphasises the high connectivity between the source Collins, A.L., Walling, D.E., 2004. Documenting catchment suspended sediment of sediment and the channel network. The in-channel sedi- sources: problems, approaches and prospects. Progress in Physical Geography – ment storage represents a small but significant part of the sed- 28, 159 196. de Vente, J., Poesen, J., Bazzoffi, P., Van Rompaey, A., Verstraeten, G., 2006. Predicting iment budget (i.e., controlling sediment dynamics and catchment sediment yield in Mediterranean environments: the importance of sed- temporal variability at the basin outlet) and, overall, accounts iment sources and connectivity in Italian drainage basins. Earth Surface Processes – for 5% of the annual sediment yield. and Landforms 31, 1017 1034. de Vente, J., Poesen, J., Arabkhedri, M., Verstraeten, G., 2007. The sediment delivery iv. A small part (1%) of the catchment's area controls most of its problema revisited. Progress in Physical Geography 31 (2), 155–178. gross sediment load. The high connectivity between sediment Dietrich, W., Dunne, T., 1978. Sediment budget for a small catchment in mountainous sources (badlands) and transfer paths (streamcourses) exacer- terrain. Zeitschrift für Geomorphologie N.F (Suppl. Bd. 29), 191–206. fl Duijsings, J.J.H.M., 1986. Seasonal variation in the sediment delivery ratio of a forested bates the in uence on the catchment sediment load of the pro- drainage basin in Luxembourg. In: Hadley, R.F. (Ed.), Drainage Basin Sediment De- duction that occurs at the local scale. The high sediment livery. IAHS Publication 159. IAHS Press, Wallingford, UK, pp. 153–164. delivery ratio of the Isábena is notable, when set against the pub- Dunne, T., 1994. Hydrogeomorphology: an introduction. Transactions Japanese Geo- morphological Union 15A, 1–4. lished literature. In addition, the in-channel sediment storage ex- Fargas, D., Martínez-Casasnovas, J.A., Poch, R., 1997. Identification of critical sediment erts a notable control on the temporal dynamics and magnitude source areas at regional level. Physics and Chemistry of the Earth 22 (3–4), of the sediment transport, showing the need of taking this key 355–359. fl geomorphic element into account in the estimation of sediment Francke, T., 2009. Measurement and modelling of water and sediment uxes in mesoscale dryland catchments. Ph.D. Thesis, Universität Potsdam, Germany. budgets of mesoscale catchments. Francke, T., López-Tarazón, J.A., Schröder, B., 2008a. Estimation of suspended sediment concentration and yield using linear models, random forests and quantile regres- Together with the Ésera, the Isábena delivers water and sediments sion forests. Hydrological Processes 22, 4892–4904. into the Barasona Reservoir. Overall, the River Isábena's suspended Francke, T., López-Tarazón, J.A., Vericat, D., Bronstert, A., Batalla, R.J., 2008b. Flood- based analysis of high-magnitude sediment transport using a non-parametric load for the study period nearly reached half a million tons, which rep- method. Earth Surface Processes and Landforms 33, 2064–2077. 3 −3 resents 0.30 hm (from a sediment density of 1.52 g cm , Mamede, Gallart, F., Llorens, P., Latron, J., Regüés, D., 2002. Hydrological processes and their sea- 2008). This value represents ca. 0.4% of the original reservoir capacity sonal controls in a small Mediterranean mountain catchment in the Pyrenees. Hy- – and 4% of the total sediment sluiced down during maintenance opera- drology and Earth System Sciences 6 (3), 527 537. Golosov, V.N., Ivanova, N.N., Litvin, L.F., Sidorchuk, A.Yu., 1992. Sediment budget in tions carried out in the 1990s (Palau, 1998; Avendaño et al., 2000). In river basins and small river aggradation. Geomorphologiya 4, 62–71. their middle sections, both catchments shear a highly active badland Hooke, J.M., 1979. An analysis of the processes of river bank erosion. Journal of Hydrol- – strip that forms on exposed Eocene marls. Thus estimates of sediment ogy 42, 39 62. Lambert, C.P., Walling, D.E., 1988. Measurements of channel storage of suspended sed- production, transfer, and yield obtained in the Isábena can be extrapo- iment in a gravel-bed river. Catena 15, 65–80. lated to the Ésera with confidence and can help explain the historical Lawler, D.M., 1986. River bank erosion and the influence of frost: a statistical examina- siltation observed in the Barasona Reservoir. tion. Transactions of the Institute of British Geographers 11, 227–242. Lawler, D.M., 1993. The measurement of river bank erosion and lateral channel change: a review. Earth Surface Processes and Landforms, Technical and Software Bulletins Acknowledgements 18, 777–821. Lawler, D.M., 1999. Downstream change in river bank erosion rates in the Swale-Ouse – fi system, northern England. Hydrological Processes 13, 977 992. The rst author has a grant funded by the Catalan Government Liaw, A., Wiener, M., 2002. Classification and regression by randomForest. R News 2 and the European Social Fund, while the third author has a Ramon y (3), 18–22. Cajal Fellowship (RYC-2010-06264). Research has been carried out López-Tarazón, J.A., Batalla, R.J., Vericat, D., Francke, T., 2009. Suspended sediment transport in a highly erodible catchment: the River Isábena (Southern Pyrenees). within the framework of the project ‘Sediment Export from Large Geomorphology 109, 210–221. Semi-Arid Catchments: Measurements and Modelling’ (SESAM), López-Tarazón, J.A., Batalla, R.J., Vericat, D., Balasch, J.C., 2010. Rainfall, runoff and sediment funded by the German Science Foundation (Deutsche Forschungsge- transport relations in a mesoscale mountainous catchment: the River Isábena (Ebro – meinschaft, DFG). The authors wish to thank the Ebro Water Authori- basin). Catena 82, 23 34. López-Tarazón, J.A., Batalla, R.J., Vericat, D., 2011. In-channel sediment storage in a ties for permission to instal our measuring equipment at the Capella highly erodible catchment: the River Isábena (Ebro basin, Southern Pyrenees). gauging station and for providing hydrological data. Special thanks Zeitschrift für Geomorphologie 55 (3), 365–382. doi:10.1127/0372-8854/2011/ are due to Álvaro Tena for his assistance in the field and in the labo- 0045. fi Loughran, R.J., 1989. The measurement of soil erosion. Progress in Physical Geography ratory. Authors are indebted to Chris Gibbins who undertook a rst 13, 216–233. revision of the manuscript. Comments provided by two anonymous Lu, H., Moran, C.J., Sivapalan, M., 2005. A theoretical exploration of catchment-scale referees allowed improving the paper at the final stage. sediment delivery. Water Resources Research 41, W09415. Mamede, G., 2008. Reservoir sedimentation in dryland catchments: modelling and management. Ph.D. Thesis, Universität Potsdam, Germany. References Mamede, G.L., Bronstert, A., Francke, T., Müller, E., deAraújo, J.C., Batalla, R.J., Güntner, A., 2006. 1D Process-based modelling of reservoir sedimentation: a case study for Alatorre, L.C., Beguería, S., 2009. Identification of active erosion and erosion risk areas using the Barasona Reservoir in Spain. Conference Proceeding River Flow 2006. Interna- remote sensing in a badlands landscape on marls in the central Spanish Pyrenees. Catena tional Conference on Fluvial Hydraulics, Lisbon, September 06–08, 2006. 76, 182–190. Mathys, N., Klotz, S., Esteves, M., Descroix, L., Lapetite, J.M., 2005. Runoff and erosion in Alatorre, L.C., Beguería, S., García-Ruiz, J.M., 2010. Regional scale modelling of hillslope the Black Marls of the French Alps: observations and measurements at the plot sediment delivery: a case study in Barasona reservoir watershed (Spain) using scale. Catena 63, 261–281. WATEM/SEDEM. Journal of Hydrology 391, 109–123. Meinshausen, N., 2006. Quantile regression forests. Journal of Machine Learning Re- Alatorre, L.C., Beguería, S., Vicente-Serrano, S., 2011. Evolution of vegetation activity on search 7, 983–999. vegetated, eroded, and erosion risk areas in the central Spanish Pyrenees, using Meinshausen, N., 2007. quantregForest: quantile regression forests; R package version 0.2-2. multitemporal Landsat imagery. Earth Surface Processes and Landforms 36, Mosley, M.P., McKerchar, A., 1993. Streamflow. In: Maidment, D.R. (Ed.), Handbook of 309–319. Hydrology, 8. McGraw-Hill, New York, pp. 1–8. 39. Avendaño, C., Sanz, M.E., Cobo, R., 2000. State of the art of reservoir sedimentation Nadal-Romero, E., Regüés, D., Martí-Bono, C., Serrano-Muela, P., 2006. Dinámica esta- management in Spain. Proceedings of the International Workshop and Symposium cional de los procesos de meteorización en cárcavas del Pirineo Central. Cuater- on Reservoir Sedimentation Management, Tokyo, Japan, pp. 27–35. nario y Geomorfología 20 (1–2), 61–77. Batalla, R.J., Sala, M., Werrity, A., 1995. Sediment budget focused in solid material trans- Nadal-Romero, E., Regüés, D., Martí-Bono, C., Serrano-Muela, P., 2007. Badlands dy- port in a subhumid Mediterranean drainage basin. Zeitschrift für Geomorphologie namics in the Central Pyrenees: temporal and spatial patterns of weathering pro- 39 (2), 249–269. cesses. Earth Surfaces Processes and Landforms 32 (6), 888–904.

Navratil, O., Legout, C., Gateuille, D., Esteves, M., Liebault, F., 2010. Assessment of inter- Swanson, F.J., Fredriksen, R.L., 1982. Sediment routing and budgets: implications for judg- mediate fine sediment storage in a braided river reach (southern French Prealps). ing impacts on forestry practices. In: Swanson, F.J., Janda, R.J., Dunne, T., Swanston, Hydrological Processes 24, 1318–1332. D.N. (Eds.), Sediment Budgets and Routing in Forested Drainage Basins, USDA Forest Nelson, E.J., Booth, D.B., 2002. Sediment sources in an urbanizing mixed land-use wa- Service General Technical Report PNW-141, pp. 129–137. tershed. Journal of Hydrology 264, 51–68. Thorne, C.R., 1981. Field measurements of rates of bank erosion and bank material Palau, A., 1998. Estudio limnológico del ecosistema fluvial afectado por los vaciados del strength. Erosion and Sediment Transport Measurement, Proceedings of the Florence embalse de Barasona. Limnética 14, 1–15. Symposium, 133. IAHS Publication, pp. 503–512. Parsons, A.J., Wainwright, J., Brazier, R.E., Powell, D.M., 2006. Is sediment delivery a fal- Trimble, S.W., 1995. Catchment sediment budgets and change. In: Gurnell, A.M., Petts, lacy? Earth Surface Processes and Landforms 31, 1325–1328. G. (Eds.), Changing River Channels. Wiley, Chichester, UK, pp. 201–215. Peart, M.R., Walling, D.E., 1988. Techniques for establishing suspended sediment sources Trimble, S.W., Crosson, P., 2000. U.S. soil erosion rates: myth and reality. Science 289, in two drainage basins in Devon, UK: a comparative assessment. In: Bordas, M.P., 248–250. Walling, D.E. (Eds.), Sediment Budgets, IAHS Publ. No. 174. IAHS Press, Wallingford, Verdú, J.M., Batalla, R.J., Martínez-Casasnovas, J.A., 2007a. Estudio hidrológico de la cuenca UK, pp. 269–279. del río Isábena (Cuenca del Ebro). I: Variabilidad de la precipitación. Ingeniería del Phillips, J.D., 1991. Fluvial sediment budgets in the North Carolina piedmont. Geomor- Agua 13 (4), 321–330. phology 4, 231–241. Verdú, J.M., Batalla, R.J., Martínez-Casasnovas, J.A., 2007b. Estudio hidrológico de la Porto, P., Walling, D.E., Callegari, G., 2011. Using 137Cs measurements to establish cuenca del río Isábena (Cuenca del Ebro). II: Respuesta hidrológica. Ingeniería del catchment sediment budgets and explore scale effects. Hydrological Processes Agua 13 (4), 331–343. 25, 886–900. Walling, D.E., 1983. The sediment delivery problem. Journal of Hydrology 65, 209–237. Regüés, D., Balasch, J.C., Castelltort, X., Soler, M., Gallart, F., 2000a. Relación entre las Walling, D.E., 1984. Dissolved loads and their measurements. In: Hadley, R.F., Walling, tendencias temporales de producción y transporte de sedimentos y las condiciones D.E. (Eds.), Erosion and Sediment Yield: Some Methods of Measurements and climáticas en una pequeña cuenca de montaña mediterránea (Vallcebre, Eastern Modelling. Geo Books, London, UK, pp. 111–177. Pyrenees). Cuadernos de Investigación Geográfica 26, 24–41. Walling, D.E., 1999. Linking land use, erosion and sediment yields in river basins. Regüés, D., Guardia, R., Gallart, F., 2000b. Geomorphic agents versus vegetation spread- Hydrobiologia 410, 223–240. ing as causes of badland occurrence in a Mediterranean subhumid mountainous Walling, D.E., Quine, T.A., 1993. Using Chernobyl-derived radionuclides to investigate area. Catena 25, 199–212. the role of downstream conveyance losses in the suspended sediment budget of Roehl, J.E., 1962. Sediment source areas, and delivery ratios influencing morphological the River Severn, United Kingdom. Physical Geography 14, 239–253. factors. IAHS Publication 59, 202–213. Walling, D.E., Owens, P.N., Leeks, G.J.L., 1998. The role of channel and floodplain storage R-Team Development Core, 2010. R: A Language and Environment for Statistical Com- in the suspended sediment budget of the River Ouse, Yorkshire, UK. Geomorphol- puting. R Foundation for Statistical Computing, Vienna. ogy 22, 225–242. Sanz-Montero, M., Cobo-Rayán, R., Avendaño-Salas, C., Gómez-Montaña, J., 1996. Influ- Walling, D.E., Collins, A.L., Jones, P.A., Leeks, G.J.L., Old, G., 2006. Establishing fine- ence of the drainage basin area on the sediment yield to Spanish reservoirs. Pro- grained sediment budgets for the Pang and Lambourn LOCAR catchments, UK. ceedings of the First European Conference and Trace Exposition on Control Erosion. Journal of Hydrology 330, 126–141. Schick, A.P., 1967. Gerlach troughs, overland flow traps. Field methods for the study of Williams, J.R., Berndt, H.D., 1972. Sediment yield computed with universal equation. slope and fluvial processes. Revue de Géomorphologie Dynamique 4, 170–172. Journal of the Hydraulics Division 98 (12), 2087–2098. Schnabel, S., Maneta, M., 2005. Comparison of a neural network and a regression model Wolman, M.G., 1959. Factors influencing erosion of a cohesive river bank. American to estimate suspended sediment in a semiarid basin. In: Batalla, R.J., Garcia, C. Journal of Sciences 257, 204–216. (Eds.), Geomorphological Processes and Human Impacts in River Basins, 299. Zimmermann, A., Francke, T., Elsenbeer, H., in press. Forests and erosion: insights from IAHS Publication, pp. 91–100. a study on suspended sediment dynamics in an overland flow-prone rainforest Selby, M.J., 1982. Hillslope Materials and Processes. Oxford University Press. 264 pp. catchment. Journal of Hydrology (accepted, under revision). Slaymaker, O., 2003. The sediment budget as conceptual framework and management tool. Hydrobiologia 494, 71–82.

Please cite this article as: López-Tarazón, J.A., et al., The sediment budget of a highly dynamic mesoscale catchment: The River Isábena, Geo- morphology (2011), doi:10.1016/j.geomorph.2011.08.020