Temporal distribution of suspended sediment transport in a humid Mediterranean badland area: The Araguás catchment, Central Pyrenees

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Geomorphology 97 (2008) 601 – 616 www.elsevier.com/locate/geomorph

Temporal distribution of suspended sediment transport in a humid Mediterranean badland area: The Araguás catchment, Central Pyrenees E. Nadal-Romero a,⁎, J. Latron b , C. Martí-Bono a , D. Regüés a a

Pyrenean Institute of Ecology (CSIC), Campus de Aula Dei, Avenida Montañana 1005, 50192, Zaragoza, Spain b Soil Science Unit, University of Girona, Campus de Montilivi 17071, Girona, Spain Received 23 May 2007; received in revised form 5 September 2007; accepted 6 September 2007 Available online 20 September 2007

Abstract This paper analyses the temporal patterns of suspended sediment yield in the Araguás catchment, Central Spanish Pyrenees, a small experimental catchment with extensive badlands. The catchment has been monitored since 2004 to study weathering, erosion, and hydrological and sediment responses to understand the superficial dynamics of a badland area in a relatively humid environment. The development of badlands in the Central Spanish Pyrenees is favoured by the presence of marls and a markedly seasonal climate. The continuous observation of selected physical parameters and environmental variables enables us to establish seasonal patterns of weathering processes and identify those factors that control regolith development. Freeze–thaw cycles in winter and wetting–drying in spring–summer are the main processes involved in regolith weathering, thereby controlling slope development in combination with rainfall-related erosion processes. The 64 floods recorded during the study period (December 2005 to January 2007) were used for a hydrosedimentological analysis. The main observed features indicate that the Araguás catchment reacts to all rainfall events, resulting in steep rising and recession limbs on the hydrograph and a very short time lag. Floods show high suspended sediment concentrations and a heterogeneous temporal distribution related to seasonal variations in surface runoff production. These differences increase the degree of complexity involved in studying sediment response. Suspended sediment concentration and transport mainly depend on rainfall volume, maximum rainfall intensity, peak flow, and runoff occurrence. Finally, the similarities among the obtained hydrographs, sedigraphs, and hyetographs, in combination with the rapid response of most of the floods, suggest a large contribution of overland flow, derived mainly from infiltration excess runoff upon badland areas. Accordingly, the significant correlations obtained between rainfall intensity and sediment concentration (mainly during the dry season), which suggest a single source area for both runoff and sediment, also support the hypothesis of Hortonian hydrological response within badland areas. © 2007 Elsevier B.V. All rights reserved. Keywords: Badlands; Suspended sediment concentration; Sediment transport; Mediterranean catchment; Central Pyrenees

1. Introduction

⁎ Corresponding author. Tel.: +34 976716034; fax: +34 976716019. E-mail address: [email protected] (E. Nadal-Romero). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.09.009

The term badlands is used to describe areas of unconsolidated sediments or poorly consolidated bedrock that contains little or no vegetation (Gallart et al., 2002). Such

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areas are commonly affected by intense processes of soil erosion, including gulling, rilling, and sheet wash erosion. Badlands develop within a wide range of climatic environments, particularly in semiarid areas and, to a lesser extent, in humid and subhumid regions (Bryan and Yair, 1982; Campbell, 1989; Regüés, 1995; Regüés et al., 1995; Pardini, 1996; Torri and Rodolfi, 2000). In subhumid areas, the development of badlands is favoured by lithological and topographical factors, as well as seasonal climatic variability; the latter effect is especially pronounced in areas characterized by strong intra-annual contrasts in temperature and rainfall distributions. When these factors are coupled with rocks that are highly susceptible to erosion, the resulting geomorphological dynamics are extremely active. Badlands usually consist of bedrock covered by a layer of disturbed rock known as regolith, which can be extremely susceptible to erosion. Rapid and deep weathering (Schumm, 1956), together with intense soil erosion, explains the fact that erosion rates in badlands are much higher than those in surrounding areas underlain by differing lithologies; in some cases, the erosion rate exceeds 10,000 t km− 2 (Clotet et al., 1988; Benito et al., 1992; Sirvent et al., 1997). In humid Mediterranean badland areas (e.g., Eastern Pyrenees, Southern French Alps) similar to that of the present study area, annual erosion rates can exceed 50,000 t km− 2 (Brochot, 1993; Regüés et al., 2000a). Suspended sediment transport has been identified as the main global mechanism of fluvial sediment transport. Walling and Webb (1986) estimated that the global amount of suspended sediment transport is about 3.5 times higher than that of solutes, while bedload represents only a small component of fluvial transport. Dietrich and Dunne (1978) estimated that global-scale bed load transport represents 5–20% of the total transport amount. Bedload dynamics in badland streams are of interest in order to understand channel evolution, but the study of bedload in these environments presents several problems, since the difficulty of field measurements increase with the size of the particles. Furthermore, in recent decades, interest in suspended sediment dynamics has increased because an appropriate assessment of suspended sediment concentration is of particular importance in estimating sediment yields (Alexandrov et al., 2003). In Mediterranean areas where the sediment budget is mainly dominated by suspended sediment (Webb et al., 1995), the transport patterns of suspended sediment is a key issue in understanding the geomorphological functioning of these areas; however, field measurements and data collection in terms of suspended sediment are generally difficult tasks, rarely achieved over long timescales. Our

understanding of the catchment-scale dynamics of suspended sediment is limited by this lack of data and the high spatial and temporal variability of sediment output, which in turn is associated with variability in factors such as precipitation characteristics, the connectivity of sediment sources, changes in contributing areas, and hydraulic boundary conditions (Schmidt and Morche, 2006). Consequently, there is no general consensus on the factors governing the dynamics of suspended sediment transport at the catchment scale, even though regolith development and rainfall intensity and amount have been identified in previous studies as being the most important factors in determining the sediment response in catchments with significant badland areas (Mathys et al., 2005). This marked spatial and temporal heterogeneity, characteristic of badland areas, increases the variability and complexity of hydrological and sedimentological responses and their interactions (Regüés et al., 2000a; Regüés and Gallart, 2004). In the Central Spanish Pyrenees, Eocene marls are the most important erodible rock substratum (Beguería, 2005), particularly within the Inner Depression where the present study area, the Araguás catchment, is located. This small research catchment has been the site of weathering studies since 2004, with previous studies documenting the above-mentioned strong seasonality of weathering processes and regolith development (Nadal-Romero et al., 2006, 2007). As with results reported from clayey badland areas in the Eastern Pyrenees (Regüés et al., 2000b), the marls of the present study area are intensely affected by processes of physical weathering, especially shrinking–swelling and freeze–thaw, mainly in winter. Nadal-Romero et al. (in review) investigated the hydrological response of the Araguás catchment in order to facilitate the later analysis of the temporal distribution of suspended sediment within the catchment, which is the main objective of the present paper. More specific objectives are to (i) determine the seasonality of suspended sediment; (ii) analyse the relationships among precipitation, discharge, and suspended sediment; (iii) examine the relationship between suspended sediment and temporal patterns of regolith development; and (iv) determine the factors that explain the variability in suspended sediment response. 2. Study area The study was carried out in a representative research catchment (the Araguás catchment) within the Inner Depression of the Central Spanish Pyrenees, located 9 km NW of Jaca (Figs. 1 and 2).

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The Araguás catchment has an area of 0.45 km2 and varies in altitude from 1100 m asl at the highest point to 780 m asl at the outlet (Fig. 1). Bedrock in the upper part of the catchment is Eocene flysch, consisting of thin alternating layers of sandstone and marl. Sheet wash erosion is one of the most active geomorphic processes in the area, although the flysch is generally affected by shallow landslides that evolve into debris flows (Lorente et al., 2002). The bedrock in the lower part of the catchment is Eocene marl, dominated by massive marls and decimetre-scale interbedded sandy layers. The mineralogical composition of the marls (determined via X-ray diffraction) is clay-rich, being dominated by illite and chlorite (44%), carbonates (41%; mainly calcite and dolomite), and quartz (15%). Parent material is essentially silt dominant (silt-loam, texture), with clay as the second particle size, while sand is very poorly represented. Badlands are exclusively developed in the marl formation in the lower part of the catchment (Figs. 1 and 2), where regolith development and physical and chemical weathering processes appear to differ between north- and southfacing slopes; in general, badlands are more extensively developed on north-facing slopes.

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The climate of the study area is defined as “SubMediterranean mountain” type (Creus and Gil, 2001), being largely Mediterranean climate with some continental and Atlantic influences, and characterized by marked seasonality. Average annual precipitation is around 720 mm. In general, the wet seasons are autumn and spring, with rainstorms being relatively frequent in summer (Beguería and Vicente-Serrano, 2006). Snowfall occurs occasionally during winter, but the snow melts rapidly over several hours or days following the snow event. The mean temperature of the catchment is 10 °C approximately, with a minimum temperature of − 14 °C and maximum temperatures in excess of 30°C. Three different land covers are recognized in the Araguás catchment (Figs. 1 and 2). First, the upper part of the catchment was cultivated until the middle of the twentieth century, but is now covered by dense plantation forest (Pinus sylvestris) that covers about 30% of the total catchment area. Second, grasslands, meadows, and open shrubs dominate several sloping terraced areas in the central part of the catchment, with Genista scorpius, Buxus sempervirens, and Rosa gr. canina being the most common species. Finally, badland areas (representing

Fig. 1. Map of the Araguás catchment showing the different land cover units and the locations of the main monitoring instruments used in this study.

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Fig. 2. (A) Araguás catchment, badlands view from the south; (B) outlet sector and the gauging station with the instrumentation.

around 25% of the catchment area) dominate the lower part of the catchment. This landscape resulted from strong human pressures in recent centuries, including deforestation, frequent wildfires, overgrazing, and the cultivation of steep slopes; these factors had a particularly strong effect upon the lower and mid-level parts of the mountain (Lasanta et al., 2006). Most of these activities were abandoned during the middle of the twentieth century, resulting in important hydrological and geomorphic changes in the Central Pyrenees (Beguería et al., 2003). The Araguás catchment drains into the Rebullesa ravine, which is a tributary of the Lubierre River. The flow is perennial, although it may be reduced close to exhaustion in summer and occasionally disappears completely for short periods. The mean annual discharge is about 3 l s− 1. 3. Data and methods 3.1. Instrumental set-up The Araguás catchment was first monitored in 2004 to study physical and chemical weathering processes (Nadal-Romero et al., 2006, 2007). In 2005, further instrumentation enabled the study of hydrological response and processes of erosion and transport.

Three tipping bucket rain gauges (Davis Instruments) connected to dataloggers were installed in the badland areas of the catchment at 780, 800 and 1000 m asl to obtain reliable data on the spatial distribution of rainfall. Air temperature and regolith temperature (within deep profiles on north- and south-facing slopes) were recorded every 30 min to study the effect of temperature oscillations on weathering dynamics within the regolith (Nadal-Romero et al., 2007). The catchment contains a gauging station (Figs. 1 and 2) composed of a rectangular concrete weir equipped with an ultrasound sensor (Pepperl + Fuchs) and a pressure-based probe (Keller DCX-22 AA) to measure water level. Suspended sediment load at the outlet was quantified from water turbidity measurements using a back-scattering turbidimeter (Endress + Hauser) and combined with an automatic water sampler (ISCO 3700) that collected samples during flood events. Samples used to determine suspended sediment and solute concentrations and to estimate water conductivity were also used to calibrate the turbidity sensor. All the sensors and a pluviometer were connected to a datalogger (dataTaker DT50) that scanned the information every 10 s, recording average values every 5 min. This design enabled the recording of a continuous series of discharge, sediment concentration and rainfall.

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Following the recommendations made in previous studies (Gippel, 1995; Regüés et al., 2002), the turbidimeter was calibrated both in the laboratory and in the field, using local samples with varying sediment concentrations. Calibration has been checked periodically since the installation of the turbidimeter. As a result of the calibration process, two different relationships were obtained to convert turbidity values to suspended sediment concentrations. As shown in Fig. 3, a linear regression was used for relatively low sediment concentrations (up to 12 g l− 1), whereas for higher values (Hyperconcentrated flows (nonNewtonian) up to 2000 g l− 1) a polynomial regression was used to obtain sediment concentrations. During the study period, several technical problems (turbidimeter buried and broken during large flood events) led to occasional difficulties in measuring suspended sediment concentrations. 3.2. Characteristics of the regolith Studies of weathering processes require continued follow-up of the physical properties of regolith (Regüés et al., 1995). In the Araguás catchment, two representative slopes with opposite exposures (north- and south-facing) were selected to study regolith development and the dynamics of weathering processes (Nadal-Romero et al., 2006, 2007). In addition to the monitoring of air and regolith temperature, temporal patterns of regolith dynamics were obtained by periodically (every 1–3 weeks) sampling the following superficial physical indicators of weathering and regolith development: bulk density, as the

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main indicator of regolith weathering; and surface mechanical resistance, to evaluate crusting development. Superficial regolith moisture was also measured periodically at 0–5 and 5–10 cm depths (Nadal-Romero et al., 2006, 2007). 3.3. Data analysis Despite the technical problems with the turbidimeter mentioned above, we obtained reliable water and sediment data for a total of 64 floods between December 2005 and January 2007. In analysing the temporal distribution of suspended sediment load at the timescale of flood events in the Araguás catchment, floods were identified in cases where the increase in stream discharge exceeded 1.5 times the baseflow recorded at the beginning of the rainfall event (García-Ruiz et al., 2005; Lana-Renault et al., 2007). For each hydrograph, baseflow and storm flow were distinguished using the constant slope method (Hewlett and Hibbert, 1967) with the modified slope value of 1.83 l s− 1 km− 2 day− 1 suggested by Latron et al. (in press). For rainfall–runoff–sediment events, the variables considered in the analysis were ultimately (i) the rainfall depth (mm), (ii) the maximum rainfall intensity over a 5-min period (mm h− 1), (iii) the rainfall depth during the previous day (mm), (iv) the runoff depth (mm), (v) the runoff coefficient, (vi) baseflow at the start of the flood (l s− 1), (vii) the peak flow (l s− 1), (viii) the mean suspended sediment concentration (g l− 1), (ix) the maximum suspended sediment

Fig. 3. Experimental turbidimeter calibration curve, showing linear and polynomial adjustments for low (A) and high (B) suspended sediment concentrations.

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concentration (g l− 1), and (x) the amount of transported suspended sediment (t). 4. Results 4.1. Regolith development and dynamics of weathering processes The results obtained over the 3-year period since January 2004 (more than 50 sampling periods) show that maximum values of bulk density (2.1 g cm− 3) and surface mechanical resistance (4 kg cm− 2) were recorded in summer, whereas the lowest values (0.9 g cm− 3 and 0.2 kg cm− 2, respectively) were recorded in winter (Fig. 4). The opposite seasonal trend was recorded for moisture oscillations: All moisture measurements (crust and 0–5 depth) showed minimum values (2.5 cm3 cm− 3) during summer periods and maximum values (around 25 cm3 cm− 3) during winter (Fig. 4). These results indicate that physical weathering dynamics showed a clear temporal variability, with relatively active processes in winter when high values of regolith moisture and lower temperatures favoured frequent freeze–thaw cycles (Nadal-Romero et al., 2006, 2007). In north-facing slopes, the higher moisture content and lower temperatures probably acted to increase the weathering efficacy of freeze–thaw processes (cryoclast-

ism, cryosuction, and ice growth), whereas in southfacing slopes the most effective weathering process was thermoclastism related to the high daily thermal variation (Nadal-Romero et al., 2006, 2007). 4.2. Weathering dynamics and erosion rates Fig. 5 shows the temporal evolution of erosion rates compared to that of bulk density, taken as an indicator of regolith development. Erosion rates are represented as a lowering of the soil surface, as determined from the suspended sediment loss measured at the outlet of the catchment (converted to centimetres) and taking into account the specific weight of the marl (2.75 g cm− 3). Because erosion in the Araguás catchment almost exclusively affects the badland areas, the total sediment output (suspended sediment) was considered as being derived from these unvegetated and highly eroded parts of the catchment. During the study period, intervals in March and September 2006 were particularly active in terms of sediment transport (Fig. 5), recording equivalent reductions in the soil surface of 13 and 8 mm, respectively, during periods of maximum rainfall amounts. During the rest of the year, moderate erosion rates were observed, with equivalent regolith lowering by b2 mm (commonly 0–0.5 mm) each month. Fig. 5 also reveals temporal differences between bulk density and erosion dynamics (i.e., suspended sediment transport), as well as nonlinear relationship between the two variables. In fact, bulk density is closely related to trends in temperature and regolith humidity (Nadal-Romero et al., 2007), whereas sediment transport is directly related to the occurrence of rainstorms and floods. Consequently, a more complete understanding of catchment-scale suspended sediment dynamics requires a detailed analysis of discharge–sediment relationships at the outlet in order to explain the observed temporal discontinuities between weathering dynamics and erosion rates. 4.3. General characterization of water and sediment volumes over the study period

Fig. 4. Seasonal trends in surface soil moisture, regolith bulk density, and surface mechanical resistance within the Araguás catchment.

The total rainfall in the studied catchment was 702 mm in 2006, 2% less than the 30-year mean annual rainfall (718 mm) recorded at Jaca, 9 km to the east. Over the study period, precipitation occurred mainly in March 2006 (90 mm) and September 2006 (147 mm). Spring and winter of 2006 were drier than average and summer was characterized by a large number of convective storms. The monthly values of precipitation, runoff, and suspended sediment transport recorded during the study period are shown in Fig. 6. The results show that precipitation had

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Fig. 5. Monthly trends in regolith bulk density and erosion in the Araguás catchment.

a direct and simple influence on runoff, but a more complex relationship with suspended sediment transport. At the monthly scale, there was no significant linear relationship between runoff/precipitation depths and suspended sediment transport. Over the study period, suspended sediment transport was mainly concentrated during March and September of 2006, simultaneous with high monthly totals of precipitation and runoff. During the rest of the study period, however, suspended sediment transport showed relatively little variation, regardless of varying monthly totals of rainfall and runoff. To study the relationship between monthly suspended sediment transport and rainfall and runoff depths, we assessed the importance of individual flood events on the export of sediment from the catchment. The 64 flood events were ranked from largest to smallest magnitude of suspended sediment transport and the respective amounts of sediment, rainfall, and runoff were calculated to obtain the percentage contribution of each event. The results, shown in Fig. 7, reveal that a large proportion of the total observed runoff and suspended sediment volumes were produced by a small number of flood events. Indeed, 80% of the total rainfall recorded during the study period fell during the 32 largest rainfall events (i.e., half of the total observed events), whereas in terms of runoff depth the same proportion of rainfall was produced by just the 11 largest floods. When considering suspended sediment transport, the importance of individual events was even greater, as 80% of the total volume was produced by the four largest events. This dominance of the largest events in terms of runoff and suspended

sediment transport clearly emphasizes the high variability of runoff and sediment production at the scale of a small catchment. The results also indicate the necessity of a more detailed study at the timescale of individual flood events to improve our knowledge of rainfall–runoff– sediment dynamics within the Araguás catchment. 4.4. Suspended sediment concentration and transport at the flood timescale 4.4.1. General description of observed events Of the 64 floods recorded in the Araguás catchment during the study period, 9 occurred in spring, 17 in summer, 28 in autumn, and 10 in winter. Following the classification scheme of García-Ruiz et al. (2005) and Lana-Renault et al. (2007) developed for a neighbouring catchment, 30 of the floods occurred in the wet season (November–May) and 34 in the dry season (June– October). Tables 1A and 1B summarizes the main characteristics of rainfall, runoff, and suspended sediment transport associated with all observed floods and differentiating wet and dry seasons. Most of the rainfall events were of small magnitude: more than 80% were b15 mm and only 11% (7 events) were N 20 mm. At the event scale, the maximum precipitation depth was 50 mm (23 September 2006). The maximum event 5-min rainfall intensity ranged from 1 to more than 62 mm h− 1, but only 12% of the events reached a maximum intensity N30 mm h− 1. Rainfall intensity was higher during dry season. The values of antecedent rainfall during the day prior to the event varied from 0 to 24 mm (Table 1A and 1B).

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Fig. 6. Monthly evolution of rainfall, runoff, and suspended sediment transport in the Araguás catchment from December 2005 to January 2007.

Considering all floods, peak flow ranged from 4.4 to 2046.2 l s− 1. Sixteen floods exceeded 100 l s− 1, but only three attained peak flows 1000 l s− 1. Runoff depths ranged from close to 0 to 19.1 mm and were lower in spring and summer than in autumn and winter. The runoff coefficient, indicative of the catchment response to a rainfall event regardless of its magnitude, varied between 0.002 and 0.49. A significant quantity of suspended sediment was transported during all floods, indicating the large amount of transportable sediment within the catchment. Suspended sediment sample size ranged between 0.5 and 2 mm. However the presence of larger particles (b40 mm) during extreme events can be highlighted. Maximum suspended sediment concentration at the outlet ranged from 4.2 to 1231.1 g l− 1, with 27 events (42% of the floods) exceeding 100 g l− 1. In terms of sediment transport, more than half of the events (33 floods) exported b10 t, 11 floods (18%) exported more than 100 t, and 3 events exported more than 1000 t (i.e., more than 2000 t km− 2 when taking into account the total area of the catchment and more than 8000 t km− 2 when taking into account only the badland area).

4.4.2. Weathering-related factors that influence suspended sediment A linear correlation analysis was performed to explore the possible influence of seasonal weathering dynamics on suspended sediment concentration and transport measured at the outlet. This analysis examined the relationships between regolith characteristics (bulk density, surface mechanical resistance, and surface moisture) measured in the field and suspended sediment concentration and transport recorded during the next flood event that followed field measurements. Correlations were first obtained on the basis of all observed events; events were then separated into those that occurred during the dry and wet seasons, as described above. The results shown in Table 2 demonstrate the absence of any significant correlation between weathering-related factors and mean or maximum suspended sediment concentration or suspended sediment transport. Surface mechanical resistance, regolith bulk density, and regolith crust moisture failed to show a significant correlation with sediment concentration or transport. Correlations for dry and wet seasons were equally weak, and in all cases explained b35% of the variance. The absence of

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Fig. 7. Percentage of accumulated precipitation, runoff, and sediment transport in the Araguás catchment in relation to the number of events. Events were independently ranked from largest to smallest for each of the three variables.

significant correlations suggests that the seasonality of the regolith weathering dynamics (see Fig. 4) did not lead to comparable seasonal dynamics in suspended sediment yield at the catchment outlet; indeed, seasonal patterns of suspended sediment transport were not observed at the monthly scale (Fig. 5). 4.4.3. Hydrological factors that influence suspended sediment To further identify factors that might explain the measured variability in suspended sediment loads at the flood scale, relationships were investigated between suspended sediment transport and rainfall maximum intensity (5-min periods), runoff depth, peak flow value, and baseflow at the start of the event (Fig. 8). The relationship between suspended sediment transport (t) and runoff depth (mm) was linear (Fig. 8A) and a power function was statistically significant (p b 0.01) when considering all cases. This type of relationship, most commonly used as sediment rating curves (e.g., Walling, 1977), explained 77% of the variance. Alternatively, equally significant power relationships were obtained for the dry and wet seasons (R2 = 0.821 and 0.711, respectively). The increase in suspended sediment transport with runoff, however, was subjected to a high degree of scatter; for example, values around 20 t were associated with runoff depths ranging from 0.2 mm to almost 3 mm (i.e., different by an order of magnitude). The relationship between suspended sediment transport and peak flow (l s− 1) was also linear in the log domain (Fig. 8B); in this case, a power fit was statistically sig-

nificant (pb 0.01) and explains almost 70% of the observed variance. The large amount of sediment available within the catchment means that relatively small floods with low peak flow values (around 10 l s− 1) were able to mobilize large volumes of suspended sediment, usually close to (or even higher than) 1 t. At the event scale, the relationship between peak flow and suspended sediment transport was much stronger for the dry season (R2 =0.924) than for the wet season (R2 =0.470). Considering the maximum 5-min rainfall intensity, no significant linear correlation was found in the log Table 1A General characteristics of the rainfall–runoff events recorded in the Araguás catchment Minimum Maximum Mean Rainfall depth (mm) Maximum rainfall intensity (mm h− 1) Runoff (mm) Runoff coefficient Peak flow (l s− 1) Baseflow (l s− 1) Mean suspended sediment concentration (g l− 1) Maximum suspended sediment concentration (g l− 1) Suspended sediment transport (t) Rainfall 1 day before the event (mm)

0.8 1.2

49.8 62.4

9.6 14.2

Std. dev. 9.1 13.8

0.005 0.002 4.4 0 1.6

19.1 0.49 2064.2 26.4 506.9

1.3 3.4 0.08 0.10 150.3 370.7 4.0 4.2 47.4 85

4.2

1231.1

212.1

278.2

0.0002

3053.4

142.3

493.6

24.4

5.2

6.2

0

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Table 1B General characteristics of the rainfall–runoff events separated into two groups according to the season (total events, n = 64; wet season (November to May), n = 30; dry season (June to October), n = 34) Wet season

Rainfall depth (mm) Maximum rainfall intensity (mm h− 1) Runoff (mm) Runoff coefficient Peak flow (l s− 1) Baseflow (l s− 1) Mean suspended sediment concentration (g l− 1) Maximum suspended sediment concentration (g l− 1) Suspended sediment transport (t) Rainfall 1 day before the event (mm)

Dry season

Min.

Max.

Mean

Std. dev.

Min.

Max.

Mean

Std. dev.

1.4 1.2 0.005 0.01 4.71 0.04 1.61 4.2 0.002 0

36 24 16.95 0.47 497.83 26.38 506.92 1231.05 3054.4 16.2

9.29 7.16 1.13 0.07 57.11 3.38 59.2 231.3 143.42 3.73

7.91 5.24 3.07 0.09 102.18 4.65 114.32 339.94 564.69 4.14

0.8 2.4 0.01 0.002 4.44 0.8 2.25 4.24 0.12 0

49.8 62.4 19.07 0.49 2046.23 16.93 243.06 645.8 2237.78 24.4

9.95 20.47 1.51 0.09 232.65 4.48 37.41 195.67 141.35 6.49

10.19 15.89 3.7 0.12 488.2 3.81 47.68 216.33 432.5 7.44

domain (Fig. 8C) when considering all cases. Rainfall intensity, when considered in isolation, did not appear as a relevant factor in determining the transport of suspended sediment; indeed, similar transport values (1 t approximately) were associated with highly variable maximum rainfall intensity (1–40 mm h− 1), and similar values of intensity (5 mm h− 1) were associated with highly variable values of suspended sediment transport (0.1–1000 t). The role of maximum rainfall intensity was especially irrelevant for wet season events (R2 = 0.039); for dry seasons a power relationship between rainfall maximum intensity and suspended sediment transport was significant, but explained only 42% of the observed variance. Finally, the relationship between suspended sediment transport and baseflow at the start of the event (Fig. 8D), indicative of the catchment wetness conditions, demonstrated the absence of any significant power relationship between these two variables (R2 = 0.037). No significant relationship was found when considering the dry and wet seasons in isolation. To analyse in greater detail the influence of the different variables on the magnitude of the sediment response and to

enable a comparison of the influence of several hydrological variables related to rainfall and runoff, a linear correlation analysis was performed with the mean and maximum suspended sediment concentrations, as well as with suspended sediment transport (Table 3). Table 3 reveals that the mean suspended sediment concentration did not show any significant relationship with rainfall or runoff when considering all 64 events. The correlations were also insignificant for wet season events, but weak correlations with rainfall maximum intensity and peak flow were obtained when only considering dry season events. These significant correlations explained less than half of the observed variance. Given the wide range of observed sediment concentrations, the mean value is unlikely to be representative at the event scale; this may explain the absence of significant relationships with most of the hydrological variables. When taking all events into account, the maximum suspended sediment concentration showed significant but weak statistical correlations with rainfall depth, peak flow, runoff, and the runoff coefficient. Stronger correlations were obtained for the dry and wet seasons when considered individually. For the dry season,

Table 2 Linear correlation coefficients between mean suspended sediment concentration (SSC), maximum suspended sediment concentration and suspended sediment transport, and different physical indicators of regolith weathering dynamics and environmental variables Total events

Surface mechanical resistance (kg cm− 2) Bulk density 0–5 cm (g cm− 3) Crust moisture (cm3 cm− 3)

Dry season

Wet season

SSC mean (g l− 1)

SSC max (g l− 1)

SS transport (t)

SSC mean (g l− 1)

SSC max (g l− 1)

SS transport (t)

− 0.288

0.002

0.210

− 0.195

− 0.062

0.112

0.047

0.310

0.361

0.454

0.205

0.098

0.457

SSC mean (g l− 1)

SSC max (g l− 1)

SS transport (t)

0.204

0.343

0.551

0.628

0.387

0.616

−0.033

− 0.378

− 0.008

0.501

0.461

0.051

− 0.404

− 0.437

Events are separated into two groups according to the season (total events, n = 64; wet season (November to May), n = 30; dry season (June to October), n = 34). No correlations were significant at p b 0.01.

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611

Fig. 8. Relationships among suspended sediment transport and (A) runoff depth, (B) peak flow, (C) maximum 5-min rainfall intensity, and (D) baseflow. Dotted lines show significant (p b 0.01) fits with a power function.

significant and stronger correlations were found with rainfall depth, rainfall intensity, and peak flow; whereas wet season events only yielded significant correlations with runoff and the runoff coefficient. These seasonal differences in the variables that show correlations with the maximum sediment concentration may be indicative of the contrasting dynamics of sediment production between the dry and wet seasons. The total suspended sediment transported during each flood showed the strongest correlations with rainfall and runoff. In considering all events, significant correlations were found with rainfall depth, peak flow, runoff, and the runoff coefficient. As already observed in the log domain, the strongest correlations were found with runoff and peak flow (Fig. 8A,B). In considering the dry and wet seasons individually, all of the correlations were maintained, with the correlations with runoff and peak flow becoming stronger. A weak but significant correlation between suspended sediment transport and maximum rainfall intensity was also found for the dry season.

4.4.4. Types of hydrological and sediment responses Fig. 9 shows four examples of hydrological and sediment responses observed in the Araguás catchment. The first flood (Fig. 9A) occurred during the wet season (5 August 2006), while the second (Fig. 9B) and third (Fig. 9C) occurred during the dry period (25 June 2006 and 15 August 2006). The last flood (Fig. 9D) occurred during the transitional period between the dry and wet seasons (29 October 2005). These four examples, representative of the floods observed at the outlet, are presented in order of their total suspended sediment transport. Despite the observed variability in rainfall events (in terms of rainfall depth and intensity), all hydrographs showed similar shapes, with short response times, relatively narrow flood peaks, and steep recession limbs. The obtained sedigraphs showed similar features, with steep rising and falling limbs; the peak suspended sediment concentration generally coincided with the peak flow. A characteristic feature of the hydrological response of the Araguás catchment is the generally

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Table 3 Linear correlation coefficients between mean suspended sediment concentration (SSC), maximum suspended sediment concentration and suspended sediment transport, and different variables for the rainfall–runoff events Total events

Rainfall depth (mm) Maximum rainfall intensity (mm h− 1) Runoff (mm) Runoff coefficient Peak flow (l s− 1) Baseflow (l s− 1) Rainfall depth 1d before the event (mm)

Dry season

SSC mean (g l− 1)

SSC max (g l− 1)

0.210 0.083

0.429⁎ 0.313

0.638⁎ 0.261

0.172 0.195 0.254 0.131 0.110

0.391⁎ 0.357⁎ 0.377⁎ 0.124 0.125

0.793⁎ 0.675⁎ 0.695⁎ 0.239 − 0.026

SS transport SSC mean (t) (g l− 1)

Wet season SSC max (g l− 1)

SS transport SSC mean (t) (g l− 1)

SSC max (g l− 1)

SS transport (t)

0.373 0.459⁎

0.481⁎ 0.694⁎

0.655⁎ 0.521⁎

0.170 − 0.055

0.436 0.067

0.662⁎ − 0.083

0.320 0.301 0.681⁎ 0.433⁎ 0.260

0.288 0.219 0.590⁎ 0.195 0.116

0.660⁎ 0.544⁎ 0.945⁎ 0.530⁎ 0.009

0.135 0.198 0.110 0.045 0.120

0.527⁎ 0.553⁎ 0.474 0.097 0.160

0.978⁎ 0.890⁎ 0.831⁎ 0.030 − 0.090

Events are separated into two groups according to the season (total events, n = 64; wet season (November to May), n = 30; dry season (June to October), n = 34). ⁎Correlations are significant at p b 0.01.

marked similarity between the shapes of the hyetograph, hydrograph and sedigraph. Fig. 9A shows that a small rainfall event (4.2 mm) that occurred during wet conditions produced a limited hydrological response (peak flow Qp = 31 l s− 1) with a runoff coefficient of 0.04. The peak suspended sediment was also small compared to values usually observed for the Araguás catchment (maximum suspended sediment concentration SSCm = 51 g l− 1), but confirmed that the badland area of the Araguás catchment always yields significant sediment, even during short and lowintensity rainfall events. Fig. 9B shows successive small rainfall events (3.4 and 2 mm) of contrasting maximum rainfall intensity (27 and 10 mm h− 1, respectively). The catchment response to the second event (Qp = 35 l s− 1; SSCm = 100 g l− 1) was similar to that shown in Fig. 9A, even though the runoff coefficient was higher because of the high baseflow discharge following the first hydrograph. The response to the first event was, however, much larger (Qp = 108 l s− 1; SSCm = 561 g l− 1) than that to the second event, even though it recorded a similar rainfall amount. The difference in the magnitude of hydrological and sediment responses for the first event in Fig. 9B is clearly related to the high rainfall intensity of the rainfall event, thereby providing evidence of the high responsiveness of the badland area to intense storms in terms of water and sediment production. The rainfall event shown in Fig. 9C was larger (22 mm) and of higher maximum intensity (65 mm h− 1). Under these conditions, the hydrological response of the catchment was significantly higher (Qp = 610 l s− 1), but maintained a similar shape to the hyetograph shown in Fig. 9B, as well as a similar runoff coefficient; however, during the flood, the suspended sediment concentrations

remained similar to those of the first event in Fig. 9B, leading to a suspended sediment transport value of almost 300 t. The simultaneous conservation of the rainfall– runoff relationship (similar runoff coefficients) and nonconservation of the relationship between discharge and sediment concentration (similar sediment concentrations for different discharges) between the events shown in Fig. 9B,C clearly illustrates the complex dynamics of the relationships among rainfall, runoff, and sediment transport within the Araguás catchment. Finally, Fig. 9D corresponds to a relatively longduration rainfall event with a total rainfall depth of 28.4 mm and moderate maximum rainfall intensity (38 mm h− 1). This event occurred in the transitional period between the dry and the wet conditions, but with a baseflow of 17 l s− 1 approximately; consequently, the observed hydrograph was one of the largest ever observed at the outlet, with a peak flow of 2046 l s− 1 and a runoff coefficient of 0.35. The hydrograph shape is still similar to that of the hyetograph, with many irregularities around the peak flow that correspond to varying rainfall intensity. The main difference between this and other events is that the recession limb has a relatively shallow gradient, thereby indicating an apparent contribution from runoff generated in the forested area in the middle and upper parts of the catchment. Although peak flow was much higher than that for the other events (i.e., 20 times higher than that for the June 2006 event shown in Fig. 9B), maximum sediment concentration remained of the same order; however, the suspended sediment concentration showed a much shallower recession limb, suggesting the supply of suspended sediment from the channel itself, probably removed by the clean water flowing from the forested area. Consequently, suspended sediment transported

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613

Fig. 9. Hyetographs, hydrographs, and sedigraphs for four representative events of different magnitude within the Araguás catchment.

during the event was 2237 t, almost 10 times more than recorded during the event in August 2006 (Fig. 9C). 5. Discussion and conclusions The Araguás catchment is characterized by the presence of a Mediterranean humid badland area, with active processes of weathering, hillslope erosion, and sediment transport. This badland area is located in the lower part of the catchment, close to the outlet, thereby enabling monitoring of the stream and a study of

sediment exportation and dynamics. The relatively high values of annual and seasonal precipitation, in combination with contrasting trends in temperature and moisture at different temporal scales and easily weathered bedrock (Eocene marls), explain the capacity of the catchment to yield and deliver high quantities of sediment. This characteristic of humid badlands was also deduced from studies undertaken in the upper Llobregat basin, eastern Pyrenees (Regüés et al., 2000a), with a faster evolution than badlands in dryland areas (Cerdá, 1999; Cantón et al., 2001).

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Clear seasonal weathering dynamics were observed, with relatively active weathering processes in winter when hillslope regolith becomes increasingly susceptible to erosion. The weathering processes that take place during the cold and humid season are strong enough to supply large quantities of fine material to the channel, but the absence of a direct relationship between total sediment output and regolith bulk density dynamics suggests the existence of a lag between weathering, erosion, and transport processes, with the latter being directly dependent on the occurrence of rainstorms and floods. Annual erosion resulted mainly from a small number of intense events, as observed previously in badland areas (Wainwright, 1996; Regüés et al., 2000a; Mathys et al., 2005). High suspended sediment concentrations were also observed during minor floods, but their influence on the total sediment balance of the catchment was moderate compared with that of the larger floods. This finding corroborates the results of studies undertaken in different environments throughout the Mediterranean area, whereby sediment haphazard transport tends to occur in an irregular, compulsive way (Romero et al., 1988; Woodward, 1995). Marls are subjected to strong erosion rate, close to 29 mm year− 1. Nevertheless the erosion rate in badland areas is very heterogeneous. Yair et al. (1980) registered low erosion rates in Zin Valley, 0.48 mm year− 1, due to low frequency of rainfall events. On the other hand, Schumm (1956) registered a loss of 17.93 mm year− 1 in South Dakota Badlands, and Lam (1977) obtained 17.36 mm year− 1 in Hong Kong Badlands. Furthermore, higher values were registered on marly lithologies, with erosion rates of 30 mm year− 1 (Chodzko et al., 1991; Lecompte et al., 1996). The existence of a lag between bedrock weathering, erosion, and sediment transport is also a feature of other badland areas. For instance, Descroix and Olivry (2002), Gallart et al. (2002), and Regüés and Gallart (2004) all concluded that physical weathering is highly active from mid-autumn to the end of winter (regolith humidity and freeze–thaw cycles), whereas erosion and sediment transport are active from spring to mid-autumn. At the event scale, flood characteristics (runoff, runoff depth, and peak flow) and rainfall depth (also rainfall maximum intensity for dry season events) were the most relevant factors in controlling the maximum concentration and transport of suspended sediment. Differences observed between events in the dry and wet periods suggest potential seasonal variations in the hydrological and sediment responses of the catchment, even though in the log domain relationships between suspended sediment transport and flood characteristics were maintained throughout the year.

The portion of the catchment occupied by badlands (25% approximately of the total area) reacted immediately to any precipitation in terms of both hydrology and geomorphology. Large amounts of water and sediment were produced independently of pre-existing favourable conditions, as they depend only on the characteristics of the rainstorm. This behaviour is a peculiarity of badland areas, where the impervious nature of bare regolith leads to almost immediate infiltration excess flow (Cerdá, 1999, 2002; Cantón et al., 2002) and soil erosion. Such behaviour contrasts with that in more highly vegetated areas of neighbouring catchments in the study area, in which the antecedent conditions, caused by precipitation prior to the flood, explain to a large extent the hydrological response being dominated by saturation excess flow (GarcíaRuiz et al., 2005; Lana-Renault et al., 2007). In the Araguás catchment, floods were generally flashy, with steep rising and recession limbs of the hydrograph and a similar sedigraph. Both, hydrograph and sedigraph, are closely related to the shape of the hyetograph, further suggesting the importance of bare areas and Hortonian flow in terms of runoff generation and sediment sources. The availability of sediment in such environments is so high that even short, low-intensity rainfall and flood events record high suspended sediment concentrations. It is also interesting to note that small increases in the discharge resulted in large increases in suspended sediment concentration, to over 500 g l− 1; this indicates an almost inexhaustible supply of sediment in the hillslopes and channel. The consequence of this sediment supply was a total loss of 6900 t of sediment during the study period; that is, around 15,300 t km− 2 based on the total area of the catchment, or about 57,500 t km− 2 based solely upon the extent of badland areas. These values are two orders of magnitude higher than those simulated by Bathurst et al. (2007) for a basin in the Flysch Sector of the Spanish Pyrenees under severe shallow-landslide activity and relatively dense grassland cover. Such behaviour contrasts with that in more highly vegetated areas of neighbouring catchments in the study area. The Arnás and San Salvador catchments located in the Central Spanish Pyrenees (García-Ruiz et al., 2005; Serrano Muela et al., 2005; Lana-Renault et al., 2007), in the Flysch sector, present different land uses and plant covers. The Arnás catchment (248 ha) is characterized by shrub and open forests in old abandoned fields; while the San Salvador catchment (98 ha) is scarcely affected by human activities and is characterized by the presence of a dense forest cover. The maximum value of suspended sediment concentration recorded in each catchment is a good element for comparison: 1.9 g l− 1 in San Salvador

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catchment and 15 g l− 1 in the Arnás catchment (LanaRenault et al., 2007) where the total suspended sediment transport recorded was about 41.46 t km− 2. Acknowledgements This research was financially supported by the following projects: “Characterization and modelling of hydrological processes and regimes in gauged basins for prediction in non-gauged basins” (CANOA, CGL 200404919-C02-01) and “Processes and sediment balances at different spatial scales in Mediterranean environments: Effects of climate fluctuations and land use changes” (CGL2006-11619/HID), both funded by the CICYT, Spanish Ministry of Education and Science, and “Validation of a coupled model for the simulation of hydrological and hydraulic processes using data from experimental catchments in a Mediterranean mountain” (PM088/2006), funded by the DGA. Monitoring of the badland catchment was also funded by a consortium between the CSIC and the Spanish Ministry of Environment (RESEL Project). The first author is grateful to the CSIC for financial support within the framework of an I3 Ph.D. grant. J. Latron has benefited from a research contract (Juan de La Cierva programme) funded by the Spanish Ministry of Education and Science. We would like to express our gratitude to Eugenio de Mingo for his collaboration in the fieldwork, and to Noemí Lana-Renault, Pilar Serrano Muela and Silvia Presa for their collaboration in the fieldwork and laboratory. Special thanks go to Dr. Ignasi Queralt for the mineralogical analysis. The authors would also like to acknowledge the helpful comments made by reviewers. References Alexandrov, Y., Laronne, J.B., Reid, I., 2003. Suspended sediment concentration and its variation with water discharge in a dryland ephemeral channel, northern Negev, Israel. Journal of Arid Environments 53, 73–84. Bathurst, J.C., Moretti, G., El-Hames, A., Beguería, S., García-Ruiz, J.M., 2007. Modelling the impact of forest loss on shallow landslide sediment yield, Ijuez River catchment, Spanish Pyrenees. Hydrological and Earth System Sciences 11 (1), 569–583. Beguería, S., 2005. Identificación y Características de las Fuentes de Sedimento en Áreas de Montaña: Erosión y Transferencia de Sedimento en la Cuenca Alta del Río Aragón. Instituto Pirenaico de Ecología, Zaragoza. Beguería, S., Vicente-Serrano, S.M., 2006. Mapping the hazard of extreme rainfall by peaks over threshold extreme value analysis and spatial regression techniques. Journal of Applied Meteorology and Climatology 45 (1), 108–124. Beguería, S., López-Moreno, J.I., Lorente, A., Seeger, M., GarcíaRuiz, J.M., 2003. Assessing the effects of climate oscillations and land-use changes on streamflow in the Central Spanish Pyrenees. Ambio 32 (4), 283–286.

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