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A suspended sediment budget for the agricultural Can Revull catchment (Mallorca, Spain)

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A suspended sediment budget for the agricultural Can Revull catchment (Mallorca, Spain)
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   69 Suspended sediment budget A suspended sediment budget for the agricultural Can Revull A suspended sediment budget for the agricultural Can Revull catchment Mallorca, Spain)  Joan Estrany, Celso Garcia, Núria Martínez-Carreras and Desmond E. Wallingwith 8 figures and 3 tables Abstract.   Extensive rainfed herbaceous crops are one of the most representative agricultural ele-ments of Mediterranean region, which should be one of the major factors affecting erosion processes. Although land use is commonly seen as resulting in increased sediment yields, the implementation of soil and water conservation practices can have the reverse effect. A sediment budget has been estab-lished in Can Revull, a small agricultural catchment (1.03 km 2 ) on the island of Mallorca, by using 137 Cs measurements, sediment source fingerprinting and more traditional monitoring techniques to quantify the individual components of the budget. A large proportion of the material mobilized from cultivated fields without conservation practices (gross erosion of 775 t yr  1 , 1,270 t km  2  yr  1 ) was subsequently deposited either within the field of srcin (112 t yr  1 , 180 t km  2  yr  1 ) or at intermediate locations between the source field and the channel network (field-to-channel conveyance loss was 454 t yr  1 , 744 t km  2  yr  1 ). The estimates of sediment accumulation rates on the floodplain in the lower reaches of the catchment indicate that the mean sedimentation rate was 0.14 g cm -2  yr -1 . This value was extrapolated to the total area of the floodplain to estimate a total annual conveyance loss or storage of 445 t yr  1 . Monitoring at the catchment outlet over the study period (2004–2008) indicated a mean annual suspended sediment yield of 7 t km -2  yr -1 . The sum of the estimates of sediment yield and floodplain storage (452 t yr -1 ) was taken to represent the total annual input of suspended sediment to the channel system. This value was subsequently apportioned using the information provided by the fingerprinting investigation, to estimate the mass of sediment reaching the channel network from cultivated fields and from eroding channel banks. Thus the annual contribution from channel banks was estimated to be 242 t yr -1 . In the case of the contributions from cultivated fields, the estimates obtained were, as expected, significantly less than the values of net soil loss from these zones pro-vided by the 137 Cs measurements due to conveyance losses associated to field-to-channel conveyance loss. The overall sediment delivery ratios (< 1 %) indicate that approximately 99 % of the sediment mobilized by erosion within the Can Revull catchment is subsequently deposited before reaching the monitoring station. As such, the low sediment outputs from the study catchment should be seen as reflecting the importance of conveyance losses and storage rather than a lack of sediment mobiliza-tion from the catchment surface, although part of the catchment headwaters was modified historically by means of terraces and transverse walls to prevent erosion. Key words:  suspended sediment, sediment budget, 137 Cs, traditional soil conservation practices, agri-cultural land, Mediterranean, Mallorca 1.  Introduction The Mediterranean region of Europe has a long history of human settlement and human impacts. The very high spatial and temporal variability of fluvial processes in the region also creates problems for measurement and monitoring and for assessing the effects of human activity. These effects have included deforestation and other land use changes, agricultural terracing on a wide scale, water transfers and irrigation schemes ( Hooke  2006). Extensive   Zeitschrift für Geomorphologie Vol. 56, Suppl. 3, 169-193  Article Stuttgart, October 2012 © 2012 Gebrüder Borntraeger Verlagsbuchhandlung, Stuttgart, Germany www.borntraeger-cramer.deDOI: 10.1127/0372-8854/2012/S-00110 0372-8854/12/S-00110 $ 6.50   7  Joan Estrany et al. cultivation of rainfed herbaceous crops is one of the most representative agricultural ele-ments of Mediterranean-climate regions and should be one of the major factor affecting erosion processes ( Douglas  1993). In Spain, this type of crop occupies more than 40 % of agricultural land ( INE  2003). Although land use is commonly seen as resulting in increased sediment yields, the implementation of soil and water conservation practices can have the reverse effect, thus resulting in reduced sediment yields ( Estrany  et al. 2009). These prac-tices have been applied since the Roman Age in Mallorca, although they have become par-ticularly important during the Modern and Contemporary Ages, because the growth of the island’s population necessitated clearing more fields for agriculture from marginal areas with steeper slopes and this required the construction of dry-stone structures ( Grimalt  et al. 2002).Likewise, an increase in the area of agricultural land also required the control of excess water, mainly the impervious, flat areas of central Mallorca. This involved the development of drainage systems promoting that subsurface-tile drainage systems discharge to a net-work of artificial channels and then to a stream, which was often artificially straightened. This provided further fertile agricultural land and limited erosion by reducing the connec-tion between the slopes and the channels.Although geomorphology and traditional conservation practices can generate different dynamics, small headwater catchments are frequently seen to represent important sources of sediment input to the downstream river system, due to the lack of shallow concave slopes, which commonly represent important sediment sinks in larger catchments, resulting in relatively high connectivity between the slopes and the river channel. Catchment sedi-ment budgets offer a means of assessing the sources, storage, rates of transport, yields, and efficiency of delivery of sediment for a range of catchment scales (e.g., Reid  & Trustrum  2002, Wasson  2002, Slaymaker  2006, Brown  et al. 2009). Establishment of a sediment budget can both provide an improved understanding of catchment functioning and assist the design and implementation of effective sediment management strategies ( Slaymaker  2003, Walling  & Collins  2008). An improved understanding of the sediment budgets of Mediterranean agricultural headwater catchments is therefore seen to be an important requirement for developing sediment control and management strategies for larger river basins, considering that sediment mobilisation is usually accepted as the main factor con-trolling sediment transport in upstream catchments ( Milliman &  Syvitsky  1992), whilst sediment delivery processes become more important in controlling sediment transport in larger basins, where sediment deposition and storage assume increasing importance (cf. De Vente  2007). To date, most of the sediment budget studies undertaken in Mediter-ranean catchments have specifically focused on a single river reach as a “black-box” (e.g., Schick  & Lekach  1993, Trimble  1997, Inman  &  Jenkins  1999, Rovira  et al. 2005, López-Tarazón  et al. 2012). More recently, however, Porto  et al. (2011) reported the results of a study aimed at exploring the use of caesium-137 ( 137 Cs) measurements to establish sediment budgets for three Mediterranean catchments of different sizes and contrasting land use. The results of this study demonstrate a reduction in the sediment delivery ratio from 98 to 2 % as catchment area increased from 1.47 ha to 31.2 km 2 .A research project has been in progress since 2004 in the Na Borges basin, an agricul-tural lowland catchment, located in the island of Mallorca, aimed at establishing a com-prehensive water and sediment budget for the catchment ( Estrany  & Garcia  2005). In this context, the Can Revull sub-catchment is a representative area selected to study the   7 Suspended sediment budget hydrology and sediment dynamics of headwater catchments and their contribution to the larger Na Borges basin. This paper reports the results of an investigation aimed at estab-lishing a detailed sediment budget for the small Can Revull catchment (1 km 2 ), based on the approach described by Walling  & Collins  (2000). This involves the integration of three separate approaches: i) continuous river monitoring to document the sediment flux at the catchment outlet, ii) sediment fingerprinting to establish the relative importance of the major sediment sources, and iii) the use of environmental radionuclides ( 137 Cs) to assemble representative information on rates of soil erosion and deposition and overbank accumu-lation within the catchment. This integrated approach produces detailed information on individual components of sediment delivery, and synthesis of the resulting information provides the basis for establishing a meaningful suspended sediment budget.2. Study area The Can Revull catchment has an area of 1.03 km 2  (i.e., at the outlet gauging station, see Fig. 1) and drains into the Torrentó de Boscana (7.9 km 2 ), a headwater tributary of the Na Borges River, a lowland agricultural catchment (319 km 2 ) located in the north-eastern part of Mallorca, Spain (Fig. 1). The geology of the Can Revull catchment is character-ised by the structurally gentle alpine relief of the Central Ranges of the island composed of Miocene calcarenites, which rest discordantly over a deformed Mesozoic-Cenozoic substratum. The maximum altitude of the catchment is 144 m a.s.l. The channel length is 2.4 km and the average channel slope is 4.7 % (10 % in the first 400 meters, and 2 % down-stream).The climate in the catchment area can be classified as sub-dry Mediterranean ( Thorn-thwaite  1948), with a mean temperature of 16.5 °C and a mean annual rainfall of 517 mm (1974-2006, data from the Boscana Nou  rainfall station, located 1.5 km from the Can Revull gauging station). Autumn is the rainiest season, followed by winter, spring and summer with an inter-annual variability of 23 %. The main characteristic of the rainfall is its tor-rential nature, especially during late summer and autumn when the daily intensity can reach 100 mm (i.e., 25-year recurrence interval). During the winter, mid-latitude westerlies and associated frontal systems bring general rainfall ( Romero  et al. 1998). The flow regime is intermittent with flow normally occurring from November or December to March or April. The mean daily discharge during the study period 2004–2008 was ca. 5.6 l s -1 . Mean annual PET estimated using the Thornthwaite method (1948) is 1010 mm.The main bedrock of the catchment is limestone and this is reflected in the soil type. Using the Soil Taxonomy System, the soils of the catchment can be classified as Entisols in the catchment headwaters and Alfisols in the flat downstream areas. In the flat areas the soils are deep and well developed and are characterized by a “silty-clay-loam” soil tex-ture ( Díaz Palmer  et al. 2006). However, they overlay Quaternary alluvial sediments sup-ported by an impervious lower-middle Miocene (Burdigalian) layer and these impervious sediments prevent percolation into deep aquifers. Uniquely, a shallow unconfined aquifer is exists in the soils characterized by a high field capacity > 40 % ( Diaz-Palmer  et al. 2006), and the installation of an under-drainage system is the only way to drain these areas which were formerly natural wetlands. Drainagehas been practiced since Roman times and the perched water-Tables enhance the streamflow. The soils together with the low relief and the very humid winters are the main factors requiring the construction of the under-drainage   72    Joan Estrany et al.   73 Suspended sediment budget network, which occupies 75 % of the catchment. The tile drains are installed at a depth of approximately 1 m and backfilled with pervious material, and follow a herringbone system, with laterals laid diagonally across the slope, intersecting a main drain running downslope to the outlet in the bank of an artificial stream. An orthogonal network of artificial channels was built to conduct the flow to a natural stream (see Fig. 1).The areas of steep convex topography in the study catchment are terraced by dry-stone walls, a historical management practice in the island ( Grimalt  2002) and in the Na Borges basin ( Estrany  et al. 2010). Rainfed herbaceous crops represent the main land use (91 %) in the catchment. They are grown on the flat areas with subsurface drainage and the main crops are cereals, such as wheat and oats, which have their highest water needs during April and May (65 %). This fact indicates that these species are well adapted to this type of soils and climatic conditions. Rainfed tree crops i.e., almond trees cover 6.2 % and are found in the steepest areas. The remaining land (2.7 %) is occupied by forest, where holm oak ( Quercus ilex ) is the climax vegetation, and pine trees ( Pinus halepensis ) occupy the clearer and dryer areas.3. Methods Caesium-137 ( 137 Cs) measurements have been used to provide representative information on sediment mobilization and redistribution rates on the slopes of the study catchment, over-bank sedimentation rates on the floodplains and channel aggradation, sediment fingerprint-ing procedures have been employed to establish the relative importance of major sediment sources to the sediment output from the catchment, and conventional river monitoring provides information on the suspended sediment flux at the catchment outlet. Synthesis of the information assembled for the individual budget components, including fine sediment mobilization, storage and delivery, provides the basis for establishing the catchment sus-pended sediment budget. Channel bed sediment storage was not included as a storage term in the sediment budget for the Can Revull catchment, because the slope erosion and flood-plain sedimentation data were obtained using 137 Cs measurements. As a result the sediment budget was established for a medium-term timescale (ca. 50 years) and it was assumed that there was no net change in channel storage. The assumption was supported by the lack of field evidence (e.g., from bridges and culverts) that the channel was aggrading. 3.1 Use of Use of 37 Cs measurements to quantify sediment redistribution on the slopes and sediment Cs measurements to quantify sediment redistribution on the slopes and sediment storage in floodplains storage in floodplains The 137 Cs approach possesses a number of key advantages including the potential for obtain-ing retrospective medium-term ( ≈  40 yr) information on the basis of a single site visit, the provision of spatially distributed information on rates of erosion and deposition and its Fig. 1. Location of the Can Revull catchment. Inset maps show: (a) Mallorca and the Na Borges catchment locations, (b) the Can Revull location in the Boscana catchment, (c) the relief and drainage network of Can Revull, (d) infrastructures and land uses, (e) source sampling sites and 137 Cs soil cor-ing related with erosional and depositional environments within the study catchment.
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