Impacts of upland land management on sediment dynamics in the Pontbren catchment, mid-Wales, UK
Shortcut URL: https://serc.carleton.edu/48229
Location
Continent: Europe
Country: United Kingdom
Setting
Climate Setting: Temperate
Type: Process
Figure 1. Location map of the Pontbren catchment. Details
Figure 2. Grazed, agriculturally-improved upland pasture at Pontbren, mid-Wales. Details
Figure 3. Drainage ditch alongside an agriculturally-improved upland pasture. Details
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Figure 5. Planted woodland strip in an agriculturally-improved upland pasture at Pontbren. Details
Description
Changes in vegetation, soil structure, surface topography and drainage associated with land management practices can affect the intensity and spatial distribution of runoff generation, erosion and sediment supply within river catchments, and modify flow regimes, sediment yields and the morphology of rivers as a result. Improved understanding of how land management practices affect the transfer of water and sediment through the landscape can help river managers to mitigate environmental problems (e.g. aquatic habitat degradation due to excessive fine sediment loads) and threats to people, property and infrastructure (e.g. flooding, channel migration). These points are illustrated herein using the results of research undertaken in an experimental upland catchment at Pontbren in mid-Wales (Figure 1).
The Pontbren study site is located in the headwaters of the River Severn, the UK's longest river, and has an area of approximately 12 km2. Land within the catchment ranges in elevation from approximately 250-450 m and receives over 1500 mm of precipitation annually. Like many parts of the British uplands, the Pontbren catchment experienced significant agricultural changes during the mid-late twentieth century. Moorland (unenclosed land characterised by low, shrubby vegetation and acidic soils) dominated the area during the early twentieth century with some mixed purpose (arable and pastoral) farming at lower altitudes. However, the introduction of government incentives to encourage domestic livestock farming after the Second World War led to the conversion of large areas of moorland and rough grazing land in the catchment to permanent pasture through drainage, ploughing, lime and fertiliser application, and reseeding with improved grass varieties (Figure 2). This widespread grassland improvement, combined with further subsidies to encourage intensive livestock production, led to a six-fold increase in sheep numbers in on farms in the catchment between the 1960s and 1990s (Henshaw, 2009). Nevertheless, these changes were not spatially uniform, allowing comparisons of runoff characteristics and sediment yields to be made between the Nant Pen-y-cwm (which drains an area that experienced widespread agricultural intensification) and the Nant Melin-y-grug (which drains an area of undisturbed moorland). This spatial variation is clearly visible in Figure 1, where areas of improved grassland appear dark green and areas of moorland appear brown.
The mean annual bedload and suspended sediment yields of the Nant Pen-y-cwm were found to be approximately 12 and 5 times greater, respectively, than those of the Nant Melin-y-grug (Henshaw, 2009). Monitoring of potential sediment sources within the two subcatchments indicated that the higher sediment yields of the Nant Pen-y-cwm could be partly attributed to the presence of agricultural ditches (Figure 3) and subsurface field drains in agriculturally-developed areas (Henshaw, 2009). These features act as both sources and delivery pathways for coarse and fine sediment, and their influence on sediment dynamics has been noted in studies of other upland land management practices (e.g. Newson, 1980). However, the majority of the additional sediment transported by the Nant Pen-y-cwm is derived from its bed and banks via fluvial erosion (Henshaw, 2009). The stream has experienced widespread recent destabilisation in comparison to the Nant Melin-y-grug (Figure 4) and this may reflect a change in the hydrological response of the catchment as a result of recent agricultural intensification in the area. Increased levels of livestock grazing and associated changes in vegetation can have a major impact on the physical properties of British upland soils and encourage widespread surface runoff. Observations of flashier hydrographs and higher peak runoff rates in the Nant Pen-y-cwm compared to the Nant Melin-y-grug during simultaneous high-flow events (Marshall et al., 2006) suggest that these factors may have increased the speed at which water is delivered to the stream and may explain the observed changes in morphology.
A recent scheme by local farmers in the Pontbren catchment may help to lessen some of these impacts. Strips of woodland and hedgerows have been planted within agriculturally-improved grazed pastures to provide livestock with shelter and act as a sustainable source of wood for use as animal bedding (Figure 5). Field experiments at the site have shown that soils beneath these strips of trees have significantly lower soil bulk densities, and higher soil moisture storage capacities and hydraulic conductivities, than those underlying adjacent grazed pastures (Wheater et al., 2008; Marshall et al., 2009). Infiltration rates are up to 60 times greater within the tree strips as a result (Carroll et al., 2004), and they generate substantially lower volumes of surface runoff than neighbouring areas of improved grassland during rainfall events (Marshall et al., 2009). These effects became apparent within 2-6 years of planting (Carroll et al., 2004). Hydrological modelling exercises indicate that the tree strips are highly effective at reducing overland flow within pastures by increasing subsurface storage and infiltration rates, thereby causing a significant reduction in hillslope runoff peaks (Jackson et al., 2009). They also suggest that planting undertaken so far has reduced peak flows for frequent high-flow events by 13%, and extreme events by 5% (Jackson et al., 2008). Corresponding further reductions of 29% and 5% could be achieved, respectively, with strategic planting through the catchment (Jackson et al., 2008). Hydrological effects of this magnitude could be expected to have significant geomorphological implications, dramatically reducing the potential for erosion in headwater channels at Pontbren and slowing the transfer of sediment to reaches downstream (Henshaw, 2009).
The Pontbren study site is located in the headwaters of the River Severn, the UK's longest river, and has an area of approximately 12 km2. Land within the catchment ranges in elevation from approximately 250-450 m and receives over 1500 mm of precipitation annually. Like many parts of the British uplands, the Pontbren catchment experienced significant agricultural changes during the mid-late twentieth century. Moorland (unenclosed land characterised by low, shrubby vegetation and acidic soils) dominated the area during the early twentieth century with some mixed purpose (arable and pastoral) farming at lower altitudes. However, the introduction of government incentives to encourage domestic livestock farming after the Second World War led to the conversion of large areas of moorland and rough grazing land in the catchment to permanent pasture through drainage, ploughing, lime and fertiliser application, and reseeding with improved grass varieties (Figure 2). This widespread grassland improvement, combined with further subsidies to encourage intensive livestock production, led to a six-fold increase in sheep numbers in on farms in the catchment between the 1960s and 1990s (Henshaw, 2009). Nevertheless, these changes were not spatially uniform, allowing comparisons of runoff characteristics and sediment yields to be made between the Nant Pen-y-cwm (which drains an area that experienced widespread agricultural intensification) and the Nant Melin-y-grug (which drains an area of undisturbed moorland). This spatial variation is clearly visible in Figure 1, where areas of improved grassland appear dark green and areas of moorland appear brown.
The mean annual bedload and suspended sediment yields of the Nant Pen-y-cwm were found to be approximately 12 and 5 times greater, respectively, than those of the Nant Melin-y-grug (Henshaw, 2009). Monitoring of potential sediment sources within the two subcatchments indicated that the higher sediment yields of the Nant Pen-y-cwm could be partly attributed to the presence of agricultural ditches (Figure 3) and subsurface field drains in agriculturally-developed areas (Henshaw, 2009). These features act as both sources and delivery pathways for coarse and fine sediment, and their influence on sediment dynamics has been noted in studies of other upland land management practices (e.g. Newson, 1980). However, the majority of the additional sediment transported by the Nant Pen-y-cwm is derived from its bed and banks via fluvial erosion (Henshaw, 2009). The stream has experienced widespread recent destabilisation in comparison to the Nant Melin-y-grug (Figure 4) and this may reflect a change in the hydrological response of the catchment as a result of recent agricultural intensification in the area. Increased levels of livestock grazing and associated changes in vegetation can have a major impact on the physical properties of British upland soils and encourage widespread surface runoff. Observations of flashier hydrographs and higher peak runoff rates in the Nant Pen-y-cwm compared to the Nant Melin-y-grug during simultaneous high-flow events (Marshall et al., 2006) suggest that these factors may have increased the speed at which water is delivered to the stream and may explain the observed changes in morphology.
A recent scheme by local farmers in the Pontbren catchment may help to lessen some of these impacts. Strips of woodland and hedgerows have been planted within agriculturally-improved grazed pastures to provide livestock with shelter and act as a sustainable source of wood for use as animal bedding (Figure 5). Field experiments at the site have shown that soils beneath these strips of trees have significantly lower soil bulk densities, and higher soil moisture storage capacities and hydraulic conductivities, than those underlying adjacent grazed pastures (Wheater et al., 2008; Marshall et al., 2009). Infiltration rates are up to 60 times greater within the tree strips as a result (Carroll et al., 2004), and they generate substantially lower volumes of surface runoff than neighbouring areas of improved grassland during rainfall events (Marshall et al., 2009). These effects became apparent within 2-6 years of planting (Carroll et al., 2004). Hydrological modelling exercises indicate that the tree strips are highly effective at reducing overland flow within pastures by increasing subsurface storage and infiltration rates, thereby causing a significant reduction in hillslope runoff peaks (Jackson et al., 2009). They also suggest that planting undertaken so far has reduced peak flows for frequent high-flow events by 13%, and extreme events by 5% (Jackson et al., 2008). Corresponding further reductions of 29% and 5% could be achieved, respectively, with strategic planting through the catchment (Jackson et al., 2008). Hydrological effects of this magnitude could be expected to have significant geomorphological implications, dramatically reducing the potential for erosion in headwater channels at Pontbren and slowing the transfer of sediment to reaches downstream (Henshaw, 2009).
Associated References
- Carroll, Z.L., Bird, S.B., Emmett, B.A., Reynolds, B. and Sinclair, F.L. (2004) Can tree shelterbelts on agricultural land reduce flood risk? Soil Use and Management, 20, 357-359.
- Henshaw, A.J. (2009) Impacts of land use changes and land management practices on upland catchment sediment dynamics: Pontbren, mid-Wales. Unpublished PhD thesis. University of Nottingham. Available online at: http://etheses.nottingham.ac.uk/889/
- Jackson B.M., Wheater H.S., McIntyre N.R., Francis O.J., Frogbrook, Z., Marshall, M., Reynolds, B. and Solloway, I. (2008) Upscaling runoff from hillslope to catchment scale: a case study in an upland Welsh catchment. Proceedings of BHS 10th National Hydrology Symposium, Exeter, 2008, 268-274. Available online at: http://www.hydrology.org.uk/Publications/exeter/42.pd
- Jackson, B.M., Wheater, H.S., Mcintyre, N.R., Chell, J., Francis, O.J., Frogbrook, Z., Marshall, M., Reynolds, B. and Solloway, I. (2009) The impact of upland land management on flooding: insights from a multiscale experimental and modelling programme. Journal of Flood Risk Management, 1, 71-80.
- Marshall, M.R., Frogbrook, Z.L., Francis, O.J., Reynolds, B., McIntyre, N. and Wheater, H.S. (2006) The impact of upland land management on flooding: preliminary results from a multi-scale experimental programme. Proceedings of BHS 9th National Hydrology Symposium, Durham, 2006, 85-90. Available online at: http://hydrology.org.uk/Publications/durham/bhs_13.pdf
- Marshall, M.R., Francis, O.J., Frogbrook, Z.L., Jackson, B.M., McIntyre, N., Reynolds, B., Solloway, I., Wheater, H.S. and Chell, J. (2009) The impact of upland land management on flooding: results from an improved pasture hillslope. Hydrological Processes, 23, 464-475.
- Newson, M.D. (1980) The erosion of drainage ditches and its effects on bed-load yields in mid-Wales: Reconnaissance case studies. Earth Surface Processes, 5, 275-290.
- Wheater, H., Reynolds, B., Mcintyre, N., Marshall, M., Jackson, B., Frogbrook, Z., Solloway, I., Francis, O. and Chell, J. (2008) Impacts of upland land management on flood risk: multi-scale modelling methodology and results from the Pontbren experiment. Manchester: Flood Risk Management Research Consortium. Available online at: http://www.floodrisk.org.uk/