The influence of grazing on vegetation, soil properties and stream discharge in a small Dartmoor catchment, southwest England, UK

Abstract

Intensive grazing has been responsible for environmental degradation of moorland landscapes over the last 50 years. Anecdotal evidence on Dartmoor, UK, for example, has suggested that it is particularly associated with a greater frequency of large floods, lower baseflow and a greater occurrence of erosion. A study was therefore conducted at Holne Moor, a small catchment on east Dartmoor, to investigate the impacts of grazing animals on hillslope hydrology and stream discharge. The vegetation in the study area ranged from heather (Calluna vulgaris), in areas with low grazing intensity, to short grass with occasional erosion scars visible associated with larger sheep numbers. The physical characteristics of the topsoil beneath heather species were very different compared with short grass. This was not due so much to the direct physical impact of grazing animals, but indirectly to the chemical and biological differences brought about by changes in vegetation response and soil structure. The experiment at Holne Moor indicated that soil water content on the hillslope was linked directly to the stream's rainfall-runoff response via the soil water pathways. In relatively dry conditions, the soil water content is mainly determined by local factors such as vegetation. In wet periods, non-local influences such as slope and contributing area are more important. A soil water content threshold of around field capacity separated the two conditions. A comparison between soil physical properties and soil moisture under different vegetation types indicated that this 'runoff' threshold between dry and wet states is lower in heavily grazed areas. It is argued that more intense grazing causes conditions suited to the increased delivery of soil water to rapid flowpaths such as sheep tracks and other rapid routes which connect the source of this hillslope water to the contributing areas during large floods. Any alteration of vegetation and soil properties due to management causes a substantial increase in 'active' source areas and therefore discharge during large storm events even before any erosion occurs.

Rural land management impacts on catchment scale flood risk

“…Meyles et al, (2006) found that the land cover the highest stock densities (short grass = 1.24 sheep / hectare) (Figure 8.5a) had the lowest soil porosities (Figure 8.5b) and the highest soil bulk density (Figure 8.5c) for all soil depths.…”

“…This highlights the importance of predicting soil moisture dynamics as a diagnostic tool for flood generation. Meyles et al, (2006) found that changes to the physical characteristics of the soil (decreased organic content, increased bulk density, decreased porosity) caused the wetness threshold between dry and wet states was lower in heavily grazed area, as field capacity was reached more readily.…”

“…Also they introduced an impermeable clay layer at 0.3 m depth to represent the effects of compaction in another approach. (Meyles et al, 2006) The effects of compaction are thought to penetrate up to the depth of 60 cm (Flowers and Lal, 1998), although the greatest effects are seen in the top 10 cm.…”

“…Light compaction results in a maximum dynamic layer moisture content of 84%. Meyles et al, (2006) found that it was the top 10 cm of the soil that were most important in controlling the time it took for overland flow to be initiated. …”

Erosion of Uncultivated Land

“…Runoff is greater from heavily grazed land (Evans, 1990b(Evans, , 1998, especially from those areas particularly favoured by sheep (Meyles et al, 2001), because the vegetation cover is very low or thin and soils are more compact. Furthermore, tracking and channeling of water downslope speed runoff into channels.…”