Field-based determination of controls on runoff and fine sediment generation from lowland grazing livestock fields

Abstract

Compared with arable land, there is a paucity of field-based measurements of erosion rates and controls for lowland temperate grassland supporting ruminant agriculture. Despite this evidence gap, reducing diffuse fine sediment pollution from intensively farmed grassland has been recognised as essential for improving compliance with water quality targets. Improved information on erosion rates and controls within intensively managed lowland grazing livestock systems are prerequisites for informing best management practices for soil and water resource conservation. Accordingly, this study assembled such information using the North Wyke farm platform in south west England where flow, suspended sediment concentration, rainfall and soil moisture are monitored quasi-continuously in 15 hydrologically-isolated (1.54-11.12 ha) catchments. This region of the UK is representative of temperate lowland ruminant grazing landscapes with semi permeable soil drainage. Catchment area was the major control on both water and sediment flux. When normalised to catchment area, sediment yields were controlled by the erodibility of the catchment's soils. Ploughing for re-seeding of grass swards was the major factor that affected this. Whilst total rainfall had a small effect on sediment yields, slope and the damage of soils by livestock had no significant effects. This finding may be due to the overriding effects of ploughing and re-seeding of some fields during the study period. Detachment by impacting raindrops mobilised sediment particles across the entire field with diffuse saturation-excess overland flow responsible for their transport. The majority of erosion occurred during the rising limbs of storm events when there is an abundance of easily detached soil particles. Given that erosion and sediment transport are driven mechanistically by processes affecting the entire field areas, a reduction in sediment yield through the implementation of highly spatially-targeted in-field management such as that for feeder ring use, troughs, poached tracks or gateways would likely be very challenging. Instead, stocking density and grazing regime management, as well as carefully planned ploughing and re-seeding will be more beneficial for erosion control.

Keywords: Erosion; Farming; Grassland; Lowland grazing; Rain splash; Saturation-excess runoff.

Fig. 1

The North Wyke farm platform; each hydrologically-isolated catchment is marked and labelled with its flume number to correspond with the background information in Table 1.

Fig. 2

The percentage of total water and sediment flux taking place under each percentile flow rate in Flume 4.

Fig. 3

Flow, rainfall and SSC time series for Flume 4 from the 23/12/2012 to the 31/12/2012.

Fig. 4

Rainfall, soil moisture, flow, sediment flux and SSC time series for Flume 10.

Fig. 5

Relationships between flow and SSC during winter – spring 2012–2013 and 2013–2014.

Fig. 6

The relationship between SSC and flow before and after ploughing in Flume 8.

Fig. 7

The relationships between total water (a) or sediment (b) flux and catchment area.

Fig. 8

The relationship between the total sediment flux and mean SSC for each flume.

Fig. 9

The relationships between the total water yield and percentage of rainfall delivered to the catchment outlets (a) and the total specific sediment yield and mean SSC sampled for the flume catchments (b).