Is landscape organisation preventing excessive sediment losses from agricultural land? Sherriff, S.1,2, Rowan, J.S.2, Melland, A.R.3, Fenton, O.1, Murphy, P.N.C.3, Jordan, P.4, Ó hUallacháin, D.1 1
Teagasc, Johnstown Castle, Wexford, Ireland 2 University of Dundee, School of the Environment, Dundee, UK 3 Agricultural Catchments Programme, Teagasc, Johnstown Castle, Wexford, Ireland 4 University of Ulster, Environmental Sciences Research Institute, Coleraine, Northern Ireland
Background
Introduction
Intensive agriculture can contribute large amounts of sediment to watercourses, leading to degradation of water quality and aquatic ecosystems. Excessive delivery of sediments to river systems can cause ecological degradation due to infilling of spawning gravels and reduced light penetration. Farming practices, where dominant in a river catchment, can therefore directly impact the likelihood of meeting Water Framework Directive targets for ecologically and chemically “good” water status by 2015. Targeted management of soil loss through erosion of bare or unstable soils and their subsequent transportation into watercourses is paramount. Connectivity of the land surface and erosion ‘hot spots’ will alter according to catchment conditions; soil type, rainfall, extent of artificial drainage. Additionally, land-use practices will vary widely according to farming systems (arable/grass-based) and by uptake of environmental schemes by farmers e.g. the Nitrate Action Plan (NAP) or agri-environment schemes. A lack of empirical sediment data currently exist in Ireland, where 64% of the country is utilised for agriculture. Relatively, sediment yield is lower in Ireland than in other European countries with similar levels of production. Under Food Harvest 2020, Irish farming is set to increase productivity by 33%, with a 50% targeted increase in milk output. The environmental implications of this is unknown and, keen to maintain improvements in environmental characteristics, efficient management of agricultural inputs to water and protection of the soil resource is desirable.
Landscape complexity is a metric of the variability, frequency or density of features such as hedgerows, roads, track, drains and land-use type in a river catchment. In intensive agricultural catchments, larger field sizes, decreased hedgerow densities and homogenous land-use may result in the increased vulnerability of soil to erosion. This is due to increased cultivation resulting in greater proportions of bare soils and reduced fallow periods. Hydrological transport pathways would additionally be accelerated whereby artificial ditch and drain networks will efficiently transport sediment from fields into watercourses. Protection of landscape complexity could therefore provide a mitigation technique to increase landscape resilience to soil erosion as opposed to response based reestablishment techniques such as riparian buffers and re-plantation of hedgerows. The combination of sediment protection and sediment acceleration features that occur in catchments will alter according to specific intra- and inter- catchment conditions. Analysis of sediment flux according to contributing catchment complexity is considered in this study in three intensive agricultural catchments.
Figure 1: Location of three monitoring catchments (in red) Printed under License No. 6155 from the Ordnance Survey Ireland
Complexity data Inter-catchment complexity Land use specific field size is shown to vary greatly between catchments. Castledockrell has the average largest arable and average largest total field sizes compared to the other catchments. There is a distinct difference here however between the average arable and grassland fields, which is reflected to a lesser degree in the Ballycanew and Dunleer catchments. For sediment generation, this may therefore reflect a greater vulnerability of Castledockrell soils to sediment erosion, however the well-drained nature of the soils could be increasing soil resilience.
Intra-catchment complexity Road and hedgerow densities are hypothesised to have opposite impacts on sediment generation/transport. Roads may create an additional transportation network of accelerated hydrological pathways, therefore increasing sediment supply to the stream. Hedgerow densities may conversely represent the binding of soil by plant roots therefore a soil protection feature. Results show these vary greatly within a catchment and therefore the partitioning of features such as these may be critical for understanding the dynamics of sediments in agricultural catchments.
Methods Complexity data are provided by the Agricultural Catchments Programme (ACP) database, providing field resolution data in ArcGIS. Information from the database were developed into a complexity dataset and additional computed metrics were included. Instream sediment sampler locations determined eighteen sub-catchments which were used to clip the spatial database. Sediment data were collected using time-integrated sediment samplers ( Figure 2). The devices represent a small proportion of the cross-section, constantly collecting sediment suspended in the river flow. Inside the sampler, sediments settle due to decreases in water velocity due to the increased cross-section size inside between the inlet (4 mm) and sampler body (98 mm ID). Samplers were emptied every six to eight weeks and the contents weighed. Sediment mass was converted into flux using discharge data. Stage monitoring occurred at three positions in each catchment (outlet, mid-catchment and upper-catchment). Event data were selected for comparisons reflecting hydro-sedimentological Figure 2: Time-integrated suspended sediment samplers responses.
Preliminary sediment results Preliminary sediment results show proportional outputs from each sub-catchment site in Ballycanew (figure 4). Generally, scale dependency may lead us to expect an increase in sediment flux with sub-catchment size, however two sub-catchments contradict the expected trend. Sub-catchments one and five show larger and smaller fluxes than expected respectively. When referring to complexity features, sub-catchment one has the largest % arable land-use, and lowest hedgerow density. Sub-catchment five features the lowest % arable and highest hedgerow densities. Figure 5: Preliminary sub-catchment sediment flux data from Ballycanew
Summary
Figure 3: Variation in land-use specific field size in the three study catchments
Figure 4: Variation in road and hedgerow density for Ballycanew subcatchments. Sub-catchments are displayed in order of size.
Better understanding of sediment dynamics in agricultural systems will develop targeted and cost-effective management techniques. Such management techniques which concentrate on prevention of soil and water degradation may be more popular in future years for the sustainable intensification of agricultural ecosystems. Complexity is one such management technique that will encourage soil resilience to erosion and therefore decrease the vulnerability of the soil system to benefit good food production and protect water quality for the future.
Contact details: Teagasc, Johnstown Castle, Wexford, Ireland. Email: sophie.sherriff@teagasc.ie Tel: +353 (0) 53 9171333 Mobile: +353 (0) 85 2705252 University of Dundee, School of the Environment, Dundee. Email: s.c.sherriff@dundee.ac.uk Mobile: +44 (0) 7730 160660