Where re-wetting is considered, the aim is to combine water control and appropriate land management to produce the desired wetland habitat, usually wet grassland.
Several wetland birds of conservation concern, notably wading birds, breed on farmland.
They may nest in wet grassland, in-bye pasture or marshy areas. Drainage and agricultural improvement of grassland to provide better grazing and forage has, however, greatly reduced suitable areas for feeding and nesting birds.
The wetness of an area of land through the year is the result of the interactions between rainfall, summer moisture loss and the soil conditions.
- Targeting certain fields or areas to provide particular feeding or nesting habitat can provide considerable wildlife benefit.
- Management to boost the availability of seeds and insects will be particularly valuable.
Benefits to wildlife
By raising water levels, or re-wetting grassland habitats on a farm, ideal feeding and nesting conditions may be created for wading birds such as lapwings, redshanks, snipe and curlews.
Waders have specific requirements for nesting and feeding and their chicks require high protein invertebrate food during the breeding season. Suitable habitat may also be created for yellow wagtails and reed buntings, while ditches may attract feeding water rails and grey herons.
Re-wetting also provides valuable feeding habitat for other farmland birds. A rich supply of insects will help species such as the tree sparrow, which rely heavily on insect food for their chicks. Appropriate management is essential to benefit any particular bird species.
What do different species require?
Nesting requirements: Tussocky, damp grassland or heathland
Feeding requirements: Pastures, damp fields, particularly with wet flushes
Nesting requirements: Short grass (0-12 cm) with some tussocks, spring tillage or bare ground
Feeding requirements: Short vegetation and wet mud in damp grassland and water margins
Nesting requirements: Short (5-15 cm) damp grassland with tussocks, close to standing water
Feeding requirements: Damp grassland, marginal vegetation, mud and shallow pools
Nesting requirements: Wet pastures and boggy heaths with a tussocky sward of 10-30 cm
Feeding requirements: Soft damp ground or shallow muddy-bottomed pools, close to cover
Nesting requirements: Damp meadows or cereal fields
Feeding requirements: Insects from grazed pasture and short, sparse marginal vegetation around pools
Nesting requirements: Trees, hedges or scrub
Feeding requirements: Invertebrates, especially earthworms and snails in damp grassland, and, in autumn, fruit
Nesting requirements: Dense ground cover, including tall, marshy vegetation
Feeding requirements: Mainly insects picked from the ground or from vegetation
Nesting requirements: Tall, marshy grassland, ditch edges, crops and set aside. Occasionally in hedges
Feeding requirements: Insects and weed seeds
Understanding your land
Many farms will have had patches of wet grassland in the past, but most will since have been drained. Where re-wetting is proposed, the area targeted should generally have been wetter in the past.
Knowledge of the past management history, notably drainage, will be valuable and in some cases, simply reversing or controlling the outflow of water from a drainage ditch will bring the desired result.
Knowledge of habitats and species present in the locality, both past and present, should be used to inform decisions on the desired end-result.
Understanding how water will behave in a particular soil type and the rate at which it will move through the soil is important in any re-wetting proposal.
Water moves faster through soils composed of large particles - sand or peat - and well-structured soils such as clays which have not been ploughed. Water moves more slowly through soils composed of small particles - clay or silt - particularly where the soil structure is poor. Some clays may be virtually impermeable.
Assessing the soils within the project area is desirable. This may be achieved by digging a pit (12 spade widths by 12 spade depths) and looking for the different layers of soil present, the structure and composition of the soil in each, and the presence of the water table. This should be repeated at several locations to assess variability of the soils.
A basic requirement for a re-wetting project is a field with the right topography to hold water. On a large scale, a topographic survey measures the height of ground level, water level and water control structures, thus providing information on slopes, flows and likely areas of flood.
The simpler method of recording a winter puddle map, perhaps assisted by the placing of a few sandbags in a ditch to allow water to back-up, will allow the water to do the levelling work.
On a smaller scale, variation within a field can affect the plant community and water regime in directly adjacent areas: for example, within ridge and furrow grassland, or on land with a complex pattern of rills. Such variability, best judged by eye, is extremely important in achieving ideal conditions for wet grassland species.
Managing water levels
- Re-wetting may be undertaken in a range of soil and hydrological conditions and the simplest schemes are often no more complicated than reversing or reducing the drainage function in a particular area.
- In many areas with a wetter climate, the rainfall is sufficient to keep sites wet into June, and reducing the rate of run-off is sufficient. Assess the soils and drainage patterns for the area and, if necessary, block any drains that take water away or redirect others to drain into it.
- In drier locations, it will be necessary to direct water to a chosen location and/or install water control structures in reduce losses.
- Water control structures, such as sluices, enable the ability to control the inflow or outflow of water and thus the water level in the field.
- The provision of shallow water and muddy margins are important to feeding waders and without control the grassland may dry out too soon in early dry weather, while a wet spring may result in levels remaining too high.
- Restoration of wetland habitats usually depends on raising water levels, so the availability of water is a crucial factor.
- Water input will depend on rainfall, surface water (river/stream flows) and groundwater levels. Water loss will depend on evapo-transpiration (combined loss from both vegetation and open water surface) and seepage (groundwater seepage, drainage or runoff).
- Groundwater sources may be utilised through the inflow of water from springs or seepages or by installing water control structures on outflows to retain the groundwater height. Surface water can be re-directed from ditches or streams to the desired location.
- Consider any likely impacts created up-stream by blocking or diverting drainage and consult with the necessary statutory agency for further advice, for example The Environment Agency in England and Wales.
- The most cost effective sluice is likely to be constructed with a length of plastic piping, either rigid pipe with a swivel end or flexipipe, laid through an earth dam in the outflow ditch. Each end extends beyond the dam, and the upstream end is held at the desired level. Flexipipe will normally need weighting to keep the lip submerged and require a length of rope to hold the upstream end at the desired level. Adjusting the upstream end (by swivelling the pipe or raising or lowering the rope) will set the desired water levels.
The water regime
- Maintain a high water table from December to March with splashy conditions over the whole field and/or shallow flooding from one to 30cm over up to 30 per cent of the field. Allow water levels to fluctuate to avoid stagnation.
- Retain a high, but reduced, water table from March to May, over 30 per cent of the field and/or shallow flooding on five to 10 per cent of the field.
- From May to July, retain the water table within 20cm of field level on average, shallow pools will dry out, with muddy edges.
- From July, reduce the water table to at least 40cm below field level to enable management to be undertaken.
How to manage water levels to benefit birds and other wildlife. PDF, 128KbManaging Water Levels To Benefit Birds Advisory Sheet (England)
Examples of water management structures or techniques on a variety of wetland sites to further nature conservation objectives. PDF, 1.0MbWater Management Structures For Conservation
Case studies to help understand, identify and modify lowland agricultural land drainage systems for wetland conservation. PDF, 1.3MbLowland agricultural and drainage systems - function and modification for wetland conservation
How to manage water levels to benefit birds and other wildlife. PDF, 283KbManaging water levels to benefit birds advisory sheet (Scotland)
How to create and manage scrapes to benefit wading birds. PDF, 794KbScrape creation for waders advisory sheet (Scotland)
Best practice considerations for reversion of arable land to wet grassland for breeding waders. PDF, 85KbArable Reversion
Best practice design, creation and management of wader scrapes. PDF, 159KbWader scrapes
How to create and manage scrapes to benefit wildlife. PDF, 1.2KbScrape creation for wildlife
Best practice techniques for the re-wetting of grassland for breeding waders. PDF, 108KbRe-wetting grassland to benefit birds
Wet grasslands are the products of agricultural management, usually by grazing, mowing or haying.
Grazing with livestock, usually cattle, at a moderate intensity is ideal as it creates a mosaic of tussocks and short turf used for nesting by a range of wader species. It also augments the invertebrate population through dunging.
To avoid trampling of nests, maintain light grazing between mid-March and June. Although stocking rates generally in the region of one cow per hectare are recommended, it is best to use species objectives rather than set prescriptions to create the correct habitat for target species.
Heavy grazing from late summer onwards will restore the required sward heights for the following year.
If grazing is not possible, cutting will be essential and should be timed for suitable dry periods after the end of the breeding season, usually between August and October. Expect to cut at least twice per year if grazing is not possible.
Small areas of the wettest land, or some of the ditch margins, may be left to develop taller swamp vegetation of sedge or reed. This will benefit birds such as reed buntings and water rails.
A potential problem associated with damp grassland is rush encroachment. Find out more about Rush management.