Research Themes The development of GIS based methodology for the identification of potential wet grassland restoration sites

Transcription

1 Catchment and Coastal Environments Research Group Research Themes The development of GIS based methodology for the identification of potential wet grassland restoration sites Dr Andrew Williams Partners Dr Andrew Williams Dr Francien van Souest Dr Rob Parkinson Dr Martin Kent Ben Meredith Introduction Van Soest (2002) established that the use of a decision support system (DSS) (Figure 1) based on a geographic information system (GIS) could be used to identify suitable areas for culm grassland restoration and/or recreation. Figure 1. Decision Support System by Van Soest (2002). The pilot study focused on the Wolf, Thrushel, Carey and North Lew river catchments in the south of the culm natural area (Figure 2) and high resolution data were used compared with what was available for the culm natural area in general (3500km2). The method applied by Van Soest (2002) required testing at a larger, regional level using data which would be easily and or cheaply obtained by conservation organisations. 1

2 Figure 2. Location map showing the Culm Natural Area, Carey, Wolf, Thrushel and North Lew catchments. Sourc: NSRI National Soil Map, Devon Wildlife Trust (1998) 2. Methodology 2.1 Culm Natural Area and culm grassland sites The Culm natural area as defined by English Nature (Hughes and Tonkin, 1997) formed the focus of study. The Culm areas were defined by the culm grassland inventory data compiled by Devon Wildlife Trust (1998). Figure 3. Digital Elevation Model of the CNA showing the 500m buffer of the coastline. Crown Copyright Ordnance Survey. An EDINA Digimap/JISC supplied service. 2

3 2.2 Elevation and Hydrology The digital elevation data for the culm natural area was obtained from Edina s digimap online resource (a service provided by Ordinance Survey and Edinburgh University), the 1:50,000 Landform Panorama Contour map was used in the original study. For the extension of the DSS to cover a wider area it was decided that the digital elevation data used should be of increased precision as computer power would now facilitate such amounts of data and would provide amore accurate model. The 1:10,000 Landform Profile data was therefore selected and transferred as digital elevation data into ArcGIS 9 and mapped using a 10m x 10m grid/cell size. The DEM was first made hydrologically correct by filling in any sinks within the elevation data. A sink is defined as a cell which is surrounded by cells of a greater elevation, this would therefore trap water and any flow calculations done by the GIS would be affected (see glossary). No user defined value was applied and the computer generated correction was accepted. To investigate the hydrology of the culm natural area, flow direction and flow accumulation (contributing area) layers were calculated using arc toolbox functions. Flow direction gives a raster layer showing the direction in which water will flow from each cell. In the flow accumulation function cells are assigned with values that represent the number of cells that provide them with water. 2.3 Soils Soil data from the National Soil Research Institute was used to investigate the relationship between soil type and culm grassland. The data set selected was the National Soil Map (Natmap). This resource is available from the NSRI, the data set can be purchased by conservation agencies and its digital format allowed accurate incorporation into the DSS. For this project English Natures NATMAP data was used under an extended license agreement with NSRI. 3.0 Analysis The relationship between the presence of culm grassland, soil type, slope and contributing area were all investigated within the culm natural area to devise appropriate groups of values for inclusion/exclusion in the suitability grading of the DSS. Chi square tests were carried out on the soil, slope and contributing area data to test the hypothesis that their distributions within areas of culm grassland had occurred by chance. These relationships were then utilised to create a decision support system in the form of a suitability map of sites for restoration and recreation covering the culm natural area and taking account of hydrology, topography and soils. 3.1 Soils From Figure 4 it is clear that the majority of culm grassland occurs on the Hallsworth I/II association (712d/e), a gley soil consisting of slowly permeable clays. The Sportsmans soil association (713b) and Onecote soils (721b) are other gley soils supporting culm grassland, these also experience seasonal water logging but have a clay and loam texture rather than the clay dominated Hallsworths. While the Neath (541h) association is mostly free draining there are local areas of the soil which exhibit a slowly permeable horizon and hence experience seasonal water logging (Soil Survey of England and Wales, 1983). 3

4 Distribution % % Distribution Distribution of soils within the CNA and present culm sites 70% 60% 50% 40% 30% 20% 10% 0% culm soils % CNA soils% 541h 421b 561b 611c 712d/e 713b 711f 721b water Soil Association Figure 4. Graph showing the distribution of soils in the culm natural area and under existing culm grassland. 541h Neath, 421b Halstow, 561b Teme, 611c Manod, 712d/e HallsworthI/II 713b Sportsmans, 711f WickhamII, 721b Onecote. 3.2 Slope Groups of slopes were analysed to investigate the relationship between culm grassland and slope in the CNA. Culm areas were then clipped from this layer. While slopes between 0 and 4 accounted for 67.7% of the culm sites there was a further 18.7% of culm grassland sites found on slopes between 4 and 6 (Figure 5). The chi square test statistic for the slope distributions was significant at the p<0.001 level proving that the distribution of culm across the slopes in the CNA was not random. Slope distribution within culm grassland sites and the CNA CNA Culm 5 0 Slope (degrees) Figure 5. Showing the distribution of slope under existing culm grassland and the entire Culm Natural Area. 4

5 3.3 Contributing Area The flow accumulation to all of the cells within each culm area was compared to the values of the contributing area from all cells in the CNA using the methods described above. It is evident that 45.5% of the land that currently supports culm grassland has areas with a contributing area of over 5,000m 2 (0.5 Ha). While contributing area is an important factor for wetland existence the GIS model suggests that over 50% of the culm sites receive water from an area no bigger than 5,000m 2. On close inspection of the GIS images it is apparent that most culm grassland sites contain many different values for contributing area, especially if they are adjacent to a water course. 3.4 Topographic Index This index provides a value for the likelihood of saturation at a point within a river catchment. The relationship between slope and contributing area was calculated: Topographic index: ln (a/tan b) Where a= contributing area (m 2 ) and b= slope. Van Soest (2002) found that a value of >8 was needed to support culm grassland. However, because such a huge area of land was above this topographic index its use in selecting an area for culm restoration/recreation was limited. 3.7 Land cover Plate 1 Typical wet grassland found in the Culm Natural Area Data on land cover for the CNA is available as the Land Cover Map 2000 data set created through interpretation of satellite imagery. It defines over 50 land cover types with 45 of them present in the CNA. Following the same clipping procedure as the soils and slope analyses the areas of each and the incidence of culm grassland on the cover types was investigated. It was found that culm grassland occurred on many land cover types including woodland, arable and more suitable acid grasslands (bogs) (see Table 1). 5

6 Landcover Number of Cells Percentage (%) Arable Woodland deciduous + scrub Conifers Improved grassland Semi-natural grassland Urban Dwarf shrub Acid grassland molinia + festuca Acid grassland rough + juncus Bare ground Water Table 1. Showing the land uses assigned to culm grassland by the Land Cover Map Source: Land Cover 2000 National Environmental Research Centre. 4. Decision Support System and Suitability Map 4.1 Creation of the suitability map Soil, slope and contributing area were selected for incorporation in the decision support system as they were the most important variables studied. It was decided that the weightings in table 5 would be used to create a grading between 0 and 5 where 0 is unsuitable and 5 is the most suitable grade for culm restoration. Figure 6. Decision Support System for the Culm Natural Area 6

7 Variable Threshold Weight Soil 712d/e & 713b, 721b. 2 Table 2. Selection limits and weights for soil type, slope and hydrology 541h 1 Other 0 Slope <4 2 <6 1 >6 0 Contributing Area <5000m^2 0 >5000m^ Use of the suitability map Figure 7. Map showing the suitability of land for the restoration/recreatio n of culm grassland within the CNA. Crown Copyright Ordnance Survey. An EDINA Digimap/JISC supplied service. NSRI National Soil Map. The suitability map (Figure 7) should only be used as an indication of areas for culm restoration or recreation. Once an area is identified from the map the soils map should be referenced to confirm the soil association (as the grading scores hide this data), reference should also be made to an Ordinance Survey or similar map to confirm the present land cover. Once these factors have been investigated a site visit should follow to look into the 7

8 grazing pressure, plant communities, surrounding land use (including human drainage) and soil conditions (hydrology, permeability, ph and chemical composition). Surrounding land use may be one of the most important factors for consideration Careful field investigation is required to assess the suitability of the area selected in terms of the hydrology, soils, topography, surrounding land uses, soil chemistry and grazing pressures. 5. Field Verification To verify the validity of the decision support system field testing was required. Areas of the suitability map representing a range of suitability grades were selected that were owned by well known landowners. Field visits were then conducted to the sites shown in Plates A to I. Plates 2 9 Selected sites visited to verify the DSS 8

9 6. Conclusions The differences in data resolutions were investigated and it was found that large scale data such as the 1:250,000 soil data could be used successfully to create a decision support system for culm restoration and recreation in the Culm Natural Area. The requirements for an area to be deemed as suitable for culm grassland restoration (suitability classes 4 and 5) must include:- 1. A soil with a slowly permeable horizon, the Hallsworth I&II (712d/e), Sportsmans (713b), Onecote (721b) and water logged areas of the Neath (541h) soil associations. 2. A slope angle below 6 o is needed to provide the correct hydrological conditions. Ideally the slope should be below 4 o however if the area has slowly permeable gley soils and displays a large contributing area a slope angle below 6 o is acceptable. 3. The contributing area for a culm grassland site must be in excess of 5000m 2. Culm grassland will be more successful with a large contributing area. The size of this contributing area may also determine the size limits of the culm grassland however more work is needed to prove this relationship. 9