The development of GIS based
methodology for the
identification of potential wet
grassland restoration sites
Dr Andrew Williams
Dr Francien van Soest
Dr Rob Parkinson
Dr Martin Kent
Ben Meredith
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1. Introduction and Aims
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.
START
HOST (hydrology of soil types)Classification 9 or 24
Slope angle <= 4o
Upslope Contributing Area > 1 ha
Curvature = flat/concave
Topographic Index > 8
SUITABILITY
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.
Aim: to test the DSS model using readily available data sets for the entire Culm
Natural Area.
2
Figure 2. Location map showing the Culm Natural Area, Carey, Wolf,
Thrushel and North Lew catchments. Source: NSRI National Soil Map, Devon
Wildlife Trust (1998).
3
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.
2.2 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.
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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
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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).
70%
60%
50%
40%
30%
20%
10%
0%
culm soils %
r
at
e
w
1f
1b
72
71
71
2d
/e
71
3b
1c
61
1b
56
42
54
1b
CNA soils%
1h
% Distribution
Distribution of soils within the CNA and present culm sites
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.
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Slope distribution within culm grassland sites and the
CNA
Distribution %
25
20
15
CNA
10
Culm
5
20+
19-20
18-19
17-18
16-17
15-16
14-15
13-14
12-13
11-12
10-11
9-10
8-9
7-8
6-7
5-6
4-5
3-4
2-3
1-2
0-1
0
Slope (degrees)
Figure 5. Showing the distribution of slope under existing culm grassland and the
entire Culm Natural Area.
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 5,000m2 (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
5,000m2. 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 (m2) and b= slope.
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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).
Landcover
Arable
Woodland deciduous + scrub
Conifers
Improved grassland
Semi-natural grassland
Urban
Dwarf shrub
Acid grassland molinia + festuca
Acid grassland rough + juncus
Bare ground
Water
Number of Cells
10762
15709
168
4459
5743
2040
517
1599
86
74
77
8
Percentage (%)
26.10
38.10
0.41
10.81
13.93
4.95
1.25
3.88
0.21
0.18
0.19
Table 1. Showing the land uses assigned to culm grassland by the Land Cover Map
2000. 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.
Start
Soil Association
712d/e, 713, 721b
541h
Slope Angle
<4o
<6o
Contributing Area
>5000m2
Suitability Grade
Figure 6. Decision Support System for the Culm Natural Area
Variable
Soil
Slope
Contributing
Area
Threshold
712d/e & 713b,
721b.
541h
Other
Weight
2
1
0
<4˚
<6˚
>6˚
2
1
0
<5000m^2
0
9
>5000m^2
1
Table 2. Selection limits and weights for soil type, slope and hydrology
4.2 Use of the suitability 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 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
Figure 7. Map showing the suitability of land for the
restoration/recreation of culm grassland within the CNA. © Crown
Copyright Ordnance Survey. An EDINA Digimap/JISC supplied service. NSRI
National Soil Map.
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4.2 Use of the suitability map
The suitability map 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 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.
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Plates 2 – 9 Selected sites visited to verify the DSS
6. Conclusions
1.
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.
2.
The requirements for an area to be deemed as suitable for culm
grassland restoration (suitability classes 4 and 5) must include:-
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.
A slope angle below 6o is needed to provide the correct hydrological
conditions. Ideally the slope should be below 4o however if the area
has slowly permeable gley soils and displays a large contributing area
a slope angle below 6o is acceptable.
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The contributing area for a culm grassland site must be in excess of
5000m2. 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.
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