SPATIAL DEPENDENCY OF BURULI ULCER ON POTENTIAL … · Buruli Ulcer (BU) is an endemic prevalent...
Transcript of SPATIAL DEPENDENCY OF BURULI ULCER ON POTENTIAL … · Buruli Ulcer (BU) is an endemic prevalent...
Jan. 2014. Vol. 3, No.5 ISSN 2307-2083 International Journal of Research In Medical and Health Sciences © 2013-2014 IJRMHS & K.A.J. All rights reserved http://www.ijsk.org/ijrmhs.html
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SPATIAL DEPENDENCY OF BURULI ULCER ON POTENTIAL
SURFACE RUNOFF AND POTENTIAL MAXIMUM SOIL WATER
RETENTION
Saviour Mantey
1, Richard K. Amankwah
2 and Alfred Allan Duker
3
1Department of Geomatic Engineering, University of Mines and Technology, Tarkwa, Ghana 2Department of Mineral Engineering, University of Mines and Technology, Tarkwa, Ghana
3Department of Geomatic Engineering, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
Email addresses: [email protected],
ABSTRACT
Buruli Ulcer (BU) is an endemic prevalent disease in Ghana and other West African countries including; Cote
d´Ivoire, Benin and Togo. Despite recent upsurge of research in Buruli Ulcer, the natural reservoir and mode of
transmission of Mycobacterium ulcerans (MU) have not yet been determined. However, all major foci are found
in wetlands of tropical and subtropical countries. In this study, a landscape spatial hydrological modeling
approach based on Potential Maximum Soil Water Retention (PMSWR), Soil Conservation Service Curve
Number Grid (SCS CNGrid) and empirical evidence from field research were applied to understand their
relationship with BU disease in two districts of Ghana.
Landuse data, Hydrological Soil Groups (HSGs), Landsat images and Digital Elevation Models (DEMs) were
used to generate the SCS CNGrid and the PMSWR of the study areas. The results of the SCS CNGrid and
PMSWR maps linked BU endemic areas to low to moderate surface runoff potential and high to moderate
PMSWR. BU endemic communities in the two districts were also mostly enclaved by galamsey (illegal) mining
activities and farms. This study proved that the PMSWR and SCS CNGrid values are important hydrological
parameters to determine surface runoff potential and thus delineate BU disease prone areas.
Key words: BU, SCS CNGrid, PMSWR, Potential Surface Runoff, DEM, Ghana
INTRODUCTION
BU is an endemic disease which destroys the skin,
underlying tissues, muscles and bones when not
treated early. It is caused by MU. The disease has
been known in endemic communities in Ghana for
years with an overall prevalence rate of 20.7 per
100 000 [1]. But in recent years, cases have
increased, prompting renewed interest in BU
research especially in parts of West Africa (Cote
d´Ivoire, Ghana, Guinea, Liberia, Benin, Burkina
Faso and Togo) [2-7]. Despite the renewed research
interest in BU disease, the natural reservoir and
mode of transmission of MU is still unclear.
However, research has pointed to wetlands of
tropical and subtropical areas as potential reservoirs
of the MU [3, 7-13]. It is also widely believed that
BU disease mostly occurs in people who live and
work close to stagnant bodies of water [14].
Additionally, research has shown that new cases of
BU have been reported after flood events [5, 7, 13,
15-18]. For example, Bainsdale in Australia,
recorded the first case of BU in 1939 after the worst
floods in 1935 [19-21]. BU cases were also
reported after flood events along the settlements of
Sepik and Kumusi Rivers in Papua New Guinea in
1957 [22]. This flooding was in concert with the
explosive eruption of Mt. Lamington in 1951 [23,
24].
Case and control studies have linked landscape
disturbances to BU disease [16, 25]. Furthermore,
surface runoff from landscape disturbances also
lead to solubilisation of exposed reactive minerals
causing pollution of rivers, streams and soils by
sulphuric acid and harmful elements such as
arsenic, cadmium and mercury [26, 27].
Pollution of rivers, streams and soils usually occurs
when mined materials from uncontrolled and
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unscientific mining (e.g. galamsey) activities are
exposed to oxygen and water [28]. For example,
spatial analysis performed by [9], detected that
some 250 cases of BU were within a 1000 m buffer
zone around mine sites, compared to 62 cases
elsewhere within his study area. Likewise, Kumi-
Boateng et al. [29], observed that improper mining
methods by galamsey operators and lack of post
mining treatment and management further alters the
already fragile ecosystem. The gravity of the
impact, nonetheless, depends on the techniques
used and the size of the Mine [30]. A study by
Salvarredy-Aranguren et al. [31], in the Milluni
Valley, Bolivia, further revealed that potentially
harmful elements (e.g. Cd, Zn, As, Cu, Ni, Pb and
Sn) exceeded the recommended World Health
Organization (WHO), limits in surface waters
downstream of galamsey mining sites.
Duker et al. [9] also linked arsenic levels in soil and
illegal gold mining activities with increased BU
disease risk in Ghana.
Thus, landscape alteration, can lead to increased
surface runoff and contamination of surface waters
[32-35]. Landscape alteration may also increase the
risk of flooding [36-39] as a result of increased
surface runoff and subsequently induce the
outbreak of BU disease due to contamination of
surface waters [9].
Surface runoff, however, varies depending on the
nature of the soil and its management [40]. Surface
runoff and PMSWR are undoubtedly important to
the understanding of BU prevalence. Though, a
number of surface runoff models have been
developed, the SCS CNGrid method stands-out,
produces better results and is well acclaimed [41-
47]. The SCS CNGrid considers major surface
runoff watershed characteristics such as soil type,
landuse, digital elevation models and antecedent
moisture conditions (AMCs) to estimate the loss
and runoff volume [41, 48, 49], which can be
obtained from optical and microwave remotely
sensed data.
The fundamental premise of the SCS CNGrid
method is that, for a single storm, the ratio of the
actual soil retention after runoff begins to the
PMSWR is equal to the ratio of direct runoff to
available rainfall. This relationship is expressed as
[41]
{ ( )
.………………….. (1)
where is the actual direct runoff (mm); P is total
precipitation (mm); is PMSWR after runoff
begins (mm); is initial abstraction (mm) [41]. To
simplify equation one (1), Ia is fixed at = 0.2S
[41]. Thus, equation one (1) can be expressed as:
{ ( )
………………… (2)
The SCS CNGrid is then related to the PMSWR as
(
) ..…..……… (3) [41]
SCS CNGrid can be found in the Technical Release
55 (TR-55) Table or can be calculated as the
composite CNGrid [41]. Equation three (3) shows
that the CNGrid decreases as the PMSWR (S)
increases.
The objective of this research therefore is to use the
SCS CNGrid model, which thoroughly considers
physiographic heterogeneity (e.g. soil, landuse and
digital elevation models) and empirical evidence
from field research to determine the relationship
between surface runoff potential, PMSWR and BU
disease.
MATERIALS AND METHODS
Study areas: The study areas are Amansie West
District (AWD) and Upper Denkyira West District
(UDWD). These study areas were selected due to
their similarities in socio-economic activities (e.g.
illegal “galamsey” mining and farming activities)
and BU prevalence.
The AWD lies between latitudes 6°07' N and 6°35'
N and longitudes 1°42' W and 2°08' W (Fig. 1). The
district is bounded by UDWD, Amansie Central
district, Bibiani-Anwiaso-Bekwai district, Atwima
Mponua district, Atwima Nwabiagya district,
Atwima Kwanwoma district and Bekwai
Municipal. The district is a tropical rain forest area
of about 1 320 km2 with an estimated population of
134 331 (2010 Population and Housing Census). It
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Fig. 1: Map of Amansie West District
Fig. 2: Map of Upper Denkyira West District
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is approximately 60 km southwest of Kumasi, the
regional capital of Ashanti. AWD has the highest
BU prevalence rate of 150.8 per 100 000 in the
country [50-52] and the second highest reported
active cases across the country [52]. The main
occupation of the people is subsistence farming and
small scale “galamsey” mining. The AWD is
drained by the Offin and Oda rivers and their
tributaries (Fig. 1).
UDWD is the second study area and lies between
latitudes 5° 54' N and 6° 18' N and longitudes 1° 49'
W and 2° 12' W of the Greenwich Meridian (Fig.
2). The district is bounded to the south-east by
Upper Denkyira East district, Amansie Central
district and Obuasi Municipal. Bounded to the
south-west are Wassa Amenfi East and Amansie
West districts. The district is also bounded to the
north-west by Bibiani-Anwiaso-Bekwai and Wassa
Amenfi West districts. UDWD covers an area of
about 1 700 km2 with an estimated population of
about 132 864 (2010 Population and Housing
Census). While the Central Region has the highest
overall prevalence rate of active cases [52], UDWD
is among the most endemic districts in the Region
and has the third highest prevalence rate of 114.7
per 100 000 nationwide together with Upper
Denkyira East District after Asante Akim North
district (prevalence rate of 131.5 per 100 000) and
Amansie West district (prevalence rate of 150.8 per
100 000) [52]. Subsistence farming and small scale
“galamsey” mining are the main occupation of the
people in the district. UDWD is drained by Offin,
Dia and Subin rivers as well as their tributaries
(Fig. 2).
Data Acquisition: Data for the study include;
DEMs, Epidemiological data of BU for the two
districts, Landuse and Soil data for the two study
areas. Landsat images (path 194/rows 056 and 057)
were acquired on January 01, 1991, February 01,
2008 and May 31, 2013 for this study. The DEMs
were obtained from the Advanced Space borne
Thermal Emission and Reflection Radiometer
Global Digital Elevation Models (ASTER-GDEM).
The Epidemiological data was obtained from the
National Buruli Ulcer Control Programme and from
field investigation based on clinical case definition
of WHO [1, 53]. The Landuse data was acquired
from the Centre for Remote Sensing and GIS
(CERSGIS) at the University of Ghana. The Soil
data was also procured from the Soil Research
Institute of the Council for Scientific and Industrial
Research (CSIR-SRI). The Landsat images were
made available by the Earth Resources Observation
Systems Data Centre, United States Geological
Survey (USGS) and were rectified to the Universal
Transverse Mercator projection system.
Methods Used: BU disease prone areas were
delineated from the study areas using SCS CNGrid
and PMSWR maps. The procedure adopted to
achieve this objective is summarized in the flow
chart as shown in Fig. 3.
Fig. 3: Flow chart for Generating SCS CNGrid
and PMSWR
Selecting Soil, Impervious Surface and
Vegetation (S-I-V) Fractions
Composite CNGrid values were computed from S-
I-V fraction which was derived from normalized
spectral mixture analysis (NSMA) and linear
spectral mixture analysis (LSMA). The NSMA was
applied to estimate and normalize the percentage of
impervious surfaces by modelling a mixed
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spectrum as a linear combination of three end-
members; soil, impervious surface and vegetation.
The normalization process of NSMA was used to
reduce the within-class radiometric variations and
therefore facilitate the separation of S-I-V land
cover types. Two key steps were employed to
perform the NSMA. Firstly, the original Landsat
TM/ETM+ images were normalized to reduce
within-class radiometric variations using equation
four (4) [54].
……………… (4)
where
∑ and is the normalized
reflectance for band in a pixel, is the original
reflectance for band , is the most probable
reflectance for that pixel and n is the total number
of bands. Secondly, LSMA was applied to calculate
the fractions of the three end-members within the
normalized spectra. Clusters of each of the end-
members were identified by visual interpretation of
the Landsat images and the feature space images of
a principal component transformation. The fraction
of S-I-V end-members in a pixel was derived with a
least squares method in which the residual, is
minimized (Equation 5):
∑ ……………… (5)
where i = 1,…m (represents number of spectral
bands); = 1,… (represents number of end-
members); Ri is the normalized spectral reflectance
of band i which contains end-members; is the
proportion of end-member within the pixel;
is the known normalized spectral reflectance of
end-member within the pixel on band i; and is the residual for band i. A constrained least-
squares solution is used with the assumption that
the following two requirements are satisfied
concurrently (Equation 6):
∑ ………..(6) [55-60].
Computing Composite CNGrid Values
To calculate the composite CNGrid values, four
steps where used (Fig. 3). The first was to classify
vegetation types based on NDVI values and assign
initial CNGrid values with reference to TR-55
Table. Secondly, the S-I-V fraction images were
extracted from Landsat images of the study areas
using the NSMA and LSMA models. Thirdly,
hydrological soil groups of the study areas were
assigned initial CNGrid values with reference to the
TR-55 Table. Finally, the composite CNGrid is
calculated as the weighted average of the initial
CNGrid values of S-I-V fractions, which were then,
keyed into the Look-Up Table for generating the
final CNGrid and PMSWR maps of the study areas.
Processing DEM and Generating SCS CNGrid
The DEM used to create the SCS CNGrid was
processed by performing “fill sink”. This was
necessary to minimize sinks in the DEM which
were due to errors.
In order to generate the SCS CNGrid, the corrected
DEMs, reclassified landuse and HSGs were
merged. The original landuse data of the study
areas were reclassified into four major landuse
categories namely; water, medium residential,
forest and agricultural. The reclassification was
based on Anderson’s land use classification scheme
[61] and empirical field evidence (groundtruthing).
The reclassification was to normalize the different
categories of the dataset for analyses. The
reclassified landuse data were then converted to
polygons.
The diverse characteristics of the soil data such as
rate of infiltration of the study areas were used to
generate the HSGs for all polygons in the study
areas. The three HSGs identified in the study areas
indicated the amount of infiltration rates as follows
[45]: A-soil with high infiltration rates (lowest
CNGrid values); B-soil with moderate infiltration
rates (moderate runoff values) and C-soil with slow
infiltration rates (high runoff values).
The percentage of each soil group in a particular
polygon was also calculated. After processing the
HSGs, the soil and landuse were then merged to
create polygons. A Look-Up Table was also created
to contain SCS CNGrid for all landuse and soil
groups.
The merged HSGs and the landuse data, was
converted to polygons to represent the merged
HSGs and landuse. Composite SCS CNGrid values
for each reclassified landuse were assigned in the
Look-Up Table [62]. HEC-GeoHMS, a SCS
CNGrid generating tool in ArcGIS was used to
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generate the SCS CNGrid maps using the HSG
map, the landuse map, the corrected DEM and the
Look-Up Table. The PMSWR maps were generated
using equation six (6) from the SCS CNGrid.
(
)……………….(6)
The PMSWR maps are related to the soils and land
cover conditions of the study areas through SCS
CNGrid, which is dependent on landuse, HSGs and
DEMs. The landscape parametres from the SCS
CNGrid and PMSWR maps were extracted using
kriging method of gridding. Kriging method was
chosen because it yielded the best correlation
coefficients for all the data sets.
Hydrological Soil Groups (HSGs): The HSGs
found in the study areas include A, B and C. Soils
are classified into HSG’s to indicate the rate of
infiltration obtained for soils after prolonged
wetting. Group “A” soils have low runoff potential
due to high infiltration rates (7.62–11.43 cm/h).
Group “B” soils have moderate runoff potential due
to moderate infiltration rates (3.81–7.62 cm/h) and
group “C” soils have a moderate to high runoff
potential due to slow infiltration rates (1.27–3.83
cm/h) [63]. The initial SCS CNGrid values for each
HSGs and corresponding landuse classes adopted
from the description of HSGs for Ghana [45, 62]
are presented in Table 1.
Table 1: CNGrid Look-Up Table for Landuse and
HSGs of Study Area
(Source: Anon., 1986; McCuen, 1982)
RESULTS AND DISCUSSION
HSG “A” leads to low CNGrid values while the
HSGs “B” and “C” lead to moderate and moderate
to high CNGrid values respectively in the study
areas. Maps showing the SCS CNGrid are
presented in Fig. 6 to 7b. The SCS CNGrid values
ranging from 80-100 have the highest runoff
potential while SCS CNGrid values from 30-49.99
and 50-79.99 have low to moderate runoff
potentials respectively. The landuse types in the
SCS CNGrid values from 30 to 79.99 are mostly
forest, agricultural and galamsey activities with
HSGs of mainly A and B. The landscape
parameters extracted from communities in the two
districts and their respective BU cases are shown in
Tables 2 and 3 and Fig. 4a to 5b. Correlation
coefficients (R) between CNGrid, PMSWR and BU
cases were determined to examine their statistical
relationships. There was a strong positive
correlation between CNGrid and BU cases (R =
0.83) as well as a strong negative correlation
between PMSWR and BU cases (R = -0.80) for
AWD (Fig. 4a and 4b). Correlation coefficients
between CNGrid and BU cases (R = 0.74) as well
as PMSWR and BU cases (R = -0.67) for UDWD
were also strong (Fig. 5a and 5b). The positive
correlation between CNGrid and BU cases link low
to moderate potential surface runoff areas to
increasing BU cases whilst the negative
correlations between PMSWR and BU cases show
that, as PMSWR decreases, BU cases increase. BU
cases generally decline with CNGrid values of
eighty (80) and above. CNGrid values from 80 to
100 represent high surface runoff potential areas.
Table 2: Landscape Parameters from AWD
Landuse
Description
Curve Number Grid
(CNGrid)
A B C
Water 100 100 100
Medium Residential 57 72 81
Forest 30 58 71
Agricultural 67 77 83
Community CNGrid PMSWR (mm)
BU Cases 1999 -2012
Dawusaso 68 119.53 62
Abore 71 103.75 62
Ayiem 84 48.38 35 Tontokrom 78 71.64 121
Bonsaaso 78 71.64 82 Akyekyewere 30 592.67 43
Watreso 30 592.67 45
Essienkyiem 58 183.93 55
Agoroyesum 78 71.64 120 Datano 78 71.64 76
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Fig. 4a: CNGrid and BU Cases (1999-2012) Relationships for AWD
Fig. 4b: PMSWR and BU Cases (1999-2012) Relationships for AWD
Table 3: Landscape Parameters from UDWD
WatresoAkyekyewer
eEssienkyiem Dawusaso Abore Datano Bonsaaso Tontokrom Ayiem
CNGrid 30 30 58 68 71 78 78 78 84
BU Cases 45 43 55 62 62 76 82 121 35
0
20
40
60
80
100
120
140
Watreso Akyekyewere Essienkyiem Dawusaso Abore Datano Bonsaaso Tontokrom Ayiem
BU Cases 45 43 55 62 62 76 82 121 35
PMSWR 592.67 592.67 183.93 119.53 103.75 71.64 71.64 71.64 48.38
0
100
200
300
400
500
600
700
Community CNGrid PMSWR (mm) BU Cases
1999 -2012
Diaso 79 67.52 47
Moadaso 67 125.10 30
Agona 67 125.10 35
Ntom 67 125.10 41
Adeade 67 125.10 47
Subin 67 125.10 42
Ameyaw 85 44.82 37
Ampabena 72 98.78 55
Nyinwonsu 30 592.67 22
Domenase 30 592.67 22 Ayanfuri 79 67.52 43 Nkotumso 55 207.82 30 Abora 72 98.78 56
Treposo 72 98.78 56 Adaboa 30 592.67 23
Bethlehem 67 125.10 45
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Fig. 5a: CNGrid and BU Cases (1999-2012) Relationships for UDWD
Fig. 5b: PMSWR and BU Cases (1999-2012) Relationships for UDWD
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Fig. 6: CNGrid Map of AWD
Fig. 7a: CNGrid Map of UDWD
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Fig. 7b: CNGrid Map of UDWD
Potential Maximum Soil Water Retention (PMSWR): The PMSWR were generated based on the SCS
CNGrid Method. The maps of PMSWR for the study areas are shown in Fig. 8 to 9b.
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Fig. 8: PMSWR Map of AWD
Fig. 9a: PMSWR Map of UDWD
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Fig. 9b: PMSWR Map of UDWD
CONCLUSIONS
This research presents a new approach to
delineating BU disease prone areas and will be
useful for identifying new areas where humans may
be at high risk to the disease. The study observed
that BU endemic communities were mostly
surrounded by farms and galamsey activities. The
endemic communities in the study areas correlated
with high to moderate infiltration rates as well as
moderate to high PMSWR.
Low to moderate runoff potential areas have SCS
CNGrid values which range from 30 to 79.99. The
landuse are forest, agricultural and galamsey
activities with HSGs of A and B.
CNGrid values from 80 to 100 also correlate with
high runoff potential areas with landuse of mostly,
agricultural and galamsey activities and HSG of
mainly C.
Low to moderate surface runoff potential and
moderate to high PMSWR of between 67.52 mm -
592.67 mm correlate with BU disease prone areas.
This study has demonstrated that PMSWR and SCS
CNGrid values are important hydrological
parameters for the delineation of BU disease prone
areas.
ACKNOWLEDGEMENTS
This research is part of a larger project funded by
the National Science Foundation (CNH Award
#0909447: Research and Education on Buruli ulcer,
Inundations, and Land Disturbance – ReBUild,
rebuildghana.org) involving geographers, earth
scientists, medical professionals, and education
specialists from several US and Ghanaian
universities and other research partners.
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