Geography Reference
In-Depth Information
This study contributes to a better characterization of the region allowing the preparation of
regional plans to control the processes of soil degradation, with an indication of possible
uses and restrictions.
2.2. Study area
The Ria Formosa catchment is limited by the WGS84 coordinates 37º 15' N to 36º 57' N and
7º 28' W to 8º 4' W. It has an area of 864 km 2 and a perimeter of 166 km, including a shallow
coastal lagoon with an area of about 16,000 ha. It is protected by EU and Portuguese Laws,
and is classified as a Wetland of International Importance under the RAMSAR convention
(PORTUGAL Ramsar Site 212). It covers the municipal areas of Tavira, Faro, Olhão, São
Brás, Loulé, Vila Real de Santo António and Castro Marim. The topography of the region is
regular and continuous without abrupt changes in altitude. The average slope is 11% and
the elevation varies between 0 and 530 meters above sea level. Mean annual rainfall of the
catchment ranges between 400 and 800 mm. The mean annual temperature is 17 ° C.
2.3. Methodology
The DEM has been used as a basic source of information on the catchment of the Ria Formosa
and was obtained from a geostatistical study with a resolution of 10×10 m 2 , which was based on
cartography at the scale 1:25,000 from the Geographical Institute of the Portuguese Army (IGeoE,
2004). From this model, other information about other terrain features were obtained. The
analysis and mapping of the data has been performed with the IDRISI Taiga software (Eastman,
2009). With this software, several terrain variables such as slope, curvature, relative position, and
local relief were modelled for each point relatively to the DEM. Finally, the automatic
classification of landforms was carried out, as established by Jordan et al. (2005).
2.3.1. Slope and curvature
Maps of slopes and curvatures are commonly used to describe the hydrologic drainage
structure of a region or a catchment. Soil properties and the characteristics of the hillslopes
are factors that combined determinate a higher or lower resistance to soil erosion, especially
due to rainfall. The inclination, the length and the shape of a slope are associated to the
velocity of runoff and to the water infiltration into the soil.
The slope at point ( M , P ), in the azimuth direction A , is given by calculating the inner
product between the gradient of the surface H and the unit vector
w with the components
A
(
SinA
;
CosA
):
2
2
HH
H
H
(1)
Hw
|
 
H
A
XY
X
Y
When the ground is represented by a matrix H (m, n), X and Y are coordinate axes, and the
gradient of the surface H represents the topography of a square that includes point ( M , P )
defined by a bi-linear polynomial expression:
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