USE OF MODELS
GIS-based spatial modelling
techniques such as neighborhood
operations, map overlays, reclassification
with attribute data or
proximity analysis are increasingly
used in land degradation studies.
Digital terrain models, generated
by interpolating contour data from
topographic maps, allow automatic
calculation of slope, aspect and
curvature gradients (Figure 4). Using
curvatures, concavity and convexity
can be calculated, which indicate
diverging and converging overland
flows, useful in predicting erosion
and deposition processes. There is
also possibility of hill-shading and
generating perspective views of
terrain, on which land degradation
features can be displayed (Figure 5).
More recently, terrain analysis
on digital elevation data has been
increasing applied to derive various
parameters useful for soil erosion
and hydrologic studies. Hengl et
al. (2003; 2004) describe terrain
analysis techniques and provide a
user guide for generating various
terrain parameters. Catchment area
can be generated by (1) computing
slope lengths for each diagonal and
cardinal direction (total 8 directions)
and their sum, (2) computing the
drainage fraction out of each cell
for each direction, (3) calculating
the drainage fraction into each cell
for each direction as a fraction of the
contributing cell, and (4) propagating
the total number of contributing
cells using iterations, starting with
a map showing 1 in each cell. This
process is illustrated in Figure 6.