Geoscience Reference
In-Depth Information
h e plot displays areas with high l ow accumulation in shades of blue and
areas with low l ow accumulation, usually corresponding to ridges, in shades
of red. We used a logarithmic scale for mapping the l ow accumulation
in order to obtain a better representation of the results. h e simplii ed
algorithm introduced here for calculating l ow accumulation can be used to
analyze sloping terrains in DEMs. In l at terrains, where the slope approaches
zero, no l ow direction can be generated by our algorithm and thus l ow
accumulation stops. Such situations require more sophisticated algorithms
to perform analyses on completely l at terrain. h ese more advanced
algorithms also include sink-i lling routines to avoid spurious sinks that
interrupt l ow accumulation. Small depressions can be i lled by smoothing,
as we did at the beginning of this section.
h e i rst part of this section was about primary relief attributes. Secondary
attributes of a DEM are functions of two or more primary attributes. Examples
of secondary attributes are the wetness index and the stream power index.
h e wetness index for a cell is the log of the ratio between the area of the
catchment for that particular cell and the tangent of its slope:
h e term 1+ l owac avoids the problems associated with calculating the
logarithm of zero when flowac=0 . h e wetness index is used to predict the
soil water content ( saturation ) resulting from lateral water movement. h e
potential for waterlogging is usually highest in the lower parts of catchments,
where the slopes are more gentle. Flat areas with a large upslope area have
a high wetness index compared to steep areas with small catchments. h e
wetness index weti is computed and displayed by
weti = log((1+flowac)./tand(slp));
h = pcolor(weti);
colormap(flipud(jet)), colorbar
set(h,'LineStyle','none')
axis equal
title('Wetness index')
[r c] = size(weti);
axis([1 c 1 r])
set(gca,'TickDir','out');
In this plot, blue colors indicate high values for the wetness index while red
colors represent low values (Fig. 7.15 f ). In our example soils in the south-
east are the most likely to have a high water content due to the runof from
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