162 
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15 
1 1 
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25 
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Figure 9 Figure 10 Figure 11 
In Figure 9, the slopes from the centre cell to the two lower neighbours are about the same. Since 
the slope cannot be exactly the same to two neighbouring pixels the user has to set a threshold value. The 
threshold is defined as the maximum deviation (%) from a perfect straight slope (same elevation from 
centre cell) to consider the slope straight. If the slopes are straight, the form is considered convex, and 
the flow is divided equally between the two lower neighbour cells. They receive 50% percent of the flow 
from the centre cell each. 
In Figure 10, the distribution of pixel values represents a concave topographic form. The slope to 
the cell above the centre cell is steeper than the other, and 100% of the flow from the centre cell will be 
drained to the ‘upper' cell. 
In Figure 11 we have a cell as high as the centre cell between the two lower neighbour cells. This 
indicates a ‘double concave situation', and the flow are divided between these two cells proportionally to 
their gradient values. The flow distribution for Figure 11 will be: 
Distribution (%) to upper cell = gradient (centre cell - upper cell) / (gradient (centre cell - upper 
cell) + gradient(centre cell - left cell)) * 100 
Distribution (%) to left cell = gradient (centre cell - left cell) / (gradient (centre cell - upper cell) + 
gradient (centre cell - left cell)) * 100 
Case 5: Three neighbour cells lower than the centre cell, and the rest equal or higher. 
Like in case 4, this distribution of elevation values represents either convex or concave topographic 
form/forms. If we have higher or equal cells between the lower cells, the distribution is divided into two 
or three groups of cells. Every group of more than one cell can represent a convex, concave, or ‘multiple 
concave' topographic form (see case 4). The flow between the groups are divided proportionally 
according to the sum of gradients for flow receiving cells in each group (i.e. if we have a ‘concave' 
group, only the gradient to the steepest cell will be taken into account). 
Case 6: Four or more neighbour cells lower than the centre cell, and the rest equal or higher. 
By testing the topographic form for groups of cells the drainage distribution within the group is 
estimated. The flow distribution between groups are estimated according to case 4 and case 5. It should 
be noted that one group of lower cells could contain more than one concave topographic form. In these 
cases the flow is divided proportionally according to slope between the receiving cells.