  The flow accumulation problem for grid terrains takes as input a matrix of
flow directions, that specifies for each cell of the grid to which of its eight
neighbours any incoming water would flow. The problem is to compute, for each
cell c, from how many cells of the terrain water would reach c. We show that
this problem can be solved in O(scan(N)) I/Os for a terrain of N cells. Taking
constant factors in the I/O-efficiency into account, our algorithm may be an
order of magnitude faster than the previously known algorithm that is based on
time-forward processing and needs O(sort(N)) I/Os.
