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I noticed something today that I don't understand. When I run r.watershed and calculate the flow accumulation raster, I expect that along each stream, in the direction of flow, accumulation values will always be higher from one cell to the next. But I see this is not the case. Sometimes, along a stream, the accumulation will drop for one cell, then "jump up" again a few cells downstream. See the attached image. The green squares are clipped from a flow accum grid. And the numbers are flow accum values. The arrow is general flow direction, and the circles show examples of a sudden drop in flow accum. I'm aware of the "edge of map" behavior where out of region accum gets a negative value. That's not the issue here, since the sample in the attached image is from right in the center of the region, no off map flow is involved. This problem surfaced in a script I've prepared to calculate total flow accumulation for each stream reach. After running the addon r.stream.order I add columns to the streams vector map for X-Y of the end points and total flow for each reach, then I use v.what.rast to get the flow accum at each stream reach end-point. But the values I'm getting are strange. Sometimes a downstream reach shows lower total accum than the previous, upstream segment. Maybe someone can shed some light? Thanks, Micha |
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