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From high resolution Digital Elevation Models. They first compute the flow direction from every pixel; from flow direction, the compute flow accumulation at eve

by driggs 2y ago

From high resolution Digital Elevation Models. They first compute the flow direction from every pixel; from flow direction, the compute flow accumulation at every pixel. They then filter based on some threshold for catchment area, and vectorize the flow paths. The USGS NHD+ Users Guide includes a very detailed description of the technical process: https://pubs.usgs.gov/publication/ofr20191096 https://pubs.usgs.gov/publication/ofr20191096

SpicyUme 2y ago
Weird, just spot checking a few places I know relatively well I see a few issues that seem like they would show up from that method. I wonder if there are issue in build up environments? Unfortunately that link is down for me. A collection of streams go uphill in this map where in reality I believe they flow into an underground pipe or something that drains into the lake in a different place. I wouldn't expect the underground section to be perfect, but the hill the streams ascend is ~50 feet. In another place two ponds are not shown, maybe due to dense tree cover? I believe both have water year round, but one was fenced off after a kid drowned and the owners may have filled part of it in.
- driggs 2y ago
  Hydrological modeling gets tricky when water flows underground, either naturally or unnaturally. A pixel surrounded entirely by higher elevation pixels is a "sink", and if you want to model the overland flow of water a preprocessing step is often to fill sinks and apply a slight gradient to them. I believe that the NHD+ dataset uses an alternate strategy; it burns an artificial channel into a sink, connecting it to a lower-elevation outlet. The dataset includes attributes that identify these "underground flow" segments. I didn't examine ESRI's Living Atlas site closely enough to see if they've used a different symbology to represent these segments, but your comment makes it sound like they did not. A pond often has no inlet and no outlet (as opposed to a lake), so it makes sense that some ponds may not show up in this dataset.
  - SpicyUme 2y ago
    Thanks, that's interesting and makes sense that there are alternate strategies with different tradeoffs. I checked the Lost River and some of the other areas on the Snake river plain. I can see how it would be hard to deal with water flowing underground. I noticed that the lava flows have effectively no streams, which I guess shouldn't be a surprise but it was.

by driggs 2y ago

SpicyUme 2y ago

Weird, just spot checking a few places I know relatively well I see a few issues that seem like they would show up from that method. I wonder if there are issue in build up environments? Unfortunately that link is down for me. A collection of streams go uphill in this map where in reality I believe they flow into an underground pipe or something that drains into the lake in a different place. I wouldn't expect the underground section to be perfect, but the hill the streams ascend is ~50 feet. In another place two ponds are not shown, maybe due to dense tree cover? I believe both have water year round, but one was fenced off after a kid drowned and the owners may have filled part of it in.

driggs 2y ago
Hydrological modeling gets tricky when water flows underground, either naturally or unnaturally. A pixel surrounded entirely by higher elevation pixels is a "sink", and if you want to model the overland flow of water a preprocessing step is often to fill sinks and apply a slight gradient to them. I believe that the NHD+ dataset uses an alternate strategy; it burns an artificial channel into a sink, connecting it to a lower-elevation outlet. The dataset includes attributes that identify these "underground flow" segments. I didn't examine ESRI's Living Atlas site closely enough to see if they've used a different symbology to represent these segments, but your comment makes it sound like they did not. A pond often has no inlet and no outlet (as opposed to a lake), so it makes sense that some ponds may not show up in this dataset.
- SpicyUme 2y ago
  Thanks, that's interesting and makes sense that there are alternate strategies with different tradeoffs. I checked the Lost River and some of the other areas on the Snake river plain. I can see how it would be hard to deal with water flowing underground. I noticed that the lava flows have effectively no streams, which I guess shouldn't be a surprise but it was.