Ayúdame a entender -- Definición de cuencas/definición de stream/segmentación de stream

Question

<\/HEAD>He estado usando ArcHydro durante 6-8 años, realizando modelado de cuencas para proyectos de drenaje. Aunque entiendo la mayoría de lo que el software hace mientras trabajo en el proceso, nunca he podido comprender completamente un par de los pasos, principalmente el proceso de definición de stream/segmentación de stream, y cómo eso se relaciona con las cuencas finales.<\/SPAN> Entiendo que en el proceso de definición de stream, un número más pequeño que uses para "número de celdas para definir stream" resultará en una red de stream más definida, trazando más arriba los streams 'alimentadores'; mientras que un número mayor creará una red de stream solo con las áreas de flujo más grandes. Creo que donde finalmente me confundo es cuando llego a generar cuencas.<\/SPAN> Para el trabajo que hago, normalmente trabajo con cuencas en el rango de 5,000-20,000 acres, y debido al tipo de trabajo que realizamos, cuando llego al paso de definición de stream, uso un área de definición de 0.008 km^2 (~2 acres) para obtener que la red de stream se genere con el detalle que queremos. Ahora, desde mi perspectiva, esperaría que con una definición de ~2 acres usada en la definición de stream, cuando se generen cuencas, en un terreno dado de 40 acres encontraría alrededor de 20 cuencas. En cambio, estoy encontrando media docena o algo así de cuencas, variando en tamaño, pero algunas tan grandes como 15-18 acres. No puedo entender cómo reconciliar esto en mi cabeza - o peor aún, cómo explicárselo a mi jefe.<\/SPAN> Creo entender correctamente que el umbral de definición del stream establece el área mínima que fluye hacia un punto dado para iniciar un stream -- pero ¿cómo continúa la segmentación del stream después de eso? ¿Cómo se dividen las cuencas a lo largo de un camino largo de flujo?<\/SPAN><\/BODY><\/HTML>

PhilPinhati · Answer

In simple terms, the answer for the original question is this:

The user controls the minimum drainage area required to form the beginning of a reach. This is the same as defining the average areas of the headwater sub-basins. This is done when you set the threshold in the stream definition.

After that, the user does not have control over the size of the downstream sub-basins, because the distance between reaches, or streams, is what drives it. Hence, the downstream sub-basins can be much larger, or much smaller than the initial threshold set on the stream definition.

MalcolmNunn · Answer

This post was from a few weeks ago but since it is something I've just been trying to deal with, thought I'd attempt to give an explanation.

My understanding of this is that while FAC determines what gets classified as a stream, stream segmentation (and thus the generation of drainage points and catchments) is essentially a function of the branching of that network.

You might set a relatively large FAC threshold so that only major rivers are defined - but if that major river network has segments that are quite dendritic in a short stretch, you end up with a drainage point at every convergence, and many catchments that are very small compared to the drainage area used to initially define a stream.

The flip side of this is that if the generated drainage network has no feeder streams or convergences for a long stretch, there will be no stream segmentation along it, no drainage points midway, and Arc Hydro will delineate you one main, large catchment - instead of nice equal area divisions of your watersheds!

EDIT: I did this example before I re-read the post and realised that you said you were getting surprisingly few catchments large catchments, not many small ones. Will leave it for illustration of the segmentation and catchment delienation process however.

For example, in the image attached I used a large FAC threshold to define streams (24 000 cells at 93 x 93 m, so ~ 207 km2. Segmentation is performed at each convergence and a drainage point generated (red dots). Segments one and two converge first, then the third happens to converge a short way downstream. The result is catchments of reasonably appropriate size upstream of segments 1, 2 & 3 before they converge. However, just upstream of where 1 & 2 converge with 3 is a tiny catchment < 1 km2 - which is an area far far below the sort of drainage area used to define the streams initially.

[ATTACH=CONFIG]33316[/ATTACH]

Hope this helps - note I could be wrong about how it works but this seems to make sense in the context of results I've had.
To avoid or control this, I guess you would need to manually select or set drainage points rather than Arc Hydro delineate automatically on all drainage points. If you wanted to break up your large catchment into smaller ones, then I'd suggest setting more drainage/delineation points along the stretch of stream that needs to be broken up. As Mark said above - this is where setting batch points or using other tools comes in handy, rather than using the automatically generated catchments/drainage points as your final delineation.

Cheers
Mal

archydro_smallcatchment_example.JPG

MarkBoucher · Answer

As I understand it, the purpose behind catchments, drainlines, and adjointcatchments isn't to give you the right size of watersheds for your study. Their purpose is to be a network to make the delineation of watersheds faster.

Before use of the network databases, each watershed was delineated just using the flow direction grid. On the then much slower computers, this was painfully slow. Once someone figured out how to use the network functions for hydrology, the delineation of watersheds was amazingly fast compared to the old way. I heard that when it was demonstrated at a conference, it was so much faster than normally experienced, the audience thought it was a trick! The HydroID, DrainID, and NextDownID in the catchments, drainlines, and adjointcatchments are all related to the network database .

Instead of focusing on the catchments, etc., you could assume they are a background "model" on which you then set batch points at the specific locations you need to study. Then the batchpoints, subwatersheds/watersheds etc. become the working layers you do you analysis with.  HEC-GeoHMS has some nice tools that compliment Arc Hydro.

That's my understanding and approach.

Btw, the stream definition has been puzzling to me too. I think the problems come because the flow accumulation can jump dramatically at "junctions" in the flow direction grid. For example, if you have one subshed with 299 cells, and another with 299 cells and you set number of cells to define the stream at 300, the number jumps from 299 to 599 (including the cell you are now on) and you miss the target.