{"id":5521,"date":"2018-06-20T20:17:41","date_gmt":"2018-06-20T18:17:41","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=5521"},"modified":"2018-10-17T12:49:10","modified_gmt":"2018-10-17T10:49:10","slug":"hydrology-with-a-gis-for-the-nulls-that-we-are-calculation-of-the-flow-3","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/hydrology-with-a-gis-for-the-nulls-that-we-are-calculation-of-the-flow-3\/","title":{"rendered":"Hydrology with a GIS, for the Dummies (that we are): calculation of the flow (3)"},"content":{"rendered":"

In the two previous articles (\u00a0Hydrology with a GIS, for Dummies (that we are): calculation of the flow (1)<\/a>\u00a0\u00a0\u00a0and\u00a0\u00a0\u00a0Hydrology with a GIS, for the nulls (that we are): calculation of the flow (2<\/a>\u00a0) we have approached three methods to calculate the flow of flow: two one-dimensional methods (D8 and Rho8) and a two-dimensional method (MFD).<\/p>\n

Now we will discuss a variation of the most widespread method (D8) which we addressed first.<\/p>\n

Fourth Method: Infinite Deterministic (D\u221e)<\/strong><\/p>\n

The difference between this method and the D8 method is that the flux is distributed in the two lowest adjacent cells and not just in the lowest cell.\u00a0It is a two-dimensional method.<\/p>\n

In\u00a0QGis\u00a0you will use, in the processing\u00a0window-\u00a0> SAGA -> Terrain\u00a0analysis-Hydrology\u00a0->\u00a0Catchment\u00a0area (parallel)<\/p>\n

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\nThe result of the calculation of the flow with this method, on the DEM involves a semi-sphere, as it follows:<\/p>\n

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\nIf we compare this with the result of the D8 method:<\/p>\n

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\nWe can see that, although the problem of the overrepresentation in the 8 directions (0\u00a0, 45,90\u00a0, …) is always very marked, the differences are dimmed.<\/p>\n

The ratio of cells with less surface area to those with larger surface area is from 1 to 700 in the case of the D8 method, and only from 1 to 22 in the case of the infinite deterministic method.<\/p>\n

Fifth method: kinematic routing algorithm (KRA)<\/strong><\/p>\n

In this section we can mention the KRA method: the kinematic routing algorithm.\u00a0This flow \u00a0algorithm is one-dimensional.\u00a0The flow behaves like a ball rolling on the DTM, without restricting its position in the centre of the cells.\u00a0This result is intermediate compared to the two previous methods.\u00a0The ratio among the cells with less surface area to those with larger surface area is from 1 to 45.<\/p>\n

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We will discuss briefly the remaining methods since they do not provide much more information.<\/p>\n

Sixth<\/strong>\u00a0method: Braunschweiger<\/strong>\u00a0Digital<\/strong>\u00a0Reliefmodell<\/strong><\/p>\n

This is a multidimensional method.\u00a0It is similar to the MFD method.
\nIt is available in\u00a0QGis\u00a0in the Treatment\u00a0Window-\u00a0> SAGA -> Terrain\u00a0analysis-Hydrology\u00a0->\u00a0Catchment\u00a0area (\u00a0parallel\u00a0)
\nThe result of the calculation is as follows:<\/p>\n

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This result has to be compared with the result of the MFD method:<\/p>\n

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\nThe result appears much better than those produced by the one-dimensional methods, but compared to the MFD method we can see a certain radial bias.<\/p>\n

Seventh method: DEMON (Digital<\/strong>\u00a0Elevation<\/strong>\u00a0Model Network)<\/strong><\/p>\n

It is a two-dimensional method, but rather complicated.\u00a0It is based on the creation of an upstream and downstream watershed matrix for each pixel.\u00a0Impossible to explain it here.\u00a0If you are interested, it is better to go to the source (\u00a0Costa-Cabral, MC;\u00a0Burges\u00a0, S.\u00a0J. Digital\u00a0elevation\u00a0model networks (DEMON): A model of flow over\u00a0hills\u00a0for computation of\u00a0contributing\u00a0and dispersal areas, Wat\u00a0Resour\u00a0.\u00a0Res\u00a030: 1681-92 (1994<\/a>\u00a0)
\nJust remember that it takes a long time to calculate the result.\u00a0In the case of our example, which is very simple, it takes about 8 minutes to calculate the result.\u00a0So with real MNTs\u00a0…
\nIn\u00a0QGis\u00a0it is available in\u00a0QGis\u00a0in\u00a0Process Window-\u00a0> SAGA -> Terrain\u00a0analysis-Hydrology\u00a0->\u00a0Catchment\u00a0area (flow\u00a0tracing\u00a0)
\nHere is the result:<\/p>\n

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This result is very close to the infinite deterministic method (see the beginning of this article), which is normal since both are two-dimensional methods.<\/p>\n

We have gone through the main methods for calculating the flow.\u00a0It remains to discuss in more detail the problems posed by the main methods.\u00a0This is what we will discuss in the next and last article on this subject<\/p>\n