{"id":5509,"date":"2018-06-19T03:59:21","date_gmt":"2018-06-19T01:59:21","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=5509"},"modified":"2018-10-17T12:49:29","modified_gmt":"2018-10-17T10:49:29","slug":"hydrology-with-a-gis-for-the-dummies-that-we-are-calculation-of-the-flow-2","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/hydrology-with-a-gis-for-the-dummies-that-we-are-calculation-of-the-flow-2\/","title":{"rendered":"Hydrology with a GIS, for the dummies (that we are): calculation of the flow (2)"},"content":{"rendered":"

This article is the continuation of the previous article:\u00a0\u00a0Hydrology with a GIS, for dummies (that we are): calculation of the flow (1)<\/a><\/p>\n

Now, we will discuss the result of the application of another flow calculation method.<\/p>\n

Second method: Rho8<\/strong><\/p>\n

This is a modification of the previous method.\u00a0It’s always a one-dimensional flow: all the water in the cell goes to one and only one other cell.\u00a0If in the D8 method, the flow is calculated according to the largest slope between the cell considered and its 8 contiguous cells, in the Rho8 method we will introduce a random factor, calculated as of the orientation of the central cell and \u00a0the direction of the two contiguous cells located in the same direction.<\/p>\n

With\u00a0QGis\u00a0you will use, in the processing\u00a0window-\u00a0> SAGA -> Terrain\u00a0analysis-Hydrology\u00a0->\u00a0Catchment\u00a0area (\u00a0parallel\u00a0)<\/p>\n

\"\"<\/p>\n

The result of this method on the DEM in semi-spherical form, for the\u00a0catchment\u00a0area\u00a0layer\u00a0is as follows:<\/p>\n

\"\"
\nIf you now compare this image with the result of method D8:<\/p>\n

\"\"
\nYou can observe that the concentration on the 8 main directions (0, 45, 90,135 …) is much less marked.\u00a0On the other hand, since we are on\u00a0a\u00a0perfect sphere, water should flow in a homogeneous way and not in the form of\u00a0\u201cstring\u00a0\u00bb.<\/p>\n

To better understand this comment, we will move to another method of calculation.<\/p>\n

Third method: Multiple Flow Direction (MFD) also called FD8<\/strong><\/p>\n

Unlike the two previous methods, the flow from a cell is not in the direction of one and only one cell.\u00a0According to the slope, all cells\u00a0located\u00a0below the cell concerned will receive a portion of the flow.<\/p>\n

The following figure is sourced from the original publication of this method.<\/p>\n

\"\"
\nYou can consult this publication at: Quinn, P.F.; Beven, K.J.; Chevallier, P.; Planchon, O.; The prediction of hillslope flow paths for distributed hydrological modelling using digital terrain models, Hydrological Processes, 5:59-79.1991.<\/em><\/p>\n

To\u00a0implement\u00a0this method, with\u00a0QGis\u00a0, you will use the treatment\u00a0window-\u00a0> SAGA -> Terrain\u00a0analysis-Hydrology\u00a0->\u00a0Catchment\u00a0area (\u00a0parallel\u00a0)<\/p>\n

\"\"<\/p>\n

The result of this method on the DEM of semi-spherical form, for the\u00a0catchment\u00a0area\u00a0layer\u00a0is as follows:<\/p>\n

\"\"
\nHere we have a globally uniform flow, on a uniform semi-sphere.\u00a0If we zoom in, we observe that the structure at 8 prints some screening.\u00a0Indeed, even if the flows are multiple,\u00a0they are calculated from the centre of the central cell to the centres of the contiguous cells.\u00a0The frame 0, 45, 90 … appears here, but when we look at a level of greater detail it disappears when we look at the phenomenon from above.<\/p>\n

In the next article we will discuss the remaining methods of calculations.<\/p>\n