{"id":5456,"date":"2018-06-06T02:00:26","date_gmt":"2018-06-06T00:00:26","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=5456"},"modified":"2023-04-28T14:56:43","modified_gmt":"2023-04-28T12:56:43","slug":"hydrology-with-a-gis-for-dummies-who-we-are-calculating-the-runoff-1","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/hydrology-with-a-gis-for-dummies-who-we-are-calculating-the-runoff-1\/","title":{"rendered":"Hydrology with a GIS, for dummies (who we are): calculating the runoff (1)"},"content":{"rendered":"

A quote to start this article:
\n\u00a0\u00bb\u00a0\u00a0\u00a0In the same way that a text editor’s manual does not teach you how to write a short story or a CAD tutorial does not tell you how to calculate the size of a beam for a building, this guide does not teach you anything about spatial analysis.\u00a0Instead, it will show you how to use the QGIS Processing Environment which is a powerful tool for performing spatial analysis.\u00a0It is up to you learning the concepts that will be essential to understanding this type of analysis.\u00a0Without them, you will not be able to use the environment and its algorithms even though you may be tempted to do so.\u00a0\u00a0\u00a0\u00bb(\u00a0QGis documentation<\/a>\u00a0)<\/p>\n

In this series of articles, we will not try to teach you hydrology.\u00a0We will just open a few doors, usually closed.\u00a0If this does not teach you hydrology, at least we hope to sow the doubt about what you are doing, doubt that will lead you to learn (if you wish to do so).<\/p>\n

In the articles devoted to the calculation of watersheds (\u00a0 ArcHydro: determination of the watersheds of a territory (1<\/a>\u00a0), A\u00a0 rcHydro: determination of the watersheds of a territory (2)<\/a>\u00a0) you will notice that the basis of almost all the calculations we performed is the layer of the runoff direction (flow direction)<\/p>\n

This layer is the key element of hydrological analysis.\u00a0Many alternatives exist, each with its advantages and disadvantages.\u00a0Explaining them in detail would take too much time, and we will simply present the ideas that are interesting from a practical point of view, so that you can get a general idea of \u200b\u200bthe difference between using one method or another.<\/p>\n

Basically, the methods can be divided into two groups: those which consider the flow as a movement between the centres of the cells and those in which the flows move \u00ab\u00a0freely\u00a0\u00bb in the DEM (algorithms \u00ab\u00a0Flow tracing\u00a0\u00bb).\u00a0Those in the first group are related to the D8 method (the oldest and the only one that you will find in ArcGis), while those in the second group are more complex and its use is rather limited.<\/p>\n

Another classification can be made between those that consider a one-dimensional flow (commonly called single flow directional algorithms) and those that consider a two-dimensional flow (multiple flow direction algorithms).\u00a0In other words, the first considers that the flow of a cell is on one, and only one, other cell, while the\u00a0second considers that the flow can take place to more than one contiguous cell of the considered cell.<\/p>\n

In order to stay within the known software, we will discuss the different options offered by QGis, taking into consideration that \u00a0ArcGis does not offer any choice.\u00a0Other methods can be used with specific software for hydrological calculations.<\/p>\n

Using QGis, you can choose among 8 different methods to calculate the direction of the flow:<\/p>\n

\u0095\u00a0Deterministic 8 (D8): The classic method, implemented in ArcGis.\u00a0The flow goes from the centre of a cell to the centre of one (and only one) of the surrounding cells.\u00a0The flux directions are therefore limited to multiples of 45 \u00b0, which is the main reason for the disadvantages of the method.\u00a0(\u00a0 O’Callaghan, JF and Mark DM The extraction of drainage networks from digital elevation data.<\/a>\u00a0
\n<\/a>Vision, Graphics and Image Processing 28: 323-44.<\/a>\u00a0 1984<\/a>\u00a0).<\/p>\n

\u0095\u00a0Rho8: Similar to the previous one, but with a stochastic component that should improve the results.\u00a0The direction of movement is determined by a random parameter that depends on the difference between the orientation (\u00ab\u00a0aspect\u00a0\u00bb) and the direction of the two neighbouring adjacent cells.\u00a0(\u00a0Fairfield, J. Leymarie P. Drainage networks from grid digital elevation models, Wat Resour Res 27 (5): 709-717, 1991\u00a0).<\/p>\n

\u0095\u00a0Infinite deterministic (D\u221e): The flow passes from one cell to two adjacent neighbouring cells, thus constituting a two-dimensional flow, which makes it possible to overcome the disadvantages of method D8.\u00a0(Tarboton, DG, Shankar, U. (1998), The Identification and Mapping of Flow Networks from Digital Elevation Data, Invited Presentation at AGU Fall Meeting, San Francisco, 1998).<\/p>\n

\u0095\u00a0Braunschweiger Digitales Reliefmodell: Another multiple-direction flow algorithm.\u00a0The flow is divided among the surrounding cell whose orientation is closest to the orientation (\u00ab\u00a0aspect\u201d)\u00a0of the central cell and its two adjacent cells.\u00a0(Bauer, J .; Rohdenburg, H. Bork, H.-R. Ein Digitales Reliefmodell als Vorraussetzung fuer ein deterministisches Modell der Wasser- und Stoff-Fluess, Landschaftsgenese und Landschaftsoekologie, H.10, Parameteraufbereitung fuer deterministische Gebiets-Wassermodelle, Grundlagenarbeiten zu Von Analysis
\nAgrar-Oekosystemen, (Eds .: Bork, H.R., Rohdenburg, H.), p.1-15, 1985).<\/p>\n

\u0095\u00a0FD8: a two-dimensional flow path calculation algorithm derived from the D8.\u00a0(Quinn, PF, Beven, KJ, Chevallier, P. Planchon, O. The Prediction of Hillslope Flow for Distributed Hydrological Modeling Using Digital Terrain Models, Hydrological Processes, 5: 59-79, 1991).<\/p>\n