Stream and catchment delineation with GIS (theory)

hello my name is Hans from the crust lecturer at ihe Delft Institute for water education in this video I'm going to explain how streams and catchments can be delineated from a digital elevation model using GIS after this lecture you'll be able to define what is a catchment to describe the generic GIS procedure to delineate streams and catchments from a digital elevation model to explain how slopes are calculated in GIS from digital elevation models to explain corrections that have to be made to use a DM for hydrological applications and to explain the Strela order method there is some confusion in the terminology of the word catchment in English in British English the word catchment refers to the surface area for which all the water drains to one outlet this is also called a drainage basin the term watershed refers to the drainage divide which is indicated with the red dotted line in the picture in US English this is a bit different watershed refers to the area for which all the water drains to one outlet and the red dotted line is called the drainage divide a drainage basin which is used in in both British and US English can be defined as an area of land where all the water that originates from different kinds of precipitation converges to one single outlet and this can be another river a bigger river or it can be a lake reservoir or wetland or the sea or the ocean because a lot of GIS software is in US English you will often see the word watershed in the tools for what we would call catchments in British English here we see a catchment is the faster catchment in Belgium and the size of a catchment depends much on the the ruggedness of the area of the elevations here we exaggerated a bit the elevations and so in British English we would call the area inside the red polygon the catchment and recall the red line itself the watershed so how do we derive streams and catchments from a digital elevation model using GIS on this slide I'm going to show you the general workflow that's applicable to many GIS desktop software in the next slide I'm going into detail of every step in the procedure first we need to acquire the DM tiles then we need to mosaic the DM tiles if our study area is spread over different tiles we need to rip react the DM we need to subset then if necessary interpolate the voids we need to fill the things and remove the spikes you need to burn in the stream network if we have a stream network available we calculate the flow direction map we derive the streams define the outlet outlet of the catchment and then derive the catchment and in the end we can convert the data sets to the format that our hydrological model needs the first step is to download the DM normally it comes in tiles and if we don't have access to local data because of the cost or unavailability we can use global open access data as explained in another video so most of the time we will use the srtm one arcsecond global product which is around 30 meters on the equator there are also some other products srtm a void filled product around 30 meters for the USA and 90 meters globally and there's the experimental aster global DM or GM product which is also around 30 meters spatial resolution you have to keep in mind that resolution is not the same as accuracy this term should not be confused not the higher the resolution the higher the accuracy it is a different concept so you can have a very high spatial resolution but then even the error per pixel can be also very high and it's also not the case that you always need the highest resolution for each purpose so for catchment modeling generally the SRTM one arcsecond global product is fine but if you want to plan a dam in a river you probably need very high spatial resolution in both cases you need a good accuracy you can download the SRTM DM from the USGS earth Explorer website or you can use a plugin in QGIS the SRTM download plugin then usually your study area is not located nicely in the center of a tile but is spread over two or more tiles so the next step is to mosaic the TEM tiles which means that you simply merge them the next step is to reproject the e/m this is necessary because the global data sets are usually in the geographic coordinate system let it ute longitude and the units are in degrees for DM analysis we need to use a metric projection so we need to reproject to another coordinate system if we cannot use the local projection of a country because the area is too large its trans boundary catchment then we can use a another more global projection such as the UTM now I'm going to show you how slope is calculated and why it does matter that the units are in meters instead of degrees like we have in latitude longitude coordinates in this graph we see on the x-axis the distance and on the y-axis the elevation the graph shows how the elevation changes with the distance if we want to know the slope at this point we know from mathematics that we have to draw the tangent and then the slope can be calculated as Delta Z over Delta X and if we take the arctangent then we have it in degrees you can imagine with this equation if the x coordinates are in degrees and the Z coordinates in meters the software will still calculate the slope by applying this equation but the result does not make sense the zet units need to be the same as the X and the y unions now this is for a graph but how do we calculate the slope in a raster therefore we need to use a so called moving window because we need to calculate the slope to the surrounding pixels so we consider the pixel in the middle of a 3x3 moving window and we assign the steepest slope to the pixel that we consider and then we move the window to the next pixel and we calculate again the slopes to all the directions and we assign the steepest slope every time to the center pixel that we consider so this is a focal operation that we discussed in another video now there's one problem with this method you can think of what would happen to the first row and column in the last row and column there's not enough information to calculate the slope in a kernel or in a window and that will result in no data at the borders so if you want to calculate slopes of a study area be sure that you make your study area a little bit larger than strictly your area of interest because otherwise you will lose the borders the next step is to subset the DM sub setting is a technical term for clipping remember that in the previous steps we mosaics two different tiles but our study area is often not covering the whole mosaic but only part of it the consequence is that if we keep all the pixels of the mosaic in the calculations that the calculation times will be too long and sometimes even your computer will run out of memory so it's better to subset the area to your study area but it should also not be too small because then it will cut off the boundaries of your catchment in the delineation so a good way is to understand a little bit the elevation differences in your area of interest to determine where the outlet is and to determine where the source areas are and where the divide could be by looking at the elevations and then you make it a little bit larger to have a safety margin the next step is to interpolate the voids voids are no data pixels in your DM which can be a result of the acquisition procedure some procedures don't handle well areas that are covered by snow or that are in the shade of other mountains and therefore we need to use the surrounding information to interpolate these areas and this is the end result we can never create the real data but we can make it possible to continue with the data set after interpolation of the the voids the next step is to fill the sinks you should not be confused with the voice voids are no data and things are depressions artificial depressions in the landscape that are caused by the DM creation process so it's one or more cells which don't have a downstream cell around it so the water is not able to escape to the outlet and we can remove those using functionality and GIS software which is called fill sink algorithms if the landscape contains real things such as lakes or other depressions they need to be added after the removal of the sinks and things are also called bits here we see a digital elevation model where water is trapped in the center cell which is much lower than the surrounding cells so there is no possibility to drain towards the outlet the fill sinks algorithm will either fill up those depressions or cut true so here you see the site of elevation a transect and the outlet is downstream of the flow direction and the algorithm can either increase the surface of the depression or decrease the surface after the depression so the water is routed to the outlet sometimes it's necessary to burn in the stream network to force the water to follow the real rivers this can only be done of course when the river network layer exists the next step is to calculate the flow direction most often we use the d8 algorithm similar to the slope algorithm it will look in a three by three window and will calculate the slope to all the directions around the cell that we consider but instead of assigning the slope it will assign the direction of the steepest slope an alternative is 2d infinite algorithm which uses not the eight surrounding cells but it will use continuous directions of course that is more calculation intensive and it will not always result in better results let's have a closer look how to calculate the flow directions using the d8 algorithm on the left side we see the elevations each pixel has the elevation in metres and we are looking at the center cell and we need the information of the surrounding cells because it's a focal operation like with the slope calculation the spatial resolution is 30 meters so the pixels are 30 by 30 meters now the question is which direction are we going to assign to the center pixel we know that we can calculate the slope by evaluating the surrounding pixels of the center pixel by calculating the Delta Z over Delta X the drop divided by the distance and for the d8 algorithm we assign then the direction of the steepest slope so you need to look at this and select one of the direction values that are displayed on the right side of this slide let's do the calculation if you would consider the cell to the south so direction number for the calculation will be 67 minus 52 divided by the distance 30 equals 0.5 if we would consider the diagonal the drop will be 67 minus 48 but the distance is not anymore 30 because we have to apply Pythagoras the diagonal distance is longer than a horizontal vertical distance therefore we use 30 square root 2 and this results in 0.45 so the correct answer of the flow direction was number 4 is the direction to the south so we do that for every cell and then we can link the lines that go flow in the same direction and in this way we can construct the stream network the map on the right side is called the stream link map now the stream links are for every pixel but not every pixel contributes to a river not all water that falls on a pixel becomes a river so we have two methods to derive the stream we can look at the flow accumulation or we can look at the Strela order let's have a closer look at the flow accumulation in this map we see the amount of precipitation in each cell we can superimpose the stream link that has been derived in a previous step your stream link determines based on the flow direction how the amount of precipitation is moving through the catchment when the precipitation follows the stream link towards the outlet the flow is accumulating and we can define a threshold above which we consider the accumulated flow as being part of a river let's assume that we consider 10 units as the threshold in this case so that means that everything that accumulates larger than 10 is considered a river and this is the way that the stream network can be derived to determine this threshold value is a bit difficult and it needs a bit of trial and error there is no rule of thumb and it depends on other properties of your catchment so here on the Left we see the example if we have 500 units as a threshold and on the right side we see thousands so we see if we make the threshold lower it results in more tributaries and sub catchments than if we make it higher the second method to derive streams is to use the stroller order this method orders the reaches and we can set a threshold for which we consider it as a stream it starts with the smallest ones and they are ordered with number one when two of the same order join it increases the order so two of one join into order two when two of order to join it becomes order three however when a lower order joins a higher order the order is not changed so here it remains three then two of order three become four and when we have order one joining again it stays order four based on this ordering we can determine a threshold value for which we consider the reaches as being part of a river for example all the orders larger than three to determine this threshold we need to do a calibration we can use a satellite image or an existing map and see which threshold value fits best with the knowledge we have of the area of course this will not always be a perfect fit because we filled the DM which makes the DM a model and there can be a lot of human influences such as mining or urbanization or channels the next step is to define the outflow point of our catchment we need to define this on the delineated River so we cannot use a background map where River has been defined in a different way because our DM and the delineator streams are now part of a model and if we don't define the outlet on the model it will not result in the catchment now what can we define as an outlet that is a location in a river where we have discharge measurements or the outlet of a tributary so here we define the red cell as the outlet and the algorithm can then define the catchment based on the flow direction the stream link so the blue area as the catchment area and orange the drainage divide and all the water that falls within that area drains towards the outlet to the red cell so you've just learned the general procedure for catchment and stream delineation which consists of downloading the DM tiles to mosaic the tiles if they if your study area is spread over multiple tiles then reproject the DM to subset the DM interpolate the voids if necessary fill the things and remove the spikes burn the stream network if you find it necessary and if you have a stream network available calculate the flow Direction map derive the streams define the outflow point and then derive the catchment and in the antis normally feeds into your model so it needs to be converted to your model Oriol tool for which you want to use this data there are some limitations to this method the methods are based on elevation differences so in areas that are flat this does not work also in areas where there is no gravity natural gravity flow but are more human control this does not work so for the Netherlands this doesn't work because we don't have much elevation difference and also the flow is very much human controlled in a large part of the country where we pump up the water to keep us dry so keep that in mind when you use this method eventually you will use this data for hydrological modeling together with other GIS data so the catchment delineation procedure provides very important inputs to catchment models the delineation of the study area the streams and the flow direction can be used

Channel: Hans van der Kwast

Views: 14,100

Rating: 4.9849625 out of 5

Keywords: GIS, QGIS, ArcGIS, catchment, watershed, Strahler order, DEM, fill sinks, D8, flow direction

Id: ZLUjSEK-nbg

Length: 21min 13sec (1273 seconds)

Published: Tue Mar 05 2019