Catchments (watershed) or drainage basins:
An extent or an area of land where surface water from rain, melting snow, or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, sea, or ocean.
Workflow for the catchment delineation:
1) Dowload and import DEM tiles
2) Mosaic DEM tiles
3) Reproject DEM
4) Subset DEM
5) Interpolate voids
6) Fill sinks/remove spikes
7) Burn-in the stream network
8) Calculate the flow direction map
9) Derive streams
10) Define outflow points
11) Derive catchment
12) Convert dataset to model format
2) Mosaic DEM tiles
In a grid, slope is calculated as a focal operation. The steepest slop in the moving window or Kernel (3x3) is assigned to the cell. The problem with this method is that, for the first and last rows and columns there are no enough information to calculate the slope in the Kernel and that will result in no data in the borders. So, if you want to calculate the slope in your study area, be sure that you make your study area a little bit larger than your area of interest, because otherwise you will lose the borders.
3) Reproject DEM tiles
The global dataset are usually in the geographic coordinate system EPSG:4326 (Latitude/Longitude) and the units are in degrees.
For the DEM analysis we use a metric projection, so we need to reproject it to another coordinate system, a Coordinate Reference System. We can use a global projection, such as the UTM.
4) Subset the DEM
If the DEM is too large, the calculation time for the following steps can be too large or computer runs out of memory. So, it's better to subset the area to your study area. But it doesn't have to be too small because then it will cut off the boundaries of your catchment.
5) Interpolate voids
Voids are pixels with NODATA in your DEM as a result of the acquisition procedure. Voids can be interpolated using the values of the surrounding cells. In fact 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.
6) Fill sinks
Sinks are artificial depression/pits in the landscape. A pit is a set of one or more cells which has no downstream cells around it. Pits are removed using the fill sinks function in GIS software. If the landscape contains real sinks (e.g. lakes), these need to be added after pit removal.
8) Calculate the flow direction
Most ofter we use the D8 algorithm, that is similar to the slope algorithm. It uses 8 discrete directions to calculate flow direction (0, 45, 90, 135, 180, 225, 270, 315 degrees) to steepest cells downwards.
An alternative is the 2D infinite algorithm, which doesn't use the eight surrounding cells but it uses continuous directions. Its calculations are more intensive and it will not always result in better results.
9) Derive streams
There are two methods to derive the stream: we can look at the flow acculumation or at the Strahler order.
1) Flow accumulation
When the precipitation follow the stream links towards the outlet, the flow is accumulating and we can define a threshold above which we consider accumulated flow as being part of the river.
Let's assume we consider 10 units as the threshold in this case. It means that everything that accumulated larger than 10 is considered a river and this is the way a stream network can be derived.
On the let, the amount of precipitation in every cell. On the right, the stream links are superimposed.
Considering 10 units as the threshould value for the flow accumulation:
The determination of 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.
In the pictures below, the difference between a threshold of 500 cells and 1000 cells is shown:
As you can see, if we make the threshold lower it results in more tributaries and sub-catchments than if we make it higher.
2) Strahler order:
The rules are the following:
a) The first streams are of order 1.
b) When two streams of the same order join, it increases the order of +1.
c) When two streams of different order join, it doesn not increase the order and the resulting stream has the higher order of the previous two streams.
Based on this ordering we can determine a threshold value for which we consider the reaches as being part of the river, for example all the orders larger than 3.
To determine this threshold we need to do a calibration. We can use a satellite image or an existing map and see which thrashold 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 DEM, wich maked the DEM a model and there can be a lot of human influences such as mining, urbanization or channels.
10) Define outflow point
We need to define the outflow point on the delineated river. We cannot use a background map where the river has been defined in a different way because our DEM and the delineated streams are not part of a model. 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 is a location in the river where we have discharge measurements or the outlet of a tributary.
The red cell is the outlet.
The algorithm can then define the catchment based on the flow direction of the stream link: the blue area is the catchment area and the orange line is the drainage perimeter. All the water that falls in that area drains towards the outlet of the red cell.
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