We will use ArcHydro to calculate the different whatersheds of a given territory. It is not only important to know the different steps to achieve the desired result, but also the assumptions we will be using. Indeed, they are not privative of ArcHydro, however they are the only ones available.
In order to determine the watershed we must follow the following steps:
- Determination of the direction of the flow
- Determination of the network flow
- Segmentation of the network flow
- Determination of the watershed in each section of the network flow
- Fusion of the watershed for each element in order to create the watersheds of the expected size.
Determination of the direction of the flow
Each cell of the DTM is surrounded by eight other cells.
ArcHydro establishes towards which cell the water is flowing to, from the central cell by calculating the slope between the central cell and the surrounding eight. It takes into consideration that the water flows to the cell with the steepest slope.
This hypothesis is one of the pillars of ArcHydro, but, by no means, universal. Other algorithms (not available through ArcGis) consider that, even if the larger flow is to the steeper cell, there will, anyway, a flow to all the cells steeper than the central one.
In order to follow the process, we will use the corrected DEM from previous articles articles ( ArcHydro : 2- Preparing a corrected DEM for hydrology – Part 1 and ArcHydro : Prepare a corrected DEM for hydrology – Part 2 ).
To calculate the flow direction clik on “Land preprocessing»->« Flow direction «
The result shows the different direction of the flow calculated from the TAM
Determination of the network flow
The second step consists in calculating how many cells flow upstream toward each of the cells of the DTM.
When no cell flows towards the considered cell, the value is zero. When several cells flow towards the considered cell, the value is equal to the sum of the flow values of these cells.
To calculate the direction of the flow, click on » Land Preprocessing »->« Flow accumulation «
The only field to fill are the raster input (Flow direction), and the name of the raster output.
The result is presented as follows:
Segmentation of the network flow
Why the watershed flow is not an hydrographic watershed? The hydrographic watershed lists the rivers/streams. The qualification of river/stream essentially rests in the following criteria:
- The presence and the permanence of a natural bed at the origin, distinguishing, therefore, a water course in a channel or ditch manmade.
- The permanence of the flow most part of the year. As for our case study, we will not take into consideration those criteria. We will build a watershed based on the accumulation data. We will create a watershed based on an accumulation threshold, for example 150. This means that when the accumulation reaches 150 cells, we consider that it constitutes a section of our hydrographic watershed. This is completely independent of the fact that this specific cell is or not located in the “official” watercourses. It is evident that that from a certain threshold, it will, undoubtedly, included in the “official” watercourses. The threshold definition does not follow any specific rule. It depends on the objective of the study, the size of the area being studied, the ground type, in short, depends on a lot of parameters. Putting it this way does not help you in anyway. Then, how to find the adequate value?
Let’s say that by the end of the data processing, you will obtain a photo of your overall territory. You will choose a value in order to create your hydrographic watershed according the quality of your photo. It is the smallest size that will give you information. According to your objective, and being aware that when you increase the definition of your photo, the more complicated becomes the process, you will have to find a suitable number. It is not worthwhile to increase the definition if you are not aiming for a global view, and it will equally inconvenient to decrease a lot the definition is your objective is to study the details of your chosen area.
To calculate the network flow click « Terrain Preprocessing »-> « Stream definition ». The definition window opens:
You can return the number of storage cells, the average surface watershed from which we consider that there exists a real one. It is very convenient that you can return just one of the values, and ArcHydro automatically calculates the other. In our example, we return 150 cells, which corresponds to a catchment area of about 0.84 sq km.
The result is the following :
In the next article we will use this network in order to calculate the watersheds.