Watershed & Watershed Delineation | Calculating Catchment Area

What is Watershed & Watershed Delineation ?

A watershed is all the land & water which contributes runoff to a common point or outlet point. It is also called catchment or drainage area. The terms catchment & watershed are found to be used interchangeably. So, watershed area is the land area, from where all the surface runoff converges to a single point to give common discharge. Watershed delineation thus refers to creating boundary that represents the contributing area for a particular outlet point.

Why Watershed Delineation is required ?

Watershed delineation is done primarily to define the boundary of study area or to divide the study area into subareas. Also, watershed delineation is the initial job for calculating catchment area. So, watershed delineation has major importance in watershed management, soil conservation & hydrometerological studies.

How to delineate watershed ?

Delineating watershed mean you're drawing boundary lines of catchment in a map. Traditionally, this is done manually in topographic map using information from contours but some digital methods are also available nowadays. Watershed delineation can be done by following different methods:

1. Topo Map Method
2. Google Earth Pro Method
3. GIS Method

While dealing with topo map method, one should have sound knowledge of contour lines & map scale. We should be able to visualize the terrain by seeing the contour lines. We should find where ridge & valley exists and also note that the flow direction is always perpendicular to the contour line. Here are few steps for watershed delineation & calculation of catchment area from topo map:

• Mark the outlet point in topo map
• Starting from outlet point, draw lines perpendicular to the contours in such a way that the line passes through the ridges
• Following the procedure, connect all the possible extreme points whose discharge converges into the outlet & finally, end at outlet point.
• After demarcation of the catchment boundary in topo map, trace the delineated area in transparent graph paper. Count the number of squares in the graph paper that the catchment has covered & keeping in mind of map scale, determine the value of catchment area.

                    As an alternative to this graph paper method, we can use Autocad to calculate the catchment area. For this, scan the topo map catchment & import it to Autocad interface. Fix the scale of imported image & retrace the catchment boundary in Autocad using polylines. Now, area of  polygon can easily be determined in Autocad.

Google earth pro method for watershed delineation is quite famous nowadays. Using Google earth pro, we can visualize our catchment in 3D & draw catchment boundary as a polygon by eye judgement & instantly, Google earth pro shows you the area covered by polygon in required unit.

GIS method of watershed delineation is most accurate among all mentioned here. While using GIS for watershed delineation & determination of catchment area, we need survey data of our study area in terms of latitude, longitude & elevation. The data is then converted into DEM format in ArcGIS & different tools are available in ArcGIS in order to delineate watershed & determine the corresponding area. Here, ArcGIS automatically delineates catchment for you once you provide correct information to it. If survey data of study area is not available, we can download the DEM data directly from USGS Earth Explorer for free & can proceed further.