Calculating the catchment area, also known as the drainage basin or watershed, from a contour map involves identifying the geographic region where all surface water runoff converges to a specific point, such as a lake, river, or gauge station. This area is delineated based on the topography shown by contour lines.
Understanding Catchment Area
A catchment area is defined by the surrounding high ground or ridgelines that separate it from adjacent drainage basins. Water flowing within this boundary will drain towards the designated outlet point.
Standard Method: Delineating and Measuring the Drainage Basin
The most common method for determining a catchment area from a contour map involves delineating the boundaries based on elevation and flow direction.
- Identify the Outlet Point (Pour Point): Pinpoint the specific location for which you want to calculate the catchment area. This could be the outlet of a lake, a bridge crossing a stream, or any other point of interest on the watercourse.
- Trace Uphill Along Ridgelines: Starting from the outlet point, visually follow the highest elevation points that surround the area. These ridgelines act as natural divides for water flow.
- Delineate the Boundary: Draw a boundary line along these identified ridgelines. This line represents the watershed divide. The boundary line should always be perpendicular to the contour lines it crosses, as water flows downhill along the steepest slope, which is perpendicular to contours. Extend the line until it forms a closed loop enclosing the area that drains to your outlet point.
- Measure the Enclosed Area: Once the boundary is drawn, calculate the area enclosed within this line on the map. This can be done using various techniques:
- Manual Method: Use a planimeter or overlay a grid system on the map and count the grid squares within the boundary.
- Digital Method: Scan the map or use digital mapping software (GIS - Geographic Information System) to trace the boundary and automatically calculate the area.
This standard method accurately defines the area contributing surface runoff to the point of interest based on topography.
An Approach Mentioned in a Reference
A different approach described in a reference outlines specific steps for calculating catchment area, particularly in relation to a pond or lake. According to this method:
- Identify the elevation of all the points that surround the pond or lake on the contour map.
- Calculate the catchment area by adding up the areas of all the points that have an elevation that is lower than the elevation of the pond or lake.
(Note: This method, as described, deviates from the standard hydrological definition and calculation of a drainage basin which typically involves identifying the area that drains into the lake, i.e., the upstream area at higher or equal elevation boundaries).
Why is Catchment Area Important?
Understanding catchment area is crucial for various applications:
- Water Resource Management: Estimating water yield and planning water supply systems.
- Flood Prediction: Analyzing how much rainfall runoff an area can generate.
- Environmental Studies: Assessing pollution sources and habitat suitability.
- Infrastructure Planning: Designing bridges, culverts, and drainage systems.
By accurately calculating the catchment area using contour maps, engineers, environmental scientists, and planners can make informed decisions about land and water management. Modern techniques often utilize digital elevation models (DEMs) and GIS software for more efficient and precise catchment area calculations.
