Watershed Delineation Consider the following topographic map The

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Watershed Delineation Consider the following topographic map. The point of interest is labeled with

Watershed Delineation Consider the following topographic map. The point of interest is labeled with the letter A. Your goal is to draw a continuous line that identifies the area that contributes surface runoff to this point. When you are done, any drop of runoff within your defined basin should drain to the designated point.

Point A WS outlet

Point A WS outlet

One approach—ideal if you are learning watershed delineation for the first time Begin by

One approach—ideal if you are learning watershed delineation for the first time Begin by high-lighting the stream network that drains to your designated point. [shown in blue] Continue by high-lighting streams in adjacent networks. These are streams that concentrate runoff from “the other side of the ridge and channel it to other outlets. ” Use a different color to mark these streams. [red] You should now be able to visualize the general path for the watershed boundary. It is the region (corridor) between the two sets of highlighted stream networks. [indicated in yellow, but don’t highlight it on your map]

Highlight stream draining to point A Highlight streams in adjoining watersheds Visualize general path

Highlight stream draining to point A Highlight streams in adjoining watersheds Visualize general path for WS boundary

For the purposes of this Power. Point show, zoom closer onto the area draining

For the purposes of this Power. Point show, zoom closer onto the area draining to point A.

Continue to fine-tune the delineation. Create addition guidelines by marking the location of the

Continue to fine-tune the delineation. Create addition guidelines by marking the location of the high points around the stream draining to the designated point. Use these points to refine the general path of the watershed boundary. You are getting close.

Mark high points Refine general watershed boundary using the marked high points

Mark high points Refine general watershed boundary using the marked high points

How much did adding more guidelines, i. e. , the high points, revise the

How much did adding more guidelines, i. e. , the high points, revise the general watershed boundary? Overlay the two general boundaries for comparison.

First general boundary Revised general boundary

First general boundary Revised general boundary

Finalize WS boundary delineation Runoff will flow perpendicular to a contour (streamlines are perpendicular

Finalize WS boundary delineation Runoff will flow perpendicular to a contour (streamlines are perpendicular to equipotential lines). The divide will always run perpendicular to the contours. High points typically mark the divide between different drainage basins. An exception is when there is an isolated hill (mountain) inside the watershed.

Start at the watershed outlet. Go one direction from the outlet. Move perpendicular to

Start at the watershed outlet. Go one direction from the outlet. Move perpendicular to the contours. Follow the contours uphill until you reach a high point. Continue along the basin’s edge. You may have to go up and down, and across saddles to get to the next high point. If done correctly, you should end at the outlet on the opposite side of the stream where you started. The area outlined by your journey marks the drainage basin for the designated point.

Visualize drainage “on the other side of the ridge” at a closer scale.

Visualize drainage “on the other side of the ridge” at a closer scale.

Visualize drainage to the stream flowing to point A at closer scale.

Visualize drainage to the stream flowing to point A at closer scale.

Note pattern at a “saddle”

Note pattern at a “saddle”

A sometimes useful graphical aid. WS boundary goes between these two vectors, which are

A sometimes useful graphical aid. WS boundary goes between these two vectors, which are streamlines for flow on opposite sides of the ridge.

Delineate watershed boundary

Delineate watershed boundary

DOP cannot draw perfect lines with Auto. Shapes…the boundary is close…maybe not perfect.

DOP cannot draw perfect lines with Auto. Shapes…the boundary is close…maybe not perfect.

Boundary for watershed draining to point A.

Boundary for watershed draining to point A.

One Final Word of Caution and Advice After completing this process, you have successfully

One Final Word of Caution and Advice After completing this process, you have successfully delineated the watershed draining to Point A. Correct? Maybe yes, maybe no. Were there any places where you made a judgment call, such as the points where there was a local gully? Or maybe a roadway and the map doesn’t clearly indicate cross-drainage. How do you resolve such issues?

The Final Step So, how do you resolve uncertainty? And what is the final

The Final Step So, how do you resolve uncertainty? And what is the final step…with any watershed delineation? The Answer Go to the watershed, put your feet on the ground, and see (observe) the local features. Worst case, you may actually have to make on-site measurements, i. e. , survey the site.