Assignment #2: Areal Estimation of Rainfall.

Assignment #2: Areal Estimation of Rainfall.

The objectives of the assignment are to:

1. Learn and apply methods for scaling point measurements of precipitation to entire watersheds.
2. Understand why estimates may differ between these methods.

Instructions:

You will find attached a map showing the boundaries of a fictitious watershed and the locations of nine rain gages. You will be required to use this map and values to estimate “average” precipitation by three different methods.  The numbers on the map are the gage catches for a particular storm in mm. Each square on the map is equal to one hectare in area. You may wish to review your text, as well as your class notes, and the help provided to you by your TA during tutorial.

Areal Estimation of Rainfall

1. What is the total area of the watershed? No example calculation required (1 mark)

1. Compute the average rainfall falling within the watershed using the arithmetic average approach. Be sure to write out your calculation to show your work (3 marks).

1. Draw “eyeball” isohyets on the attached watershed map at a contour interval of 20 mm, and on the 20’s (e.g., 20, 40, 60, 80, etc as necessary) (8 marks).
2. Using the eyeball isohyetal method, fill in the following table and estimate the average rainfall across the watershed (6 marks).

 i pi– (mm) pi+ (mm) (mm) Fraction of Area, ai/A (ai/A)· (mm) 1 2 3 4 … Total:

1. Draw Thiessen polygons on the attached map (8 marks).

1. Estimate the area of your polygons (5 marks). Use the Thiessen polygon method to fill in the fill in the following table (4 marks).  What is the total “average” precipitation in the watershed? (1 mark)

 g pg (mm) Fraction of Area, ag/A (ag/A)· (mm) 1 2 3 4 5 6 7 … Total:

1. Write a short paragraph (<1/2 page) comparing the results of the three methods, patterns you observe, and discussing which method seems most appropriate for this situation. Given the isohyetal pattern, do you think this is frontal, convectional, or orographic lifting and why (7 marks)?

Assignment #2: Areal Estimation of Rainfall.

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