Question

A hyetograph table for a precipitation event is given as below. Calculate the total precipitation depth (in cm) for this event.

Time Interval (min)	0 - 30	30 - 60	60 - 90	90 - 120
Intensity (mm/hr)	4	8	17	7

Answer 1

Area under the hyetograph gives the total precipitation depth:

Time interval (minutes)	0-30	30 - 60	60-90	90-120
Intensity (mm/hour)	4	8	17	7

Area under the hyetograph = Area of all the rectangles (shown above)

Area of rectangle = Time interval (in hour) x Intensity(in mm/hour)

(All the time interval has been calculated and converted to hour as shown in steps below)

Area of (rectangle 1):

Thus, Area of (rectangle 1) = 2 mm................................................(A)

Similarly,

Area of (rectangle 2):

Thus, Area of (rectangle 2) = 4 mm................................................(B)

Area of (rectangle 3):

Thus, Area of (rectangle 3) = 8.5 mm................................................(C)

Area of (rectangle 4):

Thus, Area of (rectangle 4) = 3.5 mm................................................(D)

Area under the hyetograph = Area of all the rectangles

Area under the hyetograph = (A)+(B)+(C)+(D)

Area under the hyetograph = 2+4+8.5+3.5

Area under the hyetograph = 18 mm or 1.8 cm

Thus the the total precipitation depth (in cm) = 1.8 cm

HYETOGRAPH 17 Rainfallintensity (mm/hour) Time (minutes) 10-30 30-60 60-90 90-120

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