In this tutorial we work on the CSV file download with this tutorial.
This tutorial explains how to work with the downloaded CSV file to compute cumulative precipitation for each quartile of the temporal distribution and for each probability of occurrence.
1. Transform the cumulative percentage of total precipitation into a cumulative fraction
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The downloaded CSV file provides cumulative precipitation as percentages (0–100%) for each quartile.
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To work with these data in calculations, divide each value by 100 to convert it into a fraction ranging from 0 to 1.
2. Calculate the incremental fraction of precipitation for each time step
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Using the cumulative fraction table obtained in step 1, compute the incremental fraction at each time step.
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This is done by subtracting the previous cumulative fraction from the current one:
Incremental Fractioni = Cumulative Fractioni − Cumulative Fractioni-1
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Cumulative fractions: 0.10, 0.25, 0.45, 0.70
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Incremental fractions: 0.10, 0.15, 0.20, 0.25
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The first time step is simply equal to its cumulative fraction.
3. Calculate precipitation for each time step to obtain the rainfall hyetograph
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First, enter the total precipitation for the event in a cell at the top of your sheet.
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For example, enter 5 (inches) as P_Max_24h.
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In Excel, multiply each incremental fraction (from step 2) by this total precipitation value to get the precipitation for each time step:
Precipitationi = Incremental Fractioni × PMax 24h
4. Compute the cumulative precipitation from the rainfall hyetograph
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Using the rainfall values calculated for each time step (from step 3), compute the cumulative precipitation by summing the rainfall sequentially.
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