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Analyzing Precipitation Probabilities and Recurrence Intervals for Potlatch, Idaho, Lab Reports of Geography

Instructions for a lab exercise in hydrology and water resources at the university of oregon (uo), focusing on calculating and interpreting cumulative probabilities, exceedence probabilities, and recurrence intervals for precipitation data from potlatch, idaho. Students will use excel to sort and calculate the necessary metrics and create a table for analysis. The exercise aims to help students understand the potential for precipitation of different quantities and characterize the site for various purposes.

Typology: Lab Reports

Pre 2010

Uploaded on 09/17/2009

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Hydrology and Water Resources, UO
Geography 4/525
Exercise 3:
“Predicting” precipitation:
Cumulative probability, exceedence probability, & recurrence intervals
The primary goal of this exercise is to familiarize you with the most widely used technique for
characterizing and predicting hydrologic phenomenon – the recurrence interval/exceedence
probability approach. This lab also introduces you to some simple statistical concepts if you have
note yet taken classes on this topic. You will conduct a similar analysis in a later lab on river
discharge.
In this lab you will:
Calculate the cumulative probabilities, exceedence probabilities, & recurrence intervals for
precipitation events at a site.
Use your results to identify the potential for precipitation of different quantities and
characterize the site in terms of its usefulness for different purposes.
The materials you will turn in are:
A table of cumulative probabilities, exceedence probabilities, and recurrences intervals for
Potlatch, Idaho.
Typed answers to the questions at the end of the lab.
This exercise requires you to use Excel, or a spreadsheet that can read Excel files. Please turn in you
graphs and answers by the time posted on the class web site. Late assignments will be penalized
15% per day, unless you have a documented excuse. Answers to the questions must be typed to
receive credit.
Please read the entire exercise before beginning.
1. Calculating and Plotting Precipitation Probability
In this lab you will analyze yearly precipitation data from Potlatch, Idaho from 1915 to 2002, with a
few years disqualified because of missing data. Consult the class handout on statistical concepts
(also available via the “Handouts” link on the class web page) to help you perform this analysis and
interpret your results.
The precipitation data for this exercise are available in Excel spreadsheet format at the class website
(look under Exercise 03). When we first did this lab, we used data directly from the National
Climate Data Center. Since that time, the NCDC has instituted charges for data, so we now provide
data from the Western Regional Climate Center for you. If you are in need of detailed precipitation
data in the future and have some funds, check out the NCDC data sets at:
http://lwf.ncdc.noaa.gov/oa/climate/climatedata.html
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Hydrology and Water Resources, UO Geography 4/

Exercise 3:

“Predicting” precipitation:

Cumulative probability, exceedence probability, & recurrence intervals

The primary goal of this exercise is to familiarize you with the most widely used technique for characterizing and predicting hydrologic phenomenon – the recurrence interval/exceedence probability approach. This lab also introduces you to some simple statistical concepts if you have note yet taken classes on this topic. You will conduct a similar analysis in a later lab on river discharge. In this lab you will:  Calculate the cumulative probabilities, exceedence probabilities, & recurrence intervals for precipitation events at a site.  Use your results to identify the potential for precipitation of different quantities and characterize the site in terms of its usefulness for different purposes. The materials you will turn in are:  A table of cumulative probabilities, exceedence probabilities, and recurrences intervals for Potlatch, Idaho.  Typed answers to the questions at the end of the lab. This exercise requires you to use Excel, or a spreadsheet that can read Excel files. Please turn in you graphs and answers by the time posted on the class web site. Late assignments will be penalized 15% per day, unless you have a documented excuse. Answers to the questions must be typed to receive credit. Please read the entire exercise before beginning.

1. Calculating and Plotting Precipitation Probability In this lab you will analyze yearly precipitation data from Potlatch, Idaho from 1915 to 2002, with a few years disqualified because of missing data. Consult the class handout on statistical concepts (also available via the “Handouts” link on the class web page) to help you perform this analysis and interpret your results. The precipitation data for this exercise are available in Excel spreadsheet format at the class website (look under Exercise 03). When we first did this lab, we used data directly from the National Climate Data Center. Since that time, the NCDC has instituted charges for data, so we now provide data from the Western Regional Climate Center for you. If you are in need of detailed precipitation data in the future and have some funds, check out the NCDC data sets at: http://lwf.ncdc.noaa.gov/oa/climate/climatedata.html

The following steps basically ask you to sort the precipitation from lowest to highest value, then do some simple percentage calculations. The following sequence is somewhat long only because we are providing a button-pushing explanation of how to use Excel to sort data and calculate the various metrics. If you are already familiar with Excel, you could probably do all these steps in five minutes or less. To calculate cumulative probabilities, exceedence probabilities, and recurrences intervals, follow these steps: Calculating cumulative probability for the precipitation record i. Highlight all the data (not including the headers). ii. Now sort the data so it is arranged from lowest to highest value. Click on “Data” at the top of the spreadsheet (where it says File, Edit, View, etc.) and select “Sort” from the drop- down menu. The Sort window will appear. iii. Where it says “Sort by” change the selection to “Ppt. (in.)” (click on the little blue arrow to reveal options. Leave “ascending” marked. Press “OK”. This will sort all the data from lowest to highest precipitation value while keeping the year attached to the precip value. iv. In the cell to the right of the first data point enter “1” and press return. In the next cell below the “1” enter a “2” and return. Highlight both cells C2 and C3. Place the cursor on the bottom right corner of cell C3 and a little fat black “ + ” should appear. Left click on the mouse and drag down all the way to the end of the data set. This gives you the rank of the precipitation values (1 being lowest, and 85 being highest). v. Now calculate the cumulative probability as a percentage. In the cell to the right of the “1”, enter the formula “=(C2/86)100”. [Note that we are using n+1, or 85+1=86, as our denominator. This is a standard statistical procedure to remove bias from our estimate of the population.] Drag this formula down to the lower end of the data set the way you did above (except you are only dragging from one cell). Now for each year, you have a cumulative probability value (see handout for explanation). Calculating exceedence probability i. In the cell to the right of your formula enter “85”. In the cell below the 85 enter “84”. Make a column of values decreasing sequentially from 85 to 1 using the same technique as in iv above. ii. In the cell to the right of your “85” enter the formula “=(E2/86)100. This calculates the exceedance probability. Copy the formula down to the lower end of the data set as above. Refer to your handout as to what this tells you. Calculating recurrence intervals i. At the top of the next column to the right In cell G2, enter the formula “86/E2”. This calculates the recurrence interval. Copy the formula down to the bottom of the data set as above Refer to your handout as to what this tells you.

Questions Name:

Exercise 3

Please type your answer the following questions, limiting your answer to one to two paragraphs. Consult the class handout to guide your answers. Create a table that shows the precipitation amount, the rank, the cumulative probability, the exceedence probability, and the recurrence interval for all the annual precipitation amounts from Potlatch. You can create this table by cut and pasting from Excel into your word processing program. Once you have created this table, answer the following questions.

  1. We might wonder if this site is suitable for non-irrigated agriculture. What is the probability that this location will receive less than 20” annually, which is the standard amount used to determine if irrigation might be needed?
  2. What is the probability of this site receiving more than 30 inches of precipitation in a given year?
  3. For any year, what is the sum of cumulative probability and exceedance probability? Does this make sense to you?
  4. What is the approximate precipitation (in inches) associated with a 50-year storm for this site?
  5. In verbal terms, what is the meaning of the 50-yr storm? i.e. -0 how would you explain this number to a member of the public? Turn in your answers to the questions above along with your table of recurrence intervals and exceedence probabilities.