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

Download Analyzing Precipitation Probabilities and Recurrence Intervals for Potlatch, Idaho and more Lab Reports Geography in PDF only on Docsity! 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 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.