Hydrology and Water Resources, UO
Geography 4/525
Exercise 1: Spatial variations in precipitation
The goals of this exercise are to develop skills in downloading and cataloging precipitation data, learn a common approach for mapping spatial patterns in precipitation, speculate and analyze the spatial patterns in your map, and consider potential sources of uncertainty in your findings.
Specifically, in this exercise, you will:
retrieve precipitation data on the internet;
organize and analyze the data
illustrate spatial patterns of mean annual precipitation through a isohyetal contour map
evaluate your findings in the context of data uncertainty and natural variability.
The materials you will turn in will be:
A bar graph of monthly mean average precipitation for Astoria
An isohyet map of precipitation patterns across the state of Oregon based on 18 locations
Typed answers to questions 1-6 at the end of the lab
In doing this exercise, we assume that you can sort data in Excel and do simple calculations such as averaging. It is also critical that you know how to draw isolines (e.g., contours) based on point data, a skill that is covered in all the prereq classes for hydrology. We’ll be available during our office hours to help you if you have problems.
Please turn in you graphs and answers at the time posted on the class web site. Late assignments will be penalized 15% per day, unless you have a valid medical excuse. Answers to the questions must be typed to receive credit.
Please read the entire exercise before beginning.
1. Data retrieval , formatting, and data issues
Many precipitation data sets are available on the worldwide web. In this exercise you will work with precipitation data from the Western Regional Climate Center (WRCC). This page contains climatic data for fairly long time spans, for free (there are other more comprehensive precipitation data sets from the U.S. government that cost money to access). In this exercise you will copy the numeric data to your spreadsheet and manipulate them in Excel to produce your own output.
- Log on to: This takes you to a map of Oregon and surrounding areas with weather stations indicated by little red squares. Scroll down the list of stations on the left of the page, and once you find the list of Oregon stations, click on “AstoriaWsoAirport.”
- Now you should see a summary of monthly climate data for AstoriaAirporton the right side of the screen, and MANY options on a list to your left. The list on the left is divided into three categories: ‘1971-2000’, ‘1961-1990’, and ‘Period of Record.’
- Go down on the left-hand list under “Period of Record,’ find the “Precipitation’ category, and click on ‘Monthly Totals’ (right above the snowfall link).
- Open Microsoft Excel
- Go back to the WRCC page you were on and highlight all the data (drag from the year in the upper left down to very last numeric value in the bottom right hand corner. Make sure you include the summary statistics at the bottom.
- Now right-click in cell A1 of you Excel spreadsheet, and choose ‘paste’. The data should appear in your spreadsheet (2 columns per month, one with the monthly value in inches, and the second column mostly blank with an occasional letter). In some cases, you may need to ‘paste special’ or use some other option to get all the data in columns—ask us the lab assistant in SSIL if you have problems here. Save this spreadsheet on your computer or flash drive.
Now you should have precipitation data for Astoria for its period of record, along with summary statistics – all in Excel format. What we’re really after here is mean annual precipitation for sites across Oregon. This brings up some statistical considerations we would like you to think about (do NOT write down answers to these questions):
- How long a period of record should we use? The 30-year sets of data are called ‘climate normals’ because they are accepted as being from a long enough time period to be representative of the site. These are the ‘standards’ we use in climate or hydrologic analysis. There really is no magic answer for the length of a precipitation data set (or any time dependent data set)—generally it’s a matter of trading one consideration against another depending on the question at hand.
- What do we do about incomplete or missing data? At the top of each page of monthly totals, there is a list of the alphabetic symbols used to designate missing or incomplete data in the data set. Note the criteria used here to exclude data.
- How do we compare stations with different periods of record? Think about problems with matching a period of record amongst all 18 stations versus just using the longest period of record available for each individual station.
2. Calculating and graphing the mean monthly and mean annual precipitation
Take the 30 year mean from 1978 to 2007 and calculate the 30-yr average monthly and annual precipitation at AstoriaAirport (visit classmates or one of us if you don’t know how to calculate mea values in Excel). Only use the period-of-record mean values already provided at the bottom of the spreadsheet as an approximate check for your answers (the period-of-record mean is not for the period 1978 to 2007).
To graph the data, highlight the row of mean monthly precipitation values (make sure you only highlight the monthly values and don’t include the total). You should have 12 values (1 for each month). Click on the “Chart Wizard” button (about ½ across the bar at the top of the spreadsheet) and follow the steps to make a bar graph of the monthly average precipitation values for AstoriaAirport. Print off this graph along with the isohyet map you make below.
3. Making an Isohyet Map
Remember that when you first chose a station, you were given a ‘Period of Record Monthly Climate Summary’ that included a mean annual precipitation total (for Astoria Airportit was 68.33”). This value is for the entire period of record minus disqualified years. Get the mean annual precipitation value for the other 17 sites listed below and make a list. You’ll use this list to make a isohyet map of rainfall across the state of Oregon.
Astoria WsoGrants PassHood Rvr Exp Sta
Salem Wso AirportMadrasKlamath Falls Agr Sta
Tilamook 1 WEugene Wso AirportBurns Wso Ap
NewportSummit (not on map)Vale 1 W
Gold BeachDetroit Dam (not on map)Pendelton Wso Airport
Portland Wso CityBendLa Grande
Canyon City (check dates)FossilBrightwood (not on map)
The three sites not on the pdf map are on the wrcc map. Detroit Dam in on the Santiam River, east of Salem; Summit is on Mary Mountain; Brightwood is near Mt Hood. You can highlioght them on the WRCC map by moving your mouse over the site name in the left column.
On a printout of the pdf map that shows the location of most of th locations label the annual precipitation to the nearest inch. Now draw in isolines between the point locations, showing change over the state of Oregon in intervals of 5 inches. Ask if you want some help getting started drawing the lines in. It’s best to use a pencil. Remember that you can use your knowledge of Oregon’s topography to help your drawing of the isohyets. For example, how might the distribution of mountains in Oregon affect the isohyets?
Questions
Please provided typed answers to the questions on the next page and turn them in along with your isohyet map and bar graph.
QuestionsName:
Please type your answers to the following questions, limiting your answer to one to two paragraphs. As you answer these questions, think about how data error, natural variability, and your contouring skill may have affected the results.
1. How different are your values for the 30 yr average and the period-of-record average at AstoriaAirport? Now consider the 18 stations for which you used the period-of-record average to plot precipitation across Oregon. Could the use of period-or-record data rather than the more typically used 30-yr average potentially modify the spatial patterns? If so, what are the implications of such problems?
2. What sources of measurement error or bias potentially affect your data? (Focus on the data, not your analysis of it.) Discuss general categories of error and give one or two examples of each type of error.
3. What data related or cartographic factors might affect the accuracy of your the isohyet locations? Assume that the data at each point are correct.
4. Given your answers to 1, 2, and 3, describe the degree of confidence you have in your findings at: (a) different spatial scales across the state; and at (b) different temporal scales.
5. What might you do to improve your confidence in the findings? Are there reasons you would not want to take these steps to improve your answers.