Surface Water Hydrology

    Hydrology is the study of water, and surface water hydrology the study of runoff, streams, and lakes.  Runoff is the transfer of water from the ground (as rainfall or snowmelt) to streams and rivers.  Once in a stream, runoff can be measured as discharge in units of volume/time: cubic feet per second (cfs, cusecs, ft³/sec), cubic meters per second (cms, cumecs, m³s^-1), acre-feet per year...
    Please respond to the questions below emphasized in green text. You are encouraged to discuss these questions with your partner and adjacent teams.  Make sure that you understand each of the italicized words, some of which are defined here (shown in boldface).  Underlining (except for hot links) emphasizes key terms or steps in your answer process.

I)    Stream Discharge

1. Look at the Web page for the Gallatin River at Gallatin Gateway, MT.  [NOTE: if you right-click on the hot link and "Open in new Window", you can then right-click on an empty part of the Windows taskbar (at the bottom of the screen) and tile the screen to view both pages at once!]  This page and others like it are products of the United States Geological Survey.
   What is the most recent discharge?  How long ago was it measured?  How do you think they do that?
2. Look at the hydrograph - a graph of discharge with time.  The hydrograph you have displayed on your computer at this time displays data from the Gallatin River for the past week.
   What are the highest and lowest discharges over that time?  What is the pattern of discharge over the past week (rising, falling, oscillating...)?  Explain that pattern in terms of the recent weather.  Is the present discharge higher or lower than the long-term average (triangles)?  Explain that observation in terms of this year's weather.
3. Look at the graph of the river stage - its depth relative to an arbitrary starting point, or datum.
   What is the relationship between discharge and stage?  Would you have expected this?  Is this relationship true for all conceivable discharges and stages?  Explain.

    How would you have found the Gallatin River discharge without a hot link?  The USGS, among its excellent Web pages, has access to both real-time data like you saw for the Gallatin River and historic data from its Water Resources page for the entire country.

II)    Stream Hydrology

The graph below shows the daily hydrograph for Bridger Creek near Bozeman, MT for an entire year.  The data from which this graph was generated were downloaded from the USGS Historical Data page for Bridger Creek and were processed in Microsoft Excel for graphing.

Because discharge values often span several orders of magnitude (powers of ten), discharge data are often shown on a logarithmic axis.  Click here (right-click; new window) to view the same data graphed logarithmically.  This graph may help you to explain the stream discharge behavior from June to August and at low flow. A)  Examine the graph and describe the trend in daily discharge and the inferred weather and/or climate which generated the discharge pattern evident at the following times: 1/1/64 - 4/1/64 4/1/64 - 4/28/64 4/28/64 - 5/7/64 5/7/64 - 5/17/64 5/17/64 - 6/2/64 6/2/64 - 7/29/64 7/29/64 - 12/14/64 12/14/64 - 12/31/64

III)    Flood Hydrology

Discharge which exceeds the channel capacity is termed flood discharge.  Floods do much of the work of shaping river channels and valleys through erosion and deposition.  They also are among the most dangerous of natural events, resulting in tens of deaths and hundreds of millions of dollars in damage or more each year. 

A)  Use the "Available data" drop-down menu to go to "Surface water: peak streamflow" data on the USGS Gallatin River at Gallatin Gateway page.  What we will do is to download those data, import them into Microsoft Excel, and analyze them.

B)  Build a spreadsheet containing the historic annual peak discharges from the Gallatin River.  Choose "Tab-separated file"

1. Select your data.  Scroll down the page to locate the bottom of the table of discharge records.  Carefully, click and drag to highlight the entire table of data, not including the column labels and blank row at the top.  Right-click and press "Copy" (or hit Control-C) to copy highlighted block.  
2. Import your data.  Close the USGS page, open Microsoft Excel (Start, Programs, Microsoft Office, Microsoft Excel), and share screen space with this page ("Tile").  Click on "File", "New", and open a blank worksheet.  Right-click in the top left cell and press "Paste special, Unicode text".  The data may import automatically into columns.  If so, skip the next section.  
3. Parse your data.  "Parsing" is splitting text data into spreadsheet columns.  If your data was not automatically parsed (the data are not clearly split into distinct cells), follow these steps.  Select Column A (by clicking on "A").   From the menu bar at the top of the page, select "Data", then "Text to Columns".  The first few rows of your data should appear in a gray box.  Key the "Delimited" radio button, then "Next".  On the next page, check (click) in the box labeled "Space".  Your data should immediately separate into columns.  
4. Clean your data.   For example, in the Gallatin River data, 1966 has footnote symbols which take up a cell.  I would suggest deleting columns you are sure you don't need. You should end up with a list of dates, gage heights, and discharges.  
5. Label your data.  To keep things straight, insert a title row by right-clicking on the "1" of the first row and choosing "Insert".  You can now label the data columns.  

C)  Analyze your data as suggested below.

1. Is there an obvious relationship between discharge and gage height?  
   • Sort your data by decreasing discharge. 
   • Click-drag to select all of your data (and headings).
   • Click on "Data", "Sort", and select the column of discharges. (If you highlighted the column heads, make sure that the corresponding radio button is clicked at the bottom of the "Sort" screen.).  What are the largest four discharges and gage heights?  Do they make sense?
   To investigate further, graph stage as a function of discharge.
   • Highlight your columns of stage and discharge data.
   • Click on Insert... Chart.
   • Choose "XY (Scatter)" - this is important!  Click on the top box (symbols only), if not already selected.
   • On the next page, click on the "Series" tab, ensure there is only one series, and make the X values the discharge column and Y values the stage.
   • You can title your graph ("Rating Curve for the Gallatin River near Gallatin Gateway"), the X-axis ("Discharge (cfs)), and the Y-axis (Stage (ft)), and on the next page, leave it as an object on your page. 
   • Explain the distribution of discharges and stages.
2. What flood discharges can we expect, and with what frequency, in the future?  
   • What you have done is to rank-order the historic floods. 
   • To label the ranks, go to the first empty column and type "Rank" in the first cell and "1", "2", "3" in the cells in rows 2, 3, and 4 (assuming a header). 
   • Click and drag to select those three cells, then click and drag on the "handle" visible at the bottom right of the highlighted block to extend that series to the bottom of your data set.  How many years of record are there (what is the last number)?
   • The recurrence interval, or average time between floods of a given size, is calculated from the equation   R.I. = N + 1 / M, where R.I. is recurrence interval (in years), N is the number of years of record, and M is the rank within that record. 
   • In the adjacent empty column, type "R.I." in the first cell, then in the second cell (for 73 years of record) "=74/cell", where "cell" is the address of the rank for that discharge (example: D2, if the "Rank" column is column D and there is no header information). 
   • When you hit "Enter", a number greater by one than the number of years of record should appear in that cell.  That is the recurrence interval for that discharge.
   • Select that cell and drag the handle down the entire column to copy the equation.  A declining series of numbers should appear. 
   • Using the graphing technique described above, graph recurrence interval against discharge.  You will want to use a logarithmic axis for the recurrence interval - right-click on the axis labels, select the "Scale" tab, and check "Logarithmic scale".
   • Using the line drawing tool from the Drawing toolbar (if one isn't evident, click on "View", "Toolbars", "Drawing" from the top menu bar) draw a line along the straightest part of the data. 
   • If that line was extended ("extrapolated") to 100 years, what would the approximate discharge be?

 This page constructed by William Locke on February 28, 1999.  All text and visuals are copyrighted.  Permission to use for noncommercial purposes (educational/scientific) is freely granted, with the condition of a courtesy notification via e-mail to Dr. Locke.