Geography 416 - Hydrology

Problem Set 2

Due Friday, Feb. 13

1.	Open the spreadsheet file wdf77_87 . wk1 using Lotus, Quattro, or Excel. In this file you will find a listing of the mean daily streamflow records for Watts Branch in Rockville, Maryland over a ten-year period between Oct. 1, 1977 and Sept. 30, 1987. I have sorted these records and placed them in rank order from smallest to largest, and I have calculated cumulative frequencies and exceedence probabilities for each day of record. The formulas necessary to do this are built into the spreadsheet. In addition I have also used some of the statistical functions in Quattro Pro version 5 to calculate sample statistics for this ten-year record. I have also used a procedure for calculating cumulative frequencies associated with a subdivisions of the data more or less following the guidelines on p. 11-13 in the textbook. I have used another graphics package to plot the resulting flow duration curve (discharge or streamflow on the y-axis, exceedence probability on the x-axis). I am handing out two versions of this plot. Both have probability scales on the x-axis; one has a linear y-scale and the other has a logarithmic y-scale.

Your job is to perform the same kind of analysis for another ten-year period from the same stream gage: from Oct. 1, 1957 to Sept. 30, 1967. This record is contained in the spreadsheet file wdf57_67 . wk1 . You should calculate the mean, median, standard deviation, coefficient of variation and skewness of this record and compare it with the values calculated for the period 1977-1987. (If you don't know how to do this, follow the procedure in Box C-2, p. 496.) Once you have completed the analysis necessary for generating a flow duration curve, plot the results in pencil on the same graph I have provided for you. In your opinion are the two records substantially the same or are they different in significant ways? Compare the flows associated with specific quantiles , e.g. exceedence probabilities of 5, 10, 20, 50, 70, 80, 90, 95 percent. What might this comparison tell you about processes going on in the Watts Branch watershed?

You may choose to hand in your spreadsheet file as part of your homework - if you wish to do so, let me know in advance.

(20 points)

2.	A third spreadsheet file, patsav96 . wk1 , contains a record of rainfall and streamflow for the Little Patuxent River at Savage, MD (about 15 minutes from campus) for a flood that occurred about 2- 1/2 weeks ago. You need to plot both rainfall and runoff on the same plot (if you don't have a version of a spreadsheet that can do this, see me and we'll make an arrangement). Also you need to convert the instantaneous discharge record into a series of flow volumes for each time period, and to convert those into equivalent water depths over the watershed drainage area. Finally you need to plot cumulative runoff vs. time and cumulative rainfall vs. time in inches. How does the total depth of rainfall compare with the cumulative equivalent depth of runoff generated? How can you explain the observed result?

If you don't know how to perform these analyses , don't worry. The purpose of the exercise is to teach you how to do this kind of work with real-world examples. We will be discussing the details before you actually have to complete the assignment. There is also a specific point to each analysis that has to do with the actual hydrologic processes involved, and as you go along I want you to think about the meaning of the assignment and discuss it with others once the actual computations are complete.

(20 points)