Introduction to Hydrology (Geog 3511)

Fall 2012

Assignment 1: The Water Balance and Catchment Manipulation



The data (below) derive from the Wagonwheel Gap Experiment, a classic paired basin study of 75 years ago. These data come from the experimental basin only. The catchment is in the upper Rio Grande valley (between South Fork and Creede) at about 10,000 ft elevation. It has an area of about 200 acres and was studied in the period 1910-1926. In 1910, 25% of it was covered by uncut spruce-fir forest, 61% by aspen, and 14% by grass meadows. In 1919-1920, the forest was cut, all large timber removed and the slash burnt (Sept 1920 for the last). By 1926, aspen regrowth over the catchment had reached 3 ft to 6 ft in height.

Year Precip ($P$) Runoff ($Ru$) Year Precip($P$) Runoff ($Ru$)
1911 21.5 8.4 1919 21.8 8.6
1912 19.7 5.3 1920 22.5 8.3
1913 21.9 5.6 1921 20.5 8.7
1914 19.8 5.4 1922 23.8 7.2
1915 21.1 4.5 1923 16.8 8.0
1916 22.8 9.9 1924 22.4 5.0
1917 18.8 3.5 1925 18.1 5.1
1918 21.2 6.0

Years are water years, i.e. October 1 - September 30. Precipitation and runoff values for each water year are in inches.


  1. Estimate evapotranspiration ($Et$) for each year. Assuming there is not change in storage ($\Delta S=0.0$)., you can estimate $Et$ as the residual difference between Precipitation and Runoff (i.e. using a water balance model).

  2. Estimate the catchment efficiency for each year (%). In terms of water as a resource available for use, efficiency can be defined as the percentage of precipitation that was converted to runoff.

  3. Graph the changes in precipitation ($P$), runoff ($Ru$), evapotranspiration ($Et$), and efficiency through time. Mark on your graphs the 1919 -1920 period when conditions in the basin changed.

  4. Use your results to answer the question "Did clear cutting have an effect on runoff or efficiency?" Briefly (ca. 5 lines), give reasons for drawing the conclusion you have.

Hints and Procedures

Even with a small data set like this one, you will find that using a spreadsheet routine (e.g. EXCEL or QUATTRO-PRO) will prove more efficient than manual labor. By allowing you to copy operations across multiple cells and graph data as X Y Plots it will give you cleaner and more consistent results than your own calculator and pencil work.