Lab 2: Slope Aspect Lab

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Jayson Jenkins
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Lab : Slope Aspect Lab Objectives: to investigate potential differences between north- and south-facing slopes in the foothills of the Colorado Front Range to become familiar with the US Geological

1 Lab : Slope Aspect Lab Objectives: to investigate potential differences between north- and south-facing slopes in the foothills of the Colorado Front Range to become familiar with the US Geological Survey s online program StreamStats to practice plotting and interpreting data in Excel and to apply some simple statistical tests Background: Research in a variety of arid and semiarid regions indicates that north- versus southfacing slopes typically have differences in moisture retention, hillslope processes and morphology. North-facing slopes are able to retain moisture (snow and rain at the surface and in the soil) longer than south-facing slopes, particularly in winter, when north-facing slopes may be continually shaded. Greater moisture translates to differences in bedrock weathering, soil development, vegetation, surface and subsurface erosional processes and, ultimately, hillslope morphology. The photo below was taken in January 03. In this view of the foothills, looking west from Route 87 just beyond the intersection with Shields Avenue, you can clearly see that north-facing slope (north is to the right) retain snow, whereas south-facing slopes are largely free of snow. North-facing slopes are also more likely to have coniferous trees; south-facing slopes are mostly covered in shrubs and grasses. North-facing slopes usually have thicker and better developed soils (greater clay content, more clearly differentiated soil horizons) with greater infiltration capacity. At higher elevations in the Colorado Front Range, moisture becomes less of a limiting factor (i.e., moisture is more abundant), and aspect-related differences in hillslope process and form become much less pronounced. In the foothills and lower elevations of the mountains, however, up to approximately 300 m (7600 ft) elevation, north- and south-facing slopes have different geomorphic processes and forms.

2 Tasks: ) Access the interactive map on the Colorado StreamStats website ) For the following pairs of points, use the Zoom To tab and enter the latitude and longitude. The point will show up on the screen. Go to :0,000 scale view, and use the terrain profile tool to generate a profile across the drainage between each pair of points. Use the values on the profile to calculate the average gradient of each north- and southfacing slope. Before leaving each pair of points, see task #3. Labeau Gulch Well Gulch Brown Gulch Long Gulch Rist Canyon Unnamed Poudre at Greyrock Devil Gulch Empire Gulch Soldier Canyon ) For each drainage, use the Watershed Delineation from a Point tool to calculate the upstream drainage area and mean basin slope. Values are provided in square miles for the drainage area: convert to square kilometers ( km = 0.36 mi ).

3 4) Plot drainage area versus mean basin slope, drainage area versus average hillslope gradient (average north and south values), and drainage area versus difference in hillslope gradient between north and south using Excel (Insert, Chart, XY Scatter). Try these plots with and without the data point from the Poudre at Greyrock, which is an example of what is known as an outlier. When plotting data, the independent variable (drainage area) should be on the x-axis. For each plot, determine the r value for the regression (add a trendline to your plot, right click the trendline and select format trendline, go to options, and check the box that says display R-squared value on chart ), and determine whether this value is statistically significant using the following procedure: t = r n r Where r is the regression coefficient, n is the sample size (0, in this case). Compare t to the test statistic in the table below, and use a cutoff value of 0.. The degrees of freedom in the table below = (n ). Example: r = 0.7, t = 4.3. This value (4.3) is > test statistic of.860, so relationship is significant at 90% level. Are any of these relations significant? Why or why not? 3

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5) Calculate the average (AVERAGE function) and the variance (VAR function) of the hillslope gradient for two populations, the north-facing and the south-facing slope.

5 5) Calculate the average (AVERAGE function) and the variance (VAR function) of the hillslope gradient for two populations, the north-facing and the south-facing slope. Use a t-test to examine difference between the two populations: t = ( n ) s + ( n ) ( n + n ) x x nn n s + n Where x and x are sample averages, s and s are variances, and n and n are the number in each sample (here, 0). The two populations have significantly different averages if t > test statistic in the table below, or if t < -[absolute value] of the test statistic. Again, use 0. for the level of significance. The degrees of freedom = (n + n - ): in this exercise, degrees of freedom = 8. Do the two populations differ statistically? Why or why not? 5

Spatial Survey of Surface Soil Moisture in a Sub-alpine Watershed Colloquium Presentation, University of Denver, Department of Geography

Spatial Survey of Surface Soil Moisture in a Sub-alpine Watershed Colloquium Presentation, University of Denver, Department of Geography Thank you all for coming today. I will be talking about my thesis work concerning the spatial distribution of soil moisture in an alpine catchment. But first we need some background in soil moisture. Why