GEOG 566






         Advanced spatial statistics and GIScience

May 24, 2017

Tutorial 2: Nitrate Data in Relationship with Red Alder cover and Land Use

Filed under: 2017,Tutorial 2 2017 @ 11:43 am

 

  1. Research Question 

Do the nitrate values that I collected in the field have a relationship with the percent of Red Alder cover in each watershed?

                Sub question: Do the nitrate values have a relationship with land use activities in each watershed tree?

  1. Name of the tool or approach that you used.

For this analysis I utilized ARC GIS and Microsoft Excel

  1. Brief description of steps you followed to complete the analysis.

First I utilized a package in Arc GIS called NetMap to delineate the 10 watersheds. This package uses GPS points to draw watershed boundaries based on elevations and flow paths.

Next I uploaded the 2011 red alder percent cover shapefile from the Oregon Spatial Library Database, and then clipped it to the watersheds.

By joining the attributes of these two shapefiles, I was able to obtain the total area for the watershed and the total area of Red Alder cover in the watershed.  Then I divided these two areas I was able to obtain percent cover for Alder and compare that to the nitrate concentrations at each site.

Table 1 Nitrate Concentrations and Percent Alder Cover

Watershed Area (m2) Alder (m2) Percent Alder Cover Average Nitrate
Cape Creek 4353700 3537.74 0.081% 1.88
Crooked Creek 31760400 3226.39 0.010% 1.35
Cummins Creek 21388800 222.89 0.001% 1.44
Gwynn Creek 2865000 5371.57 0.187% 1.61
Little Cummins Creek 3056600 361.35 0.012% 1.72
Yew Creek 9786600 969.92 0.010% 1.01
Kiser Creek 100500 1017.97 1.013% 0.23
Rock Creek 38375500 2359.26 0.006% 0.19
Middle Fork Creek 3294000 339.23 0.010% 0.17
Griffith Creek 4434800 2021.86 0.046% 0.13

 

Once this information was obtained, I utilized a scatterplot to identify a pattern.

Figure 1 Relationship between percent alder cover and nitrate concentration

 

Though the trendline indicates a negative correlation the data point at 1 percent may be an outlier. The watershed indicated by this point is small, which could inflate the true amount of alder cover in the region. With this in mind no distinctive pattern was observed for a relationship between alder cover and nitrate concentration in the ten watersheds. This information isn’t entirely surprising as the nitrate concentrations were relatively low to begin with.

 

Landuse Analysis 

Looking further, I utilized the 2011 NLCD data to evaluate land use in the 10 watersheds. With no usual suspects (agriculture, urban etc.) being particularly dominating in the watersheds it was difficult to link increased nitrate concentrations and land use at the field sites.  Most of the land cover was predominantly evergreen and mixed forest types which are natural for the areas sampled.

 

Figure 2 Coastal Watersheds Landuse

 

Figure 3 Coast Range Watersheds Landuse

  1. Brief description of results you obtained.

Overall nitrate concentrations over the five months in the ten watersheds are relatively low (~0.1-1ppm), and the results obtained were not particularly surprising.  There was no clear pattern linking red alder cover and nitrate concentrations in these watersheds (Table 1). Additionally, there were no surprise land use features in the watersheds, and thus no apparent link to nitrate concentrations in this analysis. Most of the land use in these watersheds are occupied by mixed and evergreen forests, neither are particularly linked to elevated nitrate concentrations. I would say this analysis was consistent with my expectations, starting with low nitrate concentrations, and suspecting most of the land use would be forest cover I predicted there likely not to be a strong link between land use and nitrate concentration. However, I expected slightly more red alder cover in the coastal watersheds and was surprised how little that cover related to the concentration of nitrate.

 

  1. Critique of the method – what was useful, what was not?

This method was very useful, I was able to delineate the watersheds, clip layers to identify potential sources of nitrate and utilize excel to further the analyze relationships. I used the join feature to gather information from two attibute tables to calculate the percentage of alder cover as well. I will use the skills learned and practiced in this exercise in the future to identify statistical relationships between spatially correlated (or not) data.

 

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