Terrain Analysis of the Santa Monica Mountains
California is very diverse in vegetation, having many various different types of grasses, forest, deserts, agriculture etc. How California’s vegetation cover is laid out can be analyzed by many different factors such as elevation, slope, aspect, and the solar radiation hitting the earth’s horizontal surface, or insolation. In order to completely understand the insolation, we need to create a hillshade of the Mountains for all four seasons of the year. GIS can be a very helpful tool when analyzing the terrain of the Santa Monica Mountains. By obtaining digital elevation models of the Santa Monica Mountains from http://seamless.usgs.gov and clipping the California vegetation shape file found on the classes\\kilimanjaro.labs drive, a terrain analysis can be created and analyzed.
After obtaining the DEM of the Santa Monica Mountains and clipping the vegetation cover from California to fit the Santa Monica Mountains, I created a slope and aspect of the elevation. From there, we have the slope in degrees of the Mountain sides and also the breakdown of which sides of the mountains face north, south, etc. Next I created a hillshade for each season. Using the http://susdesign.com/sunangle/ website, I typed in each solstice and equinox occurring at noon for the year 2011 and as a result got: Vernal Equinox (March 20) – altitude 55.82 – azimuth -1.43... 358.57; Summer solstice (June 21) – altitude 79.30 – azimuth 3.08; Autumnal Equinox (Sept 23) – altitude 55.59 – azimuth 5.28; Winter solstice (Dec 22) – altitude 31.01 – azimuth -14.55... 345.45. For the negative numbers, I added 365. I put the azimuth and altitude into the hillshade calculator and got the hillshade of the Santa Monica Mountains for each season. But now I need to calculate the insolation of the mountains for each season. By using the website http://edmall.gsfc.nasa.gov/inv99Project.Site/Pages/science- briefs/ed-stickler/ed-irradiance.html, I found that the equation to find out the insolation of the sun is the hillshade of the specific season x1000/255. Therefore, I went into raster calculator and entered the calculation for each equinox and solstice. The results shown in the maps are the insolation for those seasons. In the final map, I created a table showing the mean seasonal insolation and topography for each vegetation type.
The results are shown in the final maps and tables. As shown in the resulting table, elevation has a role in what vegetation type is located at a specific elevation. Most notably, agricultural vegetation is at lower elevation because people chose to have agriculture were it is easily accessible and therefore cannot put it at high elevations. Also, most agriculture will not grow up in such high elevation. What does grow in the higher elevations is mixed chaparral and Chemise Redshank Chaparral, shown in the table. This is dry brush and trees that commonly grow in higher elevation in California’s Mediterranean climate. Coastal scrub grows at a lower elevation and the Annual grass grows at an even lower elevation. Furthermore, slope has a lot to do with where vegetation grows. Again, agricultural vegetation will not be located on high slopes because that makes it hard to grow and not easily accessible. Also, we can see that both of the Chaparral vegetations grow best on higher slopes on the Mountain. The coastal scrub and annual grass shows that it can grow at the middle elevations. When analyzing the aspect of where vegetation grows, most of the results are near the same value. Therefore, I can conclude that aspect is not a significant determination of where vegetation grows.
When analyzing the seasonal insolation, you can see right away the summer months show more of the high insolation because it gets the most direct solar radiation because of the tilt of earth’s axis. Therefore, we can see that in the table, the mean values for insolation for all vegetation types are higher in summer. Therefore, when looking at the winter solstice, the mean values are all lower. This is because the Santa Monica Mountains get the most sun light in summer and then it goes progressively down in Autumn, it is at its lowest radiation in winter, and then begins to get progressively higher in spring. In the overall seasonal analysis, the Mixed Chaparral vegetation grows where there is the most radiation except in the summer, where its mean radiation is less than the other vegetation types. This may be because it grows where there is sun all year around, and the other vegetation types are seasonal.
In conclusion, we are able to see that depending on the elevation, slope, aspect, and seasonal insolation, we can analyze where different types of vegetation types grow. Agriculture is mainly on low elevation and flat surfaces, where as the Chaparral vegetation types grow in the higher elevation and steeper slopes. The coastal scrub and grass grow on the in between slopes and elevation. Also, insolation determines where vegetation grows. Chaparral grows in places that get more sun all year around, whereas the agriculture and other vegetations grow seasonally in areas that get a lot of radiation in summer. The elevation, slope and the seasonal insolation can determine where vegetation grows on the Santa Monica Mountains and GIS technology can help project this information.
Sources:
http://susdesign.com/sunangle/ http://edmall.gsfc.nasa.gov/inv99Project.Site/Pages/science-briefs/ed-stickler/ed-irradiance.html http://gis.ats.ucla.edu/mapshare/ http://seamless.usgs.gov
By
Chelsea Kemp April 12, 2011
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