Lab 2: Santa Monica Mountain Insolation

In this week’s lab, I mapped the amount of insolation received by the Santa Monica Mountains in winter, spring, summer, and fall. The elevation data for the region was retrieved from the United States Geological Survey Seamless Server. The California state line and vegetation data were downloaded from a UCLA data set. Lastly, the sun angle data for each season was derived from the Susangle website.




Using the clip analysis tool, I narrowed the entire Santa Monica Mountain region to the area in the raster vegetation data. From the USGS DEM data, the slope and aspect maps were directly calculated using standard ARCGIS spatial analyst functions. In order to make more meaningful judgments, the exact angles from the aspect map were generalized to north, east, west, and south. In order to model the hillshade for each season, I referenced the Susangle website for the altitude and azimuth for the most representative days of each season (the solstice and equinox days). Lastly, the vegetation statistics were summarized using the ‘zonal statistics’ spatial analyst tool. The output of this function is the bar graphs seen below the seasonal insolation maps (figure 1).




The aspect map produced some interesting results: most of the sunlight seems to land on north and east facing slopes. When comparing the aspect map to the slope map, it can be seen that the eastward facing slopes seem to be only near the top of mountains while the northward facing regions are less correlated with elevation. In general, south facing slopes are the least common in the Santa Monica Mountain study area.




Comparing the amount of insolation received by vegetation type, the human dominated landscapes (urban/agriculture) received the most sunlight followed by annual grass, sagebrush, chamise, and oak. This probably occurs because people grow crops and live in the brightest areas. Although oaks have a higher canopy than the other vegetation types, they require better conditions to thrive. Therefore, it is unsurprising that other vegetation types receive more insolation.




The ratio of sunlight received by each vegetation type varies little between each season. However, the absolute amounts changed (a noticeable drop is evident in the winter). This may be caused by a flaw in the setup of this analysis. Since the vegetation boundaries change over time. A more accurate analysis could use different vegetation data for each season.