Wastewater management
of the rainfall patterns of the study area is important. A comparison of rainfall data from Vogelgat, HMO and Hamilton Russel weather stations was done. Monthly rainfall records for the three stations for the monitoring period are shown in Figure 2. The optimal acquisition dates would be at the point when groundwater dependent ecosystems areas exhibit significantly different characteristics to the surrounding land cover, usually coinciding with a period of seasonal climatic change and associated vegetative response (Jensen 1996.) For the greater Hermanus area with a winter rainfall season, images were selected from October 2008, September 2009 after a wet season and March 2009, February 2010 end of dry season, when groundwater dependent ecosystems should be wetter and therefore greener. The study acquired red, green and blue aerial-photos using Canon, while the Nikon D100 (Kodak #89B opaque infrared gel filter) was used for the near-infrared (NIR) aerial photo. A single strip of stereo aerial photography covering the study area was captured with the parameters outlined in Table 1. The camera specifications allow for images to be taken in the visible and near-infrared spectrum, which are important for vegetation studies (see Table 2 and Figure 3). In the visible spectral region (0.4 to 0.7 μm), light absorption by leaf pigments dominates the reflectance spectrum of the leaf and leads to generally lower reflectance. In the visible region of the electromagnetic spectrum, there are two main absorption bands, in blue (0.40 to 0.59 μm) and in red (0.67 to 0.89 μm), due to leaf pigments for use during photosynthesis. These strong absorptions bands induce a reflectance peak in the yellow to green (0.55 μm) band. In the NIR spectral region (0.7 to 1.3 μm), leaf structure explains the spectral properties. The internal leaf structure as well as the amount of air spaces in the mesophyll determines the interfaces with different refraction indices. NIR spectral region has two main spectral regions: between 0.7 and 1.3 μm where reflectance is high, except in two minor water-related absorption bands (0.96 and 1.1 μm), and between 1.1 and 1.3 μm, which corresponds to the transition between high NIR and the water-related absorption bands of the short wave infrared. The acquired image covers the Fernkloof nature reserve in which the recharge zone to
FIGURE 1: Project process flowchart
FIGURE 2: Monthly rainfall figures from the weather stations for 2008 and 2009
TABLE 1: AERIAL PHOTOGRAPHY SPECIFICATIONS Date of photography
29 September 2009
09 February 2010
Image Resolution (GSD – nominal)
1.25 m
1.25 m
Shutter Speed (Canon 10D)
1/4 000 sec
1/4 000 sec
Aperture Setting (Canon 10D)
f4.0
f4.0
Shutter Speed (Nikon D100)
1/4 000 sec
1/4 000 sec
Aperture Setting (Nikon D100)
f4.0
f4.5
TABLE 2: DIGITAL AERIAL PHOTO SPECTRAL BANDS USED IN THE STUDY Spectral band
Wavelength (μm)
Indicator
Band 1
0.45 to 0.49 (Blue)
Water absorption
Band 2
0.49 to 0.59 (Green)
Vegetation reflectance
Band 3
0.61 to 0.68 (Red)
Vegetation absorption
Band 4
0.78 to 0.89(NIR)
High vegetation reflectance
the gateway wellfield is located. The image also covers the Hermanus gateway wellfield area. Besides acquired seasonal digital aerial photography, the following data is also required to assist in the discrimination
of groundwater dependent systems: • 1:50 000 rivers • 1:50 000 geology contacts • 1:50 000 geological faults • 20 m DEM
RéSource August 2012 – 67