2. METHODOLOGY
2.1 Climatic Parameters
2.1.2 Reference evaporation by the Penman-Monteith Method
The reference evaporation derived by the Penman Monteith FAO 56 method described in Schulze et al., (2007) was used in modelling the canopy interception. The reference evaporation in the QnC database is calculated from inputs of daily maximum and minimum temperatures described in Section 2.1.3 over South Africa on a 1‟ x 1‟ (~ 1.7 x 1.7 km) raster grid for 50 years, based on research by Schulze and Maharaj (2004).
2.1.3 Temperature
Schulze and Maharaj (2004) developed a comprehensive temperature database for southern Africa. A 50 year (1950 to 1999) record of daily maximum and minimum temperature was generated for each of the 437 039 grid cells (1' by 1') covering southern Africa. This was achieved by selecting two independent (located in different quadrants) stations from a total of 973 stations containing observed, patched and quality controlled daily record. Regional lapse rates were used to adjust for differences between the altitude of the two selected stations and the mean altitude of each grid cell.
Schulze et al., (2009) selected a representative temperature "station" for each QnC as follows. The 200 m digital elevation model was used to calculate the spatially averaged altitude for each QnC. Grid cells with mean altitudes similar to those of the QnC means, and located as close as possible to the QnC centroid (and preferably located within the quinary boundary), were then selected to represent each of the 5838 QnC‟s.
2.1.4 Mean rainfall rate
Mean rainfall rate (R) data for South Africa are not available in the QnC database. In order to derive these, the rainfall rate was estimated by calculating and applying a seasonal average rainfall intensity correction factor which is described next.
Schulze (1980) provided a distribution of kinetic energy of rainfall based on data from fourteen rainfall stations across South Africa. Since kinetic energy of rainfall and rainfall intensity are directly related (van Dijk et al., 2002), kinetic energy was used as a surrogate to determine the relative rainfall intensity for the seasonal rainfall regions in South Africa derived and mapped by Schulze and Maharaj (2007). The average rainfall of an area may be high or low, have a high or relatively low variability of rainfall from one year to the next or its rainfall may be concentrated over a short rainy season or spread over a longer period (Schulze and Maharaj, 2007). As a result of rainfall seasonality and
concentration, the rainfall intensity for these different areas is also affected. Some areas may have high intensity, short duration thunderstorms, while others, such as the Western Cape, are dominated by low intensity winter frontal rainfall of a longer duration. Hence, the canopy interception will vary accordingly. Rainfall seasonality is also an important hydrological consideration when considering canopy interception.
In this study, data from the 14 weather stations used by Schulze (1980) were first grouped into summer and winter “envelopes”, where the maximum and minimum mean kinetic energy for these
“envelopes” was determined. This was achieved using 100 mm of rainfall as a reference value as illustrated by the example in Figure 4.1 for Johannesburg (summer rainfall region) and Cape Town (winter rainfall region). Next, these stations were overlayed onto a rainfall seasonality map of South Africa as shown in Figure 4.2 (Schulze and Maharaj, 2007). Depending on the location of the weather station, a rainfall season was assigned to each station (Table 4.1).
Figure 4.1 Monthly kinetic energy: rainfall relationship (after Schulze, 1980).
Table 4.1 Kinetic energy from Schulze (1980) using 100 mm of rainfall as a reference.
Town Seasonality Summer Winter
Max (J.m-2)
Min (J.m-2)
Mean (J.m-2)
Max (J.m-2)
Min (J.m-2)
Mean (J.m-2) Beaufort West Very Late
Summer
1600 1100 1350 700 200 450
Bloemfontein Late Summer 1200 900 1050 600 300 450
Cape Town Winter 500 300 400 900 600 750
Cathedral Peak Mid Summer 1200 800 1000 700 300 500
Cedara Mid Summer 1200 1000 1100 700 100 400
Durban Mid Summer 1200 1050 1125 800 600 700
East London All Year 1000 550 775 1400 1100 1250
Grootfontein Very Late Summer
1400 800 1100 650 200 425
Johannesburg Mid Summer 1400 1100 1250 950 600 775
Kimberly Late Summer 1350 1000 1175 750 400 575
Pietersburg Early/Mid Summer
1500 1150 1325 1050 500 775
Port Elizabeth Early Summer 1000 770 885 750 500 625
Pretoria Early Summer 1500 1100 1300 700 200 450
Upington Very Late
Summer
1400 1100 1250 900 300 600
Schulze and Maharaj (2007) derived the rainfall seasonality using rainfall concentration determined from the Markham (1970) technique, at a QnC scale. In this study, this methodology was applied to the QC database to determine an updated rainfall seasonality map using mean monthly rainfall (Schulze and Kunz, 2010). The rainfall seasonality categories are the same as those used by Schulze and Maharaj (2007).
Figure 4.2 Rainfall seasonality per quinary catchment (Schulze and Kunz, 2010) and weather stations used by Schulze (1980).
The daily rainfall cannot be multiplied by a value less than 1, as this would result in a rainfall intensity lower than the daily rainfall. Therefore, to normalize the data in Table 4.1, the lowest mean kinetic energy for summer and winter was used to divide the rest of the data, so that the lowest mean value was 1 as shown in Table 4.2.
Table 4.2 Normalized data for each of the 14 weather stations.
Town Seasonality Summer Winter
Max (J.m-2)
Min (J.m-2)
Mean (J.m-2)
Max (J.m-2)
Min (J.m-2)
Mean (J.m-2) Beaufort West Very Late
Summer
4.00 2.75 3.38 1.75 0.50 1.13
Bloemfontein Late Summer 3.00 2.25 2.63 1.50 0.75 1.13
Cape Town Winter 1.25 0.75 1.00 2.25 1.50 1.88
Cathedral Peak Mid Summer 3.00 2.00 2.50 1.75 0.75 1.25
Cedara Mid Summer 3.00 2.50 2.75 1.75 0.25 1.00
Durban Mid Summer 3.00 2.63 2.81 2.00 1.50 1.75
East London All Year 2.50 1.38 1.94 3.50 2.75 3.13
Grootfontein Very Late Summer
3.50 2.00 2.75 1.63 0.50 1.06
Johannesburg Mid Summer 3.50 2.75 3.13 2.38 1.50 1.94
Kimberly Late Summer 3.38 2.50 2.94 1.88 1.00 1.44 Pietersburg Early/Mid
Summer
3.75 2.88 3.31 2.63 1.25 1.94
Port Elizabeth Early Summer 2.50 1.93 2.21 1.88 1.25 1.56
Pretoria Early Summer 3.75 2.75 3.25 1.75 0.50 1.13
Upington Very Late
Summer
3.50 2.75 3.13 2.25 0.75 1.50
The normalized correction factors established in Table 4.2 were then averaged for each of the six rainfall seasonality regions as derived by Schulze and Maharaj (2007) to obtain rainfall intensity correction factors as shown in Table 4.3.
Table 4.3 Rainfall rate correction factors per rainfall seasonality zone derived from the data of Schulze (1980).
Season Summer Winter
All Year 1.94 3.13
Winter 1.00 1.88
Early Summer 2.86 1.54
Mid Summer 2.68 1.33
Late Summer 2.79 1.29
Very Late Summer 3.09 1.23