1. INTRODUCTION AND BACKGROUND
6.3 Methodology
As detailed in Chapter 4, the water quality modelling work in this study was conducted in the Mkabela Catchment located approximately 1km east of the town of Wartburg, near Pietermaritzburg, South Africa (Figure 6.3). The Mkabela Catchment is a subcatchment of the Nagle Water Management Unit (WMU) which forms part of the Mgeni Tertiary Catchment in KwaZulu-Natal. Commercial irrigated agriculture is the most dominant land use activity in the Mkabela Catchment.
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Various structures, which act as hydraulic controls to the movement of water and pollutants are found in the catchment, including an assortment of farm dams, wetlands and riparian buffer strips. Synthetic fertilization occurs in sugar cane, vegetable and dairy (pastures) farming systems using nitrogen (N) and phosphorus (P) based fertilizers.
The Mkabela Catchment falls within the summer rainfall region of South Africa and experiences a warm subtropical climate with distinct dry and rainy seasons interlaced with high inter-seasonal variability. The altitude in the catchment ranges from 965 m.a.s.l from the eastern escarpment to 755 m.a.s.l at the catchment outlet. The mean annual precipitation (MAP) of the catchment averages 835 mm per annum. The baseline vegetation is classified by Acocks (1988) as the Southern Tall Grassveld and the catchment relief ranges from open hills, low relief to open hills, high relief.
Figure 6.3 Location of the Nagle WMU within the Mgeni Quaternary Catchment in (a) and the Mkabela Research Catchment within the Nagle WMU in (b).
169 6.3.2 Input Data Acquisition and Processing
Since the aim of this study was to develop relevant adaptation strategies based on appropriate scientific investigation, modelling exercises were used to detail potential changes to selected WQ variables under climate change. Two types of climatic data were used as input in the ACRU-NPS model: i.e. observed climate data derived from the catchment monitoring network and climatic data derived from various downscaled GCMs. Daily observed climate data were acquired from two rainfall gauging stations located in and around the Mkabela Catchment. These were the Windy Hill Number 2 and Noodsberg-Jaagbaan rainfall stations.
The records from these stations had rainfall data available over different periods and were combined to form a single composite record for the period 1950 to 2011. The reader is referred to Chapter 4, Section 4.4 for the full description related to the treatment of the observed datasets from these stations and for the creation of the composite rainfall record.
Climate change data used in this study were derived from 7 (seven) downscaled global circulation models (GCMs) employed in the Intergovernmental Panel on Climate Change Fourth (AR4) Assessment Report (IPCC, 2007). The downscaled climate change projections, derived from the downscaled GCMs, were obtained from the Council for Scientific and Industrial Research (CSIR) and from the Climate Systems Analysis Group (CSAG) based at the University of Cape Town (UCT). The downscaled climate change projections used this study were based on the Special Report on Emission Scenarios (SRES) A2 storyline and emission scenario (Nakićenović et al., 2000), which assumes that global efforts to reduce greenhouse gas emissions are relatively ineffective. The reader is also referred to Chapter 4, Section 4.4 for the full description and treatment of these data.
6.3.3 Simulations Performed
Source-pathway-response modelling of the Mkabela Catchment with the intention of understanding the impacts of agricultural NPS pollutants on the water quality of hydraulic controls across the catchment was conducted in this study. The simulation of runoff processes (from rainfall measured within the catchment) was carried out for 3 individual Hydrological Response Units (HRUs) located in the headwaters of the Mkabela Catchment (Figure 6.4).
The general characteristics of each of these HRUs are detailed in Table 6.2.
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The HRUs are located upstream of a perennial wetland, marked as “wetland1” in Figure 6.4.
The combined runoff and streamflow from each of these HRUs form the inflow into this wetland, as indicated in Figure 6.4.
The ACRU-NPS model was selected as the primary model for use in the study simulations.
The description of this model and the rationale behind its selection are detailed in Chapter 4, Section 4.3. It is important to differentiate between simulations performed based on observed climate data and simulations performed based on downscaled GCM climate data in the context of this study. Both modes of simulation were performed using the ACRU-NPS model for the individual HRUs. However, in one instance observed/historical climate data derived from in-situ catchment observations was used and in the other, GCM climate data derived from the downscaled GCM projections was used. For instance, the 1950 to 2011 simulations (encompassing the 1971-1990 period) in the ACRU-NPS model were based on observed climate data whereas the 1961-2100 simulations (encompassing the 1971 to 1990 and 2046 to 2065 periods), also in the ACRU-NPS model, were based on downscaled GCM-derived climate data.
Table 6.2 General characteristics of the 3 Hydrological Response Units located in the headwaters of the Mkabela Catchment.
HRU Name Area (km2) Elevation (m.a.s.l) Slope (%)
Sugar Cane 6.31 965 3.5
Commercial Afforestation
0.61 965 5.5
Vegetable Patch 1.06 965 1.5
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Figure 6.4 Location of the 3 HRUs relative to the wetland. Also indicated in this Figure is the flow direction from each HRU in the Mkabela catchment headwaters into the wetland and, finally, the catchment outlet.