South Africa, and the SADC region as a whole, has a very high degree of climate variability, with some catchments experiencing wet tropical-type climate while others experience arid and semi-arid climates. This type of climate variability translates into stream flow variability as well.
In general, South Africa is a water-scarce country and, as such, it is important that this limited but vital resource be well managed to achieve maximum benefit and to maintain sustainable use for generations to come. Hydrological modelling remains one of the most valuable tools for water resource management, especially for simulating hydrological information where data collection is not feasible and to reduce both labour and equipment costs.
The hydrological modelling exercise faces numerous challenges. One of the most important is lack of, or poor hydro-meteorological records such as rainfall, evaporation and stream flow.
Such records normally require financial and political will to implement, maintain and sustain.
23 Until decision-makers recognise the importance of investing in upgrading and improving gauging and monitoring networks in the country, the problem may persist for quite a long time.
Within the context of South Africa, which might also be the case in other developing countries in southern Africa, Kapangaziwiri and Hughes (2008) contend that the main challenges that limit the applicability of hydrological models are: variation of climate and resultant river discharge in space and time; inadequate rainfall and stream flow records; general shortage of information on activities that impact on stream flow such as land use and water abstractions;
inadequate scientific knowledge of hydrological processes, and more importantly, lack of trained personnel.
Hydrological modelling has proved to be one of the most important tools in the scientific understanding of the dynamic physical processes affecting the availability of water in South Africa and elsewhere. In recognition of hydrological models as invaluable water resource management tools, there have been a number of hydrological models developed in the country (e.g. Pitman, 1973; Schulze 1994; Hughes 2004b). The structures of these models were developed to suit the unique arid and semi-arid conditions of the country and the region as well.
The conditions are commonly marked by high spatial and seasonal variations. The models were aimed at addressing some of the environmental and hydrological challenges that are faced by South Africa and the neighbouring countries, perhaps initiated and driven by development of water resources prompted by economic growth. The models were also useful for evaluating possible impacts of various water resources development options/scenarios, as well as feeling the hydrological data gaps resulting from inadequate monitoring networks, common in economically developing countries (Hughes 2004b).
Pitman Model
This model was introduced in the 1970’s by Pitman (1973). It is a semi-distributed, conceptual model, with parameters which have physical relevance and can be inferred from characteristics of a catchment. The model runs on a monthly time step and there is also a version that runs on a daily time-step, but which is rarely used for practical purposes (Hughes, 2013). Since its inception, the model has been modified a number of times to include additional physical processes such as groundwater recharge and outflow, wetland and reservoir sub-modules and water abstractions (Hughes, 1997, 2004a; Hughes et al., 2014a) which are significant and prevalent under southern African environmental conditions. The Pitman model has a relatively large number of parameters (see chapter 3 for a more detailed model structure) and might be considered to be over-parameterised (Jakeman and Hornberger, 1993). It is however,
24 considered by Hughes (2013) to be a compromise between an adequate representation of a complex reality and “mathematical simplicity”. Of all the locally developed hydrological models, the Pitman model is arguably the most applied model both in the country and the wider region (Gan et al., 1997; Andersson et al., 2003; Hughes et al., 2006a, 2006b; Hughes, 2006; Wilk et al., 2006; Tshimanga and Hughes, 2012). The commercial version of this model, named the Water Resources Simulation Model (WRSM 2000) is the model of choice of the South African Department of Water and Sanitation (DWS).
ACRU Model
The ACRU agrohydrological modelling system was developed at the University of KwaZulu- Natal by Schulze (1994). It is described by Jewitt and Schulze (1999) as an integrated physical conceptual model with a multi-purpose and multi-level capability. It is able to simulate stream flow, evapotranspiration and land cover impacts on water resources. The model runs on a daily time step and was designed for use in where there is no adequate stream flow data. This was achieved by having parameters which can be directly related to measurable physical characteristics of a catchment such as soil cover, vegetation and geology (Schulze, 1994).
ACRU model has been used in South Africa for variety of purposes including the hydrological impacts of land use changes (Jewitt and Schulze, 1999; Gush et al., 2002; Görgens and van Wilgen, 2004; Jewitt et al., 2004; Dye and Versfeld, 2007), design flood estimations (Smithers et al., 1997; Boughton and Droop, 2003), agriculture (Martin et al., 2000; Schulze, 2000) and water resource availability assessment (Schulze et al., 2001).
VTI Model
The variable time interval model is a daily step model developed by Hughes and Sami (1994), at the Institute of Water Research, Rhodes University. The model was originally aimed at studying the catchment response characteristics of a semi-arid catchment in South Africa. It has, nevertheless been successfully applied in other regions of South Africa, as well as in other countries in southern Africa (Hughes, 1995; Smakhtin et al., 1997; Hughes, 1997). As the name suggests, shorter time modelling intervals can be applied depending on the objectives of the user. Based on the paucity of published documentations available in the mainstream electronic databases, the VTI model appears not to have received as much attention of water resources managers and hydrologists alike, compared to the other locally developed hydrological models (Pitman and ACRU).
25 Internationally developed hydrological Models
In addition to locally developed hydrological models, there has been a considerable interest in the application of models developed outside the southern African region. These models were used to achieve various scientific objectives. One of the examples is the use of a physically based distributed TOPographic Kinematic Approximation and Integration model (TOPKAPI) developed by Liu and Todini (1999). The model consists of five main modules: soil, overland, channel evapotranspiration, snow and can be run at hourly time-steps. TOPKAPI was tested for performance under South African environmental conditions by Vischel et al. (2008a) and was later used to simulate soil moisture content in a few catchments by Vischel et al. (2008b) and Sinclair and Pegram (2010).
The Identification of Hydrographs and Components flow Rainfall, Evaporation and Stream (IHACRES) (Jakeman et al., 1990; Jakeman and Hornberger, 1993) is one of the simplified hydrological models that has been used in the South Africa. The model is based on the moisture deficit principles and uses a non-linear module to determine effective rainfall and a linear routing module to represent transport lags to convert effective rainfall to stream flow. Dye and Croke (2003) evaluated the model for stream flow simulation in two South African catchments.
They found that the model is able to accurately simulate stream flow over a short duration (2- 3 years), while it performed poorly for longer durations.
Govender and Everson (2005) used the Soil Water Assessment Tool- SWAT, to simulate the hydrological processes in two mountainous catchments, with different land covers, in the KwaZulu-Natal province of South Africa. The authors reported that SWAT performed reasonably well in simulating the major hydrological processes in the catchment. They however, observed that the model performed better in drier years than in wet periods. SWAT is a relatively complex, continuous daily time-step model developed at the United States Department of Agriculture (Arnold et al., 1998). It is an integrated model with a comprehensive water balance, includes flood and sediment routing, water transfers as well as agricultural management and water quality components.
Another model of international origin that was used in southern Africa is the HBV (Bergström, 1992). HBV is classified as a semi-distributed conceptual model, with three main components consisting of subroutines for snow accumulation and melt; soil moisture accounting; response and river routing. The model was originally developed by the Swedish Meteorological and Hydrological Institute with the aim of simulating runoff (Lindström et al., 1997). Lidén and Harlin (2000) applied the HBV model in Zimbabwe and Tanzania with the objective of assessing its
26 suitability and performance in catchments with varying climatic conditions. The authors reported that HBV performed better under wetter conditions and less climate variability. Their findings imply that the model might not be suitable for application in South Africa, as it was developed for humid Nordic conditions.