Regional water resources and flow modelling

During the first phase of Southern Africa FRIEND, one of the main themes was rainfall-runoff modelling, led by the Institute for Water Research at Rhodes University, South Africa (UNESCO, 1997). The focus was on testing existing models (both monthly and daily time-step) with samples of the available data in the region to determine their broad applicability.

Rainfall-runoff models were identified as potentially useful tools that could provide a vital component in the regional assessment of water resources availability. However, it was also recognised that there was a lack of experience within the region in the use of such tools, as well as inadequate access to the software required to apply models, little common understanding of the benefits of different models and their results and a lack of a common database required for setting up and running models.

In a parallel development, the need for an assessment of regional surface water resources was identified by SADC. Project Concept Note (PCN) 14 was conceived as a project proposal that would enhance the surface water resources assessment capabilities in the region (SADC, 2001; Hughes et al., 2002). The overall aim of the PCN 14 project is to improve the ability of SADC member states to make surface water resource assessments that support environmentally sustainable development through broad strategic water resource planning that is based on information and approaches that are reliable and mutually acceptable within the region. PCN 14 was the first step taken by SADC to spearhead the process of assessing

and quantifying the surface water resources of the whole of Southern Africa in a coordinated and unified manner, as well as building capacity within the region to make site-specific water resources assessments.

The ideas of Southern Africa FRIEND in the area of rainfall-runoff modelling and of PCN 14 clearly have much in common. Therefore, the Phase II work programme under this theme was designed as a contribution to the larger long-term study, focusing on providing some initial pilot results and helping to move towards the larger, eventual aims of that project. The following three focus areas were identified as being of value and achievable in the available time:

• A contribution to the development of the models and software tools required to undertake a regional water resource assessment study within SADC, coupled with a limited programme of training in the use of such tools

• A regional application of the Pitman model in the Kafue basin, Zambia, to assess the model and associated software tools capabilities, identify problems related to data availability and build capacity in the use of the model (see also Chapter 10)

• The integration of other ongoing applications of the Pitman model within the SADC region into the Southern Africa FRIEND project, in particular the Okavango basin.

The modelling work was again led by the Institute for Water Research, in collaboration with a number of other organisations. More information about the model application to the Kafue and Okavango basins, including data sources and assumptions, is provided by Hughes et al.

(2003) and UNESCO (2004).

Model and tools

The rainfall-runoff modelling work in this theme uses a modified Pitman model (Pitman, 1973). The Pitman model has become one of the most widely used monthly time-step rainfall-runoff models within Southern Africa. It formed the basis of the South African WR90 modelling study (Midgley et al., 1994), which simulated natural flows for 1946 quaternary catchments, using regionalised model parameters based on calibration against naturalised observed flows. Whilst designed largely to support strategic level water resource availability assessments and the preliminary design of abstraction schemes, additional benefits have included the estimation of groundwater contributions to surface flow, environmental requirements of rivers and estuaries and the impacts of streamflow reduction activities on water availability.

The version of the Pitman model used in Southern Africa FRIEND (Figure 5.2) includes modifications added during Phase I (UNESCO, 1997). This version has now been incorporated, together with a reservoir water balance model, into the SPATSIM (SPatial and Time Series Information Modelling) package (Hughes, 2002). SPATSIM is a software package designed as a database, data analysis and modelling system, specifically for hydrological and water resource applications. A screen dump of the main window of the software is shown in Figure 5.3.

Figure 5.2 Flow diagram representation of the Pitman model

Figure 5.3 Main window of SPATSIM package

Regional context

One of the objectives of the Phase II work under this theme was to evaluate the SPATSIM package and its version of the Pitman model for use in this type of regional hydrological modelling study. The results from the case studies showed that, despite the limitations of the data, there are clear indications that regionalised rainfall-runoff modelling of catchments in Southern Africa is feasible (UNESCO, 2004). Furthermore, whilst it was not part of this work to investigate the impacts of future changes to the rainfall and evaporation regimes of the basin due to climate change, the model has the potential to investigate such scenarios.

However, a particular challenge is presented with respect to the design of model parameter regionalisation procedures in a larger study, where several teams may be involved in modelling, if anomalous and confusing results at major basin boundaries are to be avoided.

One possible solution might involve utilising the gridded spatial datasets used by the GWAVA model of eastern and southern Africa (Meigh et al., 1998; 1999; see also section 5.3.4) and developing relationships between the abstracted basin characteristics and calibrated model parameter values.