LESOTHO
GPOW 5.0 Riparian Factor 0.5
7.2 HYDROLOGICAL AND CLIMATE CHANGE MODELLING TOOLS
172 7 DISCUSSION, CONCLUSION AND RECOMMENDATIONS
173 two hydrological models namely, Pitman and WEAP for the hydrological and water resources assessment of the basin. The Pitman model has been successfully applied in South Africa for more than 40 years (Hughes, 2013) and it incorporates the known major hydrological processes relevant to the Caledon Basin. It is also available in different formats, including those that allow for the incorporation of uncertainty ensemble modelling using simple Monte Carlo sampling of the feasible parameter space. On the other hand, the WEAP model presents a rather simpler conceptual structure with fewer parameters to quantify, but includes more components to allow for the simulation of water resources development infrastructure than are available in the Pitman model. This model has not been substantially applied in the region.
The study demonstrated the potential for the WEAP model to be calibrated based on existing Pitman model setups for the Caledon River Basin. The two models performed reasonably well against the available stream flow observations, however, it has to be recognised that these observations are very limited and therefore the quality of the simulations remains highly uncertain. The somewhat reasonable performance of the parsimonious WEAP model was not unexpected as it has been demonstrated by several studies (e.g. Jones et al., 2006) that simple models can yield comparable, and sometimes better, results than more sophisticated models.
However, the choice of a hydrological model constitutes another source of uncertainty in hydrological modelling. The use of more than one model for hydrological assessment has the advantage of offering more robust results and it has been demonstrated that multi-model predictions are more accurate than individual models (Duan et al., 2007). Establishing the WEAP model also offers an additional advantage because of its additional functionalities (which are not present in the Pitman model) that are essential from the water resources management and planning perspectives.
The 2-step approach of the uncertainty version of the Pitman model used in the current study, which is based on a progressive reduction of uncertainty, ensures that all of the simulated natural incremental flows are behavioral, according to the established local and regional constraints and reflect a catchment’s hydrological responses. Thus, the resultant downstream flows comprise only behavioral upstream ensembles. One of the advantages with the 2-step approach is that it enables the constraints can be set with very narrow uncertainty bounds where there is a good quality of observed data. However, there is still substantial uncertainty in ungauged and poorly gauged sub-basins. The approach therefore allows for different levels of uncertainty to be included in basins where the hydrological response in some areas is well understood and known. The approach yielded satisfactory results and was successful in constraining the model parameter ranges. Though the simulated natural stream flow
174 uncertainty bounds were narrow for many sub-basins, they were much wider in others. This is a reflection of the lack of observed stream flow data to represent either the natural or present day development flow regimes of some parts of the basin. One of the critical issues in the approach is the dependence on observed data to set the output constraints. Data quality, quantity and spatial representation still remain a major challenge in the basin. Most of the stream flow records are of relatively poor quality, some with a lot of missing data, most with inadequate recording of high flows and with non-stationary and largely unquantified water abstraction impacts. These limitations make it very difficult to establish appropriate constraints on the model outputs and to assess the validity of the model outputs. High flows have been identified as being poorly quantified as a result of limitations of the stream-discharge rating curves, while assessments of the simulations of low flows are affected by the large uncertainties in water uses.
In estimating the water use within the Caledon River Basin, the study carried out a detailed investigation of the extent of irrigation demands for the main crops cultivated within the basin.
This involved estimating the cultivated area using satellite imagery (from Google Earth). One of the major limitations with this approach was the lack of a means of verification and confirmation. For instance, in some areas it was difficult to distinguish between rain-fed and irrigated areas as they both appear the same. Other water abstractions were estimated based on population and assumed water demands for the towns and rural areas on both sides of the South Africa/Lesotho border. This approach is also uncertain and it could be improved by field work, data collection surveys and information gathering from the responsible local authorities.
However, this would have been expensive and time-consuming.
The study reveals that the water resources of the Caledon River are heavily impacted by a number of water use activities in the basin. The main water uses in the basin are artificial impoundments in the form of a few large dams and many ubiquitous small farm dams. Both kinds are used for various purposes such as municipal and industrial water supplies as well as irrigation. Such impoundments in the Caledon River Basin have affected the natural hydrology of the watershed as they decrease the magnitude and timing of the stream flow. The major challenge in this regard is an inadequate quantification of the amount of water abstracted from the streams and the main river channel. As a result stream flow characteristics change over time even if the meteorological inputs and other natural processes in the basin remain the same.
175 There are some situations where the observed data lie outside the simulated present day uncertainty bounds and these are mostly at very high flows or at very low flows. To improve the confidence (and reduce the uncertainty) in the model outputs, it would be necessary to obtain more observed data of flood flow events through extension of the rating curves at the gauging sites. In terms of low flows, improvements in the model outputs would mostly rely on improved quantification of the patterns of water use.