Modelling of the flooding in the Okavango Delta, Botswana, using a hybrid reservoir-GIS model

The Okavango Delta is dominated by annual flood events from the Okavango River. During such events the inundated area increases from about 5000 km(2) to 6000-12,000 km(2). Severa[ models of a conceptual character were developed previously to represent hydrological processes in that system. Although essentially successful in their applications, the models have been criticised for their conceptual simplicity and the arbitrary way of representing long-term variation in outflows. All the existing models required the use of correction factors to address the apparent non-homogeneity of the time series. This paper presents a new model that has overcome these shortcomings by introducing more physical knowledge of the Okavango Delta system into the model. In view of the spatial complexity of the system and lack of data to support spatially distributed parameterisation of hydrological processes, the semi-distributed semi-conceptual approach, based on large units, has been retained. The major improvements of the model are: a better representation of surface water-groundwater interactions and the use of measurement-based rather than model calibrated parameterisation of topographic controls of floodplain water storage. These enabled a successful representation of 34 years of observed outflows and 15 years of observed inundation area in a conceptually sound way. Additionally, a GIS model has been developed for determination of spatial distribution of the simulated floods. In this model, the within-unit flood distribution is obtained from remote sensingderived flood maps. In this way, in spite of the semi-distributed nature of the hydrological model, flood distribution maps and ecologically important flooding characteristics can be determined for simulated and predicted floods with a spatial resolution much higher than that of the computational units. The hydrological model developed forms the basis for subsequent analyses of ecosystem response to hydrological change of both floodplain and dryland ecosystems. (c) 2006 Elsevier B.V. All rights reserved.