An understanding of variations in the area extent of Lake Lyambezi: perspective for water resources management

Abstract

Lyambezi is natural lake found in the Caprivi region, of Namibia. It provides the link between the Kwando-Linyanti and Chobe Rivers. The lake dried out for nearly two decades since the mid-1980s. However, Lyambezi re-emerged and went through 7 phases of drying out and reforming but has remained robust for the past half-decade. The study focused on enhancing the understanding of the variations in the lake’s areal extent from a time series analysis of Landsat imagery. The area was quantified at intra and inter-annual temporal scales based on the analysis of satellite imagery i.e. Landsat TM, ETM+ and Landsat 8 OLI sensors. Water features were delineated using the modified normalized difference water index (MNDWI). The lake spectral classes were segmented based on five dynamic thresholds based on which the area covered by water was calculated. Results show that the lake exhibits a wide range of area variations at both inter-annual and intra-annual temporal scales. However, due to the presence of the thick vegetation in the open water body, the segmentation of spectral classes yielded poor to moderate accuracy levels (kappa=28-53). The application and success of DEM was tested and the results illustrated that the use of SRTM 3 DEM led to the successful extraction of topography-based hydrological parameters that served as inputs into various GIS-based hydrological models. These results imply that lake volume can successfully be estimated, leading to a better understanding of floods on water resources availability. On the other hand, hydrological analysis results show that flood events of a magnitude above the long-term average maximum have a 2 to 3 year return period and probability of occurrence in any given year ranges between 0.29 and 0.58. This implies that floods have the potential to inundate the lake every second year, which represents a flooding occurrence at a regular enough interval to prevent the lake from going into a drying-up phase. Therefore, rainfall variability is more linked to the inundation and hence, appearance of the lake. It as well represents the most important challenge for the water resources management of the lake. Overall, the results of this study provide a better understanding of Lake Lyambezi dynamics and therefore, enhance the understanding of the behaviour and response of previously-desiccated environments to prevailing hydrological conditions.