Abstract
The understanding of surface dynamics of Lake Ngami, a part of Okavango river system is hindered by paucity of data. The aim of this study was to assess the lake surface dynamics using satellite imagery and in-situ data.Surface dynamics of Lake Ngami were monitored using remote sensing methods to complement the available inadequate in-situ measurements. Landsat imagery of the year 1979 to year 2016 were used. The Lake boundary was mapped using the Modified Normalised Difference Water Index (MNDWI), where use was made of Histogram thresholding segmentation method in extracting the water pixels. Remotely sensed actual evapotranspiration (ETa) was estimated using Simplified Surface Energy Balance (SSEB) Modelling Approach; while the satellite derived rainfall estimates were obtained from Tropical Rainfall Measuring Mission (TRMM) 3B43V7 satellite imagery product. Precipitation and potential evaporation data from meteorological stations were used to correct the bias of the satellite products. The SSEB method involved the estimation of evaporation fraction using the MODIS Land Surface Temperature and Emissivity 8-day product (MOD11A2) and GLDAS 0.25° Potential Evapotranspiration version 2.1 for the estimation of potential evapotranspiration.The lake extraction process displayed good results with the average Kappa coefficient and overall accuracy of 0.93 and 96.6%, respectively. The maximum extent of Lake Ngami was observed in the year 2012 at 272 km2. The ETa was found to be in range of 700 mm/year in dry regions to about 2000 mm/year in Okavango delta swamps. From the study, it was observed that after the 2009 flood-event, the precipitation occurring during the rainy season (October–March) at Angola Highlands had a positive relationship with the Lake Ngami surface area, with Pearson correlation of 0.70. Similar findings were noticed when the lake surface area trend was observed against the in-situ flow data at the Mohembo hydrometric station, which is the inflow of the Okavango delta. This suggests that since the floods of 2009, the precipitation in the Angola Highlands has a major influence in the surface dynamics of Lake Ngami.
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