Assessment of lake surface dynamics using satellite imagery and in-situ data: case of Lake Ngami in North-West Botswana

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. Surface dynamics of Lake Ngami were monitored using Landsat Enhanced Thematic Mapper, Landsat Multispectral Scanner and Landsat Operational Land Imager satellite imagery. The Modified Normalised Difference Water Index (MNDWI) was used to map the water surface of the lake. Histogram thresholding segmentation method was used to extract the water pixels. Satellite imagery was further used to estimate the actual evapotranspiration (ETa) and rainfall in the Okavango basin as well as Okavango delta in order to assess the probable causes of Lake Ngami surface dynamics using Simplified Surface Energy Balance (SSEB) Modelling Approach and TRMM 3B43V7 product. The SSEB method involved the estimation of evaporation fraction using the MODIS Land Surface Temperature and Emissivity 8-day product (MOD11A2) and GLDAS 0.25degree 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 272km2.The ETa was found to be in ranges of 700mm/year in dry regions to about 2000mm/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, showing Pearson correlation of 0.70.