Assessment of trends in inundation extent in the Barotse Floodplain, upper Zambezi River Basin: A remote sensing-based approach

Abstract

Study regionThe annually flooded Barotse Floodplain in the upper Zambezi River Basin in the Western Province of Zambia, Southern Africa. Study focusDischarge variability plays a significant role in inundation extent and thus it controls habitat conditions of river channels and the linked wetlands. The linkage between discharge and inundation extent in the Barotse Floodplain allowed us to analyse the trends in extent overtime using optical satellite imagery MODIS. The Desert Flood Index, a surface water extraction algorithm, was used to generate time series of inundation extent. For validation of the inundation extent we used a flood mask extracted from a supervised classification land cover map using Landsat imagery. The land cover map was validated using the error matrix method with ground truthed data. The estimated inundation extent time series enabled us to test the inundation correlation with discharge and water level using Pearson r correlation, a parametric statistical test. Based on the established correlation we used the Mann–Kendall, a non-parametric test, to analyse trends in the inundation extent and discharge and water level time series from which we made inferences on the direction of the historical trend in inundation extent. New hydrological insights for the regionThe results revealed that there is observable inter-annual variability in inundation extent in the Barotse Floodplain with prominent differences demonstrated in both the flood ascending/peak and receding period. For the period 2003–2013 the results indicated a rising trend in inundation extent with a Mann–Kendall Z statistic of 1.71 and increase in magnitude of 33.1 km2 at significance level alpha of 0.05. Strong correlations between inundation extent and water level and between inundation extent and discharge with correlation coefficients of determination of 0.86 and 0.89 respectively were observed. For the period 2000–2011 water level time series showed a rising trend with the Mann–Kendall Z statistic of 2.97 and increase in magnitude of 0.1 m at significance level alpha of 0.05. Overall, during the period 1952–2004 discharge in the floodplain showed a declining trend with Mann–Kendall Z statistics of −2.88 and −3.38 at the inlet and outlet of the floodplain respectively. By correlation inference, the overall inundation extent trend in the floodplain was in a downward movement. Rainfall and discharge variability, high evapotranspiration and the changes in the land cover-use in the catchment of the floodplain are largely the factors affecting the observed variability and trends in inundation extent in the floodplain. The presented remote sensing based approach significantly reduces the need for the expensive and time limiting traditional physical field based wetland inundation mapping methods that form a limitation for achieving progress in wetland monitoring especially in open and sparsely gauged floodplains such as the Barotse.