Mapping spatio-temporal variations in water hyacinth (Eichhornia crassipes) coverage on Rwandan water bodies using multispectral imageries

The spatial and temporal knowledge on spread of invasive aquatic plants helps to determine their extent, abundance, and propagation rates in invaded water systems. Water hyacinth Eichhornia crassipes (Liliales: Pontederiaceae) appeared in Rwandan water bodies in 1957, and it was legislated and accepted as a problematic invasive species in Rwanda in 1999. Water hyacinth has led to a reduction in water quantity and threatened the livelihood of local communities that live off fishing. To comprehend the status of the water hyacinth invasion and assist management strategies, it is important to have detailed and consistent information on its spatio-temporal spread, magnitude, and rate of change dynamics in water systems. This was investigated using time series Landsat satellite images for the years 1989, 2002, and 2017. Image classifications using the nonparametric classifier random forest as well as change detection analysis were carried out to process the satellite data. The results revealed a fluctuation in the extent of water hyacinth over the time series of 1989, 2002, and 2017 with estimated percentage cover of 17.7%, 22.4%, and 20.8%, respectively. An annual increase of 1.9% in water hyacinth invasion was observed from 1989 to 2002; while, a decline of 1.7% per annum was observed from 2002 to 2017. The decline observed in 2017 could be due to manual control undertaken by the government since 2002. This study confirms the potential of using remotely sensed imagery as a valuable method for determining the change in the extent and distribution of invasive alien weeds over time.