Remote sensing of invasive water hyacinth (Eichhornia crassipes): A review on applications and challenges

Aquatic invasive species threaten socio-economic and ecological systems, by invading freshwater ecosystems, and influencing their functionality and productivity, as well as disturbing key hydrological processes. Ecologically, water hyacinth can impact zooplankton and phytoplankton productivity in freshwater ecosystem, modify surface water clarity and cause hypoxia or a decrease in the concentration of related nutrients and contaminants, such as nitrogen, phosphorous and heavy metals. Field surveys and water-related reports indicate that water hyacinth (Eichhornia crassipes), which is one of the most aggressive and lethal floating aquatic weed; has invaded most water bodies in the sub tropics. Its spread is largely linked to eutrophication emanating from poor land use management practices and other environmental and climatic factors. Besides its threats and effects, its distribution in streams and rivers is not yet fully understood. This gap in knowledge is due to over reliance on traditional surveys and poor financial resources and most importantly the lack of readily available or cheap satellite platforms, with optimal spatial and spectral settings that can discern water hyacinth from other co-existing plant species. In sub-Saharan Africa for instance, the use of high resolution satellite data is constrained by the acquisition costs and in some cases lack of technical expertise. There is, therefore, a need to develop new and robust methodologies that can map aquatic water weeds, especially in small freshwater bodies. The use of recently launched crop of new generation sensors like Sentinel 2 and Landsat 8 sensors, with improved sensing characteristics, unlike the previous broadband multispectral sensors provides untapped prospective alternatives.