The use of remote sensing for monitoring environmental indicators: The case of the Incomati estuary, Mozambique

The Incomati river basin is a transboundary basin shared by three countries: South Africa, Mozambique and Swaziland. To assess the water requirements of the environment, as stated in the Tripartite Interim Agreement (TIA) signed by the three riparian countries in Johannesburg in 2002, Mozambique needs to monitor the ecological state of the river, including the estuary. A monitoring system has to be established that can evaluate the environmental fresh water requirements based on appropriate indicators that reflect the health of the Incomati estuary. The estuary of the Incomati has important ecological functions but it also is an important socio-economic resource. Local communities depend on the estuary's natural resources. Modifications of the river flow regime by upstream developments impact on the productivity of the estuary, diminishing fish and shrimp production, reducing biomass of natural vegetation such as grasses, reeds and mangroves and increasing salt intrusion. A decrease in estuary productivity consequently affects the incomes and living conditions of these communities. Based on an understanding of the effects of different pressures on the estuary ecosystem some indicators for monitoring the environmental state of the estuary are suggested, including the extent and vitality of mangrove forests. This latter indicator is further elaborated in the paper. Remote sensing techniques were used to identify and quantify mangrove forests in two selected areas of the estuary (Xefina Pequena Island and Benguelene Island). Five satellite images covering a period of 20 years (1984-2003) showed that the area covered by non-degraded mangroves significantly decreased on both islands, by 25% in Xefina Pequena Island and 40% in Benguelene Island. Moreover, the study of biomass reflection using NDVI also showed a significant decline in biomass densities over the last 20 years. Possible causes of these changes are reviewed: natural rainfall trends, modifications of the river flow regime, and increasing harvesting levels of mangrove woods. The findings presented in this paper show that mangrove forests are relevant indicators of the state of the estuary. which can be assessed by means of remote sensing techniques. Follow-up research is required that will establish the relative importance of the causal factors on the vitality of the estuarine mangrove forests. It is concluded that remotely sensed images may provide important data for an environmental monitoring system. (c) 2006 Elsevier Ltd. All rights reserved.