Determining wetland spatial extent and seasonal variations of the inundated area using multispectral remote sensing

L Nhamo,J Magidi,C Dickens

doi:10.4314/wsa.v43i4.02

Abstract

Wetlands can only be well managed if their spatial location and extent are accurately documented, which presents a problem as wetland type and morphology are highly variable. Current efforts to delineate wetland extent are varied, resulting in a host of inconsistent and incomparable inventories. This study, done in the Witbank Dam Catchment in Mpumalanga Province of South Africa, explores a remote-sensing technique to delineate wetland extent and assesses the seasonal variations of the inundated area. The objective was to monitor the spatio-temporal changes of wetlands over time through remote sensing and GIS for effective wetland management. Multispectral satellite images, together with a digital elevation model (DEM), were used to delineate wetland extent. The seasonal variations of the inundated area were assessed through an analysis of monthly water indices derived from the normalised difference water index (NDWI). Landsat images and DEM were used to delineate wetland extent and MODIS images were used to assess seasonal variation of the inundated area. A time-series trend analysis on the delineated wetlands shows a declining tendency from 2000 to 2015, which could worsen in the coming few years if no remedial action is taken. Wetland area declined by 19% in the study area over the period under review. An analysis of NDWI indices on the wetland area showed that wetland inundated area is highly variable, exhibiting an increasing variability over time. An overlay of wetland area on cultivated land showed that 21% of the wetland area is subjected to cultivation which is a major contributing factor to wetland degradation. Keywords : Wetland extent, remote sensing, ecosystems, change detection, Sustainable Development Goals

Highlights

Wetlands provide a variety of ecological and economic functions that include water quality improvement, flood regulation and protection, groundwater recharge, shoreline stabilisation, fish and wildlife habitat, agriculture production, aesthetics and biological productivity (Kotze et al, 2009; De Steven and Lowrance, 2011)
These stresses are impacting negatively on the very existence of wetlands and on their delivery of services to society; yet society does not have a comprehensive grasp of the magnitude of the problem because knowledge about the importance, spatial extent and seasonal variation of wetlands remains so limited. This calls for a comprehensive method to assess the spatial and temporal changes in wetlands. This has recently become increasingly relevant as the United Nations has identified the importance of protecting and restoring wetlands by including them in Goal 6 of the Sustainable Development Goals (SDGs), which deals with water issues, and in Indicator 6.6.1, which emphasises the change in extent of water-related ecosystems over time
The National Freshwater Ecosystem Priority Areas (NFEPA) wetland map (Nel et al, 2011) has an area of 448 km2, which is almost half of the area calculated in this study and represents only 12% of the total catchment area

Summary

Introduction

Wetlands provide a variety of ecological and economic functions that include water quality improvement, flood regulation and protection, groundwater recharge, shoreline stabilisation, fish and wildlife habitat, agriculture production, aesthetics and biological productivity (Kotze et al, 2009; De Steven and Lowrance, 2011) They have been subjected to a host of stress-inducing modifications, such as dredge-andfill operations, hydrologic modifications, pollutant runoff, eutrophication, impoundment, and fragmentation by roads and ditches; more recently there has been concern about the impact of climate change on wetlands (Levin et al, 2001; Klemas, 2011). Previous studies have suggested that the world’s wetland area has declined

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Conclusion