Mapping and classification of wetlands using topographic wetness index (twi) from digital elevation models of the the gravataí river basin - Rio grande do sul state (Rs), Brazil

Abstract

Wetlands (WT) cover approximately 6% of the surface of the Earth, are productive ecosystems important in the global carbon cycle, climate regulation and nutrient cycling. Despite this importance, they historically suffer negative impacts (e.g. conversion to urban or agricultural areas). Digital Elevation Models (DEMs) can be useful in hydrological research to delineate catchment areas and identify drainage and flow patterns, particularly in flat areas, where useful relief information to identify WT is difficult to obtain. In addition, depressions in a DEM may be WT characteristic of the landscape. This work aims to compare the Topographic Wetness Index (TWI) obtained from four DEMs (Alos Palsar I DEM-12.5 m; Aster GDEM-30 m; SRTM-30 m; SRTM-90 m) in order to map and classify WT in a hydrographic basin located in Rio Grande do Sul state (Brazil). A total of 4,000 WT samples (floodplain, marshes, grasslands, depressions, rice fields) and 4,000 non-WT samples (terra firme fields and forests) were collected from landscape compartments associated with Holocene system, peat, floodplain, and alluvial and fluvial channel deposits. TWI values were extracted for each sample in the four DEMs. Classification of the samples to define a threshold was based on the Classification and Regression Trees (CART) method. The results show that the TWIs extracted from the DEMs Aster GDEM- 30 m and SRTM-30 m presented the highest accuracies to delimit the WT (71.9% and 75.1%, respectively). These DEMs were the most efficient in the spatial relationship with the occurrence of hydromorphic soils in the Banhado Grande system. These results are confirmed comparing both DEMs with geological spatial units, relative to environments of lagoon, fluvial and paludial deposits in the study area. The TWI produced from the highest spatial-resolution DEM (Alos Palsar I) presented greater detail of the drainage channels associated with irrigated rice production; however, it did not precisely delimit the different spatial units characteristic of WT. The study concludes that the TWI extracted from the DEMs Aster GDEM-30 m and SRTM-30 m can be applied with 70+% precision to WT mapping in similar topographical areas.