Wetland occurrence in relation to topography: a test of topographic indices as moisture indicators

Information about the spatial distributions of soil moisture or groundwater levels is needed for aggregation of soil-vegetation-atmosphere-transfer (SVAT) models. The possibility of predicting wetness distributions in catchments from topographic data was investigated using topographic indices, notably the TOPMODEL index. The indices were calculated from commercially available gridded data (50 x 50 m(2)) over two areas with contrasting topography: a catchment (Nasten) in the low-relief NOPEX region in southern Sweden and a group of catchments in a hilly area (Kassjoan) in central Sweden. The occurrence of mires, assumed to represent the extreme wetness end of the wetness spectrum, was used as field data. The frequency distributions of topographic indices for mire and non-mires were clearly different in Kassjoan, although there was a large overlapping, whereas the two distributions were very similar in Nasten. Prediction of mires from topographic indices was meaningful only in Kassjoan. Although it gave poor results in terms of fractions of successfully predicted mire cells out of the observed mire cells, the general spatial patterns of mires were fairly well simulated. One important reason for the failure of the indices to predict mires in Nasten, and probably also to predict other wetness classes, is that the spatial resolution in the index calculation was coarser than typical length scales of the topographic features in this catchment, being only a few tenths of meters. The importance of geologic conditions in modifying the topographic control over the wetness is exemplified from the obtained relationship between topographic indices and mire occurrence. (C) 1999 Published by Elsevier Science B.V. All rights reserved.