Modeling spatial patterns of saturated areas: An evaluation of different terrain indices

[1] A key component to understanding and predicting water fluxes and water quality in river basins is the spatial distribution of water-saturated areas. There is limited knowledge on spatial patterns of saturated areas, their relation to landscape characteristics and processes, and the ability of hydrological models to represent the observed spatial patterns, particularly at the large scales most relevant for water resources management. In this study, saturated areas were mapped in two mesoscale (18 and 40 km), humid temperate basins. Geobotanical and pedological criteria were used to achieve a consistent time-integrated delineation of saturated areas. Using commonly available spatial data on landscape characteristics, various terrain indices were evaluated for their ability to predict the observed patterns. Quantitative performance criteria describing the agreement of modeled and observed spatial patterns included cell-by-cell and cell-neighborhood approaches. Upslope contributing area was the most important single factor explaining the observed pattern. An improved pattern was obtained for the topographic wetness index (TOPMODEL index). However, the performance was markedly sensitive to the algorithms used for calculation of upslope contributing area and slope gradient. Other factors such as soil or climate were of less value for improving the predictions. The optimum spatial agreement of observed and modeled saturated areas was about 50% for a combined soil-climate-topographic index. Geological features (bedrock fractures) partly explained the residual pattern. Using an independent test catchment, it was shown that the index approach can be transferred to basins with similar physiographic characteristics for estimating the general pattern of saturated areas.