Evaluating digital terrain indices for soil wetness mapping - a Swedish case study

Trafficking wet soils within and near stream and lake buffers can cause soil disturbances, i.e. rutting and compaction. This - in turn - can lead to increased surface flow, thereby facilitating the leaking of unwanted substances into downstream environments. Wet soils in mires, near streams and lakes have particularly low bearing capacity and are therefore more susceptible to rutting. It is therefore important to model and map the extent of these areas and associated wetness variations. This can now be done with adequate reliability using a high-resolution digital elevation model (DEM). In this article, we report on several digital terrain indices to predict soil wetness by wet-area locations. We varied the resolution of these indices to test what scale produces the best possible wet-areas mapping conformance. We found that topographic wetness index (T-WI) and the newly developed cartographic depth-to-water index (D-TW) were the best soil wetness predictors. While the T-WI derivations were sensitive to scale, the D-TW derivations were not and were therefore numerically robust. Since the D-TW derivations vary by the area threshold for setting stream flow initiation, we found that the optimal threshold values for permanently wet areas varied by landform within the Krycklan watershed, e.g. 1-2 ha for till-derived landforms versus 8-16 ha for a coarse-textured alluvial floodplain.