Topographical influence on soil chemistry

Topography is one of the five basic soil forming factors together with parent material, climate, biota and time, and therefore a major controlling factor on the soil properties. Topography can be quantified numerically by topographic indices, calculated based on a digital elevation model (DEM). The topographic wetness index (TWI) is a measure of the hydrological condition of a site, determined by a combined measure of upslope area and slope. TWI is a useful estimator of hydrological, pedological and biological properties at the landscape scale. In this licentiate thesis the TWI was studied, partly as a predictor of variations in soil properties, and partly for its dependency on the used calculation method and the resolution of the underlying DEM. Several methods for calculating the TWI were tested against field measurements. No single best method was found; instead an optimal method specific to the estimated property of interest was found most effective. Correlations between TWI and soil properties were observed both for 25 km2 study sites and at a larger scale based on data from the National Inventory of Forest Soils for all of Sweden. While clear patterns were observed for several soil properties, there was also a considerable scatter in the correlations. These studies were based on DEMs with resolutions of 20 and 50 meters. In an additional study, a DEM with a grid resolution of 5 meters, derived from LIDAR data, was resampled to generate DEMs of different resolutions, which were used to test the effects of different resolutions on the TWI calculations. The distribution of TWI was scale dependent and a lot of the variation of TWI based on the 5 m resolution DEM was lost at 10 meter resolution. Terrain analysis can be used to predict hydrological, pedological and biological properties at a spatial resolution impossible to achieve through field work. By choosing an appropriate DEM resolution and adjusting the calculation methods for specific purposes, the usefulness of the TWI could be refined allowing its application over a large spatial range.