Relationships between topography and spatial variations in groundwater and soil morphology within the Taoyuan-Hukou Tableland, Northwestern Taiwan

This study examines the relationships between topography and spatial variations in groundwater and soil morphology within the multi-step Taoyuan-Hukou Tableland in northwest Taiwan. Several different geomorphological and pedological approaches were jointly undertaken, including analyses of a digital elevation model (DEM), a reclassified soil map, field survey results, engineering borehole data, and a hydrological simulation. Five major geomorphic surfaces were identified based on terrace scarps observed in the DEM; minor surfaces separated by small fluvial scarps or tectonic scarps were also mapped. The simplified soil map and field study of soil morphology reveal a number of soil-distribution patterns associated with specific topographic settings. Soil morphology changes from homogeneous red soils in the fore-edge of surfaces to mottled soils and finally to gley soils in back-edge regions. Such a pattern is more pronounced in the proximal part of a surface, while toward the distal zone the patterns tend to be obscured by increasing amounts of mottled and gley soils. The proportion of homogeneous soils increases with narrowing of the surface. For fan-shaped surfaces, soil types appear to be dominated by mottled soils except for at terrace edges, where homogeneous red soils occur. While homogeneous soils dominate tectonically elevated surfaces, a toposequence of homogeneous-mottled-gley soils occurs in the surrounded lowlands. Borehole data and hydrological models indicate that these soil-distribution patterns are strongly related to groundwater occurrence and fluvial processes, both of which are generally governed by topography. Despite the observed pedodiversity in this area, the common feature of these toposequences, i.e., the distribution of homogeneous red soils along the surface fore-edge, may serve as a basis for establishing a reliable soil chronosequence for the area.