Linking the Depth-to-Water Topographic Index to Soil Moisture on Boreal Forest Sites in Alberta

Abstract

The depth-to-water (DTW) index is defined as the cumulative slope along the least-cost pathway from any cell in the landscape to the nearest flow channel. The flow channel network is determined by the flow initiation area selected, allowing the representation of various geological and topographical attributes of the landscape. We used data from 125 plots across five locations in the boreal forest of Alberta, Canada, to evaluate the following: the relationships between soil attributes and DTW; the optimal flow initiation area; and models to map the spatial variation of soil properties. Soil moisture regime (SMR), drainage class, and depth-to-mottles were strongly related with DTW, whereas soil nutrient regime, organic matter thickness, soil texture, and coarse fragment content exhibited weak and inconsistent relationships with DTW. A flow initiation area of 2 ha yielded the best representation for SMR, drainage class, and depth-to-mottles. DTW, flow accumulation (FA) and local slope were combined in a linear model to estimate and map SMR, whereas only DTW and FA were used to model drainage and depth-to-mottles. These results suggest that the DTW index can capture soil properties closely influenced by the water table but cannot characterize site and soil factors, which are also determined by parent material, climate, and vegetation.