EXPLORING INFILTRATION PATTERNS IN URBAN SOILS USING GEOPHYSICS AND MICROTOPOGRAPHY

Understanding infiltration is critical for stormwater management and assessing green infrastructure performance. In urban watersheds, infiltration rates are highly variable and influenced by soil compaction, buried debris, and textural heterogeneity, making detailed infiltration mapping using direct measurements impractical. At a vacant lot in north Philadelphia, we compared 3D ground penetrating radar (GPR) data, topographic wetness indices (TWI), and mini-disk tension infiltrometer data with volumetric water content measurements to determine whether a single factor or a combination of factors could serve as a proxy for soil moisture. GPR was used to image subsurface objects because this grassy lot formerly supported a block of row homes that were demolished prior to the early 1990’s and buried debris such as relic building foundations can influence infiltration rates. Terrestrial LiDAR was used to develop a high resolution (5 cm) digital elevation model to generate the topographic wetness index (TWI), which predicts wet areas in a landscape based on the contributing area and slope. Microtopography is not typically considered since stormwater models are usually gridded on a coarser scale. Exploring this factor could suggest stormwater model improvements. The mini disk infiltrometer was used to collect point measurements of unsaturated hydraulic conductivity and infiltration rates. Point measurements take longer to collect and may be of more limited utility when trying to characterize urban sites. Although no single factor could predict the majority of change in soil moisture at our field site, both the presence of shallow subsurface objects mapped by the GPR and high infiltrometer values correlated with lower soil moisture. The TWI was not able to predict observed soil moisture, so at this relatively flat site, microtopography was not a dominant factor. However, relief across the study area only ranged 10-20 cm. It remains an open question if at more rugged sites, microtopographic relief effects soil moisture or whether the heterogeneity of urban soils trumps the TWI’s effectiveness.