Potential Infiltration—Improving the Siting and Design of Infiltrating BMPs
Maps of soil hydrologic groups provided by the Natural Resource Conservation Service (NRCS) are often used in site assessments for infiltrating stormwater best management practices (BMPs). These hydrologic groups were originally developed for agricultural applications and indicate how well water infiltrates from the surface. However, using these hydrologic groups may not be appropriate when siting or designing a BMP whose base exists below the surface.
Soils are composed of many horizons (i.e., layers) that have different potential infiltration rates, as represented by varying saturated hydraulic conductivities (Ksat). Horizons with low Ksats, such as hardpans, have a low potential for infiltration. However, these horizons are often shallow and stormwater BMPs can go deeper than these barriers. If the depth of a stormwater BMP can be set below a low infiltration horizon (i.e., within or just above horizons with higher infiltration potential), the BMP can achieve higher infiltration, resulting in more volume and load reductions. In addition, BMPs with higher infiltration rates can fit into smaller footprints and have a greater potential to recharge groundwater.
The Office of Water Programs (OWP) at Sacramento State processed Soil Survey Geographic Database (SSURGO) soil data from the NRCS to calculate the effective Ksat for soils at depths in 1-foot intervals ranging from the surface to 7 feet below the surface. Effective Ksat values are based on the Ksat of individual soil horizons proportional to their thickness, resulting in a potential infiltration rate for soil horizons below the depth of interest that provides much more specificity than traditional soil hydrologic groups.
These calculations were performed for the greater Sacramento region as part of the American River Basin Storm Water Resource Plan (ARB SWRP), funded by the California Water Board’s Proposition 1 Stormwater Grant Program. The effective Ksat values were established as geographic information system (GIS) layers and tables of potential infiltration rates by depths and added to an online planning tool (the ARB SWRP web map), available at www.owp.csus.edu/ARBSWRP/map.htm.
Users of this web map can select the Potential Infiltration layer and use a slider to dynamically view changes in potential infiltration rates with depth. If more details are needed, the individual soil units can be clicked on to view tables on soil horizon properties and a summary of the potential infiltration rates with depth.
The potential infiltration rates have been mapped in the Sacramento region by OWP, but the methods are repeatable and applicable for any region where the NRCS SSURGO data are available.
As part of the presentation, the speaker will demonstrate the online planning tool and potential infiltration layer to show how they can be used to improve the placement and design of low-impact development BMPs. This demonstration of the tool fits in with this year’s theme “Connecting the Drops from the Summit to the Sea” by showing the audience how stormwater that would normally run off to local streams can be infiltrated in soils traditionally seen as impermeable by looking not just at surface infiltration, but at infiltration rates deeper in the soil.