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Use of Water Content Reflectometers in Bioinfiltration/Bioretention to Measure Water Movement and Estimate Evapotranspiration - abstract
Citation:
Brown, R. AND Mike Borst. Use of Water Content Reflectometers in Bioinfiltration/Bioretention to Measure Water Movement and Estimate Evapotranspiration - abstract. Presented at 2014 ASCE-EWRI World Environmental and Water Resources Congress, Portland, OR, June 01 - 05, 2014.
Impact/Purpose:
Most bioinfiltration/bioretention models assume runoff is evenly distributed across the surface area and after the engineered fill media is no longer saturated, the volumetric water content (VWC) is constant throughout the media profile and at field capacity. Four to nine water content reflectometers were spatially distributed across the surface and installed in profile in 15 systems located in Kentucky, New Hampshire, and New Jersey, to test the hydrologic model assumptions and monitor performance. Observations and measurements from these sites contradict some commonly used modeling assumptions that could affect how bioinfiltration/bioretention systems are designed.
Description:
Most bioinfiltration/bioretention models assume runoff is evenly distributed across the surface area and after the engineered fill media is no longer saturated, the volumetric water content (VWC) is constant throughout the media profile and at field capacity. Four to nine water content reflectometers (CS616, CS650, or CS655; Campbell Scientific Inc.; Logan, Utah) were spatially distributed across the surface and installed in profile in 15 systems located in Kentucky, New Hampshire, and New Jersey, to test the hydrologic model assumptions and monitor performance. These systems (two bioretention cells, six bioinfiltration areas, and seven tree boxes) have five different media blends with textures of sand, loamy sand, and sandy loam. An initial observation was that the VWC of the media after it drained was not uniform with depth. This was expected because field capacity is not a soil-water constant; VWC varies with depth based on suction from underlying groundwater. Additionally, when the measured VWC was compared to field capacity predicted based on soil texture, the measured VWC was larger than field capacity in most soil types tested, resulting in less available storage in the media than predicted. The change in VWC before an event started and after drainage ended was used to estimate water losses between events through evapotranspiration and vertical seepage since the previous event. Passive capillary lysimeters (G2, Decagon Devices Inc., Pullman, WA) were also installed in about half of the systems to quantify vertical seepage. Finally, for a bioretention cell with loamy sand media, the spatially distributed TDRs documented that runoff from most storms less than 12 mm infiltrated in the front third of the system. Whereas for the bioinfiltration areas with sand media, larger storms (up to 50 mm) fully infiltrated near the inlet. These observations and measurements contradict some commonly used modeling assumptions that could affect how bioinfiltration/bioretention systems are designed.
