Monitoring infiltration rates with time domain reflectometers

The current study determines infiltration rates in six tree boxes located in Louisville, KY, and analyzes the effects of season, age of the tree box, and depth on infiltration. Street and parking lot runoff enter each tree box through a curb cut. Time domain reflectometers (TDRs) were embedded in the tree box media at different depths to continuously monitor volumetric soil moisture content and other parameters. The moisture content recorded by the TDRs was used to calculate steady-state infiltration rates through the media at the TDR location using the Green-Ampt equation. For each rain event, steady-state infiltration rate and drainage rate were separately calculated. For four of the six tree boxes, the steady-state infiltration rate did not vary with depth. However, the drainage rate varied with depth for five of the six tree boxes. Overall, the drainage rate through the entire depth is 1.7 times larger than in the upper layer. During the second year of the study, the steady-state infiltration rate and drainage rate decreased by 20% and 38%, respectively, from the first year. The steady-state infiltration rate and drainage rate vary with seasons. Some correlation (r = -0.5) was observed between initial soil moisture content and steady-state infiltration rate. PRACTITIONER POINTS: Initial infiltration rate and drainage rate decrease by 20% and 30%, respectively, within a year. The smallest infiltration rates were observed during winter and largest infiltration rates were observed in summer. Initial and saturated infiltration rates varied among all six tree boxes.