Vegetation Alters Soil Water Drainage and Retention of Replicate Rain Gardens

Marie R Johnston,Anita M Thompson,Nick J Balster

doi:10.3390/w12113151

Abstract

Rain gardens are residential bioretention practices widely used to manage urban runoff, yet their design as plant-soil systems lacks understanding. We hypothesized that vegetative treatment (turfgrass, prairie, and shrubs, plus a non-vegetated control) would alter the volume and rate of drainage from 12 replicate mesocosms (i.e., rain gardens) through changes to the belowground system. Roof runoff was collected on-site and distributed equally among the mesocosms following natural rain events for two growing seasons. We monitored stormwater input, drainage output, and soil moisture to assess differences in hydrology by treatment, explained by indices of soil structural development (infiltration, saturated hydraulic conductivity, soil water retention). Drainage volume and response dynamics differed as predicted by vegetative treatment in support of our hypothesis. The greatest reductions in drainage volume were observed beneath shrubs and prairie following smaller stormwater inputs, and accelerated drainage responses were observed beneath turfgrass following larger stormwater inputs. Differences in infiltration, saturated hydraulic conductivity, and plant-induced changes in antecedent soil moisture among vegetative treatments help explain these plant-mediated drainage responses. This study shows that plants can alter the hydrologic dynamics of rain gardens and thus are a critical component of the design and intent of these plant-soil systems.

Highlights

Urbanization transforms the land surface and alters the pre-development hydrologic cycle, and these changes amplify the need for surface water management
We evaluated the effect of vegetative treatment on drainage volume, the lag time to peak flow, the peak flow rate, and the response duration to inputs of stormwater
There was a consistent stacking among vegetative treatments at a depth of 0.30–0.45 m across the duration of our study, and soil moisture at 0–0.15 m depth began to diverge during periods of dry weather

Summary

Introduction

Urbanization transforms the land surface and alters the pre-development hydrologic cycle, and these changes amplify the need for surface water management. Since the 1970s, urban land development has led to declines in soil-water recharge by as much as 80% [1]. The resulting runoff typically yields increased volume, increased flow rates, and shorter flow durations than the predevelopment hydrology. A 10% increase in impervious surfaces within a watershed outside of New York, NY, yielded stormwater with a three-fold greater peak flow rate compared with a nearby undeveloped watershed; the flow duration was shortened from approximately. The remaining pervious areas, such as beneath street trees and turfgrass lawn, can exhibit decreased infiltration if soil compaction occurred during land development [3,4,5].

Methods

Results

Conclusion