Water retention by raingardens: implications for local-scale soil moisture and water fluxes

Abstract

A key feature of biofiltration systems, or 'raingardens', is the modification of the local water balance through the enhancement of the infiltration and evapotranspiration fluxes. Thus, widespread implementation of raingardens has the potential to bring the catchment water balance closer to its pre-development state. However, the magnitude of these effects is still unclear, partly because of uncertainties in the water budget of individual raingardens. Since July 2011, two raingardens have been monitored in a residential area 40 km east of Melbourne, Australia, with the aim to determine the volume and fate of infiltrated water. The two sites differ in terms of topography and surrounding vegetation, with one bordered by short lawn grass and the other featuring a mix of clovers and tall grass. We measured the soil moisture profile at different distances from the raingardens to i) determine the lateral influence of the biofiltration system on soil moisture and ii) infer the enhancement of evapotranspiration due to the increased soil moisture. During the wet season, the results suggest an absence of water stress for surrounding vegetation even in the absence of a raingarden, meaning that nearly all the infiltrated water seeped to recharge groundwater. The total evapotranspiration flux of the raingarden itself is thus very small, limited by the raingarden area. However, in drier conditions, the system may enhance the evapotranspiration rates of its surroundings, particularly where nearby vegetation has high transpirational demand.

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