Stormwater infiltration could keep streams flowing

It is well known that passive irrigation systems harvesting stormwater at the kerb can benefit street trees, but it is less well known that water sensitive urban design features can also help meet environmental water needs at a catchment scale.

Urbanisation and water resources development have greatly altered the hydrological cycles of catchments like the Mount Lofty Ranges. Key flow components, such as baseflows provided by groundwater discharge, are missing from some river reaches or have been greatly reduced in terms of the river length that receives groundwater over the warmer months. In our climate, drought refuge pools and riffle habitats sustained by baseflows are critical for survival of native species such as small-bodied native fish like mountain galaxis (Galaxis olidus) that take refuge in pools over summer and move up- and downstream during periods of connected flow. These animals can be lost from a reach in a single summer if their refuge pools dry out because they are unable to move to other habitats and their eggs must stay submerged.

^{Mountain galaxias (Galaxias olidus, left, Source: M Hammer) and obscure galaxias (G. olirus, right, Source: N Whiterod) are found in watercourses throughout the Mount Lofty Ranges and Adelaide Plains. Their survival, distribution and abundance are limited by increased cease-to-flow periods and poor water quality as a result of catchment development and a drying climate. They are no longer found downstream of the city of Adelaide.}

Urban development can limit the infiltration capacity of a catchment, resulting in recharge rates and baseflows that are much less than pre-development rates⁴, although it is not quite that simple as the impacts of urbanisation and anthropogenic land use change on groundwater dynamics are multi-factorial, often resulting in complex water balance changes³.

Direct stormwater runoff provides flows to urban streams at all times of year after even small rainfall events, due to the large area of hard surfaces. These flows are often short-lived, however, and whilst they may help top up pools over summer, additional water may be needed to sustain baseflows over the summer period.

^{Even though this creek line in Mount Barker is heavily modified and overgrown with exotic plants, the deep shade and permanent water provide suitable cool and flowing habitat for both mountain and obscure galaxias. It is not known whether groundwater contributes to this baseflow, however the presence of the fish and the deep incising suggest that it may be a groundwater discharge location. Source: K Muller, November 2023.}

The good news is that research has shown that WSUD features can improve baseflows. For example, a 100m² infiltration system with a 9800m² catchment in Victoria fully restored baseflows, four other Victorian projects showed partial baseflow restoration and a further 54 studies showed that baseflow supplementation is possible¹. Passive and active discharges from residential rainwater tanks overflowing to permeable surfaces can potentially return baseflows to within natural ecological limits, with additional benefits such as flood mitigation, for relatively little cost compared with alternative WSUD features^{6, 2}.

The not-so-good news is that there is no conclusive evidence that current WSUD practices are adequate to restore baseflows to within the limits that can be tolerated by our aquatic ecosystems. Furthermore, if WSUD features, such as a constructed wetland that intersects the groundwater table, are designed or located incorrectly, this can be detrimental to baseflows by increasing evaporative losses¹. 

The solution is to raise the bar on our WSUD objectives, to understand the role that WSUD could play in restoring critical components of catchment hydrology, and ideally increase the area of infiltration from the allotment scale upwards. Targets for groundwater recharge (locations and volumes) could be matched to environmental water demands, such as baseflows to sustain permanent pools, especially those that support threatened native fish. 

Infiltration of urban stormwater has the potential to be a critical component of environmental watering that will enable our aquatic wildlife to adapt to a changing climate. These well-watered rivers provide refuge pools for fish, plus shade, shelter and atmospheric cooling, which will positively impact on recreational values, aesthetics, and ultimately, property values along urban river corridors⁵.

References

1. Buck D, Taylor B, Fabbro L and Rockloff S (2019)  Baseflow contribution from water sensitive urban design.  Water e-journal.  Australian Water Association Vol. 4, No. 3.
2. Gee KD and Hunt WF (2016) Enhancing stormwater management benefits of rainwater harvesting via innovative technologies.  Journal of Environmental Engineering, Vol. 142, No. 8, p. 04016039.
3. Han D, Currell MJ, Cao G and Hall B (2017)  Alterations to groundwater recharge due to anthropogenic landscape change. Journal of Hydrology Vol. 554, pp. 545-557.
4. McGrane SJ (2016) Impacts of urbanisation on hydrological and water quality dynamics, and urban water management: A review.  Hydrological Sciences Journal, Vol. 61, No. 13, pp. 2295-2311.
5. Nicholls S and JL Crompton (2017) The effect of rivers, streams, and canals on property values.  River Research and Applications, Vol. 33, pp. 1377–1386.
6. Taylor B (2012)  Emulating pre-urban initial rainfall losses and restoring baseflow with rainwater harvesting.  Paper presented at the 7^th International Conference on Water Sensitive Urban Design (WSUD): Water Sensitive Urban Design: Building the Water Sensitive Community, Melbourne, Australia, 21-23 Feb 2012, as cited in Buck et al. (2019).