Think landscape complexity.
How the landscape is managed plays a large role in how the landscape functions to reduce stormwater runoff according to a study in Australia. The amount of habitat components in the landscape–soils, leaf litter, vegetation surface area, vegetation layers–determine habitat complexity and exert significant effect on stormwater infiltration.
In the article, Habitat complexity influences fine scale hydrological processes and the incidence of stormwater runoff in managed urban ecosystems in the August issue of Journal of Environmental Management, researchers in Melbourne, Australia took a look at three urban landscapes in low-complexity parks, high-complexity parks and high-complexity remnants, establishing 10 plots in each.
Low complexity parks featured Eucalyptus overstory trees with a simplified herbaceous mown (less than 5 cm tall) understory. High complexity parks also had overstory eucalyptus trees with a complex understory of shrubs, small trees and herbs. Once established, the landscape was not actively managed, vegetation naturally regrew and accumulated leaf litter. High complexity remnant plots were covered with native vegetation to provide a baseline. They are managed for conservation, receiving only sporadic weeding with supplemental native plant revegetation as required.
The researchers included several key observations in their summary:
- Landscapes can be managed to provide stormwater functionality. The authors suggest that to lessen “stormwater runoff from urban ecosystems, land managers could incrementally increase the complexity of habitat patches, by increasing canopy density and volume, preserving surface litter and maintaining soil macropore structure.” In other words: Plant more plants, add and/or keep leaf litter (a.k.a. duff) or apply mulches and maintain good soil structure so that air and water infiltrate. Low-complexity urban habitats had slower rates of soil water infiltration. As a matter of fact, this habitat type experienced stormwater runoff even in low rainfall events.
- The presence of soil macropores, rather than bulk density, drives water infiltration. Translation: As soil aggregates form from the basic building blocks of clay, sand and silt, larger pore space (>0.08 mm) that allows water and air is healthier than smaller pore space for root growth, water infiltration and greater soil invertebrates too. Invertebrates directly help soil structure by burrowing and tunneling. The authors add that more research to better understand the role of soil macropores across soil types in urban ecosystems is needed.
- Complex habitats intercept and hold more stormwater than simple habitats. Translation: Facilitating invertebrates like worms, centipedes, soil insects, etc. through landscape management also facilitates stormwater management. They theorize that including mow-free zones that allow accumulation of leaf litter would support the invertebrate communities that help develop good soil structure. Vegetation complexity in terms of types and sizes of plants were positively associated with stormwater infiltration or interception. More leaf area equates to more rainfall intercepted and less through fall. Rain that is intercepted evaporates back in to the atmosphere, or slowly drips down to the ground.
- Management can increase habitat complexity and stormwater benefits. Translation: Plant more plants at different heights to have layering. The authors also suggest that these landscapes could become “self-sustaining,” by allowing natural processes and the interplay between soils, vegetation and soil organisms, thus reducing maintenance expense.
Journal of Environmental Management Volume 159, 15 August 2015, Pages 1–10.
NewTerrain newsletter September 4, 2015.