Water catchment in cities
Home rain garden
Roadside rain gardens
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Beyond the drought - Designing For Water Scarcity
By Anno Torr
Water wise species - and most gardeners interpret this label to mean plants that survive on little water - are currently the go-to plants as gardeners respond to a drought that ranges in degree of toughness from harsh to disastrous. But, beyond the drought – what then? Focusing on drought-hardy planting suggests to me a temporary fix, a way to ease a garden through a current crisis. We live, though, in a country of low rainfall and unreliable water supply, where future droughts are a certainty. Designing for water-scarcity then requires more from us than simply a change in plant material and installation of a water tank to collect roof runoff.
For the most part, we have gardened on through recent years, planting the same kind of landscapes as if the world was not changing around us”, wrote horticultural journalist, Thomas Christopher, in a recent blog post on Garden Rant.
Decades of American and English-style garden design, plant choice and management has given South African gardeners a legacy of high-input landscapes, leaving us high and dry when this latest severe drought hit. Yet, as the rains arrived in Durban and Gauteng, volumes were heavy and concentrated, causing wide-spread damage as water flowed off hard surfaces rather than soaking into ground prepared to receive it.
So often droughts break with local flooding; how do we grow gardens to cope with these extremes? The same designs can mitigate both drought and high rain events.
It requires a 6-pronged approach:
Up until now, urban planning directs water off-site as a way to keep property and road surfaces clear of water. Now, designs must focus on keeping rain where it falls, slowing it down, spreading it out, giving it time to soak in. To do this, we need to:
Shape the ground – to direct water flow
Build living soil – to absorb and hold water and feed plants
Capture rainwater – to keep it where it provides the most benefit
Designate planting hydro-zones – matching a plant’s water needs with soils moisture levels
Choose climate and soil appropriate plants – match plant to place (climate and soil type)
Establish resilient plants
Rain garden cross section
Home rain garden
Roadside swales & rain garden
1. SHAPE THE GROUND
Meet plant water needs through contouring and drainage. Imagine your property as a mini-watershed. A watershed (or basin) is the area of land that catches precipitation and directs it to drain into a larger body of water, like a wetland, natural lake, stream or river. Topography (physical features and shape of the ground) influences the entire watershed, controlling the direction the water takes and the speed with which it moves across the ground. Plant roots and soil clean the water of contaminants as it filters through the profile to recharge underground water storage.
We can shape our properties to direct water away from areas where we don’t need it to areas where we do need it, and finally to soak into the ground rather than exit through a stormwater drain. Giving the water a chance to soak in keeps it available to the local ecosystem.
Water directed away from one area becomes a resource for other areas.
We’re not talking major earthworks here. Swales (bioswales), rain gardens (bioretention system), depressions, berms and sloping ground all help to direct, slow and hold surface water enabling percolation and preventing or significantly reducing damage from flash flood events. And, contoured ground captures water for long enough to be useful during dry periods. Make sure your re-shaping does not wash water from your property onto the neighbour’s garden; you could be liable for any damage caused!
There is a relationship between topography, the volume of water, surface material, and capacity of the ground to absorb the water.
Water moves fast on steep slopes, hard ground and impenetrable surfaces like paving and compacted soils, especially where it can gather speed over a distance.
Water slows down as the gradient decreases allowing it to spread out, as it moves across lawn, through wild grasses and groundcovers, when rain is intercepted by foliage, and where the distance it travels is short. Slow moving water causes less damage and has more time to soak in.
Follow the water on your property after heavy rainfall; knowing the current watercourses will guide your new contouring designs.
Rain gardens, bioswales:
Two factors improve the effectiveness of rain gardens and bioswales:
Increased contact time between water and soil
Increased vegetative cover
By now, we’re all familiar with rain gardens, attractive features typically used in home gardens. These are shallow to mid-depth depressions that use plants and soil to manage water redirected from roof downpipes and off driveways and other paved surfaces. They’re designed to avoid ponding for longer than 24 hours, so mosquitoes should not be a problem. Plants growing here can withstand periodic flooding, such as many wetland species, and the existing soil is amended with a top-dressing of organic matter to improve drainage if infiltration is insufficient.
Bioswales are usually larger and deeper than rain gardens – usually ditches and long, narrow channels - but with the same end-goal in mind, to slow, absorb, and filter stormwater. They’re engineered to manage known volumes of water off large impervious surfaces like parking lots, commercial properties and roads. Planted vegetation is adapted to seasonal regimes of flooding and drought.
Berms to direct flow
A berm is a small mound with sloping sides typically longer than it is wide. Often used to create a noise barrier or screen an undesirable view, in the home garden they are effective in changing the course of water flow or encouraging pooling after heavy rains. Mounds are best graded as in the image below rather than shaped like a sharp, uniform speed bump, with a gradual transition from the existing ground to sloping sides. Vegetation cover, whether lawn or plants, protects the raised soil from erosion and helps to slow down the water.
Image from: http://www.extension.umn.edu
We’ll focus on berm design and uses in another article.
Too much water? If you have water pooling where it is not feasible to contour, especially after heavy rains, strategically placed trenches filled with permeable materials - and perforated drainage pipes if needed - can intercept groundwater and direct it to where it can slowly percolate over a wide, open area.
2. BUILD LIVING, ABSORBENT SOILS
Soils are our most valuable and effective ally as we design for water scarcity for the ground we plant into is the most important ecosystem on the planet. Loose, friable (easily crumbled) soils with good structure and texture act like a sponge to absorb, hold, then release large volumes of water that keep local ecosystems hydrated. And absorbent ground helps us make the most of a deluge of rain that otherwise cause damage as they wash ocean-wards uselessly.
Water is critical to the formation of soil, and its structure and texture influence how soils hold or drain water and retain or leach nutrients. Microorganisms, bacteria, fungi, animals, plant roots, water, oxygen, and minerals, connect to influence these physical characteristics (structure and texture) all of which affect the health of the plants growing here. Without healthy living soil, our landscapes will NEVER be healthy watersheds.
Do you know how permeable your soils are? Test them. No specialist tools are needed; just dig a hole in 1 or a few places, about 30 x 15 cm, fill with water and measure how long it takes to drain. Repeat the process twice; under 30 minutes both times is ideal.
For soils to absorb as much rainwater that falls on it or pools when directed there, they must have the following characteristics:
(Download this quick reference table)
GOOD TEXTURE AND STRUCTURE:
Why: For sufficient water absorption; To aid movement of water, oxygen, plant roots, animals micro-organisms.
What hinders this: Compacted soils; Low humus content; No or little biological life that helps build soil structure; Soils bare of plant and mulch cover makes soils vulnerable to erosion and temperature extremes
What you can do: Texture is difficult to change; work with what you have; Top-dress with a layer of organic mulch; Prevent compaction – soften impact with mulch & reduce machine use. Keep off soils in beds, and never walk on wet soils. If you need to weed or prune lay down a plank to spread your weight across the ground; Throw away pesticides and chemical fertilisers; Don’t dig!
Why: Improves soil structure and texture; absorbs holds and releases water; Provides habitat and food for micro and macro organisms, bacteria and fungi; Supply plant nutrients.
What hinders this: Compacted soils; Removal of plant material; No mulch layer to be worked into the soil profile; No biological life to break down plant and animal matter that build humus.
What you can do: Top-dress with a thick mulch layer to retain moisture and encourage microbes and animal activity; Prevent soil compaction
Why: Break down animal and plant detritus into organic matter; Cycle nutrients by converting minerals into a form plants can absorb; Connect with plants to aid water and nutrient absorption; Hold nutrients so prevents their leaching from soils; Help control pathogens, diseases and plant pests; Build soil structure which improves movement of water and oxygen, animals and plant roots; Remove carbon from the atmosphere holding it in the soil.
What hinders this: Compacted soils; Reduced oxygen reduces beneficial bacteria and increases harmful bacteria; Over-use of pesticides and chemical fertilisers; Frequent digging or tilling that kills beneficial bacteria and fungi; Lack of organic matter that provides habitat and food; Bare soils affected by temperature extremes and compaction.
What you can do: Stop using pesticides and chemical fertilisers; Don’t dig as it destroys this frail, filament network; Fungi: add woody pruning’s, sawdust, shredded paper, and brown, dormant or dead plant material in the mulch and compost as food for the fungi.
Bacteria need nitrogen so add green grass clippings, fruits, vegetable skins.
Animals: provide them with habitat and food; leaves, twigs, old plant matter.
EFFECTIVE GROUND COVER:
Why: Covered soil retains moisture, is efficiently insulated against temperature extremes, reduces evaporation and wind and water erosion, intercepts raindrops that cause soil compaction; Plants help to build soil structure and feed many organisms; Enables water to percolate and recharge groundwater systems, slows water movement giving it time to soak in; Provide animal habitat
What hinders this: Large expanses of solid and impermeable surfaces; No mulch turns soil into brick;
The following prevent healthy plant growth: Soils with little or no humus; Poor water absorption; Poor air & water movement & nutrient recycling; Lack of beneficial organisms; Compacted soils that prevent root movement and beneficial fungi associations to form.
What you can do: Prevent compaction; Top-dress with a thick mulch layer; Don’t use pesticides; Reduce expanses of impervious surfaces; Don’t dig; Improve conditions to encourage healthy populations of bacteria and fungi.