In light of our surprising Level 3 drought status here in the typically rainy Pacific Northwest coast, I thought I’d write on what I’ve learned about sustainable water management in my recent University classes. I’ll break it in to three posts:
– How does water flow in an urban environment?
– Water on the property scale (individual home and business)
– Water on the watershed and city scale (protection of rivers and other bodies)
This is inspired by Dr. Hans Schreier at the University of British Columbia who presented on this topic to my class, and most of the charts and figures were graciously provided by him.
PART 1: How Does Water Flow in an Urban Environment?
There are two main water cycles: the Blue Water cycle and the Green Water cycle. The Blue Water cycle is typically what city planners have in mind when designing urban landscapes. It represents the predominantly one way trip water makes when it rains: some of it soaks into the ground and the rest is run-off into streams or canals.
This concentrates pollutants in waterways, accelerates the erosion of soils, increases potential for flooding and creates instability for infrastructure (think sink-holes and landslides). It also intensifies the ‘heat island effect,’ where the temperature of a city becomes much hotter than surrounding rural areas.
In the green cycle, rain water soaks into the ground and condenses on plants, but also returns to the air through evaporation and transpiration. This significantly reduces erosion, regenerates ground water reserves, and creates a massive reservoir for water in the form of plants and soil.
We humans truly love concrete. It makes up most of the footprint of our cities in the form of buildings, roads, sidewalks, driveways and parking lots. When a piece of green space is covered by pavement, that area entirely loses its ability to soak up water.On top of that, parking lots become covered with pollutants like leaked car fluids, and brake and tire debris. When it rains, these pollutants are washed directly into the sewer or nearby streams.
This chart is a fantastic representation of what happens to water when it rains. In a forest about 1/3 replenishes ground water reserves, 1/3 stays in soils and 1/3 ‘evapo-transpires’ (evaporates from leaves and stems). Compare this to a commercial landscape where most of the water becomes surface run-off:
Here you can see what amount of land becomes impervious when a typical suburban housing development replaces green space. Looking at it, I’d say about 30% of the land is now covered by roofs, driveways, side walks and streets. And although lawn makes up the rest, typical grass lawns have very little topsoil beneath, and since grass is relatively short, lawn does a poor job in holding in moisture compared a landscape with trees, shrubs and deep rooted plants.
An existing solution to traditional non-load bearing cement infrastructure is to replace it with porous pavement or permeable paving. This can either come in the form of pavement with high porosity, meaning there’s plenty of space for water to pass through (left photo) or paving blocks that simply have spaces between (right photo).
Although lakes and streams are what we can observe on the surface, ground water and aquifers contribute the vast amount of water that we humans use every day. And though water eventually makes its way back in to the ground, we typically suck it up faster than the speed it soaks back in.
There’s plenty of other innovative ways we can create better water systems on the individual scale and on the city scale. I’ll talk about those in my next posts!
Next week: Part 2- Water Solutions on the Property Scale