MEDITERRANEAN: Swales in Perth Western Australia
(Bullsbrook, WA, Australia)
Swale close up
Swales are level sills or banks used to slow the movement of water in the landscape, to induce more of it to either sink into the ground or be stored in vegetation and other organic matter in the swale.
Our first swales were just level sills. We didn't plant them with trees as we aren't ready to do that yet. They are showing fantastic pasture growth though, much better than anywhere else on our property or even in the neighbourhood, despite the less than perfect shallow (15-20cm) granitic gritty loam duplex soil.
One of our paddocks is quite steep so after grazing most of the long grass off it last month we set about shutting it up for agroforestry and decided to try out banked swales for the first time.
The swales were first marked out using a lazer level to ensure they were level. Usually you'd use a grader to build them, but our earthmoving guy is very skilled with his ancient dozer, and just dropped a pin on the downhill side of the bucket so that it would give him the same result. He also got bogged at one point! So one tip - build your swales before the ground gets too wet!
Once in place, we sprinkled the swales and surrounds with lupin seeds to grow ourselves some quick nitrogen fixing, then covered them with straw left over from building our strawbale shed.
On the swales themselves we alternated moisture liking long lasting nitrogen fixers (Tipuana tipu, and Honey Locust - thornless) with short lived ones (Albizia, Acacia saligna) that can be slashed and thrown into the swale recess to add organic matter high in nitrogen and build up the fertility. Immediately uphill of the swales we planted two rows of drought hardy nitrogen fixing native plants - Allocasuarina huegeliana (Rock Sheoak) that produces beautiful timber, Acacia microbotrya, a source of manna gum, and Acacia acuminata, the Jam Wattle that is a source of very long lived fence posts (Australia's rabbit proof fence was largely built of them).
In a few years, when the short-lived trees have died, conditions will be perfect to plant orchard trees in the spaces left behind.
In the meantime we will be continuing to fill the swales with whatever organic matter we can find - horse manure, grass clippings, loppings, cardboard etc - to capture water and hold it where we need it, while building fertility.