I'm after some advice on how far to space my swails.
I have already dug one swail, fairly shallow and small, it is approx 50cm wide, 20 cm deep on high side of hill, 15cm deep on low side of hill.
The site is on a modest slope. I have trees a metre from the swail cut edge just on the edge of the metre wide mound of dirt. I plan to put
veggies in as well.
What I'm trying to get an understanding is basically "soak" ie how much top soil is going to get how much water and how quickly the water is going to sink.
How far from the swail can I plant veggies (which of course have shallow roots and are vulnerable when young) before the permeating soaking water no longer reaches that
top soil due to sinking too deep into the ground?
I know that if you could cut the hill in half and then look at it from the side, you could see basically a curve from where you added
water to the swail, going down hill soaking into the hill but getting dragged by gravity into the hill away from the surface and top soil..
What would be the smartest space/gap to leave before I did the next swail? I was considering 5 metres down the hill but is this a waste as top soil would
already be sufficiently damp? 10 metres? I should add that I have heavily mulched the downhill side of the swail and will continue to do so.
Should I back fill the swail with woodchips/mulch to reduce evaporation?
Should I plant plants on the high side of the swail? How close to it's edge? Which plants?
Temperature and rainfall stats in detail further below, average rainfall 1016 to 1100 mm, Annual mean maximum temperature for 2014 = 17.4 °C.
The swail is really needed during the hot spell Nov to Dec where it is possible to get very little rain and a string of hot days (25 degrees average these months) but up to 38 degrees when hot. It's possible to get two weeks of 35-38 degrees and no rain worst case scenario.
The soil seems to be a fairly highly compacted clay/red loam. It is a red soil quite dry despite being in an mountain area
that gets fairly good rainfall higher which suggests it doesn't soak easily and the water runs off before really penetrating enough.
It took a lot of effort with the rotary hoe just working and working and really having to lean over the hoe to stop it skipping even
on a shallow setting, death by a thousand cuts to get the soil chopped up. It did produce a fairly nice friable soil for the mound
heap after all the work.
If perfect soil takes 1 effort with a mattock then this would be more like 7 effort to dig I guess.
The site is in the Dandenong Ranges, Melbourne Australia (near Cockatoo).
Temperature (using highest temperature each 24hr period)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Highest daily 38.6 37.9 30.6 29.3 19.1 14.0 14.2 17.4 21.4 27.1 29.9 28.9
Lowest daily 13.3 15.6 13.6 11.8 9.9 6.8 6.5 4.1 10.3 10.5 12.1 13.3
Monthly mean 25.0 25.6 21.8 16.9 14.5 10.7 9.7 11.5 14.6 18.1 19.8 21.1
Thanks all much gratitude for advice
Handful of soil.. more life than sum total of people on the planet. The sea is the second last frontier, soil is the last earthly frontier.
Luke, I am no expert in swale design, so I will tell you what I have learned and observed. There is no perfect set of rules for designing swales. You will need to observe your land and its contours. How does the water/rain you are getting move across the land? Are you getting runoff from an adjacent piece of property? How close or how far apart may very well depend on how much land you have to work with and what you would like to do with it. I put in about 1600 linear feet of swales on my place. Image a piece of land about 300 feet wide and 600 feet long. Due to lay of the land my swlaes kind of run diagonally across this portion of the property following the contour of the land. I put in 3 swales and each is a bit different in running length. They very in width here and there and each one is a bit different in depth. We calculate when full, all three are holding roughly a total of 640,000 gallons of water. The locations I set the three swlaes was based somewhat on where the old conservation terraces had been. I drew out what I was wanting to see and then having a good conversation with my Permaculture Mentor I plotted them out. The middle swale is about 70 feet downhill from the first swale and the third swale is between 40 and 50 feet from the middle one. Here again, no magic formula. I just observed what the land was telling me what it wanted to do. I have a good deal of clay in the soil. At first it took a long time for the water to seep into the ground. Now after almost fours years the water seeps a little be better each time we get a rain event. I think your seepage is going to depend on what type of soil you have to work with. I hope this helps. Good luck with your project. I bet you will do a great job designing yours
I like to plant nitrogen fixers and biomass producers upslope from the swale, but you could grow veg there. It just depends on what the soil type and moisture levels will be.
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Swales are mainly tree growing systems. If your climate is hot it also means that the swale mount can heat up more in the sun than the rest of the slope. Shallow rooted vegetables can have problems with that. The space between swales can better be used for veggies. What we do ourselves in our garden have small swales planted with banana, plantain and papaya to create a bit of shade and the vegetable beds terraced in between. Because here we need that shade (sun here is very bright) we space the swales only about 5 meters apart.
Distance of swales can be determined in two ways:
- create shade or avoid shade, spacing them so the top of the canopy of the lower swale aligns with the base of the upper swale creates the least amount of shade in the area in between. Obviously if you want more shade you can place them closer.
- based on swale size and water holding capacity. If you want the swale to capture all water running of the inside of the swale needs to be in proportion with the amount of rain that falls upslope in mayor rain events.
Or you can combine the wanted effects to determine what spacing works best for you.
One additional consideration to add to R Scott's instructions is if you're going to be running machinery between swales, such as a mower. If so, consider an efficient number of passes the machinery can make between swales (and easy turnarounds) so as to not have "half rows" and therefore inefficient machinery operation. I agree that canopy is the first consideration, however.