Distance Between Swales in Arid Lands?

Hi Everyone, I'm doing permaculture in Joshua Tree, CA in the Mojave desert. We get about 4 to 6 inches a year of rain (if we're lucky). I want to capture and sink water from a water run off area on my land. I'm thinking of using a network of boomerang swales with a mesquite tree planted in each. I've read a lot of conflicting information about how far apart such swale rows should be from each other in arid lands. The area I have in mind has a gentle slope with a gradual 10' fall over a distance of approximately 150'. Any feedback would be greatly appreciated!

Depends... (Does the use of that word gain me a permie gold star?)

The genus requires about 10 to 20 inches of water per year, so about 2.5 times more water than falls at Joshua Tree. If a mature Velvet mesquite tree covers an area of 700 square feet, then the collection basin should be 2.5 times that, or 1800 square feet. Screwbean mesquite trees are smaller only covering 300 square feet, ( times 2.5=800 square feet) but they need a bit more water, so...

Joseph Lofthouse wrote:
Depends... (Does the use of that word gain me a permie gold star?)

The genus requires about 10 to 20 inches of water per year, so about 2.5 times more water than falls at Joshua Tree. If a mature Velvet mesquite tree covers an area of 700 square feet, then the collection basin should be 2.5 times that, or 1800 square feet. Screwbean mesquite trees are smaller only covering 300 square feet, ( times 2.5=800 square feet) but they need a bit more water, so...

Thanks for your response Joseph. A native mesquite (Honey or Screwbean) will absolutely need supplemental irrigation until the tap roots reach the water table (hopefully in about 3 years). I will also put in deep watering tubes, sponge trenches and wicking buckets inside the boomerang. We have to go extreme here in the Mojave. My question is more about the distance between swale rows in arid climates.

Jill,

I don't have THE answer you are seeking, but have given some thought to that myself. Primarily to determine how deep my swales should be. Looking at the weather data from Twentynine Palms weather station, August is you wettest month (although that certainly is a relative term.) At a norm of .76 inches (planning for the biggest rain event) that produces 20,600 gallons of water per acre. Obviously the farther apart your swales are, the deeper they will need to be. The closer together the more shallow they can be. I can not find any data on the soil map data page for Joshua Tree, CA. (I find that odd.) I can not tell how quickly water infiltrates into the soil. The slower it infiltrates (fast run off), the closer your swales will need to be to slow and capture it.

That being said, I think you could approach the problem from a more productive direction. You get rain. Not nearly enough, so the real importance is to KEEP your rain. Evaporation is the biggest enemy. How far apart can the swales be while still sheltering the swale from the sun and wind? From a capture stand point the distance does not matter as much, as long as you are catching all of it. However, it will make a big difference if the growth can not shade the ground between and in the swale effectively. From my understanding it is all about the mulch and organic matter in the swale that can retain the moisture rather than total volume of water that makes the most difference.

I would go with as close as practical staring on the uphill grade; and mulch then ground cover, ground cover, ground cover. While over story is important and mesquite would be a good variety, the ground cover per square foot of coverage is going to cool and protect the soil better than your trees, until they are well established.

http://www.amwua.org/groundcovers.html

That is my free advise. Take it for what it is worth.

jill giegerich wrote:My question is more about the distance between swale rows in arid climates.

Perhaps it would help if I rephrase my response... The catchment area of a boomerang swale in your area aught to be not less than 2.5 times the mature spread of the mesquite tree.... It seems to me that "boomerang swales" are not "swale rows"... Either way the math is the same...

Supposing that instead of making boomerang bunds that we are making on-contour swales.... A typical orchard would space the trees along the swale so that their canopies are just touching as mature trees. For Velvet mesquite then the trees would be spaced 30 feet apart along the swale. Since the water requirement is 2.5 times what falls, then the on-contour swales would be spaced no closer than (30X2.5=) 75 feet apart. Spacing of the swales for the smaller Screwbean mesquite could be not less than 20 feet times 2.5 = 50 feet plus a bit to provide for their need for more water. We are taking the water that falls and concentrating it 2.5 times in order to meet the needs of the tree.

These estimates are for sloping ground that is mostly impervious to rain. If the catch basin absorbs half of the water that falls on it (typical for bare Earth or for deserts) then the collection area should be doubled to get sufficient runoff to the tree's roots. In working with water at my place I would consider a 7% (10:150) slope to be steep. Water rushes through that sort of a gradient.


Spacing of boomerang swales when the water requirement of the tree is 2.5 times annual rainfall, and the ground is mostly impervious.


Spacing of on-contour swales when the trees are planted so that their mature canopies just touch and when the water requirement of the tree is 2.5 times annual rainfall, and the ground is mostly impervious.

Jack Edmondson wrote:

From my understanding it is all about the mulch and organic matter in the swale that can retain the moisture rather than total volume of water that makes the most difference.

I would go with as close as practical staring on the uphill grade; and mulch then ground cover, ground cover, ground cover. While over story is important and mesquite would be a good variety, the ground cover per square foot of coverage is going to cool and protect the soil better than your trees, until they are well established.

Thanks Jack! The link is very helpful. It seems like everyone has a different approach to swale distance but as you say "It depends". I planned on mulching like crazy. I haven't yet used cover crops in the desert. It always seemed like the amount of irrigation those crops would need would be daunting but maybe it's time to try it.

Thanks Joseph. Great information. It's interesting that you would go for a large spacing and Jack (the other person responding to this post) would space them as close as possible. It makes sense that I need to allow for the mature tree canopies. I guess the bottom line is how to catch the maximum amount of run off and keep down evaporation.

I'm only addressing half of the topic: How large the collection area needs to be to meet the tree's needs. Once there is runoff, and after the runoff is collected near the tree's roots, then it needs to be saved for later use. That's where organic mater can help. It helps to infiltrate the water into the soil. Just getting rid of the 7% slope helps tremendously! Pools of water infiltrate much more than streams of water. Mulch is very pricey in the desert! Small-scale growers can afford anything. I typically make one vertical mulch pit near a tree: a hole 6" to 9" in diameter and several feet deep, and filled with compost. Then when the swale gets runoff, the mulch pit channels as much as possible as deep as possible as quick as possible. Stone mulch and stone pits are less expensive and easier to come by. In any case, in the desert the swale will be filled with a layer of clay during every runoff event after construction.

5" of water per year is hard desert! I think I have it hard with 9" of water. The desert adapted shrubs around Joshua Tree space themselves at a distance of about 3 to 6 times the diameter of the shrub. I think that mesquite trees would grow best by following the same pattern. Any ground cover plants are fleeting and ephemeral. Their residues eventually get washed off the land and end up in the swales: Self collecting mulch!

Yes, mulch is key. I've been putting in junk mail sponge trenches just outside of and on the uphill side of the root ball of newly planted trees. I make sure no color print or glossies. Seems to be some controversy about this but my research shows that all commercial printers have switched to soy based inks. In any case, it makes a huge difference to the trees who seem to love it. The trenches have stayed moist for up to 4 months.

About ground cover - I have been experimenting recently with ice plants. I saw that Geoff Lawton is using them in his desert projects. I was skeptical but I have to say they seem to be amazingly resilient with very low water needs. The rabbits ate them to the ground twice and both times they bounced back. And now, for some unfathomable reason, the rabbits are no longer interested.

Great thread! I plan to use these swale techniques in my little piece of desert.