How big and spread out should a swale be?

I was contemplating some of the swales that Geoff Lawton, The Prince of Permaculture, builds. 
He is building these things 2 yards wide and 3 feet deep.

And I was thinking, "Man, that is just way too big. Can't I just build one that is, oh, maybe a foot wide and six inches deep?"

So, I did the math. Now here in Central Texas, we have a problem of too much or too little rain. Rarely do we get a consistent rain of an inch per week for the months of summer.

It is not uncommon for it to rain like crazy for five minutes and leave behind a half inch of rain.

It is also not uncommon that it can rain for five hours solid and dump 4 inches of rain, and then not rain for two months.

A half inch of rain? That isn't going anywhere. It will wet the top 4 inches of soil.

But the 4 incher? Most of that is going down to the river.

This is where the swale come in.

In five hours, I would expect that one full inch of water will seep into the soil. But three of that will travel downhill.

To catch this in a swale, you need  a cubic foot of swale for every four square feet above the swale.

So if you have 30 feet between swales, you need 30 / 4 cubic feet of "ditch" in the swale. In this case, roughly 7.5 cubic feet of ditch. So here in CTX, I might place swales 30' apart, and make them 1.5' deep and 5' wide.

The general equation would be H x W = d / 4 where H is the height, W is the width and d is the distance between swales. The further apart the swales, the larger they should be.

I would also add that H = W / 2. This is a good rule of thumb I think.

For the math geeks out there, this makes W = sqrt(d/2) and H = W/2;

This will catch nearly all of the largest rains that we have here in CTX.
And rian bigger than this is something that we would only get every few years, and I would only want to design for biggest rain likely in a year.

For a 1/4 acre getting 24 inches of rain per year, that comes to 163,000 gallons of water that you get in a dry year. I think that is a useful quantity of water. Of course we average 30 inches, and our low is probably about 15 inches in a calendar year, but even when we got 15", we got it in 3"+ spurts.
You have got to keep that water on your property.

Note, if you have some yards uphill from yours that drain into yours, then you have a bonanza on your hands, and I would increase the size of the swales to take in that acreage that drains through your backyard.

Site location is important. Design is also important.