Swale rough calculations

So I only get 16 inches of rain per year, but when I did some rough calculations, I was astounded at how much my swales should increase available moisture. So could ya'll check these numbers and tell me if I am wrong or missing something.

1. In a pinyon-juniper forest with slight slope, the average runoff percentage is 10-20%.
2. So I should expect 1.6-3.2 inches of run off lost during a year (16 x 0.1-0.2 = 1.6-3.2 inches)
3. If my interswale space is 30 ft then a seedling will get 30 feet x 1.6-3.2 inches of run off which equals (drumroll) AN ADDITIONAL 48-96 inches of precipitation.

48-96 inches annually is reaching into some of the wettest places in the US like Florida or the temperate rainforests of Washington. Are my swales really intercepting that much water?

So my questions are
1. Am I high?
2. If it is true or even half true, then can I plant trees that would normally require more moisture? Can I go into the 30 inch/precipation trees? Or 40? or more? Peaches?? Apples? Pears??
3. 60% of the precipitation arrives during June, July and August. So whatever I pick needs to have drought tolerance, right?

The captured water gets spread across the entire root system of the tree, which approximates the width of the canopy.

If the tree is 30 feet wide then spacing the swales at 30 feet, will capture ~2 inches of water that would have otherwise ran-off.

For a smaller tree, say 10 feet wide, that's equivalent to capturing about ~6 inches of additional water.  

A boomerang swale, with small widely spaced trees (as per following diagram), would capture ~18 inches of run-off water per year.

Water captured during the summer is lovely for minimizing drought.

I understand mathematics, but you have confused me!

3. If my interswale space is 30 ft then a seedling will get 30 feet x 1.6-3.2 inches of run off which equals (drumroll) AN ADDITIONAL 48-96 inches of precipitation.

Your swale may concentrate the water flow to one spot, but it cannot increase the actual volume of water.
Runoff is water that leaves the area it landed and moves down hill.
Swales capture runoff and holds it in a 'basin' so it can soak into the soil or be held like a pond or dam.

I think you're on the right track with your mathematics but looking at the wrong conclusion. Or rather, that you're missing a step in the equation.

For instance: 1.2 - 3.4 in 9f runoff yields HOW MANY GALLONS OF WATER PER SQ FT.

Than calculate approximate square footage in the interstate

Answer is how many GALLONS of water you capture that you'd otherwise lose

Than if you determine you would like to know how many "inches" that equates to, do the formula you us3d to get to GALLONS backwards, using the square footage of the scale itself.

This gives you localized precip for the SWALE ITSELF, which will likely look impressive,  however remember you're not changing WHEN that water is collected, so you're increasing available water for deep rooted species but not exactly converting a 16inch rainfall area into rainforest level precipitation

Hope that makes sense.