Agree with the "it depends"
answer.
There is a ratio between
the amount of rainfall you need to harvest (not only yearly total, but single storm or rainy week average). This is the amount you get
the size of the
swale, both height and width. This is the amount you can store per swale.
so then you adjust the distance between the swales so they fill in a nice rain but don't get washed out in a big one.
You also have to figure on how fast your soil percolates as to how fast the swales will drain and soak in.
And what you want to grow on these swales--if they are larger than your swale-to-swale distance you could create a full
canopy and that probably isn't your goal.
"You must be the change you want to see in the world." "First they ignore you, then they laugh at you, then they fight you, then you win." --Mahatma Gandhi
"Preach the Gospel always, and if necessary, use words." --Francis of Assisi.
"Family farms work when the whole family works the farm." -- Adam Klaus