Drip Irrigation in Orchards

In a recent post someone wrote to supply "one inch of water a week in the summer" to dwarf stock apple trees.

How does one measure or know what one inch amounts to when using drip irrigation?

I know how many gallons flow over a period of time by knowing the flow rate and length of the drip tubing. But then calculating inches from that . . .?

I have read explanations of this in the past without understanding the method of calculation.

Thank you for your time,

Paul Goettlich
Rockin' Rooster Ranch
Coquille, OR
Sunset Zone 5
USDA Zone 9a

I have always used the following simple logic: There is 7.5 gal per cubic ft. 7.5/12=.625 so you need .625 gal for every sqft of root zone to equal 1 inch of rain. Here is a link that explains it better. [cals.arizona.edu]

With drip irrigation, irrigate for a predetermined amount of time, then go out with a soil probe and see if the soil is wet in the root zone, checking both the width and the depth. In sand, the water tends to go straight down. It's more difficult to use drip in sand. The water tends to spread out wider in clay. From my soils text book:

Texture, Available water per foot of soil

Coarse, .3 to 1.1"
Medium coarse 1.1 to 1.8"
Medium 2.0 to 2.9"
Medium fine 1.8 to 2.6"
Fine 1.2 to 2.0"

The "available water" would be at field capacity, which is how wet your soil would be immediately after a full irrigation. Therefore, if you soil is dry, you can expect to irrigate the amounts above to achieve field capacity.