esshup,
One inch of precipitation = .62 gal/sq ft., using Area = pi x radius squared, a 8� diameter root zone is about 50 sf. and would require .62x50 = 31 gal. of water. If you have a 25 sf root zone (about 5.6' diameter) it would require 15.5 gal, if the zone is 12.5 sf (~2' diameter)it would require 7.75 gal.
So in the 8' diameter example if you watered once a week for one hour with a combination of emitters that provides approx 31 gph to a 50 sf area, this would provide the equiv. of 1� of rain. Since most fruit and nut trees require approx 2� per week, one of the following in this scenario could provide 2� per week: double the watering time, double the emitter output or simply water 2 times per week. Note that this assumes no rainfall. Rainfall should be subtracted from the irrigation requirements. A rain gauge would be a handy tool to keep in the irrigated area.
A 38� drip line with .9 GPH emitters @ 12� spacing would provide ~34 gph. Such a drip line would make 2 loops around an 8� diameter root zone (average about 3' from the trunk) to provide the equiv of 1� of rainfall.
Note that evapotranspiration (ET) is not considered here as the 2� precipitation rate is only an approximation to begin with. For reference, ET is defined as the water lost to the atmosphere from the ground surface (evaporation) and transpiration by plants. In Georgetown, TX, in March of 2010, the ET rate ranged between .05 to .3 inches per day. In July/Aug of 2010 it ranged from .10 to .35 in.
But I digress... Hope this helps.
Hey, what will you replace the 2 dead trees with? BTW, with sandy soil, it is better to provide irrigation perhaps 3 times a week. Just water 1/3 of the time than if watering once per week to provide the same 2" of water.
