Selecting a Winning Site and Crops to Match - Growing Organic Orchard Fruits - page 37

measurements. In particularly arid regions you may need to increase irriga-
tion for drought-sensitive crops.
Evaporation pans are simply round, metal pans. While at least two differ-
ent designs exist, the U.S. National Weather Service uses Class A pans that
are 47.5 inches in diameter and 10 inches deep. A hollow cylinder equipped
with a metal needle in the center is used to mark a given water level. Each
morning, enough water is added to restore the water level to that mark. The
amount of water added is determined using a pitcher that is marked in hun-
dredths of inches of water. That represents the amount of water lost by
evaporation since the last filling. In agricultural areas, evaporation pan data
is often available from the National Weather Service, a university, or various
agricultural organizations.
Outside of these regions, evaporation pan data can be hard to locate. You
might choose to install your own evaporation pan or to install any of sev-
eral types of soil moisture monitoring devices, including vacuum tensiomet-
ers or soil moisture sensors and electronic meters. Inexpensive gypsum block
sensors and handheld meters generally work well for orchard crops. Compu-
terized systems are now available that monitor the soil moisture and trans-
mit the data to your home or office computer. Some of these systems can be
set to automatically turn on and off irrigation valves.
On my research farm, for example, we usually lost about 2 inches of
water to evaporation each week during the warmest period of the summer,
as measured with an evaporation pan. For the teaching orchard, we needed a
source that provided 2 acre inches of water weekly during July and August.
Different types of irrigation systems use different amounts of water, so
you will need to factor that in as well. Overhead sprinklers were once widely
used in orchards, but they waste much water and increase diseases by wet-
ting the foliage and fruit and increasing humidity within the canopy. If you
intend to use sprinklers to control frost during bloom, plan to apply

⁄ 4 inch

1

of water each hour to your entire orchard for as long as temperatures are be-
low freezing (see chapter 3 ➔ for more information on overhead sprinklers).
Drip or trickle irrigation systems are very efficient in applying water directly