Agriculture Reference
In-Depth Information
and Sadler 1994; King et al. 1995; Evans et al. 1996; McCann et al. 1997; Omary et
al. 1997). These initial research studies concentrated on irrigation system and control
system hardware and software needed to apply varied amounts of water according
to a predetermined static map of management zones and target application depths.
Research and development of SSI for micro-irrigation systems began more than
10 years ago (Torre-Neto et al. 2000; de Miranda 2003; Coates et al. 2004, 2005,
2006; Coates and Delwiche 2006, 2009). Similarly, these initial research studies
concentrated on irrigation system and control system hardware and software needed
to apply varied amounts of water according to a predetermined map of management
zones and target application depths. In some respects, SSI with micro-irrigation is
simplified because the irrigation system is stationary, but the number of management
zones can be greater when an individual tree in an orchard is considered a manage-
ment zone (Coates et al. 2004, 2005, 2006; Coates and Delwiche 2006, 2009).
11. 5.2 C
ONTROLLING
W
ATER
A
PPLICATION
The ability to control applied water volume to small areas of a field is a key element
of SSI systems. With a stationary irrigation system such as micro-irrigation, irriga-
tion event time is the control parameter. Each control zone is equipped with one or
more valves used to control the flow of water to the control zone area, and the emitter
or microsprinkler serves as the metering device. The design flow rate of the emit-
ter or microsprinkler multiplied by the time the control valve is actuated is used to
determine the volume of water applied. With mobile irrigation systems such as center
pivot and lateral move sprinkler systems, there are two ways to control applied water
volume. Either travel speed of the irrigation system or sprinkler flow rate can be the
control parameter, or both. Using travel speed as the control parameter allows one-
dimensional control aligned parallel with the direction of irrigation system travel.
This one-dimensional control restricts the ability of an SSI system to address spatial
variability in water requirements as management zones will not likely be aligned
parallel with the irrigation system. Center pivot sprinkler irrigation manufacturers
offer varying degrees of one-dimensional control resolution on their computerized
control panels (Kranz et al. 2010). Obtaining two-dimensional control of applied
water volume requires that sprinkler flow rate be a control parameter. However, a
truly variable flow rate sprinkler has yet to be developed. In the absence of a vari-
able flow rate sprinkler, two approaches have been used to effectively achieve vari-
able sprinkler flow. One approach is to use multiple parallel sprinkler packages each
with different nozzle sizes. In the case of two parallel sprinkler packages, selecting
nozzles sizes that provide 1/3 and 2/3 of the original single sprinkler flow rate allows
stepwise variable flow rates of 0, 1/3, 2/3, and 3/3 using control valves to control
flow through each sprinkler (King et al. 1995, 1998; McCann et al. 1997). In the
case of three parallel sprinkler packages, selecting nozzle sizes that provide 1/7, 2/7,
and 4/7 of the original single sprinkler flow rate allows stepwise variable flow rates
of 0, 1/7, 2/7, 3/7, 4/7, 5/7, 6/7, and 7/7 using control valves to control flow through
each sprinkler nozzle (Omary et al. 1997; Camp et al. 1998; Stone et al. 2006). The
primary disadvantage of this approach is the cost of multiple sprinklers and valves.
The number of valves can be reduced by grouping sprinklers on manifolds mounted
