Agriculture Reference
In-Depth Information
spatial coordinates can be assigned to each control map array element. The positions
of sprinklers along the irrigation system lateral are fixed, so it is only necessary to
know the location of the lateral in the field in order to know the location of every
sprinkler in the field. Control system software continually monitors the location of
the irrigation system lateral and determines the location of each sprinkler in the field.
When a sprinkler crosses a management zone boundary, the control system software
interprets water application control map coding and makes the prescribed adjust-
ments to each control zone flow rate.
For simplicity, the control zone dimension parallel with the irrigation system lat-
eral is often representative of the physical dimension corresponding to the number of
sprinklers controlled as a collective unit. It is possible to control every sprinkler indi-
vidually, but it is not necessarily practical or economical. Sprinkler wetted diameters
can range from 1 to 25 m, with 8 to 18 m being most common. Because sprinklers
are mounted every 1.5-3 m along the system lateral, sprinkler overlap commonly
ranges from 3 to 5 m. Hence, individual sprinkler control does not achieve the tar-
get application volume at the sprinkler location due to the water volume applied to
the location from adjacent sprinklers. From a practical and economic point of view,
controlling sprinklers in groups ranging from 3 to 10 is logical and has been used
(Fraisse et al. 1992; McCann et al. 1997; King et al. 1999; Camp et al. 1998; Han et
al. 2009; Evans et al. 2000, 2010). For lateral move irrigation systems, the control
zone physical dimension in the direction of irrigation system travel is often similar
to that parallel with the system lateral for simplicity. For center pivot irrigation sys-
tems where the control map uses polar coordinates, the physical size of the control
zone perpendicular to the direction of travel changes with radial distance from the
pivot point when a standard increment of angular measure is used as the control
dimension.
Production of the control map is normally external to the control system using
other techniques and software. It can be based on producer experience with the irri-
gated area or their perception of irrigation scheduling concepts. Soil and plant water
status monitoring and crop water use data should be incorporated into production
of the control map. The development of decision support software for production of
SSI control maps is the current focus of SSI research worldwide. Such software can
potentially be included in the control program for autonomous SSI management and
will be the focus of future SSI research.
11.5.5 D
ETERMINING
I
RRIGATION
S
YSTEM
L
OCATION
Reliable and accurate determination of irrigation system lateral location is a critical
element of SSI management. Initial research studies on SSI used various methods to
determine the location of the irrigation system lateral in the field. Cumulative time of
travel was used by King et al. (1995, 1999) and McCann et al. (1997) for a lateral move
irrigation system. An electronic compass was used by Evans et al. (1996) for a center
pivot irrigation system. King et al. (1998) used an 11-bit shaft encoder mechanically
linked to first span rotation at the pivot point for system lateral positioning on a com-
mercial center pivot irrigation system. The introduction of Wide Area Augmentation
System (WAAS)-enabled GPS receivers greatly simplified irrigation lateral field
