Agriculture Reference
In-Depth Information
positioning and has been universally adopted (Han et al. 2009; Chávez et al. 2010;
Evans et al. 2010). More than one WASS-enabled GPS receiver can be used to account
for misalignment between spans, especially with lateral move irrigation systems.
11. 5.6 C
URRENT
S
TATUS
OF
SSI
Research studies on SSI began more than 20 years ago and have concentrated on
irrigation system and control system hardware to implement SSI management on
small-scale research plots. Limited research focused on how to apply SSI manage-
ment to meet the objectives of increasing crop yield and quality, increasing water use
efficiency, increasing net return, and/or reducing environmental impact. Several eco-
nomic simulation studies have been conducted without showing convincing evidence
that SSI is advantageous from any aspect (Nijbroek et al. 2003; Oliveira et al. 2005;
Lu et al. 2005; Al-Kufaishi et al. 2006; Hedley and Yule 2009). King et al. (2006)
used a center pivot equipped for site-specific water application to compare SSI man-
agement and conventional uniform irrigation management on an 11.5-ha field site
of potatoes in Idaho over a 2-year period. Irrigation management treatments were
randomly assigned to 18 plots across half the field site in each year. Half the plots
received SSI management, and the remainder received conventional uniform irriga-
tion management. There was no significant difference in yield or water use between
irrigation management treatments at
p
≤ 0.05, but significant difference was noted at
p
≤ 0.10. Results from simulation studies evaluating the potential benefits from SSI
management have predicted water savings ranging from 0% to 26%. All simulation
studies have ignored timing limitations for site-specific water application imposed
by the irrigation system and assumed perfect knowledge of crop water response func-
tions, ET
c
, and soil characteristics on a spatial basis. In the case of SSI management
with center pivot and lateral move sprinkler systems, the irrigation system cannot
be everywhere in the field instantaneously, which is assumed in simulation studies.
It can take days between irrigation opportunities with real irrigation systems. In
practice, the lack of perfect knowledge and irrigation system limitations will likely
result in water savings less than predicted by simulation studies. Economic studies
linking water use to yield response have generally found SSI management not to be
profitable. This may result from the fact that the marginal yield response to water
near maximum yield (100% ET
c
) is small, resulting in small economic return. The
majority of simulation studies conducted to evaluate the potential benefits of SSI
management have been for humid climatic conditions. In locations where in-season
precipitation can often occur, the potential to reduce irrigation water application
with SSI management by repeated depletion of available soil water storage and more
efficient capture of in-season precipitation for crop ET exists. This opportunity is
generally limited or nonexistent in arid and semiarid locations. Ironically, SSI man-
agement may have greater relative potential to reduce irrigation water use in humid
climates when irrigating for maximum yield (100% ET
c
). Under deficit irrigated con-
ditions, SSI management may have the potential to increase water use efficiency,
increase net return, and reduce environmental impact in arid areas by managing
spatial water application for maximum economic return rather than maximum yield
(Evans and King 2010).
