Agriculture Reference
In-Depth Information
Equipment to implement site-specific irrigation with center pivot and lateral move
sprinkler irrigation systems is currently commercially available. However, current
uses of SSI management are generally on a fairly coarse scale and are often lim-
ited to site-specific treatment of noncropped areas based on physical features such
as water ways, ponds, or rocky outcrops where some sprinklers are turned off in
these areas. The use of SSI for general crop production is still limited and is mostly
directed toward treating symptoms of localized overirrigation, underirrigation, run-
off, ponding, nutrient management, and related issues under maximum yield (100%
ET
c
) scenarios, which often do not produce measureable savings in water or energy
use although total field yields may increase. In order for SSI technology to survive
and expand, the immediate need is for development and testing of easy-to-use basic,
generalized decision support systems for SSI starting with simple static scenarios for
both humid and arid regions (Evans and King 2011).
11. 5.7 S
ITE
-S
PECIFIC
C
HEMIGATION
Chemigation is a generic term used for the application of chemicals through an
irrigation system with applied water. Chemigation is widely practiced for chemi-
cals appropriately labeled for application through an irrigation system. Fertilizer
is a commonly applied chemical when compatible with the pH of the water source.
Site-specific application of fertilizer, especially nitrogen, through a center pivot or
lateral move irrigation system equipped for SSI is a logical extension of the tech-
nology that can potentially conserve both water and fertilizer and reduce environ-
mental contamination through improved crop nutrient management. Site-specific
chemigation has been demonstrated by Camp et al. (1994), King et al. (1999), and
Eberlein et al. (2000). Site-specific chemigation through a center pivot or lateral
move irrigation system equipped for SSI is accomplished by adjusting the volume
of water applied to a management zone while the concentration of chemical in the
water remains constant. This means that as the flow rate through the irrigation sys-
tem changes for variable water application, the injection rate of the chemical must
change in proportion. This is accomplished using a variable flow rate chemical
injection pump. Chemical injection rate can be controlled by adjusting the stroke
length of a positive displacement injection pump (King et al. 1999; Eberlein et al.
2000) or by changing the motor speed of a positive displacement injection pump
using a variable frequency motor drive. Either approach works well with SSI sys-
tems that use multiple sprinkler packages to control water application since the flow
rate can be adequately estimated based on which sprinklers are on in each control
zone. With these systems, water flow rate changes only occur when the system
lateral crosses a management zone boundary with differing target amounts. In con-
trast, SSI systems that use on/off pulsing to control water application have continual
changes in water flow rate. To obtain constant chemical concentration in the applied
water with these systems would require continual changes in chemical injection
flow rate. Additionally, a flow meter may be required to determine the system flow
rate at any instant in time. Thus, site-specific chemigation is more complicated to
accomplish with SSI systems that use pulsing to control water application. It may be
possible to inject chemical proportional to time-averaged flow rate of such systems,
