Abstract:
In
England and Wales, rising demands on water resources and competition between
sectors is
leading to increased pressure on field vegetable growers to irrigate more
efficiently. Approximately 40,000ha of field scale vegetables are irrigated in England
and Wales in a
dry year. Between 60% and 90% of this area is estimated to be irrigated
using hose-reels fitted with rainguns. However, despite their popularity, these systems
are
inherently non-uniform in water distribution, particularly in windy conditions.
Improving their application uniformity has therefore been identified as one of the most
practical solutions to increasing irrigation efficiency for field vegetable growers.
This thesis
develops an integrated approach to model the spatial and temporal impacts
of
irrigation non-uniformity on the yield and quality of a vegetable crop grown in the
UK. The research used carrots as a
representative crop because of their sensitivity to
irrigation and high importance within the field vegetable sector. The impacts of a range
of raingun equipment and management strategies (field orientation, lane spacing, sector
angle, night versus day irrigation) have been evaluated.
Two models were used to simulate
raingun irrigation. TRAVGUN was first used to
generate a database of wind affected wetted pattems for a typical raingun system.
'TRAVELLER' then simulated
raingun movement down and across a field, applying
these
pattems according to ambient wind conditions and a pre-defined range of
equipment and management strategies. Carrot yield response to spatially variable
irrigation was simulated using the model Carrot Calculator". A spreadsheet model was
also
developed to quantify the impacts of irrigation non-uniformity on carrot quality.
The models were calibrated and validated
using data collected during 2003 and 2004
from field sites on commercial farms in East
Anglia.
The
outputs from the research include new information, datasets and detailed maps
showing the spatial and temporal pattems of irrigation application and their consequent
impacts on crop yield and quality. The findings demonstrated that the raingun
equipment and management strategies employed by growers can have a considerable
impact on application uniformity, and hence on crop production. Of particular
importance were the closely linked variables of lane spacing and sector angle. The
I
analyses suggested that the highest application uniformity occurred using a lane spacing
of 70 m and a sector
angle of 210° where wind speeds were <2 m s". At higher wind
speeds, narrowing the lane spacing to 60 m and using a sector angle of 180° (or 210Â
°
where the wind
speed was greater than 3 m s`l) provided maximum uniformity. If the
lane
spacing cannot be altered from 70 m, increasing the sector angle to 240° at higher
wind
speeds improved uniformity. The industry recommended lane spacing of 72 m
may therefore be marginally too wide, particularly under windy conditions. The
research also confirms that
orientating fields/travel lanes perpendicularly to the
prevailing wind direction and irrigating at night when wind speeds are typically lower
can
help reduce application non-uniformity. These findings have helped to substantiate
many of the measures being widely discussed for improving irrigation efficiency. The
integrated approach has also enabled the combination of various equipment and
management strategies to be more thoroughly evaluated than was previously possible.
Irrigation uniformity was found to have a considerable impact on carrot crop yield and,
in
particular, quality. For example, in a typical dry year, simulated non-uniform
irrigation resulted in a total yield loss of 4%, a marketable yield loss of 8% and a
premium root yield loss of 11%. This could have resulted in an income loss of
approximately .E288-585 ha" (4-8%). Importantly, and contrary to grower perceptions,
this research demonstrated that a small but
appreciable crop loss (up to 1%) may occur
due to
just a single non-uniform irrigation during critical crop growth periods.
This research has
provided useful insight and new information in support of developing
recommendations to assist
growers not only in improving their crop production but also
in
demonstrating efficient irrigation both for meeting grower protocol requirements and
at abstraction licence renewal. I addition, the findings will help inform the regulatory
authorities on the
complexities and difficulties of achieving efficient irrigation. The
research
approach could also be readily utilised by manufacturers to assist in designing
and
improving raingun equipment. Although the modelling approach was developed for
raingun irrigated carrots, the methodology could be readily extended to other crops and
overhead
irrigation systems to provide tools for growers and the crop services industry
to evaluate
system performance and the impacts for crop production.