Target 10 Communicator
January 2011 Edition
FAST FLOW OR FAST TALK?
Recently, there has been a lot of interest in fast flow flood irrigation as a means of improving irrigation efficiency. The concept of increasing flood irrigation flow rates is not new. Irrigators have been steadily increasing flow rates for decades. Nowadays with irrigation modernisation, modern pumps and improved farm irrigation layout, flow rates of 20 to 30 ML/day are being achieved. This makes it possible to irrigate bays in less than an hour.
On heavy and medium soil types, high flow rates and fast irrigations may not be appropriate. There is a risk with fast irrigations that there will be inadequate water penetration, particularly at the top of the bay and on newly laser graded ground. Fast irrigations can cause an uneven wetting pattern along the length of the bay, with less water penetration at the top compared to the bottom. This is described as the ‘wedge effect’. (See the diagram immediately below.) This can result in the soil drying out at the top of the bay before the bottom, reduced plant growth and a loss in dry matter production per megalitre applied over a season.
The wetting pattern in the soil profile along the length of a 300m bay, receiving a flow rate of 20ML /day for 45 minutes, with 3% run-off on local medium to heavy soil type.(Source: Analytical Irrigation Model – DPI Tatura.)
The old rule of thumb of getting water on to bays in three to six hours allowed enough time for water penetration at the top of the bay, whilst not being that long to cause excessive water-logging. Where lighter soils in the region are flood irrigated, faster flows and quicker watering is required to minimise water losses down the soil profile due to the permeable nature of sandy soils.
An uneven wetting pattern can occur due to different ‘ponding’ times along the bay. The amount of time that water sits (or ‘ponds’) on the bay will directly affect how much water penetrates in to the soil at that particular position on the bay. For the 45 minute irrigation modelled in the above diagram, water ponded for only 45 minutes at the top of the bay, whereas it ponded for over seven hours at the bottom resulting in the uneven wetting pattern shown.
Ponding times depend on the time difference between the irrigation ‘advance front’ (when the soil first wets up – see diagram below) and the irrigation ‘recession’ (when the surface water has drained away). It is important to keep in mind the ponding requirements of both the top and the bottom of a bay when flood irrigating.
Irrigation water moving down a bay.
A convenient way to check how even the wetting pattern is after irrigating is to push a 7mm diameter metal rod in to the soil at different spots along the bay. The rod is usually easy to push through the moist part of the profile, giving an indication of the wetting depth and wetting pattern for the length of the bay. Keep in mind that soil types may vary across the farm, so due to soil type differences the wetting pattern may vary accordingly between bays and along the length of bays for the same irrigation treatment.
A large amount of research has been conducted on the management of flood irrigation on different soil types in the region in the 1990s by staff at DPI, Tatura. Due to a push for fast flow flood and a changed irrigation environment largely due to the drier conditions over the last decade, Murray Dairy and DPI are currently conducting fast flow flood trials on farms in the region. Hopefully this work will be followed up with more detailed research in future years to help answer many of the questions related to fast flow flood irrigation in the changed irrigation environment.
Summary
Flow rates that are too fast may well result in water savings, but there is a real risk of an uneven wetting pattern along the length of the bay, reduced plant growth and reduced production per megalitre. The focus needs to be on flow rates that suit soil and site specific conditions.
