Improving Border-check Irrigation Performance


        Research Findings

Background: Why this project was important

High border-check bay inflow rates have been promoted as a means of substantially improving border-check irrigation water productivity at a time when irrigation renewal projects and publicly funded water saving initiatives meant that most southern Murray Darling Basin (SMDB) irrigators faced significant irrigation redevelopment decisions. The most appropriate bay inflow rate to adopt continues to be a key issue for irrigators investing in upgraded farm irrigation infrastructure in order to capitalise on improved connection to the regional irrigation supply.
Prior to this project there has been little objective data on the effects of bay inflow rate on irrigation performance in the SMDB. The issue of appropriate bay inflow rates involves a wide range of stakeholders including irrigators and industry organisations, policy makers, irrigation service providers and equipment manufacturers, public agencies implementing infrastructure renewal and water saving programs, and proponents of high flow rate irrigation.
Field work

Border-check irrigation assessments were taken from 2010/11 to 2012/13 in the SMDB (Figure 1a). During this time, the project covered eight sites, five soil types and three crop types. A total of 104 irrigation events were measured. Field measurements were taken on a pair of irrigation bays at each site, one of which was irrigated at approximately current recommended inflow rate while the other was irrigated at twice that rate. Measurements included bay dimensions and slope, electromagnetic induction survey, climate and crop water requirement, soil moisture, watertable depth, and for each irrigation, the bay inflow hydrograph, within-bay water depth hydrographs, and runoff (Figure 1c).













Figure 1: (a) Trial sites (b) water advance and depth measurement and (c) runoff measurement


        Analysis


Fitted surface irrigation models were used to interpret the data, calculate irrigation performance measures and extrapolate irrigation performance across feasible ranges of irrigation inflow rate and duration.


Research findings

A wide range of inflow rates could have equivalent irrigation performance on all modelled soil types and crops provided the appropriate inflow duration was used.

Irrespective of the crop, no substantial irrigation performance improvements were achieved on more permeable Group 2 soils by increasing bay inflow rates above the current recommended rate of 0.2 Ml/d/m of bay width, provided that both the inflow duration and the irrigation deficit were appropriate for the inflow rate adopted.

For example, on a permeable soil growing lucerne, an inflow rate greater than 0.2 Ml/d/m can reduce deep drainage below the crop root zone for crop water deficits of less than 70 mm, but generate substantial irrigation runoff to achieve satisfactory requirement efficiency at crop water deficits greater than 90 mm.

On less permeable Group 3 soils no substantial performance improvements were achieved, irrespective of the crop or the crop water deficit, above an inflow rate of 0.1 Ml/d/m, provided that the appropriate inflow duration was selected. Deep drainage could be slightly reduced by using inflow rates of 0.2 Ml/d/m.

The reasons for this were the low final infiltration rate and very slow rate of surface water drainage, causing long durations of ponding that inflow rate did not substantially influence.

No productivity differences that could be attributed to inflow rate were measured on any site.

While there may be other reasons for adopting higher inflow systems, such as time and labour savings, the message we are able to deliver to industry as a result of this project is to not assume water and productivity benefits when assessing investment in infrastructure to achieve bay inflows greater than 0.2 Ml/d/m.
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