Water is a major input resource for irrigated agriculture and has a leading role amongst the factors responsible for infield yield variability. The uniformity of irrigation applications is commonly reported to be a key determinant of crop yields and profitability. However, improvements in irrigation application uniformity do not always improve yields or economic returns and there is some debate over the benefits of implementing site specific irrigation management under different environmental and climatic conditions.
Catch can tests to evaluate irrigation uniformity are labour intensive and time consuming. Hence, they are commonly conducted on only small areas which lead to
uncertainties over field scale spatial variations. While soil-water and crop sensing technology has enhanced irrigation research, these technologies are not currently
used to evaluate commercial irrigation performance. Hence, the objectives of this research were to: (a) quantify the spatial yield and quality variability in an irrigated
crop under specific irrigation design, management and environmental conditions, (b) evaluate the potential to use proximal or remote sensors for crop and soil-water
measurements as part of irrigation performance evaluations, (c) identify variations in the seasonal patterns of irrigated water application and the resultant impact on soilwater and crop responses, and (d) evaluate the agronomic and economic benefits of improving the irrigation uniformity for a range of environmental conditions.
A preliminary trial was conducted to evaluate the effect of non-uniform sprinkler irrigation applications on lettuce grown under commercial conditions in the Lockyer
Valley, Queensland. A three-fold variation in the depth of water applied at specific locations in the field was measured within individual irrigation events even though
selected sprinkler grids within the field were found to have a coefficient of uniformity (CU) greater than 80%. Substantial variations in sprinkler operating
pressure (303 to 372 kPa) and discharge (0.07 to 0.14 l s-1) across the field were also measured suggesting that the variation in applied depths across the field may have
been larger than measured within the grids. The variation in sprinkler pressure and flow rate was due to differences in sprinkler elevation, position along the laterals,
pipe leakage and nozzle wear. However, the depth of irrigation water applied at specific locations in the field was not correlated with lettuce head size because head
size was not very variable suggesting that a range of other factors including the presence of in-season rainfall and over-irrigation may also influence crop growth under commercial conditions.
Two trials (autumn and winter crops) were subsequently conducted to evaluate the potential to use proximal plant and soil-water measurement systems to obtain spatial
data for irrigation performance evaluations. Three sprinkler irrigation grids with different application uniformities were established within each trial plot. These trials were also used to determine the variation in uniformity of irrigation applications during the season and the consequential effect on soil moisture, lettuce growth and yield.
Thermal infrared (for the calculation of crop water stress index) measurements of the lettuce plants were generally poorly correlated with both water applications (autumn
trial R2 < 0.1; winter trial R2 < 0.54) and canopy area (autumn trial R2 < 0.02; winter trial R2 < 0.28). There were also no correlations between the multispectral
reflectance measurements (used to calculate the normalised difference vegetation index) and water application. Measurements of lettuce canopy area and head size
derived from photographs taken by a camera mounted perpendicularly (either 1.15 m or 10 m) above the ground surface were found to be well correlated (R2 = 0.35 to
0.92) with physically measured canopy area and head size measurements. The correlations generally improved throughout the season suggesting that this method
may potentially be suitable for evaluating field scale spatial yield variability in lettuce crops.
The evaluation of sprinkler irrigation uniformity using traditional catch can analyses is resource prohibitive and commonly results in only small grids being used to infer
whole field performance. Measurements of apparent soil electrical conductivity (ECa) using electromagnetic (EM) sensors have been used to measure spatial variability in soil moisture but no detailed studies have been taken to evaluate the potential to use these sensors for measuring sprinkler irrigation uniformity. Apparent soil electrical conductivity (ECa) measurements were found to be not suitable for evaluating the uniformity of individual sprinkler irrigation applications where either
the volumes applied are small or the uniformity of the application is relatively high (e.g. Christensen’s coefficient of uniformity (CU) > 75%). However, ECa
measurements may be useful to identify cumulative non-uniformities in irrigation applications later in the season where the spatial pattern of water application is
consistent throughout the season and the uniformity of the application is poor (e.g. CU < 70%). A similar relationship was found between soil tension (soil matric
potential) and water application measurements with correlation generally higher in low uniformity grids later in the season.
Substantial variations were found in the uniformity of individual irrigation applications throughout the season (e.g. CU ranged from 69 to 89% for a high uniformity grid). Similarly, the uniformity measured by catch cans at different grid locations in the same field during the same event was also found to vary widely (e.g. CU ranged from 61 to 85%). Hence, uniformity measurements taken using a limited number of grids over a single irrigation event may not adequately reflect the performance of the irrigation system over the whole season. The frequency distribution plots of the irrigation application depths were generally found to be normally distributed when the CU was greater than 75%. However, low uniformity applications (e.g. CU < 60%) were often multi-modal and generally positively
skewed towards the low application depths.
The effect of irrigation water application on crop growth and yield was evaluated in both trials. Variations in water application during the mid to late growing period
were found to affect lettuce head development and marketability more than canopy size. There was also a substantial loss in marketability due to the depth of water
application at specific locations within the grids. The proportion of marketable heads ranged from less than 20% to more than 70% within the low and high water
application areas, respectively, of the low irrigation uniformity grids. The wide variation in the proportion of marketable heads with water application across each of
the grids and trials confirms that many factors other than water (e.g. disease, fertility) may influence marketability. These factors were also responsible for the high degree of scatter in the plots relating total seasonal water (irrigation and effective rainfall) applied and yield. Both polynomial (i.e. quadratic) and exponential (i.e. plateau) functions were fitted to the data and there was no difference between the correlation for each form of equation. Hence, both forms were used in the subsequent economic analysis to evaluate the benefits of irrigation uniformity improvements.
The economic analysis demonstrated that where the existing irrigation uniformity is low, returns can generally be increased with improvements in irrigation uniformity.
However, the magnitude of the benefit is dependent on the season, nature of the crop production response and the total water applied. The benefits of system improvements are maximised when the crop has a quadratic production function and appropriate irrigation scheduling is used. However, where the crop has an exponential production function or inappropriate scheduling is used then the gains
may be small or negative. Similarly, in-season rainfall reduces the marginal benefit of irrigation system improvement with negligible increase in returns when effective rainfall meets 50% or more of the crop water requirements. The incentive for irrigation system improvement is greatest when water is limited and unable to be purchased. Periods of low product price would be expected to encourage irrigation uniformity improvements as non-uniform systems have a higher-break even price
and require increased management (e.g. scheduling) to remain viable. These results may be used by both industry and growers to develop appropriate investment
strategies to improve the performance of irrigation systems.