Development of an evaluation technique in near zero slope furrowed border systems with common water supply

Development of an evaluation technique in near zero slope furrowed border systems with common water supply

Near zero slope furrowed border irrigation systems with common water supply commonly known in Australia as bankless irrigation systems, were developed in Australia about three decades ago to reduce the labour requirements associated with siphon irrigation systems. Despite the recent trend to adopt these layouts, there is no methodology to estimate the irrigation performance of these systems. The presence of hydraulic interaction between the adjacent borders, and a considerable variation in diverging flows and water advance between furrows complicates the evaluation process. This study aims to identify and develop the appropriate measurement and analysis processes that can accommodate the special characteristics of these systems, and so allow evaluation of their irrigation performance.

A new naming convention for surface irrigation systems was developed reflecting the features of modern field designs. According to this scheme, the system under study is termed Furrowed Normal Slope BOrder with Reverse slope entry (FNSBO_Re).

A combination of conventional and nonconventional measurement techniques were trialled in two commercial cotton fields in New South Wales in Australia during two irrigation seasons (2016/2017 & 2017/2018). Flow rate measurements at the field scale were conducted using ultrasonic doppler meters. Pipe energy and weir equations were adopted in some cases due to site condition limitations. Measuring the flow rate at the furrow scale was performed using a range of techniques but the most reliable and efficient technique was using a hand-held acoustic doppler. IrriMATETM advance sensors provided a high level of accuracy but covered a small number of furrows. A GPS smartphone app captured a larger number of advance measurements, but data was of lower accuracy and required considerable time and effort to collect. A novel UAV technique provided an efficient approach to collect advance and recession at the border scale. Attempts were also made to measure furrow water depth using floats and fixed cameras but the most effective approach proved to be use of wooden stakes painted with water soluble dye. Cost, time, effort, performance, and site conditions were the main issues that influenced the choice and success of these various techniques.

The temporarily blocked calculation (TBC) technique was developed to evaluate the irrigation performance at the furrow scale for the real downstream condition (neither blocked nor free draining conditions) in the FNSBO_Re system. The TBC is a spreadsheet-based calculation based on the simulation results of the SISCO model. The TBC results for the furrow length after the sill showed high values of distribution uniformity, exceeding 96%. This high uniformity is due to a rapid water advance and low infiltration characteristics in this field. There was a considerable variation in the application efficiency (28% to 74%) and the requirement efficiency (19% to 65%) between furrows. These low efficiencies were attributed to high runoff losses caused by the large inflow rates and low infiltration characteristics. A complementary calculation process based on drain-back observations at the upstream part of the furrow was developed to evaluate that part of the furrow, which can then be combined with TBC results to find the irrigation performance for the entire length of the furrow.

A UAV technique was developed to evaluate the irrigation performance for the whole border scale. This approach involves obtaining the infiltration opportunity times from the UAV advance and recession data which is then combined with infiltration characteristics from intensive irrigation evaluation measurements. The results showed high distribution uniformity (96.6%) across the border. The results showed low values of the application efficiency (43.5% to 56.5%) and requirement efficiency (52.5% to 100%) in the FNSBO_Re system at the border scale.

Intensive measurements at the furrow scale showed a significant variation between furrows regarding the diverted flows, water advance, water depth, and cross-sectional areas. The greatest variation appeared between wheeled non-wheeled furrows. Statistical analysis using an independent-sample t-test indicated that machinery wheel traffic influences aspects of the irrigation such as peak water depth, cross-sectional area, and flow rate. Investigating the factors that influence the variability of the water advance showed the governing factors to this variability were the cross-sectional area and the flow rate with R2 of 0.7038 and 0.6477, respectively. Whilst the furrow elevation, and peak water depth have insignificant impact on the water advance.

Importantly, this study outlines a recommended procedure for measurement and analysis of FNSBO_Re irrigation systems. A key part of this is the selection of positions for intensive field measurements. A simple approach based on understanding the water advance variability at the border scale through analysis of UAV advance data was developed to guide the selection of these intensive measurements. This evaluation procedure features use of the novel TBC and UAV techniques developed in this work.