6. DISCUSSION AND CONCLUSIONS
6.4 ASFI in the Context of Other Irrigation Systems
ASFI will not outperform all other irrigation systems in all circumstances, but definitely has advantages over other irrigation systems in a variety of situations. After running numerous simulations, implementing the Ukulinga trial and completing a cost analysis on a sample large scale ASFI design, ASFI can be described as a system which has:
• low energy requirements compared to systems such as SSD and sprinkler irrigation, with energy requirements similar to conventional surface irrigation with 5 to 10 m pressure required at the field edge;
• low capital costs relative to a drip irrigation system due to cost-effective, low pressure piping; and
• a high degree of robustness when correctly designed and installed, being able to produce a highly efficient and uniform irrigation application due to accurate control through the automated valve, as well as having balanced discharges through proper hydraulic design of the water supply system, and the use of short furrows.
ASFI is well suited to flat terrain, land with a gravity supply of water and as a replacement for surface irrigation systems where the supply restrictions and the field layout result in poor uniformities and large losses due to deep percolation. ASFI should also be considered in situations where surface irrigation is used on shallow soils. On steeper land, where furrows run along the contours, there are concerns that furrows may break during large rainfall events.
However, if the system is correctly installed with adequate attention to surface drainage and waterways, ASFI may actually help reduce erosion, as each furrow acts like a small contour bank.
The optimum application depth for ASFI is approximately 10 – 20 mm, depending on the soil depth. ASFI potentially has less evaporation than drip irrigation, which applies smaller irrigation depths of approximately 5mm on a daily basis, resulting in the soil surface being
consistently wet which may increase the potential for evaporation. ASFI potentially also has less evaporation losses than sprinkler and centre pivot irrigation as only a very small portion of the soil surface is wetted.
The performance of the ASFI system was assessed relative to a reference drip irrigation system as part of the Ukulinga trial. This included: irrigation performance tests, a soil moisture monitoring analysis, a yield analysis of the harvested crop and an economic comparison of ASFI and SSD irrigation systems.
6.4.1 Irrigation performance tests
From the numerous furrow tests on the selected furrows, the Ukulinga ASFI system produced a DU of 72 % to 80 %. This excludes the results from the 15 August 2007 test on ASFI line 2, with DUs of between 58 % and 75 %, as the emitter problem was an isolated incident which should have been rectified prior to the installation of the irrigation system. The DU results of between 72 % and 80 % were high relative to the results from experiments conducted by Reinders (2001) on sprinkler and micro irrigation systems (Section 2.4), despite the furrow slope being significantly steeper than optimum (1:40 as opposed to 1:250). For the SSD tests conducted on 21 May 2008, DU results of 90 % to 91 % were obtained for randomly selected drip laterals. These high DU results are contrary to what was visually observed, with holes on laterals that were not selected for the drip tests resulting in significant over-watering in the surrounding area. If the laterals with holes had been randomly selected for the test, the results could have been significantly different.
6.4.2 Soil Moisture analysis
SAsched was a simple, accurate and important irrigation scheduling/management tool for both the Ukulinga ASFI and drip irrigation treatments. SAsched is recommended for irrigation scheduling in the South African climate. Watermark sensors are recommended when assessing trends in soil moisture and when comparing differences in soil moisture at different locations. However, watermark sensors are inaccurate in dry conditions. For the Ukulinga trial, the watermark sensors were used along with Hobo loggers to assess trends in
soil water tension and to compare the trends with those predicted by SAsched for soil moisture. Both the watermark soil tension results and the SAsched soil moisture results followed similar trends and it was therefore assumed that SAsched was sufficiently accurate for scheduling.
It was also envisaged that the watermark sensors could be used to analyse differences in trends between the ASFI and SSD soil moisture tension. However, upon analysing the results, it became apparent that the localised soil and rooting conditions substantially affected the absolute soil water tension readings. Considerably more measuring locations in the plot would be required to ascertain if there was a soil moisture difference between the ASFI and SSD irrigation methods at specific locations, relative to the furrow/lateral and the crop. There were no conclusive differences between the trends in soil water tension of the two irrigation methods.
In the Ukulinga trial, faulty connections caused problems with the Hobo data-logger and watermark sensors. Subsequent to the trial, the Chief Technician at UKZN has developed an improved connector.
6.4.3 Yield analysis
Although the DU results from the irrigation performance tests were noticeably lower for the ASFI system than the drip system, this had no significant impact on both the cane and sucrose yield results. According to a one-way statistical analysis on cane and sucrose yield, there was no significant difference between the two irrigation treatments for near equal amounts of water. This result is positive, as the ASFI irrigation performance could be significantly improved by, among other things, using more gradual slopes. This could result in improved yields under the ASFI system. However, crop growth may have been dependant on the natural nutrients of the virgin land, as the first application of fertiliser was extremely late, namely 10 months after planting. The late fertiliser application may, therefore, have negated any potential variation in the yield results. The Ukulinga trial is in an area with high rainfall relative to the irrigation requirement. It is recommended that a drier region should be selected for future comparisons of the ASFI and SSD systems
6.4.4 Economic analysis
The aim of conducting the Ukulinga trial was to compare the performance of the ASFI and reference drip systems. The trial layout was selected accordingly. System costs for the irrigation systems used in the trial were therefore not considered representative of systems that would typically be used in practice. Sample irrigation designs were developed for both the drip and ASFI systems. The lifecycle costs of each system were determined using a software tool known as Irriecon V2. The ASFI sample design was designed for the purpose of this study and the drip design was conducted by Zululand Irrigation as reported in Armitage et al. (2008). Unfortunately, these two designs were conducted simultaneously and were therefore designed with slightly different irrigation requirements. The 50 ha drip design had an irrigation design application amount of 5.83mm/day and cost approximately R3 650/ha according to a lifecycle analysis using Irriecon V2. The 10 ha ASFI design had an irrigation design application amount of 5 mm/day and cost approximately R2 300/ha according to a lifecycle analysis using Irriecon V2. The ASFI system was designed for a field which required water to be pumped. In a situation where the scheme is gravity fed, both the operating and capital costs would be less.
The sample ASFI system was cheaper than the sample drip system. However, there are three issues that could/would reduce the cost difference. The first issue is that the system variable and mainline operating costs will be slightly biased against the drip system due to the slightly higher daily irrigation design application amount, as electricity and water charges will be increased slightly. The mainline fixed costs will probably not be affected as the difference in irrigation requirements between the two systems is unlikely to affect the pump and pipe sizes.
The second factor is that the cost of land preparation was not included. Both irrigation systems require land smoothing/forming. It is likely that the ASFI would require greater precision in land smoothing/levelling, which would increase the lifecycle cost/ha slightly, depending on the available resources to the farmer. The third issue is that, for both the ASFI and SSD systems, trenching and burying of drip lines were not included in the economic analysis. Adding the trenching costs to the ASFI system would substantially increase the capital cost of the system due to the amount of pipe required. With an assumed trenching cost of R6.50/m, the capital costs of the 10 ha sample design would increase by approximately R38 000. To reduce capital costs, it is recommended that the LDPE sub-mains are left on the
surface and the laterals are buried. This would not limit machinery operation in the field as the sub-mains run parallel to the furrows.
6.4.5 Labour and maintenance requirements
For the Ukulinga trial, the irrigation installation was slightly more labour-intensive for the SSD irrigation system, as the laterals were laid by hand. However, on a larger scale, the ASFI installation is likely to be slightly more intensive than other irrigation systems due to the large number of pipes that require installation. If furrow lengths vary along a lateral, the installation process for ASFI is likely to be even more intensive as emitter pipes would have to be cut to different lengths along the lateral, to obtain the optimum flow-rate and the required irrigation depth.
In the initial stages after installation, the Ukulinga ASFI system was more labour-intensive than the Ukulinga drip system, as problems with specific furrows needed to be corrected.
However, most of these problems were due to the Ukulinga ASFI system layout and inadequate land preparation. After the initial stages, the Ukulinga drip irrigation system had a higher labour requirement as the dripper lines needed to be flushed and holes in the dripper lines needed to be repaired, whereas the Ukulinga ASFI system required mainly supervision.
From the sample design in Section 5.4.1, the maximum area from which one lateral can be irrigated at a time is 10 ha. For larger areas, two or more laterals could be irrigated simultaneously. The main irrigation task for the labourer would be to ensure that the valve does not malfunction and that emitters are correctly positioned. It is unlikely that one labourer would be able to check on two laterals concurrently. Therefore the labour requirements for ASFI would be about one labourer per 10 ha. Conventional systems such as drip and dragline sprinkler require one labourer for every 20-25 ha (ARC, 2003). However, the labour for the conventional systems would be focussed solely on tasks such as moving sprinkler stands or flushing drip laterals. The ASFI labourer would be able to do tasks such as weeding, as the irrigation requirements for the labourer are only supervisory. Therefore, the total labour required, for say a 100 ha drip/dragline system, would be similar to the labour requirement for a 100 ha ASFI system.