Performance of centre pivot sprinkler irrigation system: a case study of Ubombo Sugar Estate, Swaziland
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Date
2014
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Publisher
Sokoine University of Agriculture
Abstract
The centre pivot irrigation system is increasingly becoming popular among farmers around the world on account of the benefits the system can offer compared to other methods. Hence evaluation of system performance with the ultimate aim of attaining greater irrigation efficiency and better manage irrigation is paramount. The study evaluated the performance of centre pivot sprinkler irrigation system and its effect on sugarcane yield at Ubombo Sugar Estate in the Lubombo Region of Swaziland. Four centre pivots (EEL09, NKA21, CAS02 and SMB) were selected from 106 centre pivot machines for purposes of evaluation. Information collected included meteorological data, design parameters, water application uniformity, system operating pressure, tyre pressure,
system flow rate, sprinkler nozzle discharge and pressure, crop data and planted area, harvesting programs, pumps and motor specifications, pumping hours, cost of power and actual flow rates. Drive speeds of 15%, 20%, 30% and 40% of the centre pivot systems were examined to determine their significance on system performance parameters on the basis of common field operations. Water and power budgeting systems utilized were assessed to determine adequacy of irrigation water supply and implications on system and crop performance. Irrigation water quality, soil physical and chemical properties, soil infiltration characteristics and soil compaction were also examined to determine their influence on system performance and sugarcane yields. Data analysis was carried out using Microsoft Excel, ETo Calculator, Instat, Statistical Package for Social Sciences (SPSS), ArcGIS and HPen32 software. Results demonstrated that centre pivots were accurately designed and properly installed as they adequately supplied water to meet sugarcane demand of 7.5 mm/day. Performance indicators showed that centre pivots were relatively performing well as uniformity indices (CU and DU) were within acceptable standards, i.e. above the base values of 85% for CU and 75% for DU. Recommendedapplication efficiencies (AELQ and PELQ) were only achieved by CAS02 and SMB centre pivot systems as they were above the minimum of 90%. Speed effect results showed a general decline in CU and DU with an increase in centre pivot speed as opposed to an increase in application efficiency. CU for SMB declined from 88.9% at 15% speed to 86.9% at 40% speed setting. A similar trend was evident for both NKA21 and EEL09.PELQ for SMB increased from 93.1% to 98.0%% and AELQ from 92.6% to 100%. A parallel trend was also observed for NKA21 and EEL09. Analysis of variance for these results showed that the performance indicators were not significantly different (p<0.01) among treatments. This demonstrated the ability of the centre pivot irrigation system to apply water uniformly and efficiently across the field at different speeds. Water and power budgets were not satisfactory in meeting estate crop water demand since they did not capture and consolidate relevant parameters needed to accurately define budgets.Water supply was determined by capacities of irrigation structures used and losses were incurred during periods of low demand. Electricity budgets were based on historical power records which affected overall water supply and caused water stress to the crop. Quality of irrigation water was within acceptable levels and had no negative impact on system, crop and soil. Soil physical and chemical properties indicated that soils were inherently sandy and ideal for sugarcane growth save for shallow profiles with low water holding capacities. Soil compaction as a result of mechanical harvesting was a major cause of significant reduction in soil infiltration rates, water storage capacity and hence runoff. Significantly higher soil compaction values at 0 to 15 cm depth (p<0.0l) were associated with EEL09 centre pivot with an average of 3 210 kPa compared to those of NKA2I at 596 kPa, CAS02 at 447 kPa and SMB at 1 389 kPa. Identical observations were made at 0 to 30 cm depth where higher compaction values were associated with EEL09 and SMB. EEL09, a sandy textured field measured a basic infiltration rate of 6 mm/h as a result of compaction while the other less compact sandy textured fields had infiltration rates greater than 70 mm/h. Yields were variable as EEL09 achieved a low yield of 71 t/ha against 147 t/ha for CAS02, 124.8 t/ha for NKA21 and 106 t/ha for SMB. Findings indicate that centre pivots are performing well although potential system performance and sugarcane yields are constrained by limited water and power supply,pressure variations, clogging of sprinklers, rutting of un-gravelled tracks, soil compaction,low water holding capacities, reduced infiltration rates and game encroaching sugarcane fields. It is therefore recommended that constant system checks, improvement of traction on centre pivot wheels, development of new model for water and power budgets as well as undertaking of extensive research on the effects of mechanical harvesting be done so as to improve centre pivot irrigation system performance and sugarcane yields at Ubombo Sugar Estate.
Description
Dissertation
Keywords
Pivot Irrigation System, Ubombo Sugar Estate, Irrigation farming, Swaziland