Studies on Hydraulic Performance of Furrow Irrigation to Optimize Design Parameters Suitable to Onion Field in Hawassa, Ethiopia

Ground water is a scarce and expensive resource which needs to be utilized in a highly productive manner for agriculture. Inefficient use of ground water by surface irrigation will result in soil salinization in the long run. This demands precise application of required irrigation water with high efficiency. Furrow irrigation is most widely used among the surface irrigation methods. Furrow design parameters are inflow rate, the length of the run in the direction of the flow, the time of irrigation cutoff and soil infiltration characteristics. These parameters have been studied in order to design an optimum furrow length to achieve maximum application efficiency. Time ratio, which is defined as the ratio of the time required for infiltration of net amount of water needed for the root zone to the time when the water front reaches the end of the run, plays a key role in determining optimum furrow length to achieve maximum irrigation efficiency. In this study, using established model parameters, optimum time ratio and furrow length for maximum application efficiency in furrow irrigation were determined. The model was established from field tests conducted on onion grown furrows, 0.4 m wide and 70 m long, with two existing slopes 0.2 and 3% and each slope had three different inflow rates (0.3, 0.9, 2.7 L/s; 0.4, 0.7, 1.1 L/s respectively). Inflow rate of 0.3 L/s was very low for 0.2% slope in moderately permeable soil resulting long advancing time and less application efficiency. Under open end furrows, maximum attainable efficiency was 54.2% and the optimum furrow lengths to realize this efficiency were 32 and 74 m for 25 and 60 mm irrigation depth with 0.3L/s inflow rate. It was concluded that at higher slope of 3%, increase in the flow rate beyond 0.7L/s cause decrease in optimum length for the maximum attainable water application efficiency. When runoff is eliminated or reused, the maximum attainable efficiency of 75.9% and 71.1% can be achieved with 0.4 L/s and 0.9 L/s in 3% and 0.2% slope, respectively. For furrow slope 0.2%, peak irrigation demand of 3mm/day and a recommended furrow discharge of 0.9 L/s, suitable furrow length is 106 m length to have advance time of 10.7 min and 21.3min as total irrigation time to apply 40mm irrigation water for irrigation frequency of 7days. Optimum time ratio under different in flow rates for various irrigation depths reveals that for optimum furrow length and maximum application efficiency, the advance time should be two quarter of the total irrigation time.