Abstract
Characteristics of emitters under low pressure are essential for the design of low pressure drip irrigation systems. Few data is provided by the manufacturers for drip emitter operating under low pressures. No guidelines regarding the optimum combination of operating pressure head and lateral length available either. A laboratory test was conducted to evaluate the effects of pressure head and lateral length on water distribution uniformity of a PVC drip irrigation system. Five different lengths of lateral were considered under a pressure head of 55 kPa (8.21 psi / 5.6 m). The five lengths were 20, 40, 60, 80 and 100 m. Drip tape tested in this study is a non-pressure compensating (NPC) emiiter with 3.00 L/h discharge rate, 16 mm diameter and 35 cm emitter distance. Evaluation of 10 sampled emitters from the 20 m lateral showed a flow variation (Qvar) of 5%, uniformity coefficient (UC) of 99%, with a coefficient of variation (Cv) of 0.016. The 40 m lateral length showed a flow variation (Qvar) of 9%, UC of 98% with a Cv of 0.024. The 60 m lateral length also showed an average flow variation (Qvar) of 16%, UC of 95% with a Cv of 0.060. The 80 m lateral length showed a flow variation (Qvar) of 23%, UC of 93% with Cv of 0.08 and the 100 m lateral length showed a flow variation (Qvar) of 39%, UC of 89% with a Cv of 0.138. EU for 20, 40, 60, 80 and 100 m were 92, 90, 80, 75 and 58% respectively. Flow discharge vs. sampled emitter points (Q-E curves) was also developed for each length. Q-E curves were fitted to the data resulting in R2 values of 0.1566, 0.1202, 0.8607, 0.7904 and 0.8998 respectively for 20, 40, 60, 80 and 100m. The operating pressure was 55 kPa (8.21 psi / 5.6 m) for all the tested length. From the statistical analysis, it was observed that as the lateral length increase invariably decreases the average discharge (Qvar). It is therefore recommended to use the 60 m lateral length with a low pressure head of 55 kPa (8.21 psi / 5.6 m), since the 60 m length satisfied the uniformity distribution criteria under a low pressure. Key words: Pressure head, lateral length, irrigation uniformity.
Highlights
Water for agricultural use is becoming scarce, both in quantity and quality, in the traditionally prone arid and semi-arid zones, and in regions where rainfall is abundant
Values of coefficient of variation (Cv) are statistically determined from measured flow rate from a large (>50) sample set of emitters depending on the lateral length all subjected to the same reference pressure head and is the ratio of the standard of deviation of the measured flow rate the mean flowrate
The 20 m long at 55 kPa gave an excellent Cv of 0.016 which was reported by ASAE EP405.1,(2000), that for point-source emitters, values of Cv less than 0.05 are considered excellent
Summary
Water for agricultural use is becoming scarce, both in quantity and quality, in the traditionally prone arid and semi-arid zones, and in regions where rainfall is abundant. Energy analysis of agricultural operations has shown that irrigation system consumes a significant amount of energy as compared to other operations (Topak et al, 2005). For these reasons, there is an urgent need to use water resources efficiently by enhancing crop productivity per unit of water. The drip irrigation systems require intensive capital due to sophisticated technology. It is beyond the capacity of the most farmers under small scale farming.
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