4. RESULTS AND DISCUSSION
4.2 Performance and evaluation of a remote real-time and site-specific distributed
4.2.3 Evaluation of the modified centre pivot irrigation system…
4.2.3.3 The laboratory and field tests of drop tubes
Different performance parameters were calculated in the laboratory to illustrate the relationship between the operating pressure and discharge rate, the emitter discharge exponent, the coefficient of variation, flow variation and emission uniformity. The pressure-discharge relationships of emitters are expressed by equation 4.13. The Siplast emitter discharge was very uniformly distributed for all emitters at all operating pressures as shown in Figure 4.22. At the same time, the discharge increased linearly as the operating pressure grew because this type of emitters is a non-pressure compensating (NPC). The effect of operating pressure on the emitter discharge was highly significant and the emitter discharge was strongly influenced by the operating pressure. The discharge was about 10 l/h and 19.4 l/h at 50 kPa and 200 kPa, respectively.
Means of the measured discharge rates at different operating pressures are illustrated in Figure 4.23. The results indicated that the emitter discharge rate increased linearly with operating pressure. Except at 200 kPa, measured discharge flow rates at other pressure levels and in particular at 50 kPa did not reach the design flow rate claimed by the manufacturer. These variations are presented in Table 4.5.
8 10 12 14 16 18 20 22
0 2 4 6 8 10 12 14 16 18 20
Emitter number
Emitter discharge (l/h)
50kPa 100kPa 120kPa 150kPa 200kPa
Figure 4.22: Emitter discharge rate at different operating pressures for drop tube including 19 emitters under laboratory condition
8 10 12 14 16 18 20 22
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
Emitter number
Emitter discharge (l/h)
50kPa 100kPa 120kPa 150kPa 200kPa
Figure 4.23: Emitter discharge rate at different operating pressures for drop tube including 35 drippers under laboratory conditions
Table 4.5: Average difference between the nominal discharge indicated by manufacturer and measured discharge in laboratory
Emitter pressure [kPa]
Nominal discharge by manufacturer [l/h]
Measured discharge in laboratory [l/h]
Difference [%]
50 11.1 9.9 - 10.8
100 14.5 13.7 - 5.5
120 15.7 17.1 + 8.9
150 17.1 17.7 + 3.5
200 19.4 19.4 0.0
In addition, the hydraulic characteristics of emitter were calculated based on estimated coefficient of determination values. Ke and x were 1.3444 and 0.5128, respectively as shown in Figure 4.24. With due attention to the turbulent flow type when the emitter exponent values is higher than 0.5, the Siplast emitter is classified as NPC. The coefficient of determination (R2) is also reported. When the R2 value is very close to 1, equation 4.13 is an appropriate model for the description of the relationship between the discharge and the pressure of these emitters, but low R2 values either indicate considerable data scattering or that the model used was not appropriate.
The coefficient of determination for the Siplast emitter was 0.96 (given by Figure 4.24). Thus, equation 4.13 is an appropriate model for the description of the relationship between the discharge and the pressure of Siplast emitters.
y = 1.3444x0.5128 R2 = 0.9611
0 5 10 15 20 25
0 50 100 150 200 250
Pressure (kPa)
Emitter discharge (l/h)
Figure 4.24: Means of measured discharge rates for all tested emitters at different pressures under laboratory conditions
The coefficient of discharge variation of emitters in the sample falling within a given deviation from the mean discharge was calculated using equation 4.14. The results indicated that the coefficient of discharge variation value was followed by a normal distribution at each operating pressure. Emitter performance was classified as good based on the coefficient of variation in Table 4.2, according to ISO standards (1991). Emission uniformity was calculated using equation 4.16. The relationship between operating pressure and both emission uniformity discharge and the coefficient of variation is illustrated in Figure 4.25. The emission uniformity value was higher than 95 % at all operating pressures. At the same time, results indicated that the measured CV value at 100 kPa was less than the design CV (3 %) claimed by the manufacturer as shown in Figure 4.25. The fluctuation of the coefficient of variation with pressure may be used to define emitter discharge sensitivity to pressure. The manufacturer coefficient of variation should be 15 % or less to achieve reasonable uniformity of water application (Solomon, 1977).
The results showed that an increasing value of the coefficient of discharge variation CV leads to decreasing emission EU uniformity.
75 80 85 90 95 100
0 50 100 150 200 250
Pressure (kPa)
EU (%)
0 0,5 1 1,5 2 2,5 3 3,5
CV (%)
EU CV
Figure 4.25: Relationship between the operating pressure and both the coefficient of variation and emission uniformity of Siplast drop tube
The calculation of emitter flow variation using equation 4.17 showed that the mean value of emitter flow variation was about 10.9 % at operating pressures ranging from 50 kPa to 200 kPa and that the maximum value of emitter flow variation was reached at 120 kPa. These results are presented in Figure 4.26.
0 2 4 6 8 10 12 14 16
50 100 120 150 200
Pressure (kPa) qvar(%)
Figure 4.26: The relationship between different operating pressures and emitter flow variation
The calculation of water application rates by MDI and two drop tubes (Ne = 19, LT = 3.8 m and Ne = 35, LT = 7.0 m) at different CP speeds and different pulsing levels under field conditions showed the expected results. These tests were the suggested solution which would provide variable rate water application in the radial direction as stated in the hypothesis upon which a strategy for precision irrigation is based. These test cases represented different locations in the application map (different angles in the field). The field tests of the drop tube show:
a) The variation of the drop tube flow output is proportionate to the number of emitters
installed on the drop tube
b) The variation of the drop tube flow output is proportionate to the pulsing level
Field tests of MDI depth variation were done against pulsing level under different CP speed. An example of this variation for 20 % of CP speed is shown in Figure 4.27. In Appendix F, same curves for 10, 30, 40, 50, 60, 70, 80 and 90 % of programmed CP speed are drawn. In practice, one of these curves can be selected for each irrigation time based on irrigation duration and the maximum available flow rate for pumping.
0 3 6 9 12 15 18 21 24 27 30 33
0 10 20 30 40 50 60 70 80 90 100
Pulsing level (%) Depth of Irrigation (mm) for 20 % of CP speed
Figure 4.27: Field test variation of MDI depth against pulsing level for 20 % of programmed CP speed
With due attention to different irrigation rates needed to cover different irrigation depths, it is very important to consider that the maximum irrigation rate used within IMZs must be below the soil infiltration rate to avoid runoff. Since saturated infiltration rates on the study field are 48 mm/h according to measurements by Seibold et al. (1998), the minimum speed set at the CP control box [%] for the avoidance of runoff was calculated for different pulsing levels. This minimum speed can be calculated based on the water application rate at different pulsing levels, lengths of the drop tube and a saturated infiltration rate of 48 mm/h. CP speed must be reduced by decreasing the pulsing level as shown in Table 4.6.
The results showed that at a pulsing level of 100%, the speed set at the CP control box to avoid runoff must be more than 34.3 %. But the minimum speed set at the CP control box to avoid runoff can be improved by increasing distance between emitters on drop tubes. The results showed that the maximum pulsing level used at 10 % CP speed setting can be 68 % to avoid runoff, but at 20 % of CP speed or more, the maximum pulsing level of 100 % can be used without any runoff as shown by Figures 4.27. The average manufacturer value of discharge and the average values of emitter discharge under laboratory and field conditions at 120 kPa were 15.8, 17.1 and 15.3 l/min, respectively.
Field observation showed that in spite of some advantages of the Siplast drop tube, it has hard and inflexible material and was difficult to install and to work with. Moreover, a metallic horizontal tube was found to be better than a horizontal tube of out polyethylene shown in Figure
3.17 because of the ard sliding of two tubes out of different materials which lay one on top of the other.
Although this system had not experienced any clogging problems even though no filtration system had been installed in the summer of 2006, a DN-150 filter from the company Hüdig (www.huedig.de) was installed between the supply and the pivot point for future applications of mobile CP drip irrigation.
Table 4.6: Minimum allowed speed set at the CP control box to avoid runoff at different pulsing levels
Pulsing level [%] 10 20 30 40 50 60 70 80 90 100 speed set on CP
control box [%]
7.1 10.5 12.6 15.9 17.9 20.0 22.5 28.2 33.0 34.3 Minimum
allowed CP speed to avoid runoff
Linear speed at the end of 2nd CP span
[m/min]
0.12 0.30 0.49 0.68 0.85 1.03 1.24 1.42 1.64 1.81