Effects of Climatic and Hydraulic Parameters on Water Uniformity Coefficient in Solid Set Systems
In order to study the effects of different wind conditions, operating pressures, various sprinklers layouts and spacing on water distribution uniformity in sprinkler irrigation system a research project was conducted under 3 different wind velocities (0-5, 5-7 and > 7 m sec-1), using 3 operating pressure (35, 40 and 45 m), three spacing on the lateral pipeline (15, 18 and 21 m) and 3 different layouts (square, rectangular and triangular). Simulation experiments were conducted to estimate water distribution uniformity. The results indicated that the distribution coefficient uniformity decreased with the increase of the wind velocity. With the increase of wind velocity up to 7 m sec-1, the decrease of coefficient uniformity was not significant (the coefficient was reduced by 20% in the range of wind velocity applied). The highest water distribution coefficient uniformity was occurred on 15x5 m spacing while the lowest value was achieved for spacing of 21x21 m and sprinkler spacing to spray diameter of 0.5x0.5 with the increase of sprinklers spacing to the spray diameter, coefficient uniformity is reduced, especially at higher wind velocities. Therefore at higher wind velocities, it is recommended to reduce sprinklers spacing to spray diameter ratio and use square arrangement in order to achieve acceptable uniformity.
March 02, 2010; Accepted: May 29, 2010;
Published: June 26, 2010
In the course of development of irrigation technology, a broad range of solutions
has been applied to improve irrigation processes from the technical, organizational
and economic point of view. Since, 1970 industrialized nations have focused
on reduction of irrigation labor requirements and elimination of unacceptable
working conditions, goals that, as a general rule, can be achieved through investment
in modern irrigation equipment. Introduction of sprinkler irrigation machines
around 1950 and relevant developments in the 1960s and 1970s, represented a
decisive technological step, as did replacement of sprinkler booms by portable
aluminum pipe sprinkler irrigation systems, which provided an 80% reduction
in labor requirements (De Boer and Chu, 2001). One of
the most relevant parameters in sprinkler irrigation systems is the uniformity
of water distribution (Merriam and Keller, 1978). Field
irrigation evaluations are used to establish irrigation performance, which for
sprinkler irrigation is primarily represented by irrigation uniformity. During
the evaluation process, quantitative levels of uniformity are established. Sprinkler
irrigation systems require a minimum value of uniformity to be considered acceptable.
For solid set sprinkler systems, Bliesner and Keller (2001)
classified irrigation uniformity as low when the Christiansen Coefficient of
Uniformity (CU) is below 84%. Little et al. (1993)
reported that SCS classifies uniformity of a sprinkler irrigation system as
very good, good, poor and worst if the Christensen Uniformity Coefficient (CUC)
value is = 90%, between 80 and 89%, between 70 and 79% and > 69%, respectively.
Tari (1998) reported CUC and distribution uniformity (DUlq)
values between 58 and 82% and between 37 and 82%, respectively, in the Konya-Ilgýn
In all types of mobile irrigation machines, the characteristics of the spray
plate sprinklers, overlapping spacing and machine speed determine irrigation
performance. The precipitation rate (mm h-1) is a key factor in the
evaluation of irrigation performance. When the precipitation rate is higher
than the soil infiltration rate, water remains on the soil surface and runoff
can occur, so that to obtain an adequate performance, the precipitation rate
of the machine must be as high as possible but always lower than the soil infiltration
rate (Bliesner and Keller, 2001; De
Boer and Chu, 2001). De Boer (2002), using a catch-can
spacing of 0.25 m, found that the wetted radius of an R 3000 D-4 tended to increase
with an increase in nozzle pressure and nozzle diameter; this narrower catch-can
spacing resulted in more accurate estimations of the wetted radius. Several
authors have reported that wind is the main environmental factor affecting sprinkler
performance (Dechmi et al., 2003a). Since, most
fields are smaller than 10 ha, solid set irrigation is the most common technical
solution. Although triangular sprinkler spacings of 21x18 m were common 10 years
ago (Dechmi et al., 2003b), nowadays the most
frequently used spacings are triangular 18 x15 m and 18x18 m. Performance Assessment
(PA) of irrigation and drainage systems has been an important area of research
and debate within the irrigation community in recent years (Vincent
et al., 2001). This is recognized as the systematic observation,
documentation and interpretation of the management of an irrigation system (Bos
et al., 2005). Martinez et al. (2004)
analyzed the influence of different design and performance factors, such
as subunit arrangement, lateral spacing, working pressure, average application
rate and application efficiency of water application cost, in a permanent set
sprinkler irrigation system. The results showed that the most important factor
is sprinkler spacing. As emphasized by Frizzone et al.
(2007), the uniformity of moisture from the soil and the productivity of
irrigated crops are very dependent on uniformity of water applied during the
irrigation. To assess the effect of various factors on uniformity of application
of conventional water spray systems, equipped with hydraulic cannons, Azevedo
et al. (2000) noted that the wind speed was the factor which most
influenced in uniformity of application of water, followed by the pressure of
the sprinkler, spacing between sprinkler installations in the lateral line,
line spacing, wind direction towards lateral line and speed of rotation of the
sprinkler. The objective of the present research was to study the effects of
different wind conditions, operating pressures, various sprinklers layouts and
spacing on water distribution uniformity in sprinkler irrigation system, in
Khuzestan province, South West of Iran.
MATERIALS AND METHODS
The studies described in this research were conducted on an oat-stubble field
at the research farmland, located southeast Khuzestan province of Iran at 49°
42 30= E and 30° 50 N with a net area of 42 ha during
the period of March (2008) through February (2009). Irrigation water for the
farmland is supplied from Zoreh River which is 12 km away from this pilot site.
The commercial (jaleh model 3) with two nozzles (7.32"x3.32") impact sprinkler
was located on the lateral.
||Arrangement of rain gauges (catch-containers) around the sprinkler
Riser allowed the sprinkler to be placed 1.75 cm above the catch- can openings.
The system was operated at three pressure levels of 35, 40 and 45 m. A total
of 100 catch containers on a 3x3 m grid system were located on both side of
the lateral around the sprinkler (Fig. 1) shows an arrangement
of rain gages for such a test. The area around the sprinkler was divided into
squares of equal area. A catch-can placed at the center of each square then
represented the precipitation falling on that area and the catch-cans opening
diameter was 10 and 15 cm height. The measurable parameters in this study included:
wind velocity, operating pressure, flow discharge and volume of water from the
sprinklers accumulated in the containers. The sprinklers flow discharge
was accurately determined by using a volume meter and a chronometer. Omidieh
region is a windy area with the different wind velocities during a season three
wind velocities were occurred during the time of March 2008 through February
2009. through this period of time different wind velocities were occurred and
by each wind velocity some experiments were conducted, for example when the
wind velocity was under 5 km sec-1 (0-5 m sec-1) we conducted
some experiments and while it was between (5 and 7 m sec-1), or upper
than 7 m sec-1 some other experiments were conducted. Wind velocity
and direction at 2 m above ground were measured with a recording three-cup anemometer
and wind vane for a time period equal to the duration of a test, which was about
1 h. The present study is carried out during the period of March (2008) through
February (2009). In order to obtain logic and reliable results 75 tests were
carried out in different hours overnight, so the correlations and diagrams would
represent a wide range of hydraulic and climatic conditions. Software SPSS version
14 were used for statistical analysis. Christiansen Equation (Eq.
1) was utilized to determine CU.
where, CU is Christiansen Coefficient Uniformity, yi is water contained
is average of water sprayed on cans and N is the number tests.
RESULTS AND DISCUSSION
In order to study the effects of different wind conditions, operating pressures,
various sprinklers layouts and spacing on water distribution uniformity in sprinkler
irrigation system a research project was conducted under 3 different wind velocities
(0-5, 5-7 and > 7 m sec-1), using 3 operating pressure (35, 40
and 45 m), three spacing on the lateral pipeline (15, 18 and 21 m) and 3 different
layouts (square, rectangular and triangular). Table 1 shows
the effect of various operating pressures on uniformity coefficient of sprinkler.
As seen from Table 1, when the operating pressure moves from
35 to 40 m (an increase of 14%), the coefficient rises for 6.2%. One can infer
that the relation here is not linear and with lower pressures, the slope is
steeper. Based on Keller (1983) study, in a given sprinkler
as the operating pressure lowers, the dispersion is intensified and water drops
hit the ground with greater effect but this will decrease the water distribution
uniformity, therefore, he suggested that the lower operating pressure occurs
when sprinklers spacing is lower. He also concluded that the most effective
factor in reducing coefficient uniformity in low operating pressure condition
is the relatively excessive sprinkled water in the predefined dispersion range.
Pressure enhancement will decrease excessive sprinkled water within due range
of water dispersion leading to an improved water distribution uniformity coefficient.
If the given sprinkler has a pressure of 40 to 50 m, the coefficient will reach
beyond 80%, which is acceptable to almost all the designers and manufacturers.
Keller (1983) study also did not recommend an operating
pressure more than 45 m and the results agree with those of similar researches
about moderate and high pressures.
||Water dispersion coefficient uniformity in various operating
||Effect of different sprinklers layouts on water dispersion
It is suggested that in order to specify the optimum amount of operating pressure
a wider range of pressures is tested.
Effects of sprinkler layouts on water dispersion uniformity coefficient:
As it can be seen in Table 2, the square layout enjoys the
higher water dispersion uniformity and the rectangular layout has the lowest
coefficient. Although, this coefficient for triangular arrangement is higher
than that of rectangular arrangement, due to persisting operational problems
such as displacement of the pipelines in the semi-movable system, this configuration
is not used in this system, however, in solid-set systems it is used with better
efficiency and higher uniformity coefficient. One of the decisive factors in
raising the coefficient uniformity is the extent of overlapping of the sprinklers.
Overlapping in square layouts are almost the same in all directions while in
a rectangular arrangement they differ in latitudinal and longitudinal directions.
Tarjuelo et al. (1994) investigated this issue
in their studies and concluded that square arrangements, in comparison to rectangular
arrangements, have a higher uniformity coefficient. It should be noted that
the findings of the present study backed up the results of Tarjeulos studies
suggesting that to the possible extent square and equilateral arrangements (where
sprinklers spacing on the lateral pipes and the spacing of lateral pipes on
the main pipes are equal) be used. To find optimal pressure, it is recommended
to investigate more operating pressure on distribution uniformity. Martinez
et al. (2004) analyzed the influence of different design and performance
factors, such as subunit arrangement, lateral spacing, working pressure, average
application rate and application efficiency of water application cost, in a
permanent set sprinkler irrigation system. The results showed that the most
important factor is sprinkler spacing.
||Coefficient effects of sprinklers spacing on water distribution
||The effects of wind velocity on water uniformity distribution
Effects of sprinklers spacing on water distribution uniformity coefficient:
As it mentioned in Table 3, the highest coefficient uniformity
was obtained in 15x15 m spacing while the lowest amount was gained in 21x21
m arrangements. In general, one can conclude that by increase of the sprinklers
spacing the coefficient uniformity reduces. The main reason can be relegated
to greater overlapping of the sprinklers at shorter intervals. In order to better
determine this, the relation of the sprinklers spacing to the distribution
uniformity range was calculated and different spacing's were plotted for coefficient
uniformity (Table 4). The results showed that by lowering
the spacing of the sprinklers installed on lateral pipes a reducing the relation
of intervals to dispersion range, coefficient uniformity increased. After studying
various arrangements, proposed several optimum spacing: for square and rectangular
arrangements the optimum spacing of the sprinklers to the dispersion diameter
should be respectively 0.4x0.6 and 0.5. Keller (1983)
also suggested a general rule for arrears with moderate wind condition. According
to him, for square, rectangular and triangular arrangements the relation of
spacing to the dispersion range should be respectively 0.5, 0.4x0.67 and 0.62
at best. For low wind speed (up to 6.4 km h-1) the spacing between
sprinklers should be equal to 60% of diameter of normal spray, for medium wind
speed (6.4 to 12.8 km h-1) spacing should be equal to 50% of diameter
of normal spray and for high wind speed (above 12.8 km h-1) the spacing
should be equal to 30% of the diameter of normal spray (Shanmugam,
1990). The present study emphasized the fact that if the above spacings
are applied, coefficient uniformity will rise to approximately 80%, which is
confirmed by almost the designers and manufacturers of irrigation systems. The
results agree with those by Keller (1983) researches.
Cuenca (1989) also reported that CUC values generally can
increase when lateral spacing decreases, but results in increased capital costs.
Vories and Von Bernuth (1986) claimed that reducing
sprinkler irrigation lateral and sprinkler head spacing increases CUC.
Effects of wind velocity on uniformity coefficient: According to Table
4 as a result of wind speed increase the coefficient uniformity decreased
in all parameters (sprinklers layouts, operating pressures, etc.). Several authors
have reported that wind is the main environmental factor affecting sprinkler
performance (Seginer et al., 1991; Faci
and Bercero, 1991; Tarjuelo et al., 1994;
Kincaid et al., 1996; Dechmi
et al., 2003b). These references have led to two firm conclusions.
First, applied water is lost partially by evaporation, particularly through
drift out of the irrigated area second, under windy conditions, the water distribution
pattern of an isolated sprinkler is distorted and narrowed. Therefore, the CU
generally shows a tendency to decrease as wind speed increases. Since, it was
not possible to compare the results of different parameters at a steady wind
velocity, the comparisons were made using three different wind conditions, low
(0-5 m sec-1), moderate (5-7 m sec-1) and high (>7
m sec-1) winds. In moderate wind condition (5-7 m sec-1),
the coefficient was reduced by 2.2% compared to low wind (0-5 m sec-1).
With the increase of wind velocity up to 7 m sec-1, coefficient uniformity
in relation to the increase of wind speed reduces linearly and the slope of
coefficient uniformity and wind velocity curve goes almost steadily. However,
in high wind speeds of more than 7 m sec-1 the coefficient drops
sharply (by 17% as measured in this study). Hart (1965)
in his research found out that the effect of coefficient uniformity reduction
as a Consequence of wind velocity increase for nozzle of 3.16 mm and at 9x15,
9x18, 12x18 m intervals is linear. Dechmi et al.
(2003a) in his study have reported that wind velocity is the main environmental
factor affecting sprinkler performance. For wind speeds beyond 2.1 m sec-1
the value of CU is clearly affected by the wind speed. Urrutia
(2000), under similar experimental conditions, found a decrease in the CU
when the wind speeds exceeded 3.5 m sec-1. This value almost doubles
the threshold proposed by Faci and Bercero (1991).
Three general conclusions can be inferred, from this study:
First, as a result of wind speed increase the coefficient uniformity decreased
in all parameters (sprinklers layouts, operating pressures, etc.). Under windy
conditions, the water distribution pattern of an isolated sprinkler is distorted
and narrowed. Therefore, the CU generally shows a tendency to decrease as wind
speed increases. Second, the highest coefficient uniformity was obtained in
1x5x15 m spacing while the lowest amount was gained in 21x21 m arrangements.
In general, one can conclude that by increase of the sprinklers spacing
the coefficient uniformity reduces. The main reason can be relegated to greater
overlapping of the sprinklers at shorter intervals. Third, the square layout
enjoys the higher water dispersion uniformity and the rectangular layout has
the lowest coefficient. Although this coefficient for triangular arrangement
is higher than that of rectangular arrangement, due to persisting operational
problems such as displacement of the pipelines in the semi-movable system. One
of the decisive factors in raising the coefficient uniformity is the extent
of overlapping of the sprinklers. Overlapping in square layouts are almost the
same in all directions while in a rectangular arrangement they differ in latitudinal
and longitudinal directions.
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