INTRODUCTION
As the necessity, for conservation of water resources increases, especially in areas of limited water supply, more precise knowledge about application efficiencies of sprinkler irrigation systems is required. Irrigation principles and practices for sprinkler irrigation have advanced to the point that water application efficiency is primarily controlled by the amount of evaporation and drift losses. More knowledge about water losses associated with sprinkler irrigation can significantly help towards assessor the overall application efficiency. Water can be applied at any suitable rate by sprinkler irrigation system during the day or night. The efficiency of the sprinkler irrigation depends on the losses which take place during and following the irrigation. When water conditions, some of the spray droplets are carried away from the sprinkled area, where a portion of it is intercepted by vegetation or by bars soil outside the sprinkled area.
Losses from sprinkler irrigation in arid and semi-arid areas may amount to a considerable portion of the water discharged by sprinklers. The magnitude of evaporation and drift losses depends upon the climatic and operating conditions. To obtain an insight into the magnitude of these losses, it is of paramount importance to find the factors affecting them. If these relationships can be determined, the conditions for sprinkling can be defined and functional equipment may be designed. This report describes the results of evaporation and drift-loss experiments conducted under different operating conditions in order to determine the relationships between the losses and the factors affecting them.
Christiansen (1942) determined evaporation losses by
utilizing the catch-can method and found that losses ranged from 19 to 42%.
However, no correlations of losses with climatic variables were reported. Frost
and Schwalen (1955) found that losses vary approximately proportional to
wind velocity and operating pressure and inversely proportional to relative
humidity of the air and nozzle size. A close relationship between losses and
vapor pressure deficit of the air was also obtained by these researchers. While,
Frost and Schwalen (1955) found spray losses as high
as 45% under extreme conditions of bright sunlight, high temperatures and low
humidity prevailing in Arizona, other authors signaled maximum losses of 30%
(Yazar, 1984). Hermsmeier (1973),
using the salt-concentration technique, found that air temperature and rate
of application were better factors for estimation spray evaporation than wind
velocity or relative humidity. Yazar (1984) estimated
evaporation losses from the sprinkler irrigation under various operating conditions
from the electrical conductivity measurement of the supply water and the water
in the catch-containers and found that wind velocity and vapor pressure deficit
were predominant factors affecting evaporation losses. Scientific literature
has treated the problem of spray evaporation as one of minor relevance, mainly
attributing to wind drift the global water mass reduction occurring during the
air path of the droplet (Edling, 1985; Kincaid
and Longley, 1989; James, 1996). The experimental tests,
however, have clearly showed that aerial spray evaporation is a relevant cause
of water sink (Lorenzini, 2002). Spray evaporation from
a droplet has been attributed to air relative humidity or to water vapor concentration
and or gradient in the air with respect to the droplet position (Edling,
1985; Thompson et al., 1993; Kinzer
and Gunn, 1951; Kincaid and Longley, 1989). A study
by Lorenzini (2002) reviewed recent scientific literature
regarding experimental tests on sprinkler droplet evaporation. Among the studies
reported, the most relevant include Zanon and Testezlaf (1995)
and Zanon et al. (2000), who analyzed the problems
of experimental techniques for automatic systems of water collection at ground
level and the methods for measurement of the water collected in order to reduce
experimental and wind drift. In the average meteorological conditions of Zaragoza
(Spain), the seasonal average spray losses for the solid-set system would be
15.4 and 8.5% during day and night irrigations, respectively (Playan
et al., 2005).
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, during the period of June (2006) through February (2007).
In order to obtain logic and reliable results 75 tests were carried out in different
hour during day and night so, the correlations and diagrams would represent
a wide range of hydraulic and climatic conditions. Water was supplied from a
permanent irrigation system. The commercial (jaleh model 3) with two nozzles
(7.32x3.32) impact sprinkler was located on the lateral. Riser allowed the sprinkler
to be placed 6.562 ft above the catch can openings. The system was operated
at three pressure levels of 345.1, 394.4 and 443.7 kPa. A total of 100 catch
containers on a 9.8x9.8 grid system were located on both side of the lateral
a round the sprinkler. Figure 1 shows an arrangement of rain
gages for such a test. The area a round 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.
|
| Fig. 1: |
Arrangement of rain gauges (catch-containers) a round the sprinkler |
The measurable parameters in this study included: temperature,
relative moisture, wind velocity, operating pressure, flow discharge and volume
of water from the sprinklers accumulated in the containers. Metrological parameters were measured by installing a 3-cup anemometer and
dry and wet thermometers. The sprinkler’s flow discharge was accurately determined
by using a volume meter and a chronometer. Wind velocity and direction at 6.6
ft above ground were measured with a recording 3-cup anemometer and wind vane
for a time period equal to the duration of a test, which was about 1 h. Dry
and wet-bulb temperatures were measured with a sling psychrometer at 10 min
intervals in the upwind direction during each test. A portable indicating bridge
with a conductivity cell was used to measure the electrical conductivities of
both the water supply and that in the catch-containers. Evaporation losses were
determined from the relationship.
where, E is the evaporation losses (%) and EC1 and EC1
are the electrical conductivities of the samples of the water in the catch-containers
and of the supply water, respectively. Minimum working time of the sprinkler
during each treatment was 1 h.
RESULTS AND DISCUSSION
A total of 75 evaporation-loss tests were conducted and the results of the
selected observations are given in Table 1.
| Table 1: |
Evaporation losses under various climatic and operating conditions |
 |
Evaporation losses ranged from 4.4% at a vapor pressure deficit of 5.8 mbar
and wind velocity of 6.711 mil h-1 to 8.9% at a vapor pressure deficit
of 1.2 mbar and wind velocity of 22.82 mil h-1 multiple regression
analysis of the data was performed by using a non-linear calculating technique
and the following expression for predicting the evaporation losses from sprinkler
sprays was obtained:
where, E is the evaporation losses expressed as the percentage of the total volume discharged by sprinklers; u is the wind velocity at 2 m (mil h-1); (es-e0) is the vapor pressure deficit, in which es and e0 are the saturation vapor pressure and the actual vapor pressure of the air (mbar); Ta is the air temperature (°C) and P is the operating pressure (kPa).
When the air temperature factor is omitted, then the prediction equation becomes:
Since the air temperature term is indirectly included in the vapor pressure
deficit term, the resulting multiple correlation coefficient remains unchanged.
When the operating pressure factor is deleted, the resulting equation is given
by:
Considering only the wind velocity and vapor pressure deficit in the analysis,
the equation becomes:
The relationship between the evaporation loss and wind velocity alone, shown
in Fig. 2, is given by:
|
| Fig. 2: |
Relationship between the evaporation losses and wind velocity |
|
| Fig. 3: |
Relationship between evaporation losses and vapor pressure deficit |
The relationship between the evaporation loss and vapor pressure deficit, shown
in Fig. 3, is:
The results of the multiple regression analysis of the data indicate the wind
velocity and vapor pressure deficit are the predominant factors affecting the
evaporation from the sprinkler sprays. The operation losses, for the pressure
levels used in this study. Since, the two nozzle sizes had very similar flow
characteristics at each pressure level, the test were considered as replications.
Therefore no conclusions can be drawn with respect to the nozzle size.
The result of this study is closely in support with Yazar
(1984) and Frost and Schwalen (1955). Wind not only
causes spray droplets to be carried beyond the sprayed field but also distorts
the distribution pattern of the sprinklers; which in turn results in a very
uneven application of water over the field. Combined losses from a sprinkler
system for a given set of operation conditions have been estimated by using
the results obtained from the experiments. Combined losses ranged from 4.4 to
8.9% of the applied water.
ACKNOWLEDGMENT
The authors should thank the Islamic Azad
University of Sciences and Research Branch of Ahvaz
and Research and Standards Office for Irrigation and
Drainage Networks of KWPA for their financial support
and assistance.
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