Performance of sprinklers in irrigation

Evaluating the performance of irrigation systems, despite being a practice rarely carried out by technicians and rural producers, is important to increase productivity and adequately use water resources.

Irrigated agriculture has been an important strategy for optimizing food production, promoting sustainable development in the countryside, generating employment and income. However, the availability of water has become increasingly limiting and must be used judiciously, not only aiming to increase productivity and final quality of the product, but also the appropriate use of water resources.

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For irrigation to be efficient, it is essential that the systems present uniformity in water application, equal to or above the established limits. Once an irrigation project has been installed, it is necessary to check whether the initially predicted conditions are confirmed in the field. To do so, the pressure, flow and water depth conditions applied must be evaluated.

Evaluating the performance of an irrigation system is a fundamental step to be carried out, as it is based on these results that it will be possible to evaluate and adapt the system in relation to the water needs of the crop. However, evaluating the performance of irrigation systems in cultivated areas is a practice little carried out by technicians in the area and rural producers, due to a lack of guidance and even knowledge.

One of the ways to standardize irrigation systems is the correct choice of sprinklers, adapting the water needs of the crop to the operational characteristics of this equipment.

There are cases in which irrigating producers are guided only by the catalog provided by the sprinkler manufacturer to determine the flow rate, which can lead to the application of an excessive or even deficient water depth compared to the replacement water depth required for the irrigated agricultural crop.

On some agricultural properties, producers do not carry out inspections of their irrigation system - which is generally used for years without any or reduced inspection of its components. Studies that evaluated sprinkler irrigation systems demonstrate that the main defects in irrigation equipment occur primarily in leaks along the system's pipes, the second problem with the greatest impact is that observed in the sprinklers and finally are those found in the motor pump set.

The sprinkler flow rate is a function of the nozzle diameter and the operating pressure. When the pressure is excessive, the average droplet size will decrease due to the fragmentation of the water jet, also reducing the range of the applied blade.

The lack of review and maintenance of the system is one of the main causes of poor water distribution in the soil, resulting in losses in potential productivity when compared to cultivable areas that use adequate irrigation management, with the entire operational system functioning effectively.

In an experiment carried out at the Agricultural Hydraulics Laboratory, at the Federal University of Santa Maria (UFSM), the variation in the flow of sprinklers subjected to different service pressures was tested. This analysis was based on a technical standard (NBR ISO 3951) provided by the Brazilian Association of Technical Standards (ABNT), which standardizes the procedures that must be carried out in sprinkler flow assessments. An analysis of the variation in sprinkler manufacturing was also carried out.

The commercial sprinklers used were the P5 model from the Agrojet brand. The flow rate was determined using a water meter with a known volume, and the time for one complete revolution was measured using a stopwatch. The working pressure was monitored by a pressure gauge installed on the riser pipe next to the tested sprinkler. The expression Q = V/T was used to obtain the measured flow rate, where Q is the flow rate (L/h); V is the volume (liters) and T is the time (hours).

The tests were carried out by testing five working pressures (10mca, 15mca, 20mca, 25mca and 30mca), according to the catalog provided by the sprinkler manufacturer. Flow rate repetitions were performed for each pressure value in order to increase data reliability. A plastic box was used to contain the water when carrying out the tests.

Table 1 presents the theoretical and observed flow values ​​of the sprinkler when subjected to different service pressures.

According to Table 1, we can observe that there are small deviations between the theoretical flow and the observed flow. The biggest differences occur at the lowest pressures tested due to the fact that they are more difficult to stabilize, thus causing a greater oscillation in the flow.

The standard suggests that for sprinklers with nominal flows above 250 liters/hour, the flow variation should not exceed 5%. As a result of the test, a flow variation of 4,4%, 2,7%, 1,17%, 1,03% and 0,69% was obtained respectively for service pressures of 10mca, 15mca, 20mca, 25mca and 30mca.

Based on the classification of the standard and the results obtained, it is observed that all flows at different service pressures are in accordance with the technical regulations proposed by ABNT.

Another factor that can interfere with the flow rate of the sprinkler is the variation in its manufacturing, which can lead to differences in the flow rate obtained in the field in relation to the theoretical flow rate. This is due to the precision with which this equipment is manufactured. To evaluate the variation in sprinkler manufacturing, the methodology proposed by Solomon (1979) was adopted, using the relationship between the standard deviation of the sprinkler flow and its average flow subjected to tests with the pressure under study. The result was an average manufacturing variation of 1%, which, according to the test author, is classified as excellent, demonstrating the excellent precision with which these sprinklers were manufactured.

Figure 1 shows the theoretical (l/s) and observed flow (l/s) values. Through the visual analysis of Figure 1, it is observed that there is a high correlation between the observed and theoretical flow values, obtaining a high coefficient of determination (R²) of 0,9989, proving an excellent tendency for the theoretical flow to reflect the values of flow observed in the field.

In view of this, and based on the tests carried out experimentally, we can conclude that the use of the flow rates proposed by the sprinkler manufacturer is in accordance with the technical regulations proposed by ABNT, proving that we can use the flow values ​​shown in the catalog without prejudice to determining the flow applied in irrigation systems according to each service pressure.

It is important to highlight that maintenance and necessary repairs must be carried out periodically on irrigation equipment so that there is greater efficiency and uniformity in the application of the water depth, adjusting the use of water resources for irrigation purposes.

Anderson Crestani Pereira, Adroaldo Dias Robaina, Marcia Xavier Peiter, Marcos Vinicius Loregian, Bruna Dalcin Pimenta, Jardel Henrique Kirchner, Luis Humberto Bahú Ben, Jhosefe Bruning, Wellington Mezzomo, Rogério Ricalde Torres, Irrigation Engineering Laboratory - UFSM

 

Article published in issue 169 of Cultivar Máquinas