Efficiency in central pivot water distribution

Irrigation is a key factor for Brazilian producers to produce up to three harvests a year, as irregular rainfall distribution has a direct impact on productivity. Thus, a study evaluated the efficiency of three central pivots after 15 years of use and found the main problems that the system presented at the end of this period.

In Brazil, a large part of its territory has climatic conditions favorable to intensive agriculture, and in some places it is possible to obtain up to three harvests per year, limited only by irregularities in rainfall distribution. Irrigation, in turn, can guarantee satisfactory water conditions throughout the year, thus enabling cultivation in the driest season. Of all the existing irrigation methods, sprinkler is the most used. It has important characteristics such as high uniformity of water distribution, irrigation efficiency, good control of the volume of water applied and adaptability to different crops, reliefs and soil types.

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The increase in production in irrigated agriculture is directly related to the use of efficient irrigation systems, although in most of these areas the volume of water applied is greater than that actually necessary for the relevant food production. Therefore, it is important to use properly designed equipment that guarantees a good return on the investment applied by the producer, as it allows a better rational use of water, optimizing the quantity to be applied, as well as the right time to apply, increasing the number of harvests in a given period.

Currently, Brazil has a total irrigated area of ​​6.200.000ha, of which 1.275.000ha are centrally pivoted. This central pivot irrigation system requires not only knowledge of the crop, soil, available water resources and climate data, but also the quantification of the water to be applied and mainly its efficiency in distribution and uniformity. In view of this, the aim is to use more efficient equipment, which is essential for saving water and energy without, however, putting the yield of the planted crop at risk. The non-uniformity of water generates irregular areas, with a deficit or excess of humidity, and with the adjustment of uniformity there is an increase in expenses, such as operation and maintenance of the systems.

The elements that affect water distribution uniformity are wind, air temperature, relative humidity, evaporation, emitter height, machine travel speed, spacing and type of sprinkler. When these factors are monitored correctly, water savings of 25% can be achieved in the system, without harming the crop.

To calculate this distribution efficiency, many coefficients are used to express the water applied by this irrigation system, the most used being the one proposed by Christiansen (1942), better known as Christiansen Uniformity Coefficient (CUC). The analysis, in turn, aimed to evaluate the efficiency and uniformity of water application of three central pivot systems by comparing the date of their installation, where it is expected that there was close to 100% uniformity, after 15 years of use. intensity of machines.

The property where the test was carried out belongs to the Damha Group, in the municipality of Pereira Barreto, in the interior of São Paulo, with average annual precipitation and temperature respectively of 1.300mm and 24,1ºC, the local climate type is humid tropical with a rainy season in summer and drought in winter. The property has a complex structure of equipment and irrigation systems, whose self-propelled sprinkler irrigation system is the most accepted by producers, with the objective of meeting the food needs of the livestock activity implemented on the property, with the result of production of three harvests per year, approximately.

In 2000, during the installation of the equipment, all acquired from the same manufacturer, performance evaluations were carried out, with the objective of evaluating these when they were still new, in order to classify them according to the uniformity presented and all the equipment proved to be in accordance with what is designed and desired. After approximately 15 years, new evaluations were carried out with the aim of remedying possible problems with nozzles and tubes, which caused uniformity rates below the desired level, with the biggest problems being compromising crop productivity and not meeting demand. water. Assessments were carried out adopting the highest point, as it would be the part of the areas with the greatest pressure problems. Two radial lines of rain gauges were used, arranged at an angle of three degrees to each other, and the collectors were positioned 0,3 meters high.

The volume collected was measured as quickly as possible to reduce the effect of evaporation. Rain gauges were also installed with 10mm of water on each pivot to measure evaporation during the test and determine losses, by difference. The pivot speed was adjusted to walking by applying 10mm of blade for the test. Using a stopwatch, the time required for each piece of equipment to travel between the two lines of rain gauges was measured.

During the evaluation, readings were taken of the initial pressure (pivot base) and final pressure (manometer installed at the end of the equipment), the speed of the last tower, the wind speed, the speed (%) of the test, the blade of the project, the irrigated radius, the irrigated area and the precipitation that occurred. To determine the climatic variables, a mini agrometeorological station was used, installed at a height of one and a half meters from the ground, located one kilometer from the test site, where it was possible to determine the air temperature, relative humidity, direction and speed of the wind.

The results showed a degree of equipment wear due to intense use, mainly in pivot C, which had a uniformity of less than 60%, this fact may be related to it being the only one to have three harvests in the year, thus causing the equipment does not have adequate maintenance and repair time. When comparing the calculated design flow and that obtained in the test, it can be noted that all three pivots did not reach the desired flow, which can be justified by the time of use and some factors causing loss of flow, such as leaks and clogging of sprinklers. .

These central pivot type irrigation systems work with the same angular velocity, however the linear velocity differs in each of the lateral sprinkler line towers and so that the irrigation depth is the same throughout the area, these systems are irrigated with combinations of distinct nozzles along the lateral line. To do so, they require rigorous hydraulic sizing of the emitters, where nozzles have increasing diameters from the pivot point (Figure 1).

The pivots plants classified as A, B and C had, respectively, the following CUCs: 82%, 76% and 57%, these values ​​are classified as low by standards from ABNT. The uniformity of water application to pivots can be affected by different ways, from the water exiting the nozzle holes to the ground surface and also outside the reach of the equipment. Therefore, some equipment conservation measures are necessary so that it can be optimize application efficiency and improve the system, reducing wasting water and increasing the useful life of the machine. To the farm analyzed in question, the idea would be to exchange leaky tubes and emitters that are clogged, as these, in addition to increasing the pressure until the end of the piping, reduce the blade in some points of the irrigated area and accelerate the diffuser. Correct maintenance and handling are important to achieve even after a period of 15 years, still maintain the productivity of crops irrigated with this irrigation system.

Yane de Freitas da Silva, Univ. State Julio de Mesquita Filho

Article published in issue 171 of Cultivar Máquinas