Irrigation system increases agricultural productivity

The subsurface drip system is defined as an irrigation system that uses the soil as a means of propagating water, in which the emitters are below the soil surface, positioned within the layer where the plant root system is located.

In the past, subsurface drip irrigation was used in high-value crops (fruit trees and ornamental plants). However, with the evolution of manufacturing technology for the components of this system, there was an increase in its useful life and, therefore, it began to be used in lower-value crops (commodities).

The longevity of the system is linked to the quality of the hydraulic design, the implementation of the system and proper management. Below, we will address some relevant points for the sizing and maintenance of a subsurface drip system.

Benefits 

Water conflict is one of the factors that most concerns irrigators in arid and semi-arid areas. In view of this, subsurface drip is a viable alternative to increase water use efficiency. Water savings are generally around 40%, as the surface soil layer (0cm-15cm) remains dry, which considerably reduces water evaporation.

In addition to greater water efficiency, it is possible to reduce electricity costs, which have become more expensive for producers in recent years. This system also allows for the reuse of wastewater in a safe way for plants, producers and consumers. However, the viability of this practice must be assessed for each production scenario.

The application of fertilizers, insecticides, fungicides, nematicides and herbicides using irrigation water as a vehicle is a viable technique for subsurface drip systems, as it increases the efficiency of both use and action, since the product is applied close to the roots. Furthermore, it reduces the demand for labor and reduces soil compaction due to less machinery traffic in the area.

Subsurface drip irrigation also reduces the incidence of foliar diseases and weeds, and as a result, reduces the number of applications and the costs inherent to these practices. The drip tubes are protected by a layer of soil, which reduces deterioration from solar radiation and damage caused by crop treatments and mechanization, resulting in an increased useful life of the equipment. This greater longevity of the equipment also contributes to the amortization of investment costs.

All these advantages lead to an increase in productivity. This increase can reach 30% in industrial tomato crops, 50% in coffee, and in sugar cane, up to 12 cuts can be achieved with productivity above 180 tons/ha.

Disadvantages

On the other hand, this system has some disadvantages, including the increased investment in water filtration systems to prevent clogging. Water quality can be a limiting factor and, in some cases, require special treatment to prevent the formation of precipitates. As a result, there is an increase in special components in the irrigation system, which increases implementation costs. 

In the field, the surface layer of the soil generally remains dry and this can reduce the germination or establishment rate of transplanted seedlings, requiring rain or mobile devices for artificial water application. Another factor that can hinder plant establishment is the accumulation of salts that can occur above the emitters, depending on the quality of the irrigation water. 

Emitter application rates can exceed the capacity of some soils to redistribute water. This can result in water reaching the soil surface through preferential paths, causing undesirable wet spots in the field. Another problem is fixed spacing between irrigation lines and between emitters, as it restricts the number of crops that can be used for succession or rotation. 

Despite its many benefits, subsurface drip irrigation is still a technology that is little used in Brazil, and consequently, technicians and producers have little experience with this system. Therefore, design errors are more difficult and expensive to resolve in the field, since the emitters are buried in the soil, and the problem often appears at the end of a crop's production cycle.

Project

The sizing criteria are similar to those for the surface drip system, except for some mandatory components, which must be incorporated into the subsurface drip system. It is important to emphasize that the project must be carried out by a qualified professional so that it is possible to meet the water demand of the crop or group of crops used in succession or rotation, according to the soil attributes and the farmer's preferences.

Filtering

Subsurface drip irrigation systems require good filtration, which means that to ensure good project performance and longevity of the irrigation system, it is not recommended to reduce costs with filtration and water treatment systems. Problems with clogging can be difficult or impossible to remedy in this system.

According to drip tube manufacturers, if the irrigation water contains more than 2 ppm of sand, a hydrocyclone filter must be installed before the main filter to extract the sand. When there are solids (sand, silt and clay) greater than 100 ppm, the water must be left to settle to settle the particles. After the hydrocyclone, it is important to install sand filters to remove the solids more efficiently. And the last filter element (screen or discs) must have a filtration capacity suitable for each emitter model, which generally varies from 140 mesh to 200 mesh.

Sizing

The emitter flow rate should be selected based on the maximum water demand. Generally, most commercial flow rates and typical emitter and dripline spacings provide application rates above the reference peak (crop evapotranspiration - ETc). Emitters with higher flow rates are recommended for this system because they are less prone to clogging and allow more flexibility in irrigation scheduling. 

On the other hand, when emitters with higher flow rates are chosen, the length of the lateral line may need to be reduced to maintain good uniformity. Furthermore, depending on the type of soil, backpressure problems may occur, which generally occurs in heavy soils with low hydraulic conductivity, and the pressure against the emitter makes it difficult for water to escape or find preferential paths in the soil, the so-called “chimneys”, very common in sandy soils and/or soils with cracks.

The recommended fluid flow velocity for sizing lateral lines ranges from 0,3 m/s to 0,6 m/s. For subsurface drip systems, it is feasible to adopt values ​​close to 0,6 m/s, since a higher discharge velocity makes it possible to have better overall cleaning of the materials.

At the end of the lateral lines, it is important to install piping and discharge valves to ensure proper cleaning of the system. If it is not possible to use an automatic system, manual end-of-line devices must be installed. However, these components must be sized so that they do not significantly influence the normal process of the system. It is important to adopt the same sizes for the lateral lines and, therefore, avoid variations in operating pressure.

When it comes to automated final cleaning of lines, it is recommended to insert the discharge valve in the center of the cleaning line, as this helps to reduce pressure losses and, therefore, it is possible to opt for a smaller diameter pipe for this line.

Generally, to choose the diameter of the cleaning line, a value equal to or greater than 25% of the accumulated cross-section of the drip lines is adopted, with this the flow velocity of the lateral line is approximately 0,3 m/s, which is acceptable for the project.

At the highest points of the cleaning lines, double-acting valves should be installed to prevent crushing of the dripperline and suction of soil by the emitters under the condition of depression and to facilitate the expulsion of air from all buried piping. In addition, anti-suction emitters are also recommended to prevent soil suction.

To properly clean the subsurface system, the pump assembly must be sized with sufficient pressure and flow capacity to remove residue from the most critical sector. Therefore, it is important to choose dripper pipes with good resistance to withstand the pressure used in cleaning and to use pressure regulating valves at the beginning of the sectors to ensure normal operating pressure.

Driving

The best way to maintain a subsurface drip system with good irrigation efficiency for many years is to adopt preventive measures, that is, a properly dimensioned and implemented project. As well as the acquisition of quality material. 

There are companies operating in the country that are market leaders in localized irrigation. They offer advanced drip tube technologies that increase producers' confidence in adopting subsurface drip systems. Emitter technologies include: protection against root intrusion, self-compensating emitters and an anti-siphon mechanism, which prevents soil particles from entering the dripper.

However, not all producers have the financial means to purchase high-quality materials. Therefore, we will present some practices adopted to evaluate the irrigation system, routine management to prevent obstruction and effective unclogging actions. 

Efficiency assessment

Generally, drip tube manufacturers guarantee 90% uniformity. However, one of the main disadvantages of the subsurface system is the difficulty in assessing the uniformity of water distribution by the emitters after they have been installed. If it is carried out conventionally, by collecting the sheet, it becomes very laborious, as it will be necessary to dig trenches, making this practice unfeasible in productive areas.

Installing pressure gauges right after the pump set, at the beginning of the sectors and in the cleaning lines to check the system pressure daily, is a viable strategy to indicate the current status of the system, enabling the operator to detect problems in the pumps, leaks in the suction and main line, as well as possible blockages (increase in service pressure) or leaks in the drip tubes (reduction in service pressure). 

Another way to detect clogged emitters would be by checking plants with deficient growth, failures in the field or through UAV images, due to the high spatial resolution, which allows visualizing the heterogeneity of the crop.

Routine management

The recommended routine management for subsurface drip systems is similar to surface drip and micro-sprinkler systems. Localized systems are ideal for automation, however, the presence of the operator in the irrigation area is essential to detect problems. Therefore, it is recommended to walk the area weekly and/or before each fertigation event to check for possible leaks (surface wetting) and, if detected, use appropriate connections to solve the problem.

Weekly and/or after fertigation, it is necessary to clean the drip tubes by opening the discharge valve and applying water for ten to 20 minutes or until the discharge water is the same color as the inlet water. However, this may vary for each property, depending on the quality of the water. In some specific cases, cleaning must be carried out after each irrigation event.

Final considerations

Subsurface drip irrigation is a system that is not widely used in Brazil, and the cost of implementation, together with the lack of experience of technicians and producers, constitutes an obstacle to increasing the areas irrigated with this system. The system has been widely used for sugarcane crops and in the production of pasture seeds, with significant growth in irrigated areas for grain crops. For this, good projects and correct management are necessary to ensure increased crop productivity and greater longevity of the system, increasing the viability of its use.

Management to remedy obstructions

To prevent or reduce root growth in the emitter holes, it is recommended to apply the herbicide trifluralin at a dose of 100 ml/ha via irrigation water for sugarcane crops. For coffee crops, it is recommended to apply it twice a year, once before water stress and once after the rainy season. For tomato plants, it is recommended to apply it at the beginning of the cycle, for 20 to 30 minutes.

Additionally, some emitters are already manufactured with trifluralin or it can be incorporated into the filter components. The use of phosphoric acid (15 mg/L) or chlorine can also help prevent root intrusion or help remediate emitters that are partially clogged by root oxidation. It is worth noting that chemical remediation of clogging is not always feasible, especially when sensitive crops are grown.

*Per Edcassio Dias Araujo, Gustavo Henry da Silva, Job e Fernando France da Cunha, from UFV