Garden, advanced systems for irrigation

The irrigation methods used for green spaces are basically "sprinkler irrigation and "drip irrigation". The first type of system is mainly used for large areas, football fields, golf courses and parks, while the second is more suitable for smaller surfaces, such as hedges and flower beds. Nowadays, there are also automated systems that plan the water delivery according to the specific needs of each site

Depending on the species and variety, all plants need water, albeit to different extents. The ongoing climate change is bringing about a definite reduction in water precipitation, as well as extreme rainfall events and prolonged drought periods, both of which are harmful to crops. The water retention capacity of the soil, i.e. the reserve of water easily assimilable to plants (RFU), regardless of the nature of the soil, is unlikely to meet the water requirements of crops over the entire year. It follows that irrigation is indispensable for a good vegetative development. However, we must not forget that water is a precious and limited resource, and as such it must not be wasted. Its rational use must therefore be established in order to optimize irrigation efficiency. The market offers several system solutions; you need to know how to choose the one that best suits your needs.

In summary, it can be said that for green spaces, the choice concerns two types of systems: sprinkler and drip irrigation. Sprinkler irrigation is practiced on large surfaces, football and golf fields, parks, etc. If carried out well, it provides total and uniform coverage, even using a limited number of sprinkler heads. On the other hand, there are losses by evapotranspiration, problems in the event of wind, as well as the need for high pressures and long sprinkling throw, which involve a significant amount of power. Drip irrigation, also known as localized irrigation, is often used for limited areas, to irrigate hedges, flowerbeds, shrubs and trees, as well as to irrigate individual containers. It is impervious to wind and has no loss by evapotranspiration, it uses reduced flow rates and pressures with a prolonged watering time and short shifts (1-3 days), so as to maintain optimal soil moisture content. The irrigation efficiency is high (0.8-0.9) and the energy consumption is low. Conversely, the piping can be an obstacle to carrying out some operations.  In this article we will focus on the drip irrigation of gardens in its various forms, examining its components and its management.

Emitters

An essential component of drip irrigation systems are emitters, different depending on whether they are part of drip systems or spray systems. In drip systems, the emitters are drippers, located above or below the soil surface, distributing small volumes of water at the desired points and close to the crop’s roots. They require an operating pressures of 1-1.5 bar and deliver 2-4-8 litres per hour. In the most widespread solution, inside them the water follows a tortuous course in the form of a labyrinth resulting in a reduction in the hydraulic load that was inside the tubing, whereby the water comes out with null pressure and speed.

These drippers belong to two categories: common ones, also known as turbulence drippers, and compensating ones, also called self-regulating. In the former, the flow rate is affected by pressure changes; when the pressure increases, the flow rate increases, and vice versa. Consequently, they are not suitable for very long lines and sloping terrain. The flow rate difference between drippers should never exceed 10%. Self-compensating drippers are equipped with compensation mechanisms that allow you to have constant flow rates independent of the pressure changes. They therefore have a wide range of applications and can be used on very long lines, on sloping terrain and in large-scale systems, always maintaining uniformity of delivery.

In any case, the feature of the two types of dripper is that at the point of impact of the drop on the ground, a “bulb” of moisture is created, whose shape depends on the nature and structure of the soil. The lighter the soil is, the more the “bulb” takes on an elongated shape, while in clay and compact soils the “bulb” expands laterally and tends to assume a spherical shape. This affects the distance between the lines and the distance of the drippers on each line.

Depending on the way in which the drippers are installed, they are defined as “online” when they are installed at the end of branch lines, or “inline” when they are inserted along the tubing, as is the case for drip tubes. The former are suitable for mounting on suspended pipes, while the latter are more suitable for inserting along ground-level lines.

In spray systems, the emitters are represented by sprayers or misters. They operate at pressures of about 2 bar or even higher. The sprayers have calibrated nozzles with a diameter of 0.8 to 2.3 mm, so the flow rate ranges from 20 to 200 litres per hour, with a throw of 1 to 4 metres. They are divided into static-type, capable of circle or arc spraying, and dynamic-type, equipped with a movement element (turbine, hammer) that always waters in a circle. Sprayers too can be common or self-compensating. Misters produce very fine drops and are particularly useful in greenhouses.

Drip lines and hoses

Drip lines are tubes that can be pre-fitted within them with “inline” drippers, also known as long-run drippers. The lines can be flexible or rigid, depending on the material they are made of. The flexible ones are called hoses. The most common ones are equipped with simple holes with a diameter of 40-50 mm, with a spacing of 15-30 cm; the most advanced ones have drippers inside them. Rigid drip lines can have “inline” drippers inserted in pre-set sections, or emitters soldered inside them, which are referred to as integrated drip lines. In both cases the flow rates may be different depending on the type of dripper.

Pipes

The pipes used in the drip irrigation systems are mainly divided in two types according to the material they are made of: vinyl polycarbonate (PVC) or high or low density polyethylene (PE). Rigid PVC pipes are used to convey water under pressure and are characterized by low load losses. PE pipes are characterized by low unit weight and ease of installation, thanks to their flexibility. They are generally supplied in rolls of 50 to 500 metres long, with a wide range of diameters. High-density PE pipes are mainly used for underground lines.

Pumps

Pressurizing the water in the system is normally carried out by means of electric pumps, which can extract water from both surface water and wells. However, motor pumps are also widely used. In any case, the choice of pumping device should be based on the type of system. The technical data of a pump to be considered are: flow rate (Q), expressed in litres/hr or litres/min; power (P), expressed in watts or kWs; prevalence (H), expressed in metres.

The flow rate should be defined according to the number and flow rate of the drippers. For example, in a system with 3000 drippers with a flow rate of 2 litres/h, the pump’s flow rate should be Q = 2 x 3000 = 6000 litres/h, that is, 100 litres/min for a power of 1200-1500 watts. The market offers low-consumption pumps, which are those required for drip irrigation systems, equipped with a control system for all functions and LCD displays for easy reading of system information.

Irrigation computers

The automated systems are equipped with solenoid valves and irrigation computers, with which the irrigation can scheduled by choosing different daily delivery schedules as well as irrigation intervals with variable duration. A rain sensor and a moisture meter can be connected to the computer to avoid water waste.

Distance between the lines and the drippers

In drip systems, the goal is to bring water to the rhizosphere area in a uniform way. It follows that, in creating the system, it is necessary to consider the root development of the crop and the nature and structure of the soil. In the case of a lawn, the “bulb” of water coming out of the drippers will have to cover a depth of about 15 cm, bearing in mind what has already been stated, namely that for clay soils, the “bulb” tends to expand laterally, while in a basically sandy soil it tends to go deep. For the same flow rate, in the first case the drippers will be more distant from each other compared to the distances that will have to be used in light soil. This is because the flow rates of the drippers are fixed and not variable. Therefore, the lighter the ground is, the closer the delivery lines and drippers will have to be. In the case of a system for a grass lawn with 2 litre-per-hour drippers, the distances between the lines should be between 40 and 60 cm, with the drippers inserted 30-40 cm apart along the line. This ensures uniform irrigation throughout the surface. The length of the lines depends on the diameter of the pipe, the flow rate of the drippers, their distance along the line and the topology of the land. In other words, there are no predefined lengths because they have to be evaluated for each case. However, the installation must be carried out so that the water velocity within the pipe is approximately 1.5 metres per second. In the case of trees, irrigation of the individual plants in the garden is made possible with the installation of branch lines.

Drip systems for hanging gardens

The rules already laid out also apply to hanging gardens. In particular, in the irrigation of lawns, the system must include the use of drippers with a flow rate of 1.5-1.8 litres per hour, while the distance between the lines should be maintained within 30-50 cm. The more porous the texture of the substrate is, the closer the pipes need to be. When the lines and drippers are integrated into a geotextile fabric, the flow rate of the drippers is 0.6-0.8 litres per hour and the distance between the lines is 40 cm. This solution is often used for lawns placed on roofs.

Subsurface drip irrigation

One of the disadvantages of drip irrigation with underground lines is that certain lawn maintenance operations such as perforating, soil aeration or digging cannot be performed without damaging the system. To overcome this obstacle, subirrigation is used, using drip lines at depths of 20 to 75 cm. The depth at which the drip lines should be placed depends on the texture of the soil and the depth of the crop’s roots. In this case too, the irrigation operations must be frequent, i.e. several times a day, without ever reaching saturation of the soil. In this way, the movement of water by capillary action is promoted, avoiding percolation losses (capillary subirrigation). Specific precautions and special solutions ensure that the drippers do not get clogged because of soil particles or roots.