Water for arable land

For the farm today, it is essential to have management models capable of increasing and preserving water in the soil.

Lorenzo Benvenuti

Without water, there is no agriculture. In fact, water must be available during the entire crop cycle in increasing doses related to its development, the intensity of direct evaporation from the soil and transpiration from the plant. These processes are more intense when the air is dry, when there is wind, when air temperatures are high. It is therefore evident that climate change affects these phenomena by amplifying their effects. To these parameters, direct solar radiation must be added, which acts as an accelerator favouring the passage of the water molecule from the liquid to the gaseous state; however, this will only occur if the air is able to take in water vapour, and therefore if it has a relative humidity lower than the saturation one.

In order to counteract losses through direct evaporation from the soil, it is necessary to give up deep tillage that exposes the soil to the air, to increase the coverage of the soil with plant residues, and to modify the planting spacing of crops in order to achieve earlier coverage of the soil by the crop itself.

Obviously, action must also be taken in the sense of increasing the water reserves incorporated in the soil, keeping it permeable, by means of a few targeted decompaction operations, even deep ones, slowing down the flow of rainwater so as to increase the infiltration phenomenon, using the farm drainage network also for conservation purposes, at least in seasons when this practice does not cause disturbance to the crops present. Covering the soil with plant residue also performs a positive action in this sense, protecting the soil from the kinetic action of the raindrop and in so doing maintaining the surface porosity intact.

Once these agronomic methods have been implemented, which allow the soil water reserves to be protected and accumulated in greater quantities, when water is available for irrigation, it is a good idea to prearrange irrigation systems suited to the farm structure, to the local rainfall pattern (even though the last decade there have often been years with unpredictable rainfall patterns both in terms of quantity and seasonality) and the type of crops.

Selection Criteria

Restricting our investigation to arable crops, we can identify, among the most common irrigation systems, the most efficient ones, meaning with this term the relationship between the water distributed and that which performs an agronomic function directly useful to the crop.

Amongst these methods, phreatic irrigation, when using the tubular drainage system, is an excellent and relatively inexpensive solution given that the cost of the drainage system should be properly distributed among several expenditure items. Its limitation lies in the fact that in the initial stages of cultivation, when the plants’ root system is poorly developed, this system does not provide any support. In fact, phreatic irrigation requires water to be introduced into the drainage system until it reaches the drains along its entire length. The water then flows out of the drains and by capillarity permeates the soil without, however, reaching the surface. It works best in soils rich in micropores, such as clay soils with a good supply of organic matter. The buried hose uses the same principle, although in this case there is an active pressure that ‘pushes’ the water into the soil; moreover, in this type the depth of deposition is sometimes lower than that of the drains. The georeferencing of their deposition can allow for shallower depths, thus limiting decompaction operations in strips without a hose.

On the other hand, the irrigation systems capable of meeting the needs of the crop throughout its entire cycle are the sprinkler type and the drip type with hose placement above the soil. The latter are the most efficient in terms of water saving, but require the annual renewal of hoses: this represents a significant waste in environmental terms. However, some, unfortunately few, manufacturers have set up effective systems for recovering spent hoses based on a reward system. Drip systems require the hosepipe to be laid, usually in alternating rows, shortly after the crop emerges. The water is sent inside through pumps; the operating pressure is very low, but the water must be free of suspended materials (to avoid the clogging of the nozzles) and it is therefore subject to very accurate filtering, even with different types of filters placed in succession.

This irrigation method, which is very popular in horticulture, due to its cost and laboriousness, is less so on arable crops, where it nevertheless finds several interesting applications.

Sprinkler systems use self-propelled irrigation machines, the so-called rotary irrigation machines, or systems with a translating or pivoting wing. To the latter is dedicated the following article; here it is enough to say that they are the best solution when the farm has the dimensional and spatial characteristics to be able to install them. In fact, although today there are solutions that allow these systems to be adapted to the characteristics of cultivated areas, it is also true that these, in addition to being flat and of a good size (the larger the surface area, the lower the cost per hectare), must be characterised by a high degree of continuity, free of obstacles or in any case with obstacles placed at the edges. These conditions are not so common in our country.

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