With the term drainage, in agricultural hydraulics, we mean the ability of a soil to drain water.
Drainage, or subsurface drainage, is the complex of natural or man-made systems that oversee the drainage and disposal of excess water in the soil in the deeper layers of the soil.
The term drainage is indicated indifferently to describe the ability of a soil to drain water but also the arrangements made by farmers, in an artificial way, to facilitate the outflow of surface water.
The drainage capacity of a soil is then linked to the pedological characteristics of a soil and the relationship between the solid fraction (particles) and the aeriform fraction (porosity and microporosity).
Particle size also affects drainage capacity in the sense that, in general, soils with coarser particles (such as sandy soils) have a greater drainage capacity while soils with finer particles (such as clay soils) have a lower capacity. drainage.
The drainage capacity is also influenced, among other characteristics, by the percentage of organic substance in the soil and by the agronomic processes carried out on it.
Furthermore, a soil is naturally well drained when the water table is at an adequate depth and the porosity is such as not to hinder the percolation of excess water.
Artificial drainage methods are applied using rudimentary systems or drainage networks designed for the purpose. In the latter case, the term tubular drainage is generally used.
Artificial drainage systems, therefore operated by farmers, can be of various kinds; from the most rudimentary and provisional ones to the most technically advanced ones and of higher cost and persistence.
Rudimentary drainage systems consist in the use of scissor tools, which perform a vertical cut of the soil, provided with terminal shapes capable of tracing a modeled canaliculus (“mole plow”) at the working depth. These interstices act as drains, collecting the water that percolates and conveying it into the drains.
By their nature, a system of this type has a limited duration, for the most part, to a crop cycle.
It is also possible to create, especially before the planting of orchards or vineyards, extraction systems made with trenches that are dug in correspondence with the row, on the bottom of which a bed of stones and branches extends, so as to form a layer of greater permeability able to facilitate the draining of the percolation water and contain the stagnation in the area explored by the roots.
This technique, used mostly in the past, has a limited duration, mostly a few years, because over time the fine particles of earth tend to fill and clog the interstices at the bottom of the trench.
Today there is a tendency to use tubular drainage more. This type of drainage is longer lasting and consists in digging ditches on the bottom of which perforated pipes (drains) are spread at the desired depth.
These drains are made of plastic material and are provided with a filter coating, made with coconut or synthetic fiber, in order to prevent the fine earth from occluding the pores.
In this case, the installation is carried out by giving the drains an inclination sufficient to guarantee the flow of water towards the collector ducts. The distance between the drains is related to the porosity of the soil: the net is denser in soils with a fine texture.