To obtain the maximum agronomic benefit from irrigation, it is absolutely necessary that the water distributed uniformly reaches the plants present in cultivation. Otherwise, the different degree of humidity present in the soil can lead to situations of water stress in certain parts of the field and excess humidity in others and, therefore, to different growth conditions among the plants, very negative for production purposes:
- shading of taller plants to the detriment of less developed ones
- heterogeneous achievement of maturation with problems for manual and mechanical harvesting
- worse uniformity of color and fruit size
- highly variable product quality, etc.
In extreme cases, excess water can cause, in addition to poor aeration of the root system of plants, also inhibition of plant development, abundant waste in deep percolations, and finally the complete withering of the crop.
Uniformity and fertigation
The poor homogeneity of water distribution is then even more negative in the case of fertigation, due to the enhancement of the heterogeneity of plant growth. Dispensers that distribute too much water risk making too much fertilizer available to plants, in particular nitrogen, which is the main nutrient conveyed with water. The undesirable effect will be the washout of the same element, with consequent shortage for the crops, but with polluting accumulation in the groundwater.
Uniformity and efficiency
A little homogeneous wetting of the crop then always leads to a lowering of the efficiency in the use of water and therefore to an incorrect use of the resource, furthermore it pushes the farmer to meet the needs of the plants to which less water arrives, consequently over-irrigating the others, with further waste (such as water percolations and runoffs) and damage to the crop. In particular for sprinkler systems when the coverage is not uniform, the irrigation cycles are often activated for longer periods of time to compensate for areas with insufficient coverage. This will result in excessive watering in all other areas. Still in sprinkler irrigation, an uneven pressure in the network produces a decreasing rainfall both upstream and downstream of the wing, and from the sprinkler to the final part of the jet and therefore an irregular overlap of the areas wet by the sprinklers.
A wet bulb is defined as the volume of soil reached by the water during watering: the more this volume includes the volume explored by the roots, the more effective the irrigation will be. Due to lack of uniformity along a dispensing wing, for example, the bulb will have different dimensions depending on the position on the wing, and therefore the effect of the dispenser will be different from plant to plant. Several studies have quantitatively highlighted the extra water supplied by an irrigation system that percolates beyond the layer explored by the roots:
Figure 1 – Penetration of water into the ground along a flat dripping line
Figure 2 – Example of the waste of water related to a watering of 6 mm (60 cubic meters per hectare) caused by non-uniformity
Figure 2 represents all the extra water delivered (beyond the red line) and therefore wasted due to the lack of uniformity. For example, for an irrigation intervention of 6 mm (corresponding to a useful volume of 60 m³ / ha), in the case of a plant with little uniformity, a gross volume of water supplied was calculated in the field equal to 92.6 m³ / ha and therefore a wasted volume of 32.6 m³ / ha (the difference between 92.6 and 60 m³ / ha) by percolation in depth. In this case the efficiency is equal to 64%. The waste of water is directly proportional to the waste of energy, i.e. the increase in the electrical supply times of the pumping systems.
Two scenarios have also been calculated which involve, for efficiency increases, on the one hand a saving of water and on the other the possibility of increasing the extension of the irrigable farm area.
For what has been said so far, the hydraulic design of an irrigation system must generally be aimed at achieving the maximum values of uniformity. The ability of an irrigation system to apply water uniformly and efficiently is the factor that most influences both the productive response of crops and the possibility or not of using water and resources in the most effective way. Ultimately, by exploiting modern methods with a rigorous design, it is possible to save between 10% and 30% of water resources (as well as fertilizers) and achieve a uniformity of distribution of over 90%. With an approximate design, the uniformity drops below 60% resulting in an unacceptable lack of delivery in the different parts of the system.
The “precision irrigation” obtained through the best design is not only a purely theoretical objective, but an action that involves important and undeniable reflections from a practical point of view, which involves both the farmer and the relative agricultural production (for the principle of “good agricultural practice”) as well as the technician who installs a system. Modern agriculture is increasingly oriented towards achieving quality objectives, reducing environmental impact and homogeneous distribution of water and fertilizers. In particular, the more the quality and quantity of production involves large variations in the profitability of the farm, the greater the attention paid to these technical aspects.
Case study
From a study carried out in Emilia Romagna on 7708 farms cultivated with drip or spray irrigated orchards, differences in flow rate determine individual excesses or shortages of water on each plant, leading to reduced yields and product quality and to considerable problems of development uniformity and poor contemporaneity of ripening among the trees of the orchard. Even more pronounced problems occur in fertigated orchards: these differences cause the heterogeneity of the doses of nutrients distributed on each plant, with even greater problems in production and quality, in the release of nutrients and in unevenness between the plants in the orchard. An experimental research was carried out by the CER (consortium for reclamation of the Emilia-Romagna canal) both on horticultural crops and on tree crops: by way of example, the experience conducted by the CER on “Cooper” apple trees clearly demonstrates the decrease in the PLV (gross product salable) if in the supply of water we depart from the seasonal optimum of 1471 m3 / ha In fact, a variation in the supply of 10% with respect to the optimal quantity of water is sufficient to have variations in the PLV of 1,000,000 lire in 1986 per hectare (i.e. 882.57 € / ha in 2008 assuming the constant cost of apples and a consumer price index increased by 2.5% per year).
Although the study is not recent, the results are perfectly applicable today and testify to an irrefutable fact: varying, even slightly, the values of the flow rate with respect to the optimal one (and therefore the uniformity of delivery), there are repercussions on productivity and therefore on the profitability of crops for the farm.
