Agronomic techniques to rationalize the use of water in agriculture

Mariella Pentimalli - October 11, 2022

* L’immagine di copertina di questo report è stata presa dal sito Water Footprint Calculator, consultabile al seguente link: https://www.watercalculator.org/footprint/farmers-use-drip-irrigation/

In light of the growing problem of water resources in the world, it is appropriate to identify strategies in all fields to safeguard the available ones and use them in the most effective way possible. Considering that 70% of drinking water is used to irrigate cultivated fields (according to data made available by the FAO in 2022), it is necessary to identify and disseminate agronomic practices aimed at optimizing its use in this area. Given that each cultivation environment has peculiar characteristics, these techniques must be chosen and calibrated appropriately taking into account multiple factors that are interconnected.

Fig. 1: Una piccola pianta che riceve la preziosa dose di acqua necessaria per crescere e produrre frutti
https://www.centroverderovigo.com/it/blog/perche-innaffiare-le-piante-anche-in-inverno-48

Let's first examine the consequences that water stress has on the vital processes of plants: when plants do not find enough (free) water in the soil explored by their roots to counterbalance transpiration, they enter a phase of anabolic stasis, that is, they stop producing, they wither and finally dry up. Depending on the degree of stress to which they are subjected, the following can occur:

• Anatomical modifications. These are due to the fact that the cells do not expand completely. As a result, dwarfism occurs, internodes are shortened, the size of shoots is reduced, the root system develops more, leaves have smaller cells and thicker walls and cuticles, and lignification of tissues increases.

• Modification of development. To increase the probability of bringing their seeds to maturity in conditions of water shortage, plants try to shorten the cycle by increasing the speed of development. The effects vary depending on the extent of the deficit and the time in which it occurs. On this basis, there may be disturbances in germination, pollination, fertilization, flower, fruit, leaf drop, and so on.

• Metabolic changes. The photosynthetic fixation of carbon decreases due to the closing of the stomata that the plant creates to reduce water losses through evaporation. This results in changes in metabolism that lead, for example, to higher percentages of sucrose in sugar beet and sugar cane, nicotine in tobacco, gluten in wheat, sometimes certain fruits are more flavorful. Since protein synthesis is hindered by water deficiency, this can induce accumulation of non-protein nitrogen (nitric) in the tissues. The damage to yield is highly variable with the species and with the phase in which the deficiency strikes

Plants present great differences in tolerance or sensitivity to water deficiency. Some plants (so-called hygrophilous) even require a submerged environment (e.g. rice) and therefore have no resistance to water scarcity. Others (xerophytic plants) are equipped with resistance mechanisms such as rapid lengthening of the root system in depth and extension (possible if the soil is well worked) and the ability to reduce transpiration. Most cultivated plants belong to the group of mesophytes that cannot live in submerged soil and that reach the point of wilting after having lost 25 to 50% of water, depending on the species. Plants that have evolved in environments low in humidity have mechanisms that allow them to save water and survive periods of drought. For example, the olive tree is characterized by very particular leaves and an extensive root system: with the former it is able to carry out gas exchanges, greatly reducing water loss; and with the latter it is able to explore enormous volumes of soil and reach any water reserves in depth.

Crops with a long growing period are most affected by drought. These include sugar beets, potatoes, corn and all forage crops.

Sugar beets remain in the fields for a long time and water shortages result in lower yields. Potatoes are very demanding and require regular irrigation, otherwise they form irregular tubers or stop growing. The transition between extreme periods of drought and rain can cause re-germination or the formation of second-generation tubers. The tubers lose some of their starch, the pulp becomes glassy and they rot quickly in storage.

Corn loves warm weather, but long periods of drought slow its growth. In some areas, interest in sorghum has increased because in case of water stress it is able to remain in vegetative stasis and resume growth in case of water availability, thanks to a dense deep root system and leaves covered with a layer of wax.

Alfalfa has a dense root system and is able to access deep water reserves while ryegrass requires large quantities of water. Among the herbaceous crops most resistant to drought are durum wheat (more resistant than soft wheat), barley and early oats; among legumes, chickpeas, broad beans and lentils are more resistant than peas and beans. For horticulture, plants such as onion, garlic, spinach and potatoes are preferred, the most disadvantaged are certainly cucurbitaceae and Solanaceae. Among the arboreal plants, the most resistant are the vine and the olive tree, followed by the almond tree, the fig tree, the pistachio tree and the carob tree.

Fig. 2: Un albero di pistacchi
https://www.coltivazionebiologica.it/albero-di-pistacchio/

Choice of varieties

Furthermore, within the different species, there are differences between the different varieties and, therefore, the choice of these is a first factor to consider in dealing with any limited water availability. The lists of cereal and corn varieties contain relatively precise data on the ripening period. Early varieties have the advantage of being able to be harvested before the drought or violent storms begin and are therefore less subject to stress than late varieties, which instead have more time to ripen and take advantage of hot years. The lists of recommended varieties currently do not contain references to drought resistance, since genetic selection has taken into account above all the achievement of high yields.

In the future, however, it is estimated that drought tolerance will become increasingly important and will be an important selection objective. Scientific research is making great strides in identifying the genes that regulate the response of plants to water stress. Furthermore, the development of increasingly sophisticated computer systems is allowing the integration of the enormous amount of information continuously generated, giving great impetus to the identification of the genes most responsible for stress tolerance and to the development of genetic engineering strategies. However, it is necessary to avoid any massive introduction of modified varieties because there would be the risk of losing local varieties selected over time forever, causing the so-called "genetic erosion" and the loss of valid information to face future environmental challenges.

Choice of sowing period

In addition to the choice of crops and varieties, another tool to avoid water stress conditions for plants is the choice of sowing period in relation to the climate trend to ensure that the most delicate growth periods occur when rainfall is most likely and vice versa. In fact, more important than the sum of rainfall is its distribution over the growth period. In the case of sugar beet, for example, autumn sowing is preferable to spring sowing. Annual crops need water especially during and after flowering. During the ripening period, however, periods of drought become less important. Thanks to the relatively early harvest period, cereals and rapeseed in general are less affected.

Fig. 3: Una barbabietola da zucchero
https://www.boscodiogigia.it/storia-dell-agricoltura/la-coltivazione-della-barbabietola-da-zucchero

Among grain legumes, soybeans are the crop that benefits most from the increase in temperature and can also survive long periods of drought. Protein peas and broad beans, on the other hand, require a lot of water, especially during the flowering phase until the seeds form. In this phase, these plants are particularly sensitive to drought; in fact, the water requirement in that period is particularly high. If sown in spring, flowering takes place in April, a month in which dry and very hot periods have become more frequent in recent years due to climate change. Broad beans and peas sown in autumn flower three to four weeks earlier and are therefore less affected by drought. A further advantage of autumn sowing is the good coverage of the soil during the winter. A possible disadvantage could be represented by the damage caused by frost in exposed places and at high altitudes, which however can usually be avoided by sowing at a depth of between five and eight centimetres and around mid-October.

The optimal use of available water requires the presence of a good water retention capacity of the soil. This is achieved through the presence of organic matter which, thanks to its ability to hydrate and the binding action between soil particles, forms more stable agglomerates, preserving the porosity of the soil and creating an optimal balance between macropores and micropores. In the former there is air, in the latter the water reserve which, in reality, is a very diluted solution of mineral salts. For these reasons, all soil management practices that tend to increase the content of organic matter contribute to improving the water capacity of the soil and therefore increasing overall water savings. In recent decades, technical choices have been made aimed at obtaining maximum production yields through the use of chemical inputs that perform better in the immediate and are cheaper and easier to administer rather than systems aimed at preserving soil fertility. The latter, in fact, has been treated as a simple substrate from which to obtain maximum results in the immediate future and not as a delicate ecosystem to be preserved as well as used. Given the stress to which crops will be subjected due to ongoing climate change, it is important to adopt techniques aimed at maintaining a good water retention capacity by administering organic matter through manure, compost and green manure. It is also necessary to reintroduce the practice of rotation, introducing crops suitable for enriching the soil with organic matter. These, due to the symbiosis they establish with the bacteria responsible for the formation of nodules in their roots, increase the presence of nitrogen by capturing it in a gaseous state from the atmosphere and making it organic by then releasing it into the soil.

Soil tillage is an important practice for obtaining a structure that allows the roots to move and deepen easily and therefore also use the water present in the deeper layers. The presence of a good structure plays a very important role even during violent storms, when the flow exceeds the water infiltration capacity. In these cases, in fact, not only does the water not accumulate in the soil and is lost but phenomena of root asphyxiation and/or surface runoff and soil erosion can occur. These phenomena are becoming increasingly frequent due to climate change. The global rise in temperature, in fact, in addition to determining an increase in evaporation, determines a change in the frequency and intensity of rainfall.

Fig. 4: Strati del sottosuolo
https://digimparoprimaria.capitello.it/app/books/CPAC90_2613616A/html/240

In this context, soil tillage, by causing the breakage of the compact superficial layers, increases the porosity and therefore the storage capacity. However, it must also be considered that, when the soil is worked leaving the clods exposed to the air, a significant loss of water from the soil is determined, especially if this occurs in seasons characterized by medium-high evapotranspiration. It should be emphasized that greater infiltration through this tillage method does not always guarantee positive effects, at least in semi-arid climates. This is due to the fact that not every year the rains manage to replenish the depleted reserves. With this method, the loss of water is certain, while its restoration is uncertain. More effective in this sense are those tillage methods that leave the soil settled, as foreseen by the minimum conservative tillage. In this case, even the presence of crop residues on the surface contributes to reducing evaporation and, in the case of heavy rains, the surface flow of water. If water flows slowly over the field, it has more time to penetrate inside it. Therefore, when plowing, it is advisable to follow it with a soil settlement intervention in order to limit the surface exposed to the air and therefore to evaporation.

Surface processing, such as hoeing, limits both transpiration - since it controls weeds - and evaporation, since it positively modifies the physical structure of the soil, increasing water infiltration and drainage. Hoeing, in particular, breaks up the most superficial layers and creates a light cloddiness that has a mulching effect. Mulch is nothing more than a material (it can be gravel, bark, etc.) to be placed on the soil around the roots. Freshly hoed soil dehydrates quickly but only in the surface layer, maintaining humidity in the underlying layers. During the summer, or in any case during non-cultivation periods, it is useful to carry out very superficial tillage with the aim of interrupting the capillary rise of water, encouraging the formation of a layer of dry surface soil of a few centimetres to protect the deeper layer, still characterised by a certain humidity. To generally increase the water retention capacity of the soil, it is recommended to sow green manure and artificial lawns which serve to produce humus and give a better structure to the soil.

Fig. 5: Sarchiatura di un terreno agricolo
https://terraevita.edagricole.it/contenuto-sponsorizzato/sarchiatura-la-pratica-ideale-per-chi-sceglie-il-biologico/

In addition to acting on the structure of the soil, you can act on its surface by trying to reduce water loss through evaporation. This can be achieved in different ways, depending on the different contexts:

• Protect the soil with mulching material, thus reducing the solar energy that reaches the surface and decreasing direct evaporation. Mulching retains moisture and discourages the growth of weeds. Covering the soil with dark-colored material reduces its albedo and therefore increases the percentage of radiation stored by the soil in the form of heat and vice versa, in the case in which you want to limit evaporation, it is best to use light-colored materials. Mulching with plant material or biodegradable films, in terms of saving water, plays a very important role. In fact, in addition to preventing the growth of weeds (which could negatively affect the soil's water reserve) it greatly limits direct evaporation from the soil. It is therefore a practice that allows you to enhance the water reserves of the soil and when, as is customary, it is combined with hose irrigation systems, reducing the consumption of irrigation water.

Fig. 6: Pacciamatura su un terreno agricolo
https://ilfattoalimentare.it/agricoltura-biologica-bioteli.html

• The cultivation of increasingly lower plants in layers allows for optimal use of the soil surface and ensures that the plants in the lower layer are protected by those in the upper layer and therefore their evaporation is limited.

• Control of weeds to limit their competitiveness with respect to water. This control must be carried out considering, however, that grassing intercepts rainwater very well, reduces runoff and facilitates its infiltration into the deeper layers of the soil.

  Use windbreaks to slow down the movement of air around the vegetation, thus reducing transpiration. These can consist of trees or hedges or inert material; it has been observed, for example, that anti-hail nets reduce water loss by up to 50%.

• The wind causes alternating contractions and expansions of the intercellular spaces, and in particular of the substomatal chambers, which force the exchange of internal, saturated air with the drier external air, thus accentuating transpiration. “Dead” windbreaks consist of dry stone walls or reed mats (reed mats tied together with wire) or simple reeds stuck into the ground. In “living” windbreaks, the action is carried out by grasses, shrubs or trees both by dampening the kinetic energy of the wind and by deflecting it. Thanks to them, a strong reduction in potential evapotranspiration is achieved; in fact, an increase in dry matter production has been observed due to increased photosynthetic activity due to the increased number of hours of the day in which the stomata are open. This leads to better use of the water present in the soil. Therefore, windbreaks are now recognized as having considerable importance as improvers and modifiers of the microclimate, so it is advisable to extend their use as much as possible. In this regard, it is necessary to avoid cutting down existing trees, limiting it to what is absolutely necessary, and not transforming areas with a high tree density into large open areas without first carrying out studies on the possible imbalances in the water supply.

Irrigation systems

Irrigation efficiency refers to the ratio between the amount of water used by the crop and the amount taken by the pump. The more efficient a system is, the greater the water savings. The systems with the lowest irrigation efficiency are those for submersion (25%) and for flow (30-40%). If performed in furrows, this technique can guarantee an efficiency of 50%; these two systems also cause significant leaching of nutrients. Sprinkler irrigation systems, also called "rain", are characterized by an efficiency of between 70 and 80%. They require a lot of energy because the water is expelled at high pressure. They can be fixed systems (as in orchards, where however they also perform temperature mitigation functions) or more often mobile based on the hose reel, pivot or ranger. The most efficient irrigation systems are those that distribute water close to the plant (85-90%), near the base of this or near its roots. In this way, all the water reaches the ground instead of being deposited on the above-ground part of the plant where it evaporates very easily.

Fig. 7: Impianto di irrigazione interna “a pioggia”
https://www.casaegiardino.it/giardinaggio/come-scegliere-il-sistema-di-irrigazione-interna.php

There are many solutions that allow this system to be adopted on conventional systems and machines, replacing the sprinkler distributor with devices that deliver water under the foliage or directly to the ground. Drip irrigation is therefore the most efficient system even if it is not easy to use when the topography of the soil is irregular and the quality of the water is such that over time it generates nozzle blockages. Recently, a new technology has been developed that provides self-compensating drippers in the case of complex and sloping topography that, within a defined pressure range, maintain a constant flow rate and are equipped with a self-cleaning system that allows the use of both hard water and digestate. The latter is made up of the portion of biomass not completely absorbed at the end of the fermentation process aimed at producing biogas from organic matter. The digestate is separated into a solid fraction and a liquid fraction and can be administered to crops to provide nitrogen, phosphorus and potassium. Self-cleaning driers therefore allow simultaneous irrigation and fertilization.

Since the clarified fraction of the digestate contains a very high content of ammonia nitrogen, the risk of losses due to ammonia volatilization is very high. These losses, in addition to having an impact on the environment, significantly reduce the nutritional potential of the digestate that is distributed. In fact, ammonia losses translate into a lower availability of ammonia nitrogen in the soil for crops, drastically decreasing distribution efficiency.

Wastewater recycling

With regard to the use of residues in agriculture, in order to optimize the use of water resources, recycling urban wastewater and using it in agriculture can help alleviate problems related to water scarcity and reduce pollution. However, according to a recent FAO report, it should be emphasized that this practice is currently not as widespread as it should be. Since wastewater is derived from human activities – domestic, industrial or agricultural – it must be purified before being used for irrigation because it contains organic and inorganic substances that can be harmful to health and the environment. Safe reuse of wastewater for food production can help alleviate competition between cities and agriculture for water use.

Although the implementation of adequate wastewater treatment and recycling systems involves both upstream capital investments and ongoing operating costs, the main benefit of such schemes is expected to be the increased availability of potable water for human consumption in cities or for industrial use. The costs could be offset by reusing biogas produced by water treatment processes as an energy source, or even potentially by selling energy credits. The possibility of reusing wastewater in agriculture depends on local circumstances and conditions, which influence the balance of costs and benefits.

Productive destination of agricultural land

In view of the appropriate rationalization of the use of water resources, it is also necessary to consider the optimization of the productive destination of agricultural land, taking into account that the quantity necessary to produce vegetable proteins is far lower than that needed to produce animal proteins. In fact, to produce a kilo of corn, 900 liters of water are used, which is little compared to the 3,000 liters used to produce a kilo of wheat. To produce a kilo of chicken meat, 3,900 liters are used, for a kilo of pork meat almost 6,000 liters while for a kilo of beef, coming from an animal that grows much more slowly, as many as 15,500 are needed. In situations of water shortage, it is therefore appropriate to adequately choose the productive destination of the land, also taking these calculations into account.

Water pollution caused by agriculture

Another important aspect to consider regarding the relationship between water and agriculture is the polluting load that comes from the use of fertilizers, fungicides and their transport into groundwater and drainage channels through rainwater and irrigation. To these are added hormones, vaccines and antibiotics when livestock farming is also considered. This phenomenon causes environmental damage and the onset of diseases in the population. When calculating the costs necessary for food production, it is therefore necessary to calculate the high environmental and health cost that the use of chemical inputs determines. As seen in the previous paragraphs regarding agronomic techniques aimed at the rational use of water, also with regard to the containment of this serious problem, the use of agronomic practices that provide for a limited use of chemical inputs and the adoption of systems that allow obtaining high yields with less impacting systems that respect human health and the environment and preserve soil fertility is recommended.

From both an environmental and economic point of view, the containment of the polluting load present in these waters should be treated upstream. In fact, the further away you are from the farm, the higher the cost of their treatment. There are various techniques for purifying these waters; among these is that of the wetlands in the riverbed or outside the riverbed. This technique is not yet very widespread in Europe (some interventions exist in England and Switzerland, but an experimental case was also carried out near Chioggia in Italy) but is now consolidated in America and Australia. These systems aim to "treat" the polluting loads that have already reached the watercourse: for this reason they are generally created when it is not possible, or not sufficient, to intervene "upstream", reducing the loads at the source. The structure of the wetlands in the riverbed is made up of an initial energy dissipator, followed by a deep area of ​​free water to promote sedimentation, and a phytoremediation system that occupies most of the available surface. The phytoremediation system is generally of the “free water” type (free surface). If the main objective is, as in all the projects analyzed, to remove nitrogen and phosphorus, they can have a much simpler and more “natural” structure.

The plant species used will mainly be emerging macrophytes (Tipha spp. and Phragmites australis which are among the most widespread species and have an excellent capacity for assimilating nitrogen) or submerged. The wetlands outside the riverbed can be created either to treat a portion of the ordinary flow (in this case they are always active and receive a constant flow) or to treat only the flood flow: in the latter case their creation is generally aimed at lamination and only secondarily do they have a purifying function. The structure of the wetland is substantially similar to that “in the riverbed” but differs in the supply system, which is created in such a way as to subtract only a part of the flow from the outflow. A classification of this type of intervention can be based on the type of supply:

• Continuous feeding: wetlands are fed by introducing a fraction of the total flow of the watercourse into the system (in this case the wetland is fed constantly and the efficiency of pollutant removal, dependent on the retention time, is maximum).

• Discontinuous feeding, through a “threshold” operation (in this case, however, the wetland is largely “empty” and fills up only during floods, so the annual volumes of water “treated” by the wetland are normally much lower than in the case in which the wetland is fed continuously and, consequently, the effectiveness of pollutant removal is lower).

  Conclusions

  The agronomic techniques examined in this report, aimed at the optimal use of water resources in agriculture, must be chosen and modulated carefully in relation to the different environments in which they operate and to the productive, social and economic context. In view of the challenging challenges to be faced related to water scarcity, since there are no predefined solutions valid for each territory, it would be more than appropriate for the institutions in charge to create, in homogeneous circumscribed territories, experimental-demonstrative companies in which to develop the best techniques to be adopted[1] in that specific context and demonstrate them to agricultural entrepreneurs operating in that territory. It would therefore be appropriate to set up a network of agricultural extension workers able to manage such companies and spread the methods developed. This would also allow the coordination of operators in the sector who could share the strategies to be adopted, also reducing the cost. The world population is increasing and, at the same time, the water-agricultural challenges that we will have to face are growing; sharing knowledge and cooperation will allow us to use natural resources in an optimal way.

Bibliografia

• F. Bonciarelli 1981. Agronomia.

• S. Grillo, A. Blanco, L. Cattivelli, I. Coraggio, A. Leone 2006. Risposte genetico-molecolari delle piante a carenza idrica.

• S. Moricca, B Ginetti 2013 – Risorse idriche e salute delle piante.

• S. Grillo; A. Blanco; L. Cattivelli; L. Coraggio; A. Leone. 2006 Risposte genetico-molecolari delle piante a carenza idrica.

• S. Ceccarelli; S. Grando 2020. Evolutionary plant breeding as a response to the complexity of climate change.

• S. Ceccarelli; S. Grando 2020. Organic agriculture and evolutionary populations to merge mitigation and adaptation strategies to fight climate change. South Sustainability.

• W. Baethgen 2010. Climate risk Management for Adaptation to climate Variability and Change. Crop Science 50.L. Evans 1993. Crop Evolution, Adaptation and Yield; Cambridge University Press: New York, NY, USA.

• M. Van der Wouw; C. Kik; T. Van Hintum; R. Van Treuren; B. Visser 2010. Genetic erosion in crops: concept, research, result and challenges. Plant genetic reslutces: characterization and utilization.

• D. Renard; D. Tilman, 2019. National food production stabilized by crop diversity. Nature Plants.

• K. Zimmerer; S. De Haan, 2017. Agrobiodiversity and sustainable food future. Nature Plants.

• Rapporto FAO del 2010 sull’utilizzo delle acque reflue.

  ____________________________________________________________________________________________________________________________________________________________

[1] Epoche ottimali di semina, scelta delle varietà, metodi di lavorazione, tecniche d’irrigazione, eventuale opportunità di predisporre frangivento, eventuale uso di pacciamatura reperibile in loco ecc.