How to reduce irrigation costs

Irrigated agriculture, in general, is the dream of most farmers, simply because it can make their crops independent of the seasonality and relative unpredictability of rainfall and, thus, ensure rewarding productivity. However, an impasse occurs when requesting a quote from a specialized company, as there is widespread consensus in admitting that irrigation represents the most costly input applied in agricultural production. For this reason, before getting involved with the possibility of contracting a project, several measures can be taken with the aim of reducing fixed and variable costs, thus contributing to highlighting the economic viability of this important technological resource.

REQUIRED WATER VOLUME AND REQUIRED FLOW

The first step prior to an irrigation project is to assess whether the property's water resources are satisfactory, in terms of quantity and quality, to satisfy the water demand of the crops that will be developed in the intended areas. To do so, simply perform a calculation that, although simple, requires a responsible estimate of the water demand of the crops of interest, for the period and location where the irrigation will be carried out. In general, the usual values ​​adopted when sizing projects vary between 3mm/day and 6mm/day, with the lowest values ​​prevailing in months with lower temperatures and lower incidence of solar radiation. It should be noted, however, that the stage of crop development must be considered when managing irrigation, to rationalize the application of water, limited by the maximum demand values ​​previously adopted in the sizing.

Thus, by identifying a representative value of water demand, the volume of water required can be calculated depending on the area to be cultivated. For example, if the crop's water demand (D) is estimated at 4mm/d (which represents 4 liters per m2 of cultivated area per day) and the area (A) is 10 hectares, the volume of water required (V) will be:

V = 10 × D × H = 10 × 4 × 10 = 400m3/day

The number 10 in this equation is just a factor to adjust the dimensional units used. It is worth noting in this example that each millimeter included in the equation corresponds to 100m3 of water per day for an area of ​​just 10 hectares. Therefore, this is a number that must be adopted with great care, as it will affect all hydraulic sizing and, consequently, the cost of the project.

In conditions of water scarcity, there are alternatives that, even assuming lower than necessary allocations, can provide satisfactory productivity and rewarding economic results. These alternatives are widely recognized and are identified as “deficit irrigation” in the context of irrigated agriculture.

Now, the required flow rate (Q) can be calculated. To do so, simply provide the daily irrigation period (T) in hours, and an estimate of the water application efficiency of the irrigation system to be adopted. It is important to note that the longer the daily period of operation, the lower the flow required, with numerous advantages in sizing, such as lower power of the motor and hydraulic pump set and transformer, smaller diameter of pipes and accessories or lower loss of hydraulic head. in piping and accessories, lower capacity of operational safety devices, etc. On the other hand, water application efficiency, in general, depends on the irrigation system considered.

However, society's current demand for better performance in irrigation systems practically does not allow efficiency below 80%. This number means that for every 100m3 of water applied, 80m3 will be used by the crop and 20m3 will be lost through percolation, runoff or evaporation, alone or together. Some automated systems, well dimensioned and operated, can reach 85% to 90% efficiency. Others, neglected, cannot exceed 50%. It should be noted that there is no ideal irrigation system capable of operating with absolute efficiency, that is, without water loss. In summary, adopting a daily operating period (T) of 20h and an efficiency (E) of 80%, converted to decimal (80/100 = 0,8) in the following equation, results: Q = V/(T × E) = 400/(20 × 0,8) = 25m3/h. 

If the operational period is reduced to 10h/day, the required flow will be doubled (50m3/h) with the inconveniences already mentioned. The necessary condition to enable nighttime operation is simply to provide devices capable of ensuring operational conditions, which are available at a relatively low cost.

An affordable alternative in irrigation projects for annual crops consists of adopting a pumping capacity lower than the water demand during the critical period of crop demand. The proposal would be to use the soil as a reservoir of available water, whose capacity could meet the deficit demand during that period. This alternative is graphically represented below. 

Thus, for example, assuming that the average water demand during the critical period is evaluated at 4mm/day and adopting a pumping capacity of 3,2mm/day (20% reduction) it can be concluded that there is a demand deficit of 0,8mm/day. Assuming that the available water capacity in the soil is 32mm, a characteristic value for most soils classified for irrigation, it can be stated that this value could complement the crop's water demand for 40 days (32mm/0,8mm/ day = 40 days). The width of this interval must necessarily include the critical period of crop water demand. The necessary condition in this proposal is the replacement of the water capacity available to the soil, through rain or irrigation, in the preceding periods, in which the water demand is lower than the pumping capacity adopted in the project.

Once the quantity has been verified, it remains to assess the quality of the water in relation to the requirements of the intended irrigation system. In this case, the demand for better water quality is directed to localized irrigation, micro-sprinkler and drip systems, mainly the latter, due to the reduced size of the conduits in the drippers. In these systems, the requirement for an efficient water filtration system is mandatory. Furthermore, the presence of excessive concentrations of soluble salts or ferruginous waters also requires special care to avoid problems of precipitation and deposition in drippers, causing partial or total obstruction, a condition that is very difficult to reverse, and almost always resulting in the disposal of all equipment compromised.

Sprinkler and surface systems are less dependent on the physical and biological quality of the water. In most installations, just a screen installed at the entrance to the suction pipe may be enough to prevent sprinklers or holes and siphons from being blocked, simply because they have larger diameter holes.

DIMENSIONS AND SHAPE OF THE AREA TO BE IRRIGATED

Except for the center pivot sprinkler system, whose area is completely or partially circular, most irrigation systems favor rectangular areas. In these areas, both the design and management of irrigation are facilitated. It is often advisable to discard irregular shapes that can make sizing difficult, make the project more expensive and make irrigation management difficult.

The definition of the dimensions of the areas must also consider the characteristics of the irrigation system. Thus, for example, a DN 40 PVC pipe, specified for sanitary sewage, with an internal diameter of 37,6mm, is capable of simultaneously supplying 12 sprinklers spaced 12m apart, with an average flow rate of 500L/h, which corresponds to 1.728m2 of effective irrigated area. Admitting another lateral line supplied by the same derivation line, there is a total of 288m of irrigated strip with 3.456m2. This alternative reduces the number of records at the beginning of the side lines and favors the operation of the system. Therefore, when exploring the pipe's water-carrying capacity to define the dimensions of irrigated areas, there will always be a reduction in investment and operational costs in irrigation projects.

There are particular situations in surface irrigation in which the shape of the irrigated areas is determined by the topography of the terrain, which is predominantly flat. The structure or pipe responsible for supplying water runs along a very low or zero gradient, distributing water that moves through gravitational action on the surface of the soil. Under these conditions, in general, water does not constitute a limiting factor in the design and management of irrigation.

 

HOW TO CONSUME IT

COGENERATION

Rarely does an irrigation system operate without consuming energy. In general, the most used sources, represented by reservoirs, streams or rivers, are located at lower elevations on properties, requiring a repression action to make them available at higher elevations in irrigated areas. Some systems consume large amounts of energy, notably when they involve steep differences in level between the water source and the irrigated areas and operate at high pressures. In this case, the relationship is directly proportional, that is, the greater the head required for pumping, the greater the energy consumption.

There are several alternatives to reduce energy consumption in irrigation. Firstly, prioritize areas that are close and with a small difference in level in relation to the water source. The ideal condition would be to locate the water resource in the geometric center of the irrigated area. The success of the central pivot system began when it was possible to extract water from underground aquifers at the central point of the area, or pivot point, to supply the equipment.

Secondly, when possible, opt for surface irrigation systems, which do not depend on pressurization in the water distribution process in the irrigated area. Then, consider the option for localized irrigation systems, especially drip, using spaghetti microtubes as point emitters. Microtubes with an internal diameter of 1,5 mm can operate, in many conditions, with gauge loads of less than 2 m, which could eliminate the need for pressurization by pumping, in addition to being less susceptible to obstruction than commercial drippers. If it is sprinkler, consider opting for sprinklers with a single nozzle and reduced diameter. These conditions, in addition to reducing pressurization, offering a lower specification in the nominal pressure class of the pipes, reduce the flow in the pipes that make up the lateral lines, favoring a reduction in diameter and loss of hydraulic head, allowing for greater lengths of these pipes.

TOPOGRAPHIC CONDITIONS

Surface topography plays an important role in the economics and even viability of an irrigation project. The increase in the topographic gradient accentuates the rigor in sizing, requiring, in pressurized systems, a greater number of pressure controllers. In furrow irrigation, the increase in transverse slope increases the risk of water overflow, predisposing to the emergence of surface erosion processes, which are difficult to correct. For these reasons, transverse gradients above 10% practically make furrow irrigation unfeasible.

An uneven topography also makes it difficult to size and manage irrigation, making projects more expensive. The ideal condition would be the maintenance of reduced topographic gradients in all directions, characterizing a relatively flat and uniform condition.

FERTIRRIGATION

There is no justification for a carefully designed irrigation system to do without the application of water-soluble fertilizers together with water. The premise of a satisfactory water distribution in the irrigated area summarizes the necessary condition for the incorporation of these fertilizers in the application process. The strategy of frequently incorporating sufficient amounts of fertilizers, making them readily available to the root system of cultivated plants, is an important additional benefit that should be incorporated into irrigation projects.

It is worth highlighting the recognized versatility of fertigation in surface irrigation systems, which can successfully apply even organic fertilizers, practically prevented or limited in other irrigation systems, making them a priority in organic production systems.

CONCLUSIONS

In summary, one must admit the difficulty in finding integrated conditions that favor the economy of irrigation projects. Thus, as conditions move away from those that are more favorable, investment and operational costs will inevitably increase, justifying the option for more valued crops, capable of making the enterprise economically rewarding. In some situations, it may be recommended to start irrigation under the most favorable conditions, with the aim of accumulating financial resources to pay for the future incorporation of areas with greater restrictions on irrigated agriculture. 

Edmar José Scaloppi, Unesp