IRRIGATION MANAGEMENT

Fernando Braz Tangerino Hernandez
UNESP - Ilha Solteira

1. INTRODUCTION

Irrigation processes cannot and should not be understood solely as artificial procedures aiming to cater to soil humidity conditions and focusing exclusively on the enhancement of farm production regarding quantity, quality or opportunity.
In truth, irrigation processes encompass an array of procedures (which aggregate themselves in a system) necessary to the catering to the water requirements of plants, including eliminating excess water, which simply go beyond the relationship among soil, water and plants. Climate, human interference and the wide field of knowledge which is currently known as foundations of the environmental sciences also belong in the realm of irrigation.
As defined, the science and art of irrigation has broad and interdisciplinary foundations, passing through the fields of the agricultural sciences, the hard sciences (water, civil and electrical engineering, etc.) and the social sciences (economics, sociology, politics, etc.). None of these various roots is inherently prevalent in irrigation science, for all these factors must be taken into account in making decisions over the use of water.
A system should be understood as a group of elements which come together and act together in pursuance of a common objective. Whether they want to or not, people working with irrigation should have an eclectic knowledge base, understand the whole process from production to marketing, and be used to working in multidisciplinary teams. The irrigation professional cannot be a specialist in all subjects, but should not dispense with a solid generalist education.
It is conventional wisdom that irrigation is one of the spearheads of modern agricultural technology, for, together with balanced fertilization programs, it holds the key that allows genetic materials in the field to realize their full productive potential, which could certainly not come about without those inputs. Moreover, water and nutrients nowadays walk hand in hand, as it is possible to dispense them at the same time through fertirrigation, with its countless advantages.
Thus, the choice of species to be planted in a given area, the spacing of the planting, the fertilizers that enable high-yield production, the phytosanitary control, the erosion avoidance techniques, the correct irrigation procedures and, finally, the harvest and marketing decisions should be regarded as parts of one production process, instead of taken separately. The irrigation expert should have a solid grasp of the techniques underlying all these processes. A big question for the irrigation expert regards the decision of when and how much to use irrigation. Knowing the right moment to start and the right amounts of water to apply are the objectives of rational irrigation management. The increases in the cost of energy and the progressive scarcity of water that we experience nowadays force the expert to ponder these issues. Rational management of water necessarily deals with the economic side of the process. In this respect, there appears an issue not always diagnosed by the expert: excess of water can be as harmful to the development of crop yields as its scarcity. For instance, we know beans are not very tolerant to water excess, whereas pineapple trees can endure long periods of dryness.
Thus, before launching an irrigation management program, it is essential to know the physiology of the plant which we purport to irrigate. To know the physiology of a crop means to know the critical periods for water consumption and their relationship to productivity.

2. GENERAL ISSUES IN IRRIGATION SYSTEMS

Various systems, each with its own features, can be used in irrigation. The most common are surface, overhead and targeted irrigation systems. In surface irrigation, swamp and ditch systems are the most widely used. These systems have limitations in irregular areas and sandy terrains where the infiltration rate is high, and they are the ones that use up more water.
Overhead irrigation has three main varieties: conventional overhead irrigation systems (either over or under tree tops), auto-propelled systems and central mast systems. These systems normally use up less water than surface systems, but require investment in equipment and have higher operating costs, because they work with high water pressures and therefore need more powerful engines.
The main targeted irrigation systems are micro-sprinkler systems, drop-by-drop systems and drop tube systems. These are the ones that use up less energy and water, for they work with low water pressures and moisten but a portion of soil surface. Its use has increased considerably in the past few years; however, they require bigger initial investments. In these systems, fertirrigation practices are almost mandatory; these practices enhance economy and efficiency in fertilizing processes.

3. IRRIGATION MANAGEMENT AND CONTROL

A key term in irrigation management is irrigation frequency, which refers to the number of days between two irrigations. The frequency of irrigation can be fixed or variable, depending on the irrigation professional’s stance. Fixed irrigation frequencies enable the planning of crop irrigation activities: since the times of irrigation are known in advance, all that needs to be done is setting the amounts of water to be dispensed. On the other hand, when using a variable irrigation frequency, we do not know in advance when will irrigation take place, but we can have a reasonable idea of the amount of water that will be dispensed.
We shall thus study the issues involved in the process of irrigation control, and the mechanisms used therein. But before starting an irrigation process, we need a certain knowledge of the crop to be irrigated. Thus, crop stages (the fenologic cycle) and water requirements and critical periods should be known in advance. Irrigation should monitor three main issues: atmospheric conditions, soil water conditions and plant water conditions. Irrigation can also be monitored simultaneously in the atmosphere and in the soil.
Before choosing the control system, the irrigation expert should bear in mind that the greater the effective depth of the roots system, the better for the crop. The more humid the soil, the bigger the roots system shall be, and roots system size is directly related to productivity. Therefore, planting practices should be geared towards ensuring bigger roots systems.

3.1. Control processes based on soil conditions

O The control of irrigation through the soil requires knowledge of soil characteristics. Thus, the irrigation expert should be acquainted with notions such as apparent density, grainmetry, declivity, basic infiltration speed (BIS), disposable water capacity (DWC), saturation humidity, field capacity, permanent withering point and soil characteristics curve.
An analogy can be made between the soil and a water reservoir, in order to plan water consumption in such a way that it will not imperil plants’ future supply.
Simply put, we could identify the disposable water capacity (DWC) with the size of the “reservoir”; disposable water (DW) would be the amount of water to be used up by the plants, which should be replenished by irrigation. DWC equals the difference between field capacity humidity (q FC) and permanent withering point humidity (q PWP), multiplied by the effective depth of the roots system (EDRS).

CAD = ( q CC - q PMP ) x PESR

Thus, if a given soil has a field capacity (FC) humidity of 0,260 cm³/cm³ and a permanent withering point (PWP) humidity of 0,083 cm³/cm³, and the effective depth of the roots system is 300 millimeters (30 cm), we have a DWC of 53,1 mm. I.e. our “reservoir” should have a capacity of 53,1 mm. If we take a value of 50% for disposable water (DW), maximum irrigation will be 26,6 mm. Beyond this measure we would be substituting water for soil air, which may harm the crop by means of water excess. Saturation would be the point where all the pores of the soil are taken by water, which would leave no oxygen to be used by plants, therefore causing harm. PWP would be the plants’ water absorption limit. If soil humidity should attain that point, plants will be unable to recover.
DWC is a characteristic of the soil, and therefore changes from soil to soil, depending on grainmetry, compactness and organic matter contents. The maximum irrigation frequency to be used by a farmer will be given by the division of the DWC by the maximum evapotranspiration, which would be the highest water consumption by the crops.
The characteristic curve of the soil (DWC) is a graph that relates soil water potential to its volume-based humidity. As the greatest humidity variation of the soil comes in the range under 1000 centimeters of water column, or 1 atm, and the characteristic curve encompasses from saturation (0 atm or 0 water column centimeters) to 15 atm (15.000 centimeters of water column), it is common to represent the matrix potential as a logarithm of the absolute value of the matrix potential, which is a negative number. The logarithm of the absolute value of the matrix potential is usually noted as pF. Figure 1 shows the characteristic curve of a red yellow podzolic. The maximum depletion level of soil water can be determined on the basis of the readings of soil water matrix potential, done by means of tensionmeters, and the characteristic curve of the soil. This point is also called critical management tension; irrigation should start whenever it is attained.
Strictly speaking, the value of the matrix potential is a negative number; therefore, the smaller its value should be, the lower soil humidity shall also be. In practice, however, we take its absolute value, to simplify its use.
When the soil characteristic curve is not available, a critical matrix potential, based on surveys or the technical literature, should be used. For grape crops, a matrix potential (or tension) of management of -500 wcc (water column centimeters) can be considered critical, although this value is closely related to the disposable water capacity of the soil. When applying irrigation, the amount of water used should be such that soil humidity shall remain at field capacity. This can be considered to be the humidity corresponding to 100 centimeters of water column. In order to convert matrix potential into a percentage of DWC, it shall be converted first into current humidity.

FIGURE 1 – Schematic representation of a soil water characteristic curve.

3.2. Control processes based on atmospheric conditions

Knowledge of climatic factors is critical for rational irrigation management. Based o these factors it is possible to estimate with reasonable accuracy evapotranspiration, i.e. water consumption in a given place, through soil water evaporation and plant transpiration, which takes place during photosynthesis processes.
Reference evapotranspiration (ETo) is the name given to evapotranspiration as estimated by means of empirical formulas postulated by different authors. These formulas are based on meteorological data, and come in diverse shapes: the most simple need just a few inputs, whereas the more complex models use a considerable array of climatic elements. An easy and cheap estimate of ETo can be obtained by using a Class A Tank. This is a round evaporimeter (tank), measuring 1,21 m in diameter by 0, 254 m in height, built in galvanized plate number 22. It is seated on the ground on a bar platform measuring 0,10 x 0,05 x 1,24 meters, which is leveled on the ground. The Class A Tank is full of clean water up to a distance of 5 cm of the top of the upper rim. The minimum water level permitted is 7,5 cm from the top of the rim; i.e., after every 25 mm (2,5 cm) of evaporation the water volume in the tank should be restored. The operation of the tank is fairly simple. Changes in the water level are measured with the help of a measuring point, hook-shaped, seated on the tranquilizing well, which should also be adequately leveled. The accuracy of measurements is around 0,02 mm. Water level reading is done daily. The difference between readings represents evaporation in the period.
Daily readings, however, do not give us evapotranspiration. To get that, it is necessary to convert Class A Tank evaporation into reference evapotranspiration (ETo). ETo is defined as the water loss experienced by a surface completely covered in ground plants, in active development stage, and where humidity doesn’t preclude the plants´ optimal development. In these conditions, the process of evapotranspiration is influenced only by elements external to the ground (i.e. climatic elements). Thus, evapotranspiration can be calculated using the following expression:

ETo = ECA x Kp
Kp = f (wind, relative humidity, surrounding vegetation)

where Kp = Tank coefficient

Class A Tank coefficient (Kp) depends on wind speed, relative humidity and the size of the grass and potato plants surrounding the Class A Tank. Kp in our region gravitates around 0,75 most of the year.
Meanwhile, what we are really after is crop evapotranspiration. Therefore, we need to replenish water consumed by the crop which is economically meaningful. This consumption changes depending on the crop´s development stage, and from one crop to another. Thus, crop evapotranspiration is obtained by multiplying reference evapotranspiration by the crop coefficient.

ETc = ETo x Kc
where Kc = f (species, development stage)

The crop cycle is divided in fenologic stages. Each stage assumes different values for Kc. For grape crops in Northwestern São Paulo state these values oscillate between 0,3 and 0,7. The stages are called growing stage (or vegetative stage), crop, blossoming, harvest formation (increase in fruit size) and maturation. Kc values are multiplied by ETo in order to obtain crop evapotranspiration (ETc).

TABLE 1 – Crop coefficients, according to FAO (Doorenbos and Kassan, 1994).

3.3. Combined processes for irrigation management

When controlling irrigation by means of the combined process, all irrigation is done on the basis of evapotranspiration and monitored with tensionmeters placed on the soil. Whenever it is verified that, under given conditions, the soil has attained the critical DWC, irrigation is administered.

4. IRRIGATION SYSTEMS EVALUATION

Irrigation practices should be understood not only as an insurance against drought or Indian summers, but as a technique which may nurture the conditions that genetic materials in the field need to yield its full productive potential. Moreover, if correctly used, irrigation is a very effective tool to increase business profitability, as it allows rationalization of inputs, for instance, through fertirrigation.
Meanwhile, in order for the process to be efficient, the irrigation system should ensure a high level of uniformity in water distribution. This is achieved by means of good projects, based on the usage of the right materials and precise hydraulic calculations.
After implementing an irrigation project, it is interesting to check whether the initial assumptions are proven to be right in the field. To that purpose, a field evaluation should be carried out, in order to ascertain pressure conditions, outflows and plates applied. Regarding the irrigation plates applied, the CUC, or Christiansen’s Uniformity Coefficient, is the index which is most widely used to determine water distribution conditions in the irrigated area.

5. FINAL REMARKS

AA species´ maximum yield depends on the genetic potential of the material, of water and nutrients availability and on plant population. A rational mix of those elements will certainly result in an excellent harvest. Thus, farmers have in irrigation and fertilization two levers to increase immediately their yields. This is not to underplay, however, the importance of the need to choose quality seeds, for this initial factor is critical to the obtainment of high yields.
All processes pertaining to irrigated farming depend on the choice of an irrigation system. This choice, therefore, should be judicious, and take into account the reliability of the project design company, the project itself, the project company’s technical ability and its ability to supply technical support, for it is to be desired that the chosen irrigation system stay with the farmer for a rather considerable time.

6. LITERATURA CONSULTADA

ABID. ASSOCIAÇÃO BRASILEIRA DE IRRIGAÇÃO E DRENAGEM. Uma ABID para os novos tempos - Mais dinâmica e mais atuante. Boletim Informativo, Ano XVIII, número 158, junho-novembro, 4p. 1993.
COSTA, E.F., VIEIRA, R.F., VIANA, P.A. Quimigação: aplicação de produtos químicos e biológicos via irrigação. EMBRAPA - CNPMS, Brasília, 315p. 1994.
DOORENBOS, J., KASSAM, A.H. Efeito da água no rendimento das culturas. FAO/UFPb, Campina Grande, 1994. 306p. (Estudos FAO: Irrigação e Drenagem, 33).
DOURADO NETO, D., BOTREL, T.A., LIBARDI, P.L. Curva de retenção de água no solo: algorítmo em QuickBasic para estimativa dos parâmetrso empíricos do modelo de GENUCHTEN. ESALQ-USP, Piracicaba, 34p. 1990.
GENUCHTEN, M. Th. Van. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J., v.41, p. 892-8, 1980.
HERNANDEZ, F.B.T. Potencialidades da fertirrigação. In: Simpósio Brasileiro sobre Fertilizantes Fluidos, ESALQ-USP, Piracicaba, 1993. p. 199-210.
KELLER, J., BLIESNER R.D. Sprinkle and trickle irrigation. Van Nostrand Reinhold, New York, 1990. 651p.
LEMOS FILHO, M.A.F. HIDRISA: Contribuição à elaboração de balanços hídricos - O caso da região de Ilha Solteira. Ilha Solteira, FEIS-UNESP, 1994, 59p. (Trabalho de Graduação)
REICHARDT, K. Processos de transferência no sistema solo - água - atmosfera. Fundação Cargill, Campinas, 1985. 466p.
REICHARDT, K. Controle da irrigação do milho. Fundação Cargill, Campinas, 1993. 20p.
TOSSO, J.T., TORRES, J.J. Relaciones hidricas de la vid, bajo diferentes niveles de riego, usando goteo, aspersion y surcos. I - Evapotranspiração y eficiencia en el uso del agua. Agricultura Tecnica , v.46, n.2, p.193-98, 1986.

A N N E X E S

OVERHEAD IRRIGATION CONTROL

Reading: daily reading in Class A Tank
EAC: Evaporation in Class A Tank (mm/day)
Kp: Class A Tank coefficient
ETo: Reference evapotranspiration (mm/day) = EAC * Kp
Kc: Crop coefficient
ETc: Crop evapotranspiration (mm/day)
IT: Irrigation time (hours)
IN Acum: ETc accumulated between one irrigation and the next

ETo = ECA * Kp ETc = ECA * Kp * Kc

IT (hours) = IN acum / sprinkler precipitation

TARGETED IRRIGATION CONTROL

Reading = Daily reading in Class A Tank
EAC: Class A Tank evaporation (mm/day).
Kp = _____; Kc = _____ ; Kr = FCS = ____ Acum = Acummulated
K = (Kp * Kc * Kr * A) / (Np * Ef) = Á = Area (square meters)
Np = Number of plants in the area Ef = System efficiency
V = Volume per plant per day IT: Irrigation time (hours)

Note: Attention should be paid when using a different number of outlays for each plant.