Introduction

Irrigation scheduling is the process of defining the most desirable irrigation depths and frequencies. Scheduling provides for the optimal profit on yield of a crop, taking into consideration crop, farming, water, and environmental restrictions [1].

The use of irrigation scheduling is becoming more and more necessary because of the continuous increase in water demand, both in agriculture and in other sectors (industry, recreation, and urban use), when water resources are becoming scarcer all over the planet [2]. These techniques are useful not only to prevent water waste, but also to avoid negative effects of overirrigation on crops, both on yield and on the environment.

Although it may seem simple, irrigation scheduling is a complex problem indeed, because the satisfaction of crop water requirements must consider all the restrictions imposed on farm management [3]. Among the particular determining factors of each farm are water availability, manpower and energy availability, characteristics of the existing irrigation system and equipment, legal factors affecting the farmland, and user training [3]. Other determinants to be considered when designing irrigation scheduling are soil factors (texture, water-retention capacity, and depth), climatic factors (temperature, solar radiation, humidity, wind speed), factors related to the crop (crop type and variety, characteristics of root system, susceptibility to water stress and salinity), and cropping factors (sowing time, length of the growth cycle, critical growth stages, tillage, fertilizing, control of pests, diseases, and weeds). Many of these factors are interdependent and may vary both in space and time, thus confirming the complexity in accomplishing good irrigation scheduling. In practice and to be operative, the technique can be simplified with the aid of field sensors, computers, and automation [3].

The objectives may be of a different nature, ranging from technical (including yield) to economic and environmental aims, although they usually are combined. The selection of a particular objective for irrigation scheduling depends on specific needs in each situation, but four main strategies can be noted [4].

The first strategy consists of maximizing yields per unit of irrigated surface. To obtain this yield optimization, the user must fully satisfy crop evapotranspiration demand. This approach is becoming increasingly difficult to justify economically because of increasing water scarcity, high energy costs, and changes in agricultural policies. Its achievement could be justified in small farms where land is the limiting factor.

The second objective is to maximize yield per unit of water applied. This requires adoption of strategic irrigations, the water being applied during the critical periods of the crop cycle, when the effects of water stress could strongly affect yields. This optimization strategy is justified when water is the limiting resource and when its cost is high.

Another objective is the economic optimization by maximizing benefits in the farm production. This approach takes into account each of the farm's limitations and may be achieved when there are no marginal benefits, that is, when the cost of the last unit of water applied is equal to the benefit produced.

Other objectives relate to the environment. One strategy would be to minimize the use of energy. Its achievement is related to the application of penalties during the periods of peak energy demand. This requires not only the scheduling of irrigation but also the selection of crops, that have lower water requirements. Other strategies consist of reducing the water return flows, thus ensuring a better use of the resources available, avoiding groundwater pollution, and controlling soil salinity. This requires avoiding overirrigation and integrating irrigation scheduling with other cultivation techniques, such as soil tillage and fertirrigation (fertilizer application through irrigation system).

Adopting these strategies implies in most cases an accurate knowledge of the functions that relate yield to the volumes of water applied. These production functions usually adopt different forms and may be related to the growth and development cycles of the crop [5].

By not applying water during a given period of crop development, there may be no negative effects on final yield, particularly when the water shortage affects only nonproductive plant organs. This is the case with cereals when complete grain development has occurred. These aspects are dealt with in Section 5.5.

Production functions represent a key element for successful irrigation scheduling [4]. At present, few production functions are well defined in relation to the amount of water applied . Therefore, they must be used very carefully (see Section 5.5).

As more and better production functions become available, irrigation scheduling will be come easier to use in response to economic criteria, as well as to environmental and technical ones, including saving water during some specific periods of plant development.

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