CIGR Handbook of Agricultural Engineering, Volume I Land and Water Engineering, Chapter 5 Irrigation and Drainage, Part 5.3 Irrigation Scheduling Techniques

First paragraph: 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].