1.5: Irrigation Management Concepts

Modern irrigation management is based on the concept of soil-plant-water relations. This concept is a unification system in which all processes are interdependent. In this unified system, called the soil-plant-atmosphere relations, the availability of soil water is not a property of the soil alone, but a function of the plant, soil, and environment. The rate of water uptake by the plant depends on the root's ability to absorb water from the soil, the soil's ability to transmit water toward the roots, and the evaporative demand of the atmosphere. These, in turn, depend on: (1) characteristics of the plant such as rooting density, rooting depth, rate of root growth, and the plant's ability to maintain its vital functions under water stress; (2) properties of the soil like hydraulic conductivity, soil bulk density, soil texture, soil layers, water retention, and available water capacity; and (3) weather conditions which dictate the rate of transpiration from the crop and soil evaporation. These components of the continuum will be presented and discussed for soil water (Chapter 2) and plant water use (Chapter 3).

Irrigation scheduling is the term that describes the procedure by which an irrigator determines the timing and quantity of water application. It is possible to schedule irrigations based on monitoring the soil, the plant, and/or the microclimate. By monitoring soil moisture, the idea is to measure the reserve of water within the crop root zone as it is diminished following each irrigation to ascertain when the soil water has been depleted to a prescribed minimum level. Sensing the water status of the plant is a second method to detect the beginning of plant water stress. There are many plant sensing techniques available today to measure or infer plant water status ranging from specialized equipment to visual observation. As important as the earliest detection of plant water stress is, it does not give information on how much water to apply. A third technique is to monitor the meteorological conditions that impose the evapotranspirational demand on the crop. Accumulating the amount of water lost to the atmosphere by the crop will estimate the amount of water to apply. The timing of the irrigation is established by knowing the capacity of the soil to store water or monitoring the water status of the plant. These various measuring techniques and strategies for scheduling irrigations will be presented in detail (Chapter 6).

The later chapters of the text are devoted to descriptions of the various types of irrigation systems, emphasizing the major methods employed in irrigated agriculture (Chapters 10-14). The application of agricultural chemicals through irrigation systems is presented separately (Chapter 15). Effective management of irrigation systems also benefits from a working knowledge of the hydraulics of pipeline and pumping systems (Chapter 8).