1.2: Role of Irrigation

The irrigation process consists of introducing water to the soil profile where plants can extract it to meet their needs, mainly evapotranspiration. An important goal of irrigators is to design and manage their irrigation system to optimize placement and timing of applications to promote growth and yield while protecting against soil erosion, salination, water quality degradation, or other detrimental environmental impacts. Since physical circumstances and socioeconomic conditions are site specific, there is no single answer to designing, developing, and managing an irrigation system. In all circumstances, however, the factors and principles involved are universal.

The practice of irrigation has evolved gradually toward improved control over plant, soil, and even weather variables. The degree of control possible today is still only partial because of unpredictable extremes in the weather. Modern irrigation is a sophisticated operation, involving the monitoring and manipulation of numerous factors impacting crop production. With the continuing loss of suitable land and water and the rising demand for agricultural products, the search for new knowledge on how to improve irrigation and the need to apply this new knowledge have become increasingly urgent.

Any attempt to irrigate must be based on a thorough understanding of soil-water-plant relationships. The movement of water, once applied, consists of a sequence of dynamic processes beginning with the entry of water into the soil, called infiltration. The rate of infiltration is governed by the rate at which water is applied to the soil surface, as long as the application rate does not exceed the capacity of the soil to absorb it. An important criterion for a sprinkler or microirrigation system is to deliver water at a rate that will prevent ponding, runoff, and erosion.

After infiltration, water normally continues to move because of gravity and hydraulic gradients in the soil. Water moves downward and, with some irrigation systems, laterally in a process called redistribution. In this process the relatively dry deeper zone of the soil profile absorbs water draining from wetter zones above. Within a few days (depending on the irrigation system and management) the rate of flow becomes so low as to be negligible. The water content of the wetted zone as flow becomes negligible is termed the field capacity and represents the upper limit of the soil's capacity to store water. Field capacity is normally higher in clay than in sandy soils.

Any water draining below the root zone is generally considered to be a loss from the standpoint of immediate plant water use. It is not necessarily a final loss, however. If the area is underlain by an exploitable aquifer, the water percolating below the root zone may eventually recharge the aquifer and be recovered by pumping. Some deep percolation may later return to streams or drainage systems. This quantity of water plus surface runoff from irrigated agriculture is called return flow. Where the water table is close to the soil surface, some water may enter the root zone by capillary rise up from the saturated zone below the water table and supply a portion of the crop's water requirement. This process of subirrigation, however, may infuse the root zone with salts. Water flowing down through the root zone may leach soluble salts or crop nutrients and degrade the quality of groundwater.

Properly designed and managed, modern irrigation methods can increase crop yields while avoiding waste, reducing drainage, and promoting integration of irrigation with essential concurrent crop management operations. The use of degraded water has become more feasible, and coarse-textured soils, steeply sloping lands, and stony soils, previously considered not irrigable, are now productive. Such advances and their consequences were unforeseen only a few decades ago.