Irrigation scheduling is the process of deciding when to irrigate and how much water to
apply. The objective is to maintain optimum soil water conditions for crop growth in
order to meet crop yield and quality targets with minimum water wastage.
Although many approaches have been promoted over the years for all crops, it remains the case
that only a minority of farmers use an objective (scientific) method of scheduling
irrigation, and most still rely solely on their judgment or intuition. The concept of a soil
water deficit is an important component of irrigation scheduling, virtually independent
of the method of scheduling used.
It is a measure of how much water is needed to bring
the soil profile back to field capacity (the maximum depth of water that soil can retain
against gravity). A limiting deficit is the critical deficit beyond which a crop will
begin to suffer water stress. Deficit irrigation is when only a proportion of the water
needed to rewet the soil to field capacity is applied at each irrigation event. This can
sometimes be more economic than full irrigation.
Rising energy, labour and water costs, the need to increase water productivity, less
water available for abstraction due to expansion of cropped areas, intensification of
existing plantations, increasing competition for limited resources, climate change
risks and demands for greater environmental protection are now the driving forces
influencing technology choice in irrigated crop production. In this context a good
understanding of soil–plant–water relations is important, and more accurate scheduling
may prove to be a useful adaptation strategy.