Deficit irrigation refers to a water management strategy where, on purpose, the root zone is not refilled to field capacity by irrigation. Soils are ‘leaky systems’, which means that any water above field capacity will percolate or ‘leak’ beyond the root zone and be lost for crop uptake. Rainfalls directly after irrigation will, most likely also result in water loss due to deep drainage. If there is a good chance of rainfall occurring during the growing season, rainwater use can be optimised by not completely refilling the profile to field capacity (‘full point’) with irrigation. This is called deficit irrigation or a ‘room for rain’ strategy. The selected ‘room for rain’ amount depends on factors such as the sensitivity of the crop to water stress, the water holding capacity of the soil, the chance of rain falling and the expected quantity of rain. This strategy will only be sensible if there is a good chance of substantial rainfall events during the growing season of a crop. In the example illustrated below, a fixed irrigation interval of once every seven days is used and 5 mm ‘room for rain’ is allowed. Two instances of rainfall occurred shortly after irrigation events. Since there was provision for ‘room for rain’, the rainwater could be utilised effectively, with little or no drainage.
Applying a deficit irrigation strategy.
Irrigation scheduling where 5 mm ‘room for rain’ is kept on every irrigation event. Arrows indicate irrigation (black) and rainfall (blue) events.