Knowing the water needs of crops is the basis for rational farm water management and achieving optimum yields. These needs depend closely on the crop species and even often vary within different varieties of each species.
This knowledge is particularly useful when irrigation is carried out on the basis of evaporation calculations, where lower evaporation values, and therefore lower irrigation rates, are assumed for the early development phases, and higher for the yield-forming phases, i.e. flowering, setting and filling of grain, fruit, or tubers or roots. In the case of irrigation based on soil moisture, irrigation control is based on maintaining the appropriate level of water available in the soil for the crop. Soil moisture in the root zone of plants during growth phases that do not require significant amounts of water will not fall quickly, so irrigation will be less frequent. A separate area is frost irrigation, used to protect plants from frost, especially during the flowering phases of berry and fruit crops. Over-crown irrigation is used here and the decision to irrigate is based on the weather forecast and the temperature above ground.
In general, water requirements increase as the plant grows, up to the maturity stage. In addition, for each species there are critical periods where water shortages have a particularly negative effect on growth and yield, e.g. intensive growth phases in cereals, oilseed rape, maize and others, flowering phases in berry bushes and fruit ripening, when water availability directly affects the final yield. When selecting water rates for irrigation at different times of the growing season, it is important to take into account the growth phase of the crop and adjust the rates to their actual needs. This saves water and energy for its extraction and delivery to the field. For some crops, an element of agronomic management may be to deliberately extend the period without irrigation so that the plants develop a stronger root system. An example of this is cotton, which is not irrigated during the initial growth phases so that the plants do not develop the so-called “lazy roots”. For example, maize does not require much water after emergence, only about 100 mm/mc, but already in July, 50% more, which determines both the yield of green matter and grain. In the case of permanent crops (orchard crops), irrigation should also be controlled during the post-harvest period, sometimes stopping it to force the plants to go into dormancy. At other times, the crop should be irrigated during autumn and winter to protect it from winter physiological drought, when, with sudden warming resuming the vital processes of the plants, they are unable to draw water from the frozen or over-dried ground. Increased soil moisture during the dormant period also promotes better overwintering of plants by increasing the heat capacity of the soil.