Irrigation Conditions

factors that determine the requirement, time, and number of waterings of crops. The conditions are determined by calculations based on the biological characteristics of the crops; on the climatic, soil, and hydrological conditions of the area being irrigated; on the irrigation method and equipment; and on the methods used in growing the crops.

The irrigation requirement is determined from the equation of the water balance of the field: E = αP + ΔW + M, where E is the total water consumption of the crops (evaporation from the soil surface and transpiration) in cu m per hectare (m³/ha), αP is the precipitation used by the crops, ΔW is the supply of water used from the reference soil layer, and M is the irrigation requirement. The irrigation requirement is supplied periodically in the form of individual watering requirements (rice is watered continuously by flooding). The number of waterings is equal to the number of watering requirements, the size of which changes during the irrigation period and depends on the growth of the root system, the soil texture, the phase of crop development, and the characteristics of the crop’s water consumption. For example, before flowering, cotton consumes 10–15 percent of the water required during the growing season; it consumes 60–70 percent in the flowering phase and 15–20 percent in the ripening phase.

The watering requirement is also determined by the technical possibility of regulating the supply of water to a field. For example, in strip or furrow irrigation the minimum quantity is 600 m³ha (maximum, 1,200 m³ha), which usually exceeds the actual water requirement of the crops in the initial period of their development. In the case of sprinkler irrigation, the requirement is lower but the crops are watered more often. The watering requirement is determined from the formula m = αH (β_max – β₀)•100, where α is the density of the soil (in tons per cu m), H is the soil layer to be moistened (0.4–0.6 m for vegetables and potatoes, 0.8–1.0 m for cereals and industrial crops, and 1.0–1.2 m for orchards and vineyards), β_m is the maximum field moisture capacity of the soil layer to be moistened, and β₀ is the preirrigation moisture content of the soil layer to be moistened (expressed as a percentage of the weight of dry soil). In case of leaching irrigation on saline soils, the theoretical irrigation requirement is increased by 20–50 percent.

In addition to irrigation during the growing season, water-supply irrigation is carried out in fall and winter at the rate of 1,000–2,000 m³ha, and the layer to be moistened is 1.0–1.5 m deep; the rate of presowing irrigation is 600 m³ha, and the depth is 0.4–0.5 m. The irrigation periods are determined by the moisture content of the soil (irrigation is used when the moisture content of the active layer is close to the minimum permissible moisture content, which is equal to twice the wilting moisture), concentration of cell sap, ambient temperature, and soil temperature. Irrigation dates are usually timed to coincide with the developmental phases of the crops. Irrigation dates and requirements are also determined by analysis of graphs.

The irrigation conditions of a crop rotation are determined by addition of the irrigation conditions of the individual crops, usually by means of a graph of the specific water consumption, which is a representation of the irrigation conditions. It shows the dynamics of the water consumption required to irrigate the entire area of a crop rotation at any time during the growing season. During actual use, the graph of irrigation conditions is corrected each year in accordance with weather, economic, and other conditions.