Irrigation regime is determined by the rate, number, and time of water applications to crops. It depends on the biologic characteristics of plants as well as climatic, soil, and hydrologic conditions of irrigated lands.

Water is an indispensable component of fertility. Moisture content in the soil under natural conditions can vary from the moistening condition at the level of absorbed moisture to the level of total moisture capacity. Both moisture shortage and excess is unadvisable for crops. Normal growth conditions are created when 40-50% of interstices of soil are filled with air and the rest spaces between soil particles are filled with water. As a rule, such moistening occurs when soil is saturated with moisture up to the condition of field moisture capacity determined generally by the capillary porosity of soil.

Soil moistening does not remain unchanged over the growing season; therefore, under natural conditions it is hard to constantly maintain optimum moisture content. Plant growth intensity lowers at reduced moisture from an optimal value, but the growth does not stops at all. It is inadmissible to reduce moisture content down to a critical value called wilting point. The value characterized by soil moisture content of about one-and-a-half or double maximum hygroscopicity (water absorbency) is taken as wetness/humidity index. Wilting happens in case of sandy-loam soil at volume humidity of 4-6%; for light loamy soil – at 6-8%; for medium loamy soil – at 8-10%; and for heavy loamy soil – at 10-14%.

During the growing season, sites are repeatedly irrigated. The period during which it becomes necessary to carry out irrigation is called irrigation period. Its duration depends on the duration of vegetation of the species irrigated. The water quantity supplied per hectare of irrigated field over the irrigation period is called seasonal irrigation rate (depth). This rate is calculated as the difference between crop water consumption and natural moisture resources in the soil during the growing season. The value of irrigation rate IR is determined by the following equation:

IR = E_t – P + E_s – M_b + M_e - M_q

where:

E_t stands for crop transpiration (consumption),

P stands for amount of precipitation entering the active layer (AL) of soil over the growing season,

E_s stands for evaporation during the growing season,

M_b stands for AL moisture storage at the beginning of the growing season,

M_e stands for AL moisture storage at the end of the growing season,

M_q stands for the quantity of moisture coming to AL from groundwater.

Irrigation rate is to provide moistening of only the active layer of soil without water discharge to lower horizons, and for irrigation, on the contrary, the moisture of groundwater should be used. Therefore, irrigation rate should be reduced on lands with shallow groundwater table.

With modern water application methods, water is generally delivered to the field by separate water applications. The water volume that should be supplied per hectare of irrigated land for one water application process is called water application rate, or rate of application.

The size of water application rate depends on the degree of soil wetting by the beginning of a water application process, quantity of water consumption by crops during a given crop development phase, soil moisture-holding capacity, and depth of soil wetting layer. Water application rate is influenced by the option to control water supply and water regime, as well as irrigation technique. As was mentioned above, plant wilting begins at moisture content of about one-and-a-half or double maximum hygroscopicity (water absorbency); however, water application should be started at moisture content above this critical value. For example, after sowing of seeds in the seedling development phase, when the plants do not have developed root system and suffer acutely at drying out of topsoil, it is necessary to maintain higher soil moisture content. The plants that form the root system and aboveground part by absorbing nutrients from soil and intensely transpiring moisture cause further drying out of soil layers, but more slowly take down the growth intensity under higher degree of drying out of soil than in the seedling development phase. Soil moisture capacity and soil texture, which are determined largely by the quality of agricultural technology and characterize production standards, are of paramount importance. When setting a water application rate, it is necessary that after a water application process water should not inflow the underlying layers of soil, since this will result in irrigation water losses, may cause rise of groundwater table and contribute to salinization of soils.

When setting a wetting layer on nursery gardens (or just nursery), one should take into account that deepening the wetting depth will favor the development of the root system. When taking out planting material, a considerable part of the roots is cut. This will decrease the establishment of the planting material on a reafforesting area or when planting in transplant sections of nurseries. Hence, the soil wetting depth in nurseries of the seedlings of hardy-shrub species is taken equal to 10-30 cm depending on the development phase and type of plants. In seedlings nurseries, the wetting depth is increased from 0.4-0.5 m; in young gardens, up to 0.6-1.0 m; in parks, up to 1.0-1.2 m.

Water application rate is calculated by the formula:

m = 100 H α (r_wa – r₀)

where:

Н stands for wetting layer,

α stands for soil bulk density,

r_wa – r₀ stand for soil moisture after and before water application, respectively; in percent to absolutely dry soil.

Soil bulk density depends on the texture, which comes to 1.38 for sandy-loam soil, 1.40 for light loamy soil, 1.42 for loamy soil, 1.45 for heavy loamy soil, and 1.50 for clay. Depending on the type of wood species and phase of forest stand development, water application rate depends also on soil type.

Nursery garden irrigation regime. The number of water applications varies within a wide range depending on the type and age of woody species, climatic conditions, water and physical properties of soil, and groundwater depth. In the excessive moistening zone, water application in nursery gardens is required only after sowing and in the seedling development phase, and from time to time in dry periods and years. In the unstable moistening zone, the number of water applications may come to 5-7, and 8-10 and over in the insufficient moistening zone.

Orchard and forest stand irrigation regime. When irrigating orchards, their soil gets wetted to a depth of 1 m, because the active layer of the root system is located within that depth. In orchards, water application rate depends on the type of fruit and berry crops, their age, and soil texture. In fruit planting material nurseries, water application is executed at least 3-5 times at a rate of 150-200 m³/ha for sandy-loam soil to 250-350 m³/ha for heavy loamy soil. For young non-bearing orchards, 2-3 water applications would be enough at a rate of 300-500 m³/ha. In apple-bearing orchards, the first water application is required after dropping excessive fruit; the second water application is needed two-three weeks prior to summer fruit ripening. In the Moscow province, for example, at most three water applications are recommended at a rate of 400-600 m³/ha. The water application rate comes approximately to 1100-1300 m³/ha.

Berry fields are to be irrigated three times: in the period of green seed buds on plants; when small fruit ripens, and after harvesting for formation of fruit buds of the next year.

On tree plantations, water application is advisable when forest shelterbelts and parks are developed.

Irrigation of tree plantations of natural origin is to be carried out occasionally, and the most appropriate irrigation way is establishment of coastal lakes.

Source: www.skyrage.ru