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Irrigation and water application rates and their estimation

Irrigation rate means the water quantity required to supply for crop irrigation throughout the growing season. Irrigation rate covers the deficit in water balance on a cropped hectare, that is the difference between total consumptive water use (water consumption due to plant transpiration and soil evaporation) and natural moisture deposit in soil. Irrigation rate depends on climatic and weather conditions, soil properties, crop characteristics, and its cultivation technologies. The irrigation rate for cotton is 6-10 ths m3 of water per ha, grain crops 2.5 ths m3 of water per ha, and for alfalfa 2-12 ths m3 of water per ha. Irrigation rate is broken down into water application rates.

Water application rate implies the water quantity delivered per hectare of irrigated crops for a single water application operation. The sum of the water application rates for the growing season is to be equal to the irrigation rate. Water application rate depends on the depth of the soil root zone intended to be watered, crop characteristics and development phases, texture, and hydrophysical properties of the soil, water application methods and purpose, etc. As a rule, at gravity vegetative irrigation, the water application rate comes to 600-1200 m3/ha; at sprinkling irrigation, 300-800 m3/ha; and at water-charging irrigation, 1000-2000 m3/ha.

Source: Great Soviet Encyclopedia

Water application rate:

M = E - 10 μ Hp - (Wb - We) - Wg, m3/ha,


stands for total crop water use (total evapotranspiration), m3/ha,

= Y * Kw,


Y stands for planned yield, t/ha,

Kw stands for water-use ratio, m3/t, means the ratio of the total soil water discharge, m3/ha, (i.e. soil evaporation plus transpiration) to the yield output of the main agricultural product, t/ha,

Hp stands for amount of the precipitation fallen during a given crop growing season, mm,

μ stands for precipitation utilization ratio,

Wb stands for the deposit of moisture in a design soil layer at the beginning of the growing season, m3/ha,

We stands for the deposit of moisture in a design soil layer at the end of the growing season, m3/ha,

Wg stands for the water quantity coming to design soil layer through capillaries from groundwater during the growing season, m3/ha.

There are net irrigation rate (Mn) and gross irrigation rate (Mb).

Net irrigation rate leaves out of account the water losses to seepage through canal sidewalls and bottom, evaporation, leakage through pipe junctions, etc.; therefore, more water should be taken from an irrigation source allowing for the size of these losses.

Water losses are taken into account by means of the efficiency factor (η) of irrigation systems, which comes to 0.9-0.95 for open and 0.6-0.8 closed systems. Hence, the gross rate is given by:

Mg = Mn/η, m3/ha,

As crop water requirement throughout the growing season is non-uniform and partially covered by falling precipitation, irrigation rate should be provided during dry seasons to the field not at a time but in portions.


The rate of a single water application is equal to the difference of water reserves in a design layer before and after water application.

The water quantity required to be supplied per hectare of irrigated crops for a single irrigation operation is called water application rate (m) is defined by the following formula:

m = 100 h d (ßmax - ßmin), m3/ha,


h stands for the depth of the active soil layer, m,

d stands for the volume weight of the design soil layer, t/m3;

ßmax stands for the moisture percentage of dry soil mass,

ßmin stands for the moisture percentage of dry soil mass, which corresponds to the lower moisture limit, i.e. ßmin = (0.6/0.8) ßmax;

Water application rate and time of water application to crops are determined by the grapho-analytical method elaborated by Academician A.N. Kostyakov.

During the growing season, water application rate is supplied in portions depending on the change in the thickness of the root zone, crop water requirement, natural moistening, permissible moisture limits. The more frequent water applications and less their amount, the more precise can be the required moisture regime maintained in the design layer; however, at that more technical and organizational difficulties will arise and economic costs will rise. Uninterrupted water supply coordinated with the irrigation regime would be possible only with automated drip and subsoil irrigation systems which have not yet reached a proper level of technical excellence and extensive use. Traditional irrigation methods (surface and sprinkling irrigation) allow performing only water application periodically.


Water application rate is also influenced by the thickness of soil layer and lithologic composition of subsoil: e.g. for shallow soil, under which soil of high permeability is located, water application rate is lowered. It is influenced by local topography, too: at greater slopes, water application rates should be reduced. The water applications rates on saline and subject-to-salinity soils are higher than of non-saline ones. Based on the experience of land irrigation, limits on water application rate for different irrigation methods: for surface irrigation method 80-120 mm; for sprinkling irrigation 15-70 mm; for drip irrigation 5-10 mm; for subsoil irrigation 5-10 mm.

Water application time and rate is set by different ways. In operating irrigation systems, working irrigation regime is developed according to recommendations provided by research institutions, experience of other farms, direct field observations over roots absorption capacity, cell sap concentration, and soil moisture.

When designing and reconstructing irrigation systems, based on the characteristics of similar facilities they develop or adopt a design irrigation regime with estimated frequency of watering.

Selected bibliography