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i) Net irrigation requirement
• NIR= ET - ERAIN
• where
• NIR= net irrigation requirement,
• ET= evapotranspiration, and
• ERAIN = effective rainfall.
• ERAIN is that portion of rainfall which can be effectively used by a crop, that is,
rain which is stored in the crop root zone. Therefore, ERAIN is less than total rainfall
due to interception, runoff and deep percolation (or drainage) losses.
- The net irrigation requirement is the amount of irrigation water just required to
bring the soil moisture content in the root zone depth of the crops to field capacity.
Thus, net irrigation requirement is the difference between the field capacity and
soil moisture content in the root zone before application of irrigation water.
- Is the amount of irrigation water required in order to meet the evapotranspiration
need of the crop as well as other needs such as leaching and deep percolation.
NIR (NIWR) = Consumption - Effective rainfall (some water is lost as percolation
in satisfying others needs such as leaching)
ii) Gross irrigation requirement
- The total amount of water inclusive of water in the field applied through irrigation
is termed as gross irrigation requirement, which in other words is net irrigation
requirement plus application and other losses.
- Gross irrigation water requirement (GIWR) is the amount of water to be extracted
(by diversion, pumping) and applied to the irrigation scheme. It includes NIR or
NIWR plus water losses
where E is the global efficiency of the irrigation system.
- The gross irrigation requirement (GIR) is the amount that must be pumped. GIR is
greater than NIR by a factor which depends on the irrigation efficiency (EFF):
• GIR = NIR / EFF
• where
• GIR = gross irrigation requirement (inches),
• NIR = net irrigation requirement (inches), and
• EFF = irrigation efficiency (decimal fraction).
iii) Consumption use
- It is the quantity of water used by the vegetation growth of a given area. It is the
amount of water required by a crop for its vegetated growth to evapotranspiration
and building of plant tissues plus evaporation from soils and intercepted
precipitation. It is expressed in terms of depth of water. Consumptive use varies
with temperature, humidity, wind speed, topography, sunlight hours, method of
irrigation, moisture availability.
Mathematically,
Consumptive Use = Evapotranspiration = Evaporation + transpiration
iv) Irrigation interval
- This is the time between successive irrigations. Irrigation interval is equal to:
Readily Available Moisture or Net Irrigation divided by Evapotranspiration, ET.
The shortest irrigation interval is normally use in design. The irrigation interval
varies with ET.
NatalieR answered the question on June 30, 2022 at 05:37
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