This document discusses water requirements for crops and related concepts. It defines crop water requirement as the depth of water needed for normal crop growth. Crop water requirement considers factors like evapotranspiration, rainfall, and soil water contribution. Determining crop water requirements is important for planning irrigation. Net irrigation requirement refers to the amount of irrigation water needed to bring the soil moisture to field capacity, while gross irrigation requirement includes losses and is the total amount of water applied.

1. Water Requirement for Different Crops
and Related Problems
Md. Nurul Kadir
Lecturer
Department of Agricultural Engineering
Sher-e- Bangla Agricultural University, Dhaka
6/3/2015

2. Water Requirement
Water requirement (WR) may be defined as the quantity of water, regardless
of its source, required by a crop or diversified pattern of crops in a given
period of time for its normal growth under field condition at a place.
In short, Crop water requirement (CWR) is defined as the depth of water (mm)
needed to raise crop in a given period.
It comprises of the water lost through evaporation from crop field, water
transpired and metabolically used by crop plants, water lost during
application which is economically unavailable and the water used for salt
leaching and so on.
Determination of crop water requirement is used for the planning, design, and
operation of both rain fed and irrigated agriculture.
Therefore, WR or CWR = ET or CU + application losses + special needs
Also, WR or CWR = Irrigation requirement (IR) + Effective rainfall (ER)
+ Soil profile contribution (S)
6/3/2015 Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka

3. 6/3/2015
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Factors Affecting Water Requirements of Crop
1. Sunshine
2. Temperature
3. Humidity
4. Wind speed
5. Depth and distribution of rooting
6. Size of the crop canopy (top portion)
7. Thickness of stem
8. Texture of soil
9. Organic matter in the soil
10. Type of crop grown
11. Rainfall pattern
12. Method of irrigation
13. Quantity and type of fertilizer used
Climatic
factor

4. Evapotranspiration
6/3/2015
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Terminology
 Evaporation
Process of water movement, in the vapor form, into
the atmosphere from soil, water, or plant surfaces
 Transpiration
Evaporation of water from plant stomata into the
atmosphere
 Evapotranspiration
Sum of evaporation and transpiration
 Consumptive use (CU)
Consumptive use is the total amount of water needed
to grow a crop (the sum of the water used in
evapotranspiration plus the water stored in the plant's
tissues). The term consumptive use is generally used
interchangeably with evapotranspiration because the
amount of water retained in plant tissue is negligible
compared to the amount of evapotranspiration.

5. Evapotranspiration
6/3/2015
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Factors affecting ET
1. Solar radiation 2. Temperature 3. Wind velocity 4. Relative humidity
5. Plant characteristics and growth stages 6. Soil water regime
Evaporation essentially occurs on the surfaces of open water such as lakes, reservoirs,
or puddles, or from vegetation and ground surfaces. Transpiration involves the
removal of water from the soil by plant roots, transport of the water through the plant
into the leaf, and evaporation of the water from the leaf interior into the atmosphere.

6. Evapotranspiration
6/3/2015
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka

7. 6/3/2015
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Irrigation Water Requirement (IWR):For irrigated crops, another main
concept is the irrigation water requirement (IWR), defined as the net depth of
water (mm) that is required to be applied to a crop to fully satisfy its specific
CWR. The IWR is the fraction of CWR not satisfied by rainfall, soil water
storage, and groundwater contribution.
Net Irrigation Requirement : It is
the amount of irrigation water
requirement to bring the soil moisture
level in the effective root zone to field
capacity. Thus it is the difference
between the field capacity and the soil
moisture content in the root zone
before starting irrigation.

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Md. Nurul Kadir Lecturer, Dept. of
Agril. Engg. SAU Dhaka
The root zone

9. 6/3/2015
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Gross Irrigation Requirement : the total amount of water applied
through irrigation is termed as “gross irrigation requirement”. In other
words, it is the net irrigation requirement plus losses in water application
and other losses.
Gross Irrigation Requirement =
𝑵𝒆𝒕 𝒊𝒓𝒓𝒊𝒈𝒂𝒕𝒊𝒐𝒏 𝒓𝒆𝒒𝒖𝒊𝒓𝒆𝒎𝒆𝒏𝒕
𝑭𝒊𝒆𝒍𝒅 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 𝒐𝒇 𝒔𝒚𝒔𝒕𝒆𝒎
For example, if the net amount of irrigation is 8 cm and the field efficiency
is 75%, the gross amount of water to be applied to the field is,
8
0.75
= 10.66 cm

10. 6/3/2015
Md. Nurul Kadir Lecturer, Dept. of
Agril. Engg. SAU Dhaka