Lecture note

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. Crop Water Requirement
Reference:
Crop Evapotranspiration, FAO Irrigation and Drainage Paper 56, FAO,
Rome
Major functions of water are:
1. Cell development
2. Solvent for nutrients and for translocation within the plant
Water is adsorbed by the roots and only about 5% of the water
adsorbed is actually used by the plant itself.
Transpiration: Plant leaves contain stomata (8,000 to 120,000 per cm2).
In presence of light, the stomata opens and CO2 and HO2 react to
form starch which is used in building the plant tissue. In the
process, water escapes from the stomata which is known as
transpiration.
Evaporation: Change of water from liquid to vapour state through
transfer of heat.

3. Plant Soil
Plant Soil
Root
Root

4. Stomata
In presence of sunlight:
6H2O + 6CO2 ----------> C6H12O6+ 6O2

5. Crop Water Requirement (CWR): Rice
Crop water requirement (CWR): the quantity of water
required by a crop for its normal growth under field
condition. It includes ET and other unavoidable losses.
CWR of rice = Evapotranspiration (ET) + Seepage and
Percolation (S&P)
The main reasons for keeping standing water in rice field
are: (1) stress free high yield (2) fertilizer management (3)
reduction of weed growth and (4) easier water
management.
PET is often estimated from evaporation of US Class A pan
(125 cm in diameter and 25 cm high).
For lowland rice, which is grown with standing water the ET is
about 1.2 times pan evaporation.

6. Evaporation Pan

8. CWR: Rice
Estimation of ET:
There are a number of methods to estimate Reference Crop
Evapotranspiration (ET0). The choice of a method depends
upon the availability of data (solar radiation, temperature,
wind speed, sunshine hours, relative humidity, crop
reflectance etc.)
Blaney-Criddle Method
Modified Penman Method
Penman-Monteith Method: Recommended by FAO and
NWMP. CROPWAT (FAO software)
ET (crop) = ET0 x KC ; where ETo = Reference crop
evapotranspiration (mm/day) and KC is the crop coefficient
The crop coefficient KC is dependent on type of crop and crop
growth stage.

9. Evapotranspiration
The Penman-Monteith equation is:
Where, ET0 is the reference crop evapotranspiration, Rn is the net radiation, G is the
soil heat flux, (es - ea) represents the vapour pressure deficit of the air, r a is the mean
air density at constant pressure, cp is the specific heat of the air, D represents the slope
of the saturation vapour pressure temperature relationship, g is the psychrometric
constant, and rs and ra are the (bulk) surface, u2 is the wind velocity at 2 m height and
aerodynamic resistances.
A software called CROPWAT has been developed by FAO to calculate the ET0 by
Penman-Monteith method.
Penman-Monteith Formula:
Empirical Methods of Calculation of ET
The Blaney-Criddle formula:
ETo = p (0.46 T + 8)
ETo = Reference crop evapotranspiration (mm/day)
T = mean daily temperature (°C)
p = mean daily percentage of annual daytime hours

10. Determination of ET with CROPWAT (version 8.0)
Data Requirement
Latitude
Longitude
Altitude
Temperature
- Maximum
- Minimum
Humidity
Wind speed
Sunshine hours

11. Growth
Stage
Crop
HYV Boro HYV Aus HYV Aman Wheat Potato Maize Winter Veg.
Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days
Nursery 1.20 30 1.20 30 1.10 30
Initial stage 1.10 30 1.10 30 1.10 20 0.59 30 0.58 25 0.45 25 0.55 15
Dev. Stage 1.10 30 1.10 30 1.10 30 1.05 30 1.12 30 0.45 35 0.55 25
Mid season 1.25 30 1.10 30 1.10 40 1.10 30 1.13 45 1.10 40 1.00 35
Late
season
1.00 30 0.85 10 0.90 30 0.69 30 0.92 30 0.55 30 0.98 15
Total
duration
150 130 150 120 130 130 90
Growth stages, durations and KC of major crops
Generally: Initial and development stages are considered as vegetative stage
Mid-season as reproductive stage, and
Late season as ripening stage

13. CWR: Rice
Although ET is the true CWR, in applying it water is lost as
seepage (lateral movement) and percolation (vertical
movement). As seepage (S) and percolation (P) loss occur
simultaneously and it is difficult to measure them separately
in the field, S&P loss is considered together.
S&P loss depends upon soil texture, existence of plow pan,
depth of standing water, water table depth etc.
Methods of Measuring S&P
Sloping Gage
PVC Gage
Lysimeters have been used for the measurement of P.
Once the ET and S&P are known, the CWR can be determined
on daily, decade, monthly and seasonal basis.

17. Irrigation Water Requirement (IWR) of Rice
IWR = CWR - Effective Rainfall (RE )
= ET + S&P - RE
Effective Rainfall (RE )
Useful or utilizable portion of rainfall that is stored and
supplied to meet the crop water requirement. It excludes
deep percolation, soil storage, surface runoff and
interception.
There are a number of methods for calculating RE:
For monthly total rainfall (RT ), RE (USDA) is :
RE = RT (125 – 0.2 RT )/125 if RT is < 250 mm
RE = 125 + 0.1 RT if RT is > 250 mm

18. IWR of Rice
As in Bangladesh, the rainfall is scarce during the dry season,
dependable rainfall (80% probable) is considered as effective
rainfall.
Land Preparation (LP)
LP of rice fields means plowing and puddling, which breaks
down the soil aggregates for ease of transplantation and
reduces percolation. Puddling levels the land, incorporates
weeds and stubbles, reduces weed growth and promotes
good water management.
Water requirement for LP includes:
evaporation from wetted fields
water required to establish a water layer
water absorbed in the plow layer (land soaking)
S&P beyond the plow layer

20. IWR of Rice
About 150-180 mm of water is required for LP provided the
operations are completed within two weeks.
IWR = (CWR – RE ) + LP
= (ET + S&P – RE) + LP
No irrigation is provided during last 15 days of crop season
and the fields are drained: Terminal Drainage
Gross IWR
= IWR / Efficiency
Efficiency (%) is that part of water pumped/diverted which is
effectively used by the plants. It depends upon the
conveyance and other losses and leakages.
For lined canals the efficiency is 95%.
For unlined canals the efficiency varies from 40%-80%,
depending upon the soil type and level of maintenance and
management.

22. Months
ETo (mm/day)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ave
ETo 1.9 2.7 3.8 4.8 5.2 3.9 4.1 3.9 4.0 3.5 2.6 2.0 3.53
Table-5: ETo of Rangpur Station.
Months
Rainfall (mm/day)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Rainfall (KIPNU) 5.58 11.84 37.40 153.11 338.52 548.04 589.21 392.72 384.54 210.69 9.53 7.49 2688
Rainfall (KIPSU) 6.33 14.28 45.76 157.23 331.03 459.90 505.30 302.83 313.70 190.28 8.58 8.98 2344
Table-6: Monthly Average Rainfall of the Project Area (KIPNU & KIPSU)
Growth Stage
Crop
HYV Boro HYV Aus HYV Aman Wheat Potato Maize Winter Veg.
Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days
Nursery 1.20 30 1.20 30 1.10 30
Initial stage 1.10 20 1.10 30 1.10 20 0.59 30 0.58 25 0.45 25 0.55 15
Dev. Stage 1.10 30 1.10 30 1.10 30 1.05 30 1.12 30 0.45 35 0.55 25
Mid season 1.25 40 1.10 30 1.10 40 1.10 40 1.13 45 1.10 40 1.00 35
Late season 1.00 30 0.85 10 0.90 30 0.69 30 0.92 30 0.55 30 0.98 15
Total duration 150 130 150 130 130 130 90
Table-8: Growth Stage, Duration and Kc

23. Example
Calculate the HYV Boro command area of a STW at
Rupnagar with a discharge of 15 l/s from the following data (120 day
growth period: 15 January -15 May):
1. Conveyance efficiency is 70%.
2. Maximum pumping hours is 12 hrs/day
3. Seepage and percolation rate is 5 mm/day
4. Zero effective rainfall during the season.
ET (crop) = ET0 x KC = 5.5 x 1.25 = 6.9 mm/day (maximum calculated
ETcrop for the season)
CWR = 6.9 + 5.0 = 11.9 mm/day
IWR = 11.9 – RE (0) mm/day = 11.9 mm/day (1.377 l/ha/sec)
Gross IWR = 11.9 / 0.7 = 17 mm/day = 0.017 m/day
Volume pumped from STW = 15 x 3600 x 12 l/day = 0.648
million liters/day = 648 m3 /day
Command area = 648 / 0.017 = 38117.6 m2 = 3.81 ha
= 15 / (1.377/0.7) / 2 = 3.81 ha
Effect of rainfall:
To be deducted from all the CWR to determine the peak requirement.

24. CWR of Non-Rice Crops
Saturated Moisture Content: All voids are filled with water
Field Capacity (FC): Moisture content after draining of gravitational water;
Moisture content 1-3 days after irrigation; Moisture content at 0.1- 0.3 bar
Permanent Wilting Point (PWP): Moisture content at 15 bar
Available Water = FC – PWP
Readily Available Water = 50% of Available Water
Moisture content (by volume) cc/cc = (Dry density / Water density) x
Moisture content (by weight; gm/gm)
= bulk (dry) density x Moisture content by weight
For Non-Rice crops:
CWR = ETCROP
ETCROP = ET0 x KC where, KC is the crop coefficient which would
vary from crop to crop and for a crop upon its different growth stages.
Like rice, ET0 can be estimated by Penmen-Monteith method (Cropwat).
IWR = ETCROP – RE ; where, RE is the effective rainfall.
Irrigation is actually provided at the critical growth stages.

25. Water Holding Capacity of Soil

28. IWR of Non-Rice Crops
IWRFIELD = IWR / Field Application Efficiency (EF )
GL
Plow pan
Field Application Efficiency (EF):
60% for surface irrigation
75% for sprinkler irrigation
90% for drip irrigation
IWR= (ETCROP – RE ) / EF
Gross IWR
= IWR / Efficiency (same as rice)

29. SWD s
SWD
SWD

31. Example
Calculate the command area of a STW (discharge of 15 l/s and
maximum 12 hours of pumping) for wheat grown at
Rupnagar during 15 November-15 March in a soil with FC
and PWP of 27% and 5% (by weight) respectively. If the crop
root zone is 1 m, the dry density (bulk density) of the soil is
1.5, field and scheme irrigation efficiencies are 60% and
70%, then calculate the amount of each irrigation at field
level.
CWR = ETCROP (peak) = 4.8 x 1.2 = 5.76 mm/day (in
February)
IWRFIELD = 5.76 / 0.6 = 9.6 mm/day = 1.11 l/s/ha
IWRSCHEME = 1.11 / 0.7 = 1.586 l/s/ha
Command area = 15 / 1.586 / 2 = 4.73 ha
Root Zone Storage = 0.5 x 1000 x (0.27 – 0.05) x 1.5 =165mm
Irrigation amount = 165 / 0.6 = 275 mm

32. For the modern wheat varieties (BARI Gom 25) grown in Bangladesh, the crop
growth period is 110 days: 20 + 30 + 30 + 30