Crop Water Requirement Factors and Calculations

3. CROP WATER REQUIREMENTS (CWR)
09-May-22
Ceng5082
Mengistu .Z (MSc in Hydraulic Engineering )
Lecturer @ Hydraulic and Water Resources Engineering
department
Mizan Tepi university
Email: mengistu.zantet@gmail.com
mengistuzantet@mtu.edu.et
P.O.Box: 260
Tepi, Ethiopia
MENGISTUZANTET@MTU.EDU.ET
LECTURER@ HYDRAULIC AND WATER
RESOURCES ENGINEERING
DEPARTMENT

3. GENERAL ASPECTS OF CROP WATER REQUIREMENTS
09-May-22
3.1 Reference Evapotranspiration
3.2 Crop Water Requirements/Consumptive Use
3.3 Irrigation Efficiency and Irrigation Frequency
3.4 Irrigation Scheduling

Objective of Crop Water Requirement Study:
To decide possible cropping pattern of area
Effective use of available water
Plan and design an irrigation project
Plan water resource development in an area
Assess irrigation requirement of an area
Management of water supply from sources
09-May-22
DEPARTMENT

CROP WATER REQUIREMENTS
09-May-22
It is the total amount of water required by the crop in a
given period of time for normal growth, under field
conditions.
 It includes evapo-transpiration, water used by crops for
metabolic growth, water lost during application of water and
the water required for special operations such as land
preparation, tillage and salt leaching etc.
DEPARTMENT

09-May-22
It is expressed as the surface depth of water in
mm, cm or inches per unit cropped area.
CWR = Consumptive use (Cu) + conveyance
losses (Wu) + water required for special
operation (Ws)
Consumptive use includes water retained in
plant tissue at harvest, but this is generally
minor relative to amount of ET
DEPARTMENT

SOURCES OF WATER FOR CROP USE
09-May-22
1) Effective Precipitation (ER):
It is that part of total precipitation which is used
by crop as soil water reserve.
It is the precipitation falling during the growing
period of a crop that is available to meet the
evapotranspiration needs of the crop.
It is determined as:
ER = Total rainfall (P) – Runoff (R) – deep percolation (PW)

GROSS IRRIGATION REQUIREMENTS OF CROPS (GIR)
09-May-22
 It refers to the amount of water applied to the field from the start of land
preparation to harvest of the crop together with the water lost through
distributaries and field channels and during water application to the crop
field.
GIR = CWR – (ER + ∆SW + ∆GW)
Net Irrigation requirements
 It refers to the amount of water needed to replenish soil moisture
deficit in the crop field.
NIR = GIR x Efficiency of water application
 = Cu – ER - conveyance losses

2) SOIL WATER CONTRIBUTION FOR CROP USE (∆SW):
09-May-22
 It refers to the difference in moisture content at the time of
sowing and harvesting of the crops that may be positive or
negative. It is given as:
• Where:
• ∆SW = soil water contribution in cm
• Msi = moisture content at the time of sowing in the ith layer, %
• Mhi = moisture content at the time of harvesting in the ith layer, %
• Asi = Apparent specific gravity of soil (The specific gravity of a porous solid
when the volume used in the calculations is considered to exclude the
permeable voids)
• Di = depth of ith layer of the root zone soil, cm

3) Ground Water Contribution for Crop Use (∆GW):
09-May-22
DEPARTMENT
It refers to the water used by crops due
to capillary rise in case of shallow water
tables.
CWR = ER + GIR + ∆SW + ∆GW

EVAPOTRANSPIRATION (ET)
09-May-22
 The process whereby water is lost on one hand from the soil
surface by evaporation and on the other hand from the crop
by transpiration.
 Evaporation from the soil surface
 Transpiration by the crop
 Evaporation: Liquid water is converted to water vapour (vaporization)
and removed from the evaporating surface (vapour removal).
 Energy is required to change the state of the molecules of water from
liquid to vapour. This energy is mainly from solar radiation and, to a lesser
extent, from the ambient temperature of the air.
DEPARTMENT

 If sufficient moisture is always available to
completely meet the needs of vegetation fully
covering the area, the resulting evapotranspiration is
called Potential Evapotranspiration (PET)
 The real evapotranspiration occurring in a specific
situation is called Actual Evapotranspiration (AET)
 Field Capacity is the maximum quantity of water
that the soil can retain against the force of gravity. Any
higher moisture input to a soil at field capacity simply
drains away. 09-May-22
DEPARTMENT

 Permanent Wilting Point is the moisture content of
a soil at which the moisture is no longer available in
sufficient quantity to sustain the plants.
 At this stage, even though the soil contains some
moisture, it is so held by the soil grains that the
roots of the plants are not able to extract it in
sufficient quantities to sustain the plants and
consequently the plant wilt
09-May-22
DEPARTMENT

 The difference between field capacity and
permanent wilting point is called Available
Moisture, the moisture available for plant growth
 The field capacity and permanent wilting point are
dependent upon the soil characteristics
09-May-22
DEPARTMENT

CONSUMPTIVE USE (CU)
09-May-22
DEPARTMENT
It is the evapotranspiration plus the water used by
plants for metabolic activities which is hardly 1 % of ET
 is the water required by plants to fulfill the
evapotranspiration needs of crops. (FAO)
 is the total amount of water used by the plants in
transpiration (building of plant tissues etc) and
evaporation from adjacent soils or from plant leaves in
any specified time period. (S.K. GARG)

CLASSIFICATION OF CONSUMPTIVE USE
09-May-22
DEPARTMENT
a) Daily Consumptive use:
The amount of water consumptively used during 24-hours
b) Peak Period consumptive use:
It is the average daily consumptive use during a few days (6
to 10 days) of highest consumptive use in a season.
c) Seasonal consumptive use:
It is the amount of water consumptively used by crops
during the entire cropping season/period.

IMPORTANT TERMINOLOGY ON EVAPOTRANSPIRATION
09-May-22
DEPARTMENT
• Potential / reference crop evapotranspiration (ETo):
The highest rate of evapotranspiration (ET) by a short and
actively growing crop or vegetation with abundant foliage
(leafage) completely shading the ground surface and
abundant soil water supply under a given climate.
An extensive surface of short green grass cover of uniform
height (0.12m), actively growing, completely shading the
ground and no water shortage resembles the reference
crop.

ACTUAL CROP EVAPOTRANSPIRATION (ETC):
09-May-22
DEPARTMENT
 It refers to the evapotranspiration from a disease
free crop growing in a large field under optimal
soil conditions with adequate water and fertility
and giving full potential production under the
given environment.
Usually calculated by multiplying the Crop
Coefficient (Kc) for the period with ETrc, thus
 ETcrop = Kc. ETrc

FACTORS AFFECTING TRANSPIRATION PROCESS
09-May-22
Radiation
Air temperature
Air humidity
Wind speed
Soil water content and the ability of the soil to
conduct water to the roots
Crop characteristics
Environmental aspects and cultivation practices.

EVAPOTRANSPIRATION…
09-May-22
Leaf area Index (LAI) = Leaf area/Soil area covered by
leaf
At sowing nearly 100% of ET comes from
evaporation, while at full crop cover more than 90%
of ET comes from transpiration.
DEPARTMENT

FACTORS AFFECTING ET
09-May-22
A) Climatic (weather) parameters (Ra , T, U, RH)
B) Crop characteristics
- Crop type & Variety
- Growing length/development stages
C) Management factors
 Irrigation method
 Irrigation management
 Cultivation practices
 Fertility management
 Disease and pest control
D) Environmental (soil) factors
DEPARTMENT

FACTORS AFFECTING ET
09-May-22
DEPARTMENT

REFERENCE EVAPOTRANSPIRATION - ETO
09-May-22
is defined as ET from a hypothetical crop with
an assumed height of 0.12 m having a surface
resistance of 70 s/m and an albedo of 0.23,
closely resembling the evaporation of an
extension surface of green grass of uniform
height, actively growing and adequately
watered.
DEPARTMENT

CROP EVAPOTRANSPIRATION UNDER STANDARD
CONDITIONS - ETC
09-May-22
refers to the evapotranspiration from excellently
managed, large, well-watered fields that achieve full
production under the given climatic conditions.
 Crop Evapotranspiration under non-standard
conditions – ETc-adj.
Due to sub-optimal crop management and
environmental constraints that affect crop growth and
limit evapotranspiration, ETc under non-standard
conditions generally requires a correction.
DEPARTMENT

09-May-22
ETc = ETo * Kc
ETc –adj. = ETo * Ks*Kc
DEPARTMENT

METHODS OF ESTIMATING EVAPOTRANSPIRATION
09-May-22
DEPARTMENT
These methods are classified into three types:
1) Direct methods
• Lysimetric method
• Field experimentation method
• Soil water depletion method
• Inflow-outflow method
2) Pan evaporimeter method
• USWB class-A pan evaporimeter
3) Empirical methods
• Blaney criddle method
• Penman method
• Modified penman method
• Radiation method
• Penman Monteith equation

DIRECT MEASUREMENT APPROACHES
09-May-22
1- Lysimeter method
The crop grows in isolated tanks filled with either
disturbed or undisturbed soil. In precision weighing
lysimeters, where the water loss is directly measured by
the change of mass, evapotranspiration can be
obtained with an accuracy of a few hundredths of a
millimeter, and small time periods such as an hour can
be considered.
DEPARTMENT

2. FIELD EXPERIMENTAL PLOTS
09-May-22
Suitable for determination of seasonal water
requirements.
Water is added to selected field plots, yield obtained
from different fields are plotted against the total amount
of water used.
The yield increases as the water used increases for some
limit and then decreases with further increase in water.
The break in the curve indicates the amount of
consumptive use of water.

3. SOIL MOISTURE STUDIES
09-May-22
soil moisture measurements are done before and after
each irrigation application.
Knowing the time gap b/n the two consecutive
irrigations, the quantity of water extracted per day can
be computed by dividing the total moisture depletion
b/n the two successive irrigations by the interval of
irrigation.
Then a curve is drawn by plotting the rate of use of
water against the time from this curve, seasonal water
use of crops is determined.
DEPARTMENT

4.INFLOW-OUTFLOW METHOD
09-May-22
DEPARTMENT
Used to estimate yearly consumptive use over large area,
also called as water balance method
ET = I + P - RO - DP + CR ± ΔSF ± ΔSW
where: I - Irrigation
P – rainfall
RO - Runoff
CR – capillary rise
DP – Deep percolation
SF – subsurface flow
SW – soil water content
Change in soil water storage is considered negligible and
it is assumed that the subsurface inflow into the area is
same as subsurface outflow

5. SOIL-WATER BALANCE APPROACH
09-May-22
DEPARTMENT

2) PAN EVAPORIMETER METHOD
09-May-22
DEPARTMENT
A) USWB class-A pan evaporimeter:
There exist a close relationship between the rate of
consumptive use by crop and the rate of evaporation from
properly located pan evaporimeter.
Pan evaporation is the combined effect of all atmospheric
factors and is independent of plant and soil factors
Crop evapotranspiration rates for various crops may be
estimated from the pan evaporation rates multiplied by a
factor known as crop factor (Kcrop) which varies with the
stages of growth, extent of ground cover with foliage,
climate and geographical locations

09-May-22
DEPARTMENT
It is the most widely used evaporimeter for finding
evaporation from the free water surface
The Class A Evaporation pan is circular, 120.7 cm in diameter
and 25 cm deep. It is made of galvanized iron (22 gauge) with a
stilling pan
The pan is mounted on a wooden open frame platform which
is 15 cm above ground level to facilitate the circulation of air
beneath the pan
Daily evaporation rate is given by the fall in water level
measured in the stilling well by hook gauge

Adjustments are made to the evaporation
values if rainfall occurs during a period of
measurement
After measuring the drop in water level each
time, water is added to the pan to bring back
the water level to original position of pointer tip
level
09-May-22
DEPARTMENT

EMPIRICAL METHODS OF ESTIMATING ETO
09-May-22
The following methods are the combination of some empirical,
analytical and theoretical approaches.
1.FAO Balnney-Criddle Method
2.FAO Radiation Method
3.FAO Penman Method
4.Hargreave's Class A Pan Evaporation Method
5.FAO Pan Evaporation Method
6.FAO Penman-Monte ith Method
7.Thornthwaite Method
DEPARTMENT

1) BLANEY CRIDDLE METHOD:
09-May-22
DEPARTMENT
Developed a formula for estimating CU based
on temperature, daylight hours, and locally
developed crop coefficients
F
K
T
E 54
.
2
 
 100
/
f
h T
P
F
Where,
ET = PET in a crop season (in cm)
Ph = monthly percentage of annual day-time hours, depends on the latitude of the
place (Table 5)
K= an empirical coefficient depends on the type of the crop (Table 6)
F= sum of monthly consumptive use factor for the period
o

TABLE-5:
09-May-22
DEPARTMENT
Table 5: Monthly daytime hours percentages Ph (hours) in north latitude
for use in Blaney-criddle formula
North
Lat.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 8.50 7.66 8.49 8.21 8.50 8.22 8.50 8.49 8.21 8.50 8.22 8.50
10 8.13 7.47 8.45 8.37 8.81 8.60 8.86 8.71 8.25 8.34 7.91 8.10
15 7.94 7.36 8.43 8.44 8.98 8.80 9.05 8.83 8.28 8.26 7.75 7.88
20 7.74 7.25 8.41 8.52 9.15 9.00 9.25 8.96 8.30 8.18 7.58 7.66
25 7.53 7.14 8.39 8.61 9.33 9.23 9.45 9.09 8.32 8.09 7.40 7.42
30 7.30 7.03 8.38 8.72 9.53 9.49 9.67 9.22 8.33 7.99 7.19 7.15
35 7.05 6.88 8.35 8.83 9.76 9.77 9.93 9.37 8.36 7.87 6.97 6.86
40 6.76 6.72 8.33 8.95 10.02 10.08 10.22 9.54 8.39 7.75 6.72 6.52

TABLE-6:
09-May-22
DEPARTMENT
Table 6: Values of K for selected crops for use in Blaney-Criddle formula
Crop. Value of K Range of monthly values
Rice 1.10 0.85-1.30
Wheat 0.65 0.50-0.75
Maize 0.65 0.50-0.80
Sugarcane 0.90 0.75-1.00
Cotton 0.65 0.50-0.90
Potatoes 0.70 0.65-0.75
Natural vegetation
Very dense 1.30
Dense 1.20
Medium 1.00
Light 0.80

2) PENMAN METHOD
09-May-22
DEPARTMENT
 Developed the formula using important climatic
parameters such as solar radiation, temperature,
vapour pressure and wind velocity to compute the
evaporation from open free water surface
 ET is obtained by multiplying with crop coefficient
• it is quite satisfactory for both humid and arid regions under
calm weather conditions
• It drawback is that it uses many climatological parameters
that are difficult to obtain





A
E
AH a
n
PET

PET = daily potential evapotranspiration (in
mm/day)
A = slope of the saturation vapor pressure
versus temperature curve at the mean air
temperature (mm of Hg/oC) Table 1.
Hn = net radiation in mm of evaporable water per day
(mm/day)
Ea = parameter including wind velocity and
saturation deficit
g = psychrometric constant = 0.49 (mm of
Hg/oC)
09-May-22
DEPARTMENT

Where,
Ha =incident solar radiation outside the atmosphere on a
horizontal surface expressed in mm of evaporated
water/day. It is a function of the latitude (f) and period
of the year as indicated in Table 2
a =a constant depending upon the latitude (f) and is
given by a = 0.29Cosf
b = a constant with average value of 0.52
n =actual duration of bright sunshine (in hours)
09-May-22
DEPARTMENT
    






-
-







-

N
n
e
T
N
n
b
a
r
H
H a
a
a
n 90
.
0
10
.
0
092
.
0
56
.
0
1 4


N = maximum possible hours of bright sunshine (it is
a function of latitude as indicated in Table 3)
r = reflection coefficient albedo. Usual range of
values of r for different surface conditions are given in
Table 4
s = Stefan-Boltzman constant = 2.01 x 10-9 mm/day
ea = actual mean vapor pressure in the air (in mm of
Hg)
Ta = mean air temperature in degree Kelvin =
273+? oC
09-May-22
DEPARTMENT

09-May-22
The parameter Ea is estimated as:
Where
ew =saturated vapor pressure at mean air temperature
(in mm of Hg)
ea = actual vapor pressure of air (in mm of Hg)
u2 = wind velocity at 2 m above ground (in km/h)
 
a
w
a e
e
u
E -








160
1
35
.
0 2

TABLE-1:
09-May-22
DEPARTMENT
Table 1: Saturation vapor pressure of water
Temperature (oC)
Saturation vapor pressure ew (mm of Hg) A (mm/oC)
0 4.58 0.30
5.0 6.54 0.45
7.5 7.78 0.54
10.0 9.21 0.60
12.5 10.87 0.71
15.0 12.79 0.80
17.5 15.00 0.95
20.0 17.54 1.05
22.5 20.44 1.24
25.0 23.76 1.40
27.5 27.54 1.61
30.0 31.82 1.85
32.5 36.68 2.07
35.0 42.81 2.35
37.5 48.36 2.62
40.0 55.32 2.95
45.0 71.20 3.66

TABLE-2:
09-May-22
DEPARTMENT
Table 2: Mean monthly solar radiation at top of atmosphere, Ha
(in mm of evaporated water/day)
North Lat. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 14.5 15.0 15.2 14.7 13.9 13.4 13.5 14.2 14.9 15.0 14.6 14.3
10 12.8 13.9 14.8 15.2 15.0 14.8 14.8 15.0 14.9 14.1 13.1 12.4
20 10.8 12.3 13.9 15.2 15.7 15.8 15.7 15.3 14.4 12.9 11.2 10.3
30 8.5 10.5 12.7 14.8 16.0 16.5 16.2 15.3 13.5 11.3 9.1 7.9
40 6.0 8.3 11.0 13.9 15.9 16.7 16.3 14.8 12.2 9.3 6.7 5.4
50 3.6 5.9 9.1 12.7 15.4 16.7 16.1 13.9 10.5 7.1 4.3 3.0

TABLE-3:
09-May-22
DEPARTMENT
Table 3: Mean monthly possible values of sunshine hours, N (hours) in north latitude
North Lat. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1
10 11.8 11.8 12.1 12.4 12.6 12.7 12.6 12.4 12.9 11.9 11.7 11.5
20 11.1 11.5 12.0 12.6 13.1 13.3 13.2 12.8 12.3 11.7 11.2 10.9
30 10.4 11.1 12.0 12.9 13.7 14.1 13.9 13.2 12.4 11.5 10.6 10.2
40 9.6 10.7 11.9 13.2 14.4 15.0 14.7 13.8 12.5 11.2 10.0 9.4
50 8.6 10.1 11.8 13.8 15.4 16.4 16.0 14.5 12.7 10.8 9.1 8.1

TABLE-4:
09-May-22
DEPARTMENT
Table 4: Usual range of reflection coefficient, r values
Surface Range of r values
Close ground green crops 0.15-0.25
Bare lands 0.05-0.45
Water surface 0.05
Snow 0.45-0.95

MODIFIED PENMAN METHOD
09-May-22
DEPARTMENT
ETo
* = Refenrence crop Evapotranspiration
(unadjusted)
ETo
= Refenrence crop Evapotranspiration (adjusted)
C = adjustment factor to account for day and night
weather effect

4) RADIATION METHOD:
09-May-22
DEPARTMENT

PENMAN-MONTEITH EQUATION
09-May-22
DEPARTMENT
Where:
Rn is the net radiation,
G is the soil heat flux,
(es - ea) represents the vapour pressure deficit of the air,
ρa is the mean air density at constant pressure,
cp is the specific heat of the air,
Δ represents the slope of the saturation vapour pressure tempe
relationship,
Λ is the latent heat of vaporization and γ is psychrometric constan
rs and ra are the (bulk) surface and aerodynamic resistances.

09-May-22
DEPARTMENT
PET = mm/day
A = slope of ew vs temp in mm Hg/oC (Table)
Ea = parameter including wind vel. & saturation
deficit
Ha = incident solar radiation outside the atm (Table)
n = actual duration of bright sunshine in hours
N = max. possible hr of bright sunshine (Table)
r = reflection coef. (albedo)
C
Hg
of
mm
t
cons o
/
_
_
49
.
0
tan 



09-May-22
DEPARTMENT
day
mm
x
t
cons
Boltzman
Stefan /
10
01
.
2
tan
_ 9
-

-


Ta = mean air temp. in degree kelvin=273 + oC
ea = actual mean vapour pressure in the air in mm of Hg
ew = sat. vapour press. at mean air temp. in mm of Hg (Table)
U2 = mean wind speed at 2m above the ground in km/day
Suface Range of r value
Close ground crops 0.15 – 0.25
Bare lands 0.05 – 0.45
Water surface 0.05
Snow 0.45 – 0.95

PENMAN-MONTEITH EQUATION
 The surface resistance, rs, describes the resistance of vapour
flow through stomata openings, total leaf area and soil
surface.
 The aerodynamic resistance, ra, describes the resistance
from the vegetation upward and involves friction from air
flowing over vegetative surfaces.
 The latent heat of vaporization, λ, expresses the energy
required to change a unit mass of water from liquid to water
vapour in a constant pressure and constant temperature
process. The value of the latent heat varies as a function of
temperatur
09-May-22
DEPARTMENT

The specific heat at constant pressure Cp is the amount of
energy required to increase the temperature of a unit mass of
air by one degree at constant pressure. Its value depends on
the composition of the air, i.e., on its humidity
•The vapour pressure deficit is the difference between the
saturation (es) and actual vapour pressure (ea) for a given time
period.
•The solar radiation received at the top of the earth's
atmosphere on a horizontal surface perpendicular to sun’s
rays is called the extraterrestrial (solar) radiation, Ra
09-May-22
DEPARTMENT

•The net radiation, Rn, is the difference between
incoming and outgoing radiation of both short and
long wavelengths. It is the balance between the energy
absorbed, reflected and emitted by the earth's surface
or the difference between the incoming net shortwave
(Rns) and the net outgoing longwave (Rnl) radiation
•The soil heat flux, G, is the energy that is utilized in heating the
soil. it is positive when the soil is warming and negative when the
soil is cooling. The soil heat flux is small compared to Rn and may
often be ignored 09-May-22
DEPARTMENT

THORN THWAITE METHODS :
• This formula was developed for the data of eastern
USA and use only the mean monthly temperature
with an adjustment for the day-light. The equation
is expressed as:
09-May-22
DEPARTMENT
a
t
I
T
a
L
.
T
E 







 10
6
1
  514
.
1
12
1
5
/
T
i
i
It 
  where
49239
.
0
10
792
.
1
10
71
.
7
10
75
.
6 2
2
5
3
7




-

 -
-
-
t
t
t I
I
I
a
Where,
ET = monthly PET (in cm)
= mean monthly air temperature (in oC)
La = adjustment for the number of hours of
daylight and days in the month related to
the latitude of a place (Table 7)
It = the total of 12 monthly values of heat index
a = an empirical constant

TABLE-7:
Table 7: Adjustment factor La for use in Thornthwaite formula
North
Lat.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 1.04 0.94 1.04 1.01 1.04 1.01 1.04 1.04 1.01 1.04 1.01 1.04
10 1.00 0.91 1.03 1.03 1.08 1.06 1.08 1.07 1.02 1.02 0.98 0.99
15 0.97 0.91 1.03 1.04 1.11 1.08 1.12 1.08 1.02 1.01 0.95 0.97
20 0.95 0.90 1.03 1.05 1.13 1.11 1.14 1.11 1.02 1.00 0.93 0.94
25 0.93 0.89 1.03 1.06 1.15 1.14 1.17 1.12 1.02 0.99 0.91 0.91
30 0.90 0.87 1.03 1.08 1.18 1.17 1.20 1.14 1.03 0.98 0.89 0.88
40 0.84 0.83 1.03 1.11 1.24 1.25 1.27 1.18 1.04 0.96 0.83 0.81
09-May-22
DEPARTMENT

3.3 DUTY-DELTA RELATIONSHIP
09-May-22
Crop period and Base period
The time period that elapses from the instant of its
sowing to the instant of its harvesting is called the
crop period.
 The time between the first watering of a crop at the
time of its sowing to its last watering before
harvesting is called the base period.
.
DEPARTMENT

09-May-22
☻Duty and Delta of Crops
•Duty (D): is defined as the area of the land which can be
irrigated if one cumes (m3/sec) of water was applied to the
land continuously for the entire base period of the crop.
•Delta ( ∆ ): is the total depth of water required by a crop
during the entire base period.
Delta (∆) = Total quantity of water (ha-m)or Volume of water
Total area of land (ha)
DEPARTMENT

The relation between duty, base period and delta, can be
obtained as follows:-
Considering the area of land of D-hectares, base period
of B days
Quantity of water= 1*B*24*60*60, m3 …. 3.21
If Delta (∆ ) is the total depth of water in meters supplied
to the land of D- hectares, the quantity of water is also
given by:
Quantity of water = ( D *104)* ∆ , m3 .…3.22
09-May-22

09-May-22
Equating the volumes of water given in egn_s 3.1 and 3.2:
1*B*24*60*60* = (D*104)*∆
D = 8.64B or ∆ = 8.64B
∆ D
Where D = in ha/cumec
∆ = in m
B = in days
Different forms of Duty
1. Flow duty (m3/s)
2. Quantity of Duty (m3)

Factors affecting Duty
- Duty of water depends up on different factors.
Type of soil
Type of crop and base period
structure of soil
Slope of ground
Climatic condition
Method of application of water
Salt content of soil 09-May-22
DEPARTMENT

3.4 Determination of Crop Evapotranspiration
(ETc) Under Standard Condition
Two calculation approaches are outlined: the single and the dual crop
coefficient approach.
 In the single crop coefficient approach, the difference in
evapotranspiration between the cropped and reference grass is
combined into one single coefficient.
 In the dual crop coefficient approach, the crop coefficient is split into
two factors describing separately the differences in evaporation and
transpiration between the crop and reference surface.
09-May-22
DEPARTMENT

Calculation procedure by the crop coefficient approach:
ETc = Kc * ETo … 3.27
Where:
ETc =crop evapotranspiration [mm d-1],
Kc =crop coefficient [dimensionless],
ETo = reference crop evapotranspiration [mm
d-1]
09-May-22
DEPARTMENT

CROP COEFFICIENT
09-May-22
DEPARTMENT
 It is the ratio b/w the actual crop Evapotranspiration to the
reference crop evapotranspiration.
Kc = ETc / ETo
 It determined experimentally for various crops, Etc is
determined by Lysimeter technique and ETo by USWB class A
evaporation pan.
 Kc is different for different crop and for different crop growth
stages.
 It is mainly affected by crop type, soil type and climate of the
area.

Crop coefficient curve
09-May-22
DEPARTMENT

09-May-22
DEPARTMENT
•During the crop development and late season stage, Kc varies
linearly between the Kc at the end of the previous stage (Kc
prev) and the Kc at the beginning of the next stage (Kc next),
which is Kc end in the case of the late season stage:
Where:
I, day number within the growing season [1.. length of the growing
season],
Kc I crop coefficient on day i,
L stage length of the stage under consideration [days],
(Lprev) sum of the lengths of all previous stages [days]

EFFECTIVE RAINFALL (PEFF)
09-May-22
There are four methods for calculating the
effective rainfall from entered monthly total
rainfall data.
1. Fixed Percentage Effective Rainfall
2. Dependable Rain
3. Empirical Formula for Effective Rainfall
4. Method of USDA Soil Conservation Service
(default)
DEPARTMENT

3.5 IRRIGATION EFFICIENCIES
09-May-22
1. Water Conveyance efficiency ( Ec)
Ec = Wf * 100
Wd
Where Ec = water conveyance efficiency , %
Wf = Water delivered to the irrigated plot ( At the field
supply channel)
Wd = Water diverted from the source.
2. Water application Efficiency (Ea)
• Ea = Ws *100
wf
Ea = application efficiency, %
Ws = water stored in the rot zone of the plants.
• Wf = Water delivered to the irrigated plot

3. Water storage efficiency (Es)
Es = Ws *100
Wn
Where Es = Water storage efficiency, %
Ws = water stored in the rot zone of the plants.
Wn = Water needed in the root zone prior to irrigation
4. Field Canal Efficiency (Eb)
Ef = Wp *100
Wf
Where Ef = Field canal efficiency
Wp = water received at the field inlet
Wf = water delivered to the field channel e.t.c…
09-May-22
DEPARTMENT

5. WATER USE EFFICIENCY
09-May-22
•
DEPARTMENT

3.6 IRRIGATION SCHEDULING
09-May-22
Scheduling of irrigation application is very
important for successive plant growth and
maturity.
The scheduling of irrigation can be field
irrigation scheduling and field irrigation supply
schedules.
DEPARTMENT

FIELD IRRIGATION SCHEDULING
09-May-22
•The two scheduling parameters of field irrigation scheduling are the
depth of irrigation and interval of irrigation.
Depth of irrigation (d):
d (net) = As *D *(FC – PWP)*P, m ... 3.46
Where
As = Apparent specific gravity of soil
D = Effective root zone depth in m
FC = water content of soil at FC
PWP = Water content of soil at PWP
P = depletion factor

09-May-22
 Because of application losses such as deep percolation and
runoff losses, the total depth of water to be applied will be
greater than the net depth of water.
d (gross) = As*D(FC-PWP)*P ,m ...
Ea
Where Ea = Field application efficiency and other
parameters as defined above
 Interval of irrigation (i):
i (days) = As D(FC-PWP).P
ET crop(peak)

EXAMPLE PROBLEM 3.1
09-May-22
•The base period, duty of water and area under irrigation for
various crops under a canal system are given in the table
below. If the losses in the reservoir and canals are
respectively 15%, 25%, determine the reservoir capacity.
Crop Wheat Sugarcane Cotton Rice Vegetables
Base period B
(days)
120 320 180 120 120
Duty , D
(ha/cumec)
1800 1600 1500 800 700
Area irrigated
(ha)
15000 10,000 5000 7500 5000
DEPARTMENT

EXAMPLE PROBLEM 3.2
09-May-22
A stream size of 150 lit /sec was released from the diversion
headwork to irrigate a land of area 1.8 hectares. The stream size
when measured at the delivery to the field channels is 120lit/sec.
The stream continued for eight hours. The effective root zone
depth is 1.80m. The application losses in the field are estimated to
be 440m3.
The depth of water penetration was 1.80m and 1.20m at the head
and tail of the run respectively. The available water holding capacity
of the soil is 21cm/m and irrigation was done at 60% depletion of
AM. Find Ec, Ef, Ea, Es and Ed. The stream size delivered to the plot
DEPARTMENT

Example 3.3 Determine Kc at day 20, 40, 70 and 95 for the dry bean crop.
Crop growth stage Length (days) Kc
initial 25 Kc ini = 0.15
crop development 25 -
mid-season 30 Kc mid = 1.19
late season 20 Kc end = 0.35
09-May-22
DEPARTMENT

EXAMPLE PROBLEM 3.4:
09-May-22
A discharge of 15 cumecs is released at
the head of the canal. If the duty at the
field is 1800 ha/cumecs, and the losses in
the transit are 30%, determine the area
that can be irrigated.
DEPARTMENT

Crop Water Requirement Factors and Calculations

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