2. crop water requirements 2n3
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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
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Crop water requirement (CWR): 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; evapotranspiration, water used by crops for metabolic growth, water lost during conveyance and application of water and water required for special operations such as land
preparation, tillage and salt leaching etc.
It is expressed as the surface depth of water in mm, cm or inches.
CWR = Consumptive use (Cu) +
Conveyance losses (Wu) +
Water required for special operation (Ws)3
Approximate daily water use and total growing season water use in millimetres (mm) for some commonly grown crops in
AlbertaSource: http://www1.agric.gov.ab.ca/%24department/deptdocs.nsf/all/agdex12726 4
The crop water requirement mainly depends on: the climate: in a sunny and hot climate, crops need more water
per day than in a cloudy and cool climate
the crop type: crops like maize (Makei) or sugarcane (ganna) need more water than crops like millet (Bajra) or sorghum
the growth stage of the crop: fully grown crops need more water than crops that have just been planted.
Moreover, there are short duration crops, e.g. peas, with a duration of the total growing season of 90-100 days and longer duration crops, e.g. melons, with a duration of the total growing season of 120-160 days
Climatic factor Crop water requirementHigh Low
Sunshine Sunny (no clouds) cloudy (no sun)Temperature hot cool
Humidity low (dry) high (humid)Wind speed windy little wind
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Conveyance Losses: These losses take place from diversion structure (barrage) to
the field (outlet). Major loss of water in an irrigation channel is due to absorption,
seepage or percolation and evaporation. In an earthen channels losses due to seepage are much more
than the losses due to evaporation. The absorption losses depend upon the:
Type of soil Subsoil water Age of canal Position of Full Supply Level w.r.t to Natural Surface Level Amount of Silt carried by canal Wetted perimeter
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Irrigation water losses in the fieldIrrigation water losses in canals
1. Evaporation from the water surface, 2. Deep percolation to soil layers underneath the canals, 3. Seepage through the bunds of the canals, 4. Overtopping the bunds5. Bund breaks, 6. Runoff in the drain7. Rat holes in the canal bunds
1. Surface runoff, whereby water ends up in the drain2. Deep percolation to soil layers below the root zone
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In Pakistan the following formula can be used for obtaining the conveyance losses in earthen channels
K= 5.0Q0.625
K= absorption loss per million square feet of wetted perimeter
Q= Discharge in channel (cusecs). According to Lacey
QA=0.0133 L Q0.5625
QA= Absorption loss L= Length of channel in thousand feet Q= discharge in channel (cusecs)
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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 (IRg): 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, field channels and during water application to the crop field.
IRg = CWR – (ER + ∆GW+∆SW ) Where Ground Water Contribution for Crop Use (∆GW) Soil Water Contribution for Crop Use (∆SW)
Ground Water Contribution for Crop Use (∆GW): It refers to the water used by crops due to capillary rise in case of
shallow water tables.
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Soil Water Contribution for Crop Use (∆SW): 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
Net Irrigation requirements It refers to the amount of water needed to replenish/fill soil
moisture deficit in the crop field.IRn = IRg x Efficiency of water
application = Cu – ER - ∆SW
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Consumptive use (CU): 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
DEFINITIONS a) Evaporation: The process by which water is changed from the liquid or
solid state into the gaseous state through the transfer of heat energy.
b) Transpiration: The evaporation of water absorbed by the crop which is used directly in the building of plant tissue in a specified time. It does not include soil evaporation.
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Daily consumptive use: The amount of water consumptively used during 24-
hours.
It is usually estimated to record the peak period consumptive use rates to
formulate the cropping pattern and to decide the water supply from sources during different
periods of cropping.
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.
It occurs when the vegetation is abundant, temperature is high and the crops are in flowering stage.
It is used in the planning of an irrigation system
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Seasonal consumptive use: It is the amount of water consumptively used by crops
during the entire cropping season/period. It is used to evaluate and decide the seasonal water
supply to a command area of an irrigation project.
Rabi Season (October to March):Crop Consumptive
Use (cm) Wheat 37 Gram 30 Barley 30 Potato 60-90 Sugar cane 90 Fodder 40 Oil seed 45 Berseen 70
Kharif Season (April to September):Crop Consumptive
Use (cm)Cotton 25-40Maize 45Rice 125-150Sugar Cane 90
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Evapotranspiration: It is defined as the water transpired by crop plants and
the water evaporated from the soil in the crop field and intercepted precipitation by areal parts of plants in any specified time period
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Potential/reference crop evapotranspiration (ETo): This is the evapotranspiration rate from a reference surface
(crop) which is not short of water.
The reference surface is a hypothetical grass reference crop with an assumed crop height of 0.12m, a fixed surface resistance of 70sm-1 and an albedo of 0.23.
The reference surface closely resembles an extensive surface of green, well-watered grass of uniform height, actively growing and completely shading the ground.
Potential/reference crop evapotranspiration (ETo)
15Surface resistance describes the resistance of vapour flow through the transpiring crop and evaporating soil surface
Actual crop evapotranspiration (ETc): It is the rate of evapotranspiration by a particular crop
in a given period under prevailing soil water and atmospheric conditions.
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.
It is usually calculated by multiplying the Crop Coefficient (Kc) with ETo, thus:
ETc = Kc. ETo
Actual crop evapotranspiration (ETc) 16
Climatic factors:
Precipitation, with greater frequency and amount of rainfall, ET becomes higher.
Solar radiations, it supplies energy for ET processes.
With increasing day length or solar radiation, ET becomes more.
Temperature, Temperature of plant and soil rises because of more amount of solar radiation received from the sun and consequently increases ET.
Wind speed, ET from soil surface and plants occurs at a
higher rate on a windy day. The moist air in the immediate vicinity of a moist soil or leaf surface is swept away by wind and the dry air occupies the space.
Relative humidity, ET varies inversely with the atmospheric humidity
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Growing season: Length of growing season and the actual date of sowing
and maturing are important factors. The growing season of a crop coinciding with the hotter part of the year is expected to increase ET. Crops grown in different seasons have different ET.
Crop characteristics: Growth habit, canopy development, leaf area index,
plant density, duration and time of year when the growth is made, are important consideration to study the effect of crop characteristics on ET.
Soil characteristics: Hydraulic conductivity and water holding capacity of soil
affect ET.
Cultural Factors: Irrigation frequency, method of irrigation, depth of
irrigation, fertilizer application and mulching are the important cultural factors affecting ET. Mulching is covering of soil due to rotten vegetable matters
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Crop coefficient: It is the ratio b/w the actual crop evapotranspiration to
the reference crop evapotranspiration.
Kc = ETc / ETo
It is determined experimentally for various crops.
ETc is determined by Lysimeter technique and ETo is determined with 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.
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Water is essential to crop plants for their growth and development.
Amount of water required by the crops is influenced by the soil type.
Soil water plant relationship is a process that requires to be regulated for maximization of yields with a given unit of water.
An understanding of this relationship is essential in order that water management principles are applied to various climate, soil and cropping regions of both rain-fed and irrigated lands.
To understand this relationship, the concept of soil water/moisture and field capacity is essential.
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Gravitational moisture: When the water falls over the ground, a part of it gets
absorbed in the root zone, and the rest flows downwards under the action of gravity, and is called as gravitational moisture.
Field Capacity: Immediately after the rain or irrigation water application,
when all the gravity water has drained down, a certain amount of water is retained on the surface of soil grains by molecular attraction and by loose chemical bonds (adsorption). This water cannot be drained under the action of gravity and is called the field capacity.
Field capacity is very important because it is the water which is available in the soil for crop use
The total field capacity water is not used by the crops. The plants can extract water from the soil till the permanent wilting point is reached. 22
Fields capacity is further divided into two types: 1. Capillary moisture:
It is that moisture which is attached to the soil molecules by surface tension against gravitational forces and which can be extracted by crop through capillarity.
2. Hygroscopic moisture: It is that moisture which is attached to the soil
molecules by loose chemical bond and it is not available to the plants for use (adsorption).
Permanent wilting point: It is moisture content at which plant can no longer
extract sufficient water for its growth and wilts up.
Available moisture: It is the difference in moisture content between field
capacity and permanent wilting point.
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Crop Period: It is the time normally in days that a crop takes from the
instance of its sowing to harvesting. Base period: It is the time between first watering of crops at the time of
its sowing and the last watering of crops before harvesting
Delta of crops: Total depth of water required by the crop in unit area during
base period. In other words it is the total depth of water required for maturing the crop.
Volume of water required by the crop = Delta x Area or
Delta = Volume (acre-ft) / Area (acres)
Duty of irrigation water: It is defined as the no. of hectares (acres) of land irrigated
for full growth of a given crop by supply of 1 m3/sec (1 ft3/sec) of water continuously during the entire base period.
Crop Total growing period (days)
Crop Total growing period (days)
Alfalfa 100-365 Melon 120-160
Barley/Oats/ Wheat
120-150 Millet 105-140
Bean, green 75-90 Onion, green 70-95
dry 95-110 dry 150-210
Citrus 240-365 Pepper 120-210
Cotton 180-195 Rice 90-150
Grain/small 150-165 Sorghum 120-130
Lentil 150-170 Soybean 135-150
Maize, sweet 80-110 Squash 95-120
grain 125-180 Sunflower 125-130
Let there be a crop of base period B days.
Now the volume of water applied to this crop during B days @ 1 m3/sec = V = 1 x 60 x 60 x 24 x B) m3 = 86400 B
By definition of duty, D, is the Area in hectare (10000m2) irrigated by 1m3/S
1 m3/S of water supplied for B days irrigates D hectares (104 m2) of land.
Therefore, total depth of water required by crop per unit area (Delta)
= Volume/Area = 86400B/104D
Hence,
Delta = ∆ = 8.64 B / D (meters)Delta = ∆ = 864 B / D (centimeters)
Example: find the delta of a crop when its duty is 864 hectare/cumecs with base period of 120 days.
Full Supply Factor/(Duty): The term duty is only used for existing or running projects,
but in a proposed project it is known as full supply factor.
Intensity of Irrigation: Percentage of culturable area irrigated during a base period
or annually
Cropping Intensity/cultivation intensity: It is to the %age of area of a particular crop with respect to
culturable command area
Cropping Pattern: It means how many crops and how much area for a crop is
being cultivated.
Water Allowance: is antonymous of duty. It is the discharge in cusec required to irrigate 1000 Acres of an area and is expressed in cusec/1000 acres (or in cumec/100 ha) at outlet head, distributory head or main canal head
Water Conveyance Efficiency: It is the ratio of the water delivered to the farmer by conveyance system to the water introduced into the canal at source.
Gross Command Area (GCA): is the total amount of area which needs to be irrigated. It also includes the area which cannot be cultivated e.g., villages, roads, utility etc.
Culturable Command Area (CCA): is the effective area which is culturable or the area that is cultivated out of gross command area.
Non-Culturable Command Area (NCCA): It is the area which is not cultivated.
CCA=GCA – Non-Culturable Command Area
Find out the capacity of the reservoir if its culturable area is 65000 ha, from the following data
B=BASE PERIODD=DUTY
These methods are classified into three types: Direct methods
Lysimeter method Field experimentation method Soil water depletion method Inflow-outflow method
Pan evaporimeter method USWB class-A pan evaporimeter
Empirical methods Blaney-criddle method Penman method Modified penman method Radiation method Penman Monteith equation
Source: Irrigation water management: principles and practice By D. K. Majumdar
Lysimeter method: Used to measure ET and various components of water balance
It is a container (usually 0.5m – 2m in diameter) having an experimental soil separated from the surrounding soil in the crop field
Lysimeter are installed in fields with a large guard area having the same crop as in the lysimeter
Measurements of different components for water balance studies such as water added to lysimeter through precipitation and irrigation, change in soil water storage and water lost through evaporation, transpiration, runoff and deep percolation are made,
By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost to evapotranspiration can be calculated.
The general relationship to estimate ET is :
Lysimeters are so constructed that measurements of deep percolation and surface runoff are possible or it is possible to avoid these losses
Both weighing and non weighing type lysimeters are used for measurement of ET
For very short period (daily or hourly) estimates of ET, weighing type lysimeter is used
Field experimentation method: Field experiments with varying level of irrigation are
carried out to estimate seasonal consumptive use of irrigated crops
Measurement of water supplied to the crops through effective rainfall and irrigation and changes in the soil moisture reserves during the growing season are made
The water, thus, supplied under varying levels of irrigation is then correlated with the yields obtained
The quantity of water used to produce most profitable yield is taken as CU
Soil water depletion method: Soil water contents in different layers of root zone are
measured just before and after irrigation or rainfall and during the period between two successive irrigations as frequently as possible depending upon the degree of accuracy desired.
The soil water depletion during any short period is considered as the consumptive use for that period
The seasonal consumptive use is obtained by summing up soil water depletion or losses during the different periods of measurement in the growing season
Inflow-outflow method: It is used to estimate yearly consumptive use over
large area It is also called as water balance method
Change in soil water storage is considered negligible and it is assumed that the subsurface inflow into the area is same as subsurface outflow
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
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
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