IRR Chapter 2

8/11/2019 IRR Chapter 2

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By A.W.

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CHAPTER TWO

2. Crop water requirement2.1. General

Crop water requirement may be defined as the quantity of water, regardless of its source, required by

crop or diversified pattern of crops in a given period of time for its normal growth under field

conditions at a place. It includes the loss due to evapo-transpirtation (ET) or consumptive use (CU)

plus the losses during the application of irrigation water and the quantity of water required for special

operations such as land preparation, transplanting, leaching etc. it may those be formulated as:

CWR=ET or CU + application losses +conveyance losses+ special needs.

In other words crop water requirement can be defined as the total amount of water and the way in

which a crop requires water from the time it is sown to the time it is harvested. It is clear that the water

required will vary with the crop as well as the place. Different crops will have different water

requirement and the same crops may have different water requirement at different place dependingupon climate, type of soil method of cultivation and useful rainfall etc.

Crop water requirement serves as the basis for the design of the capacity of reservoir and canal,

irrigation scheduling and management.

Crop period and base period

Crop period: is a period elapsed from the instant of its sowing to the instant of harvesting.

Base period: is the time between the first watering of a crop at the time of its sowing to its last water

before harvesting.

Crop period is slightly more than the base period but for all particular purpose, they are taken as one

and the same thing, and generally expressed in days.

2.2. Duty and delta of a crop

Duty (D)

The duty of water is the relationship between the volume of water and the area of the crop it matures.

This volume of water is generally expressed by a unit discharge flowing for a time equal to the base

period of the crop called Base of Duty.Duty represents the irrigation capacity of a unit water(ha/m3/s).

Q

AD= ; Where A command area and Q continuous discharge required for the base period.

If 3m3/s of water is required for a crop sown in area of 5100ha continuously, the duty of irrigationwater will be 5100/3=1700ha/m3/s, and a discharge of 3m3/s is required throughout the base period.

Duty is generally expressed by D.

In a large canals irrigation system, the water from its source, first of all flows into the main canal, then

it flows into primary canal; from the primary it flows into secondary canals and from secondary to


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tertiary canals and finally in to the field. During the passage of water from those irrigation channels,

the water is lost due to evaporation and percolation. Those losses are called transit loss or transmissionor conveyance losses.

Duty of water for a crop is the number of hectares of land which the water can irrigate. Therefore, ifthe water requirement of the crop is more, less amount of hectares of land it will irrigate. Hence, if

water consumed is more, duty will be less. Therefore its clear that the duty of water at the head of the

water course will be less than the duty of water on the field; because when water flows from the head

of the water course and reaches the field, some water is lost as transit losses. Duty of water, therefore,

varies from one place to another and increases as we move downstream from the head of the main

canal towards the head of branches or water courses.

Delta ()

Each crop requires certain amount of water depending up on the area to be cultivated. If the area to be

cultivated is large, the amount of water required will be more; on the other hand if area is small theamount of water required will be less. The total quantity of water required by the crop for its full

growth may be expresses in ha-m. Thus the total depth of water (in cm) required by a crop to come to

maturity is called Delta.

Suppose certain amount of water is applied to a crop from a time of sowing till the crop matures and if

the applied water is not lost or used up by any means then there will be a thick layer of water standingall over the field. The depth or height of this water layer is known as delta for the crop.

A

V= ; where V is total volume of water required for the base period and A is command area.

If rice required about 8cm depth of water at an average interval of about 12days, and the crop period

for rice is 120days. Find out the delta of rice.

8cm of water at an average of 12 daysWater requirement = 8cm/12days = 0.6667cm/day

For 120 days =120day*0.6667cm/day

Delta () =80cmThe average values of delta for certain crops are shown below. Those values represent the total waterrequirement of the crop on the field, actually can be less depending upon the useful rainfall.

Crop Delta on field cm

Sugarcane 120

Rice 120

Tobacco 75

Garden fruit 60

Cotton 50

Vegetables 45

Wheat 40

Barly 30

Maize 25

Fodder 22.5

Peas 15


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Relation between Duty and Delta

Assume a crop of base period B in days, D duty of water in hectare per cubic meters per second and

be the delta or depth of water for a crop in meter.

From the definition of delta, duty and base period 1m3/s flowing continuously for B days mature Dhectares of land under the crop or 1m3/s continuously for B days gives a depth , over D hectares of

land.

The total amount of water applied to this crop during B days. By definition of duty:

( )3

3

*400,86

*24*60*60*1

BmV

mBV

=

=

The depth of water applied on this land 1ha = 104m

2

mD

Bm

D

B

A

V 46.8

10*

864004 ===

Where: B in days, delta in m and D in ha/m3/s

2.3. Optimum utilization of irrigation water

If a crop is sown under absolutely identical conditions, using different amounts of water depths, the

resulting yield will not be the same. The yield increases with water and reaches a certain maximumvalue and then after falls down, see figure below. The quantity of water at which the yield is

maximum, is called the optimum water depth.

Fig Delta and yield relation

Therefore, optimum utilization irrigation generally means, getting maximum yield with any amount of

water. The supplies of water to the various crops should be adjusted in such a fashion, as to get

optimum benefit ratio, not only for efficient use of available water of available water. You should be

aware that more than the optimum depth or less than quantity reduces the yield.

Water depth mm

Yield

kg

Optimum

Depth

Max. Yield


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2.4. Irrigation efficiency

Efficiency is the ration of the water output to the water input, and is usually expressed as percentage.

The design of the irrigation system, the degree of land preparation, and the skill and care of the

irrigator is the principal factors influencing irrigation efficiency. Loss of irrigation water occurs in theconveyance and distribution system, non-uniform distribution of water over the field, percolation

below crop root zone, and with sprinkler irrigation evaporation from the spray and retention of wateron the foliage. Water is lost in irrigation during various processes and, therefore, there are different

kinds of irrigation efficiencies as shown below:

Conveyance efficiency(c): it is the ratio of the water delivered into the field from the outlet point of

the channel, to the water pumped into the channel at the starting point.

Application efficiency(a): it is the ratio of the quantity of water stored in to the root zone of the

crops to the quantity of water actually delivered into the field.

Water storage efficiency (s): is the ratio of water stored into the root zone during irrigation to the

water needed in the root zone prior to irrigation (i.e field capacity existing moisture content).

Water use efficiency(u): is the ratio of water beneficially used, including leaching water, to the

quantity of water delivered.

Ex 1. 15m3/s of water is delivered to 40ha field, for 5hrs. Soil probing after irrigation indicates that

0.5m of water has been stored in the root zone. Compute the water application efficiency. Ans

(74.04%)

Uniformity coefficient or distribution efficiency(d): represents the extent to which the water haspentrated to a uniform depth, through out the field. When the water has penetrated uniformly

throughout the field, the deviation from the mean depth is zero and the water distribution efficiency is

100%.

=D

dd 1

Where D : mean depth of water stored during irrigation

d : average of absolute values of deviation from the mean

2.5. Crop water requirement (CWR)

Crop water requirement 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. It includes the loss due to evapotranspiration (ET) or consumptive use (CU) plus the losses

during the application and conveyance of irrigation water and the quantity of water required for special

operation such as land preparation, leaching etc

needsSpeciallossAonveyancelossesnApplicatioCUorETCWR +++=


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Consumptive use of water (Evapotranspiration) (CU)

Consumptive use for a particular crop may be defined as the total amount of water used by the plant in

transpiration (building of plant tissues, etc) and evaporation from adjacent soils or from plant leaves in

any specified time. The values of consumptive use (CU) may be different for different crops, and maybe different for same crop at different time and places. The combination of two separate processeswhereby water is lost on the one hand from the soil surface by evaporation and on the other hand from

the crop by transpiration is referred to as evapotranspiration (ET).

Values of monthly consumptive use over the entire crop period are then used to determine the

irrigation requirement of the crop.

Evaporation

Evaporation is the process whereby liquid water is converted to water vapour (vaporization) and

removed from the evaporating surface (vapour removal). Water evaporates from a variety of surfaces,

such as lakes, rivers, pavements, soils and wet vegetation.

Energy is required to change the state of the molecules of water from liquid to vapour. Direct solar

radiation and, to a lesser extent, the ambient temperature of the air provide this energy. The driving

force to remove water vapour from the evaporating surface is the difference between the water vapourpressure at the evaporating surface and that of the surrounding atmosphere. As evaporation proceeds,

the surrounding air becomes gradually saturated and the process will slow down and might stop if the

wet air is not transferred to the atmosphere. The replacement of the saturated air with drier air depends

greatly on wind speed. Hence, solar radiation, air temperature, air humidity and wind speed are

climatological parameters to consider when assessing the evaporation process.

Where the evaporating surface is the soil surface, the degree of shading of the crop canopy and theamount of water available at the evaporating surface are other factors that affect the evaporation

process.

Transpiration

Transpiration consists of the vaporization of liquid water contained in plant tissues and the vapour

removal to the atmosphere. Crops predominately lose their water through stomata. These are small

openings on the plant leaf through which gases and water vapour pass.

The water, together with some nutrients, is taken up by the roots and transported through the plant.The vaporization occurs within the leaf, namely in the intercellular spaces, and the vapour exchange

with the atmosphere is controlled by the stomatal aperture. Nearly all water taken up is lost bytranspiration and only a tiny fraction is used within the plant. Transpiration, like direct evaporation,

depends on the energy supply, vapour pressure gradient and wind. Hence, radiation, air temperature,

air humidity and wind terms should be considered when assessing transpiration.

Evapotranspiration (ET)

Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing

between the two processes. Apart from the water availability in the topsoil, the evaporation from a

cropped soil is mainly determined by the fraction of the solar radiation reaching the soil surface. This

fraction decreases over the growing period as the crop develops and the crop canopy shades more and

more of the ground area. When the crop is small, water is predominately lost by soil evaporation, but


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once the crop is well developed and completely covers the soil, transpiration becomes the main

process.

Fig. The partitioning of evapotranspiration into evaporation and transpiration over the growingperiod for an annual field crop

The above figure shows partition of evapotranspiration into evaporation and transpiration in

correspondence to leaf area per unit surface of soil below it. At sowing nearly 100% of ET comes from

evaporation, while at full crop cover more than 90% of ET comes from transpiration.

The evapotranspiration rate is normally expressed in millimetres (mm) per unit time. The time unit canbe an hour, day, decade, month or even an entire growing period or year .

2.5.1 Factors affecting evapotranspiration

Weather parameters, crop characteristics, management and environmental aspects are factors affecting

evaporation and transpiration.


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Fig. Factors affecting evapotranspiration with reference to related ET

Weather parameters:The principal weather parameters affecting evapotranspiration are radiation, air

temperature, humidity and wind speed.

Crop factors:The crop type, variety and development stage should be considered when assessing the

evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to

transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics

result in different ET levels in different types of crops under identical environmental conditions.

Management and environmental conditions:Factors such as soil salinity, poor land fertility, limited

application of fertilizers, the presence of hard or impenetrable soil horizons, the absence of control of

diseases and pests and poor soil management may limit the crop development and reduce the

evapotranspiration. Other factors to be considered when assessing ET are ground cover, plant density

and the soil water content.

2.5.2.Meteorological factors determining ETo

Air Temperature

The solar radiation absorbed by the atmosphere and the heat emitted by the earth increase the air

temperature. The sensible heat of the surrounding air transfers energy to the crop and exerts as such acontrolling influence on the rate of evapotranspiration. In sunny, warm weather the loss of water by

evapotranspiration is greater than in cloudy and cool weather. The average daily maximum and

minimum air temperatures in oc are required. Where only (average) mean daily temperatures are

available, the calculations can still be executed but some underestimation of ETo will probably occur.

Using mean air temperature instead of maximum and minimum air temperatures yields a lowersaturation vapour pressure, es, and hence a lower vapour pressure differences (es-ea), and a lower ETo

estimate.

The daily maximum air temperature (Tmax) and daily minimum air temperature (Tmin) are,

respectively, the maximum and minimum air temperature observed during the 24-hr period, beginning

at midnight, Tmax and Tmin for longer periods such as weeks, 10-days or months are obtained by

dividing the sum of the respective daily values by the number of days in the period. Tmean for 24-hr

periods is defined as the mean of the daily maximum (Tmax) and minimum temperatures (Tmin)rather than as the average of hourly temperature measurements.

Air Humidity

The water content of the air can be expressed as vapour pressure or relative humidity. Water vapour is

a gas and its pressure contributes to the total atmospheric pressure. The amount of water in the air is

related directly to the partial pressure exerted by the water vapour in the air and is therefore a direct

measure of the air water content.

When air is enclosed above an evaporating water surface, an equilibrium is reached between the watermolecules escaping and returning to the water reservoir. At that moment, the air is said to be saturated

since it cannot store any extra water molecules. The corresponding pressure is called the saturation

vapour pressure (eo(T)). The number of molecules that can be stored in the air depends on the

temperature (T). The higher the temperature, the higher the storage capcity, the higher its saturation


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vapour pressure. The slope of the saturation vapour pressure curve, , is an important parameter indescribing vaporization and is required in the equations for calculating ETo from climatic data.

The actual vapour pressure (ea) is the vapour pressure exerted by water in the air. When the air is not

saturated, the actual vapour will be lower than the saturation vapour pressure. The difference betweenthe saturation and actual vapour pressure is called the vapour pressure deficit and is an accurate

indicator of the actual evaporative capacity of the air.

Relative Humidity

The relative humidity (RH) expresses the degree of saturation of the air as a ratio of the actual(ea) to

the saturation (eo(T)) vapour pressure at the same temperature (T):

RH = ea/eo(T)

Relative humidity is the ration between the amount of water the ambient air actually holds and theamount it could hold at the same temperature. It is dimensionless and is commonly expresses as apercentage. Although the actual vapour pressure might be relatively constant throughout the day, the

relative humidity fluctuates between a maximum near sunrise and a minimum around early afternoon.

The variation of the relative humidity is the result of the temperature changes during the day; the

relative humidity also changes substantially.

While the energy supply from the sun and surrounding air is the main driving force for the

vaporization of water, the difference between the water vapour pressure at the evapotranspiring

surface and the surrounding air is the determining factor for the vapour removal. In humid tropicalregions, notwithstanding the high energy input, the high humidity of the air will reduce the

evapotranspiration demand. In such an environment, the air is already close to saturation, so that lessadditional water can be stored and hence the evapotranspiration rate is lower than in arid regions. The

(average) daily actual vapour pressure, ea (kpa) is required. The actual vapour pressure, where not

available, can be derived from maximum and minimum relative humidity (%).

Solar radiation

The evapotranspiration process is determined by the amount of energy available to vaporize water.The potential amount of radiation that can reach the evaporating surface is determined by its location

and time of the year. Due to differences in the position of the sun, the potential radiation differs at

various latitudes and in different seasons. The average (daily) net radiation expressed in MJ/m2 is

required. These data are not commonly available but can be derived from the (average) shortwaveradiation measured with a pyranometer, or from the (average) daily actual duration of bright sunshine

(hours per day) measured with a (Campbell-Stokes) sunshine recorder.

Wind Speed

The process of vapour removal depends to a large extent on wind and air turbulence which transfers

large quantities of air over the evaporating surface. When vaporizing water, the air above theevaporating surface becomes gradually saturated with water vapour. If this air is not continuously

replaced with drier air, the driving force for water vapour removal and the evapotranspiration rate

decreases. The (average) daily wind speed in m/s measured at 2 m above ground level is required. It is


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important to verify the height at which wind speed is measured, as wind speeds measured at different

heights above the soils surface differ.

The evapotranspiration demand is high in hot dry weather due to the dryness of the air and the amount

of energy available as direct solar radiation and latent heat. To adjust wind speed data obtained frominstruments placed at elevations other than the standard height of 2 m, the following equation is used:

2.6. Reference Crop Evapotranspiration (ETo)

To define unique evaporation parameters for each crop and stage of growth, the concept of a reference

surface was introduced. Evapotranspiration rates of the various crops are related to the

evapotranspiration rate from the reference surface (ETo) by means of crop coefficients. The reference

surface closely resembles an extensive surface of green grass of uniform height, actively growing,

completely shading the ground with adequate water. By defining the reference crop as 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 extensive surface of green grass of uniform height, activelygrowing and adequately watered, the FAO Panman-Monteith method was developed. The evaporation

from a reference surface, not short of water, is called the reference crop evapotranspiration or

reference evapotranspiration and is denoted as ETo.

The only factors affecting ETo are climatic parameters. Consequently, ETo is a climatic parameter and

can be computed from weather data. ETo expresses the evaporating power of the atmosphere at a

specific location and time of the year and does not consider the crop characteristics and soil factors.The FAO Penman-Monteith method is recommended as the sole method for determining ETo. Themethod has been selected because it closely approximates grass ETo at the location evaluated, is

physically based, and explicitly incorporates both physiological and aerodynamic parameters.

Typical ranges for ETo values for different agroclimatic regions are given in Table below. These

values are not intended for direct application.

Table . Average ETo for different agroclimatic regions in mm/day

Mean daily temperature (C)Regions

Cool

~10C

Moderate

20C

Warm

> 30C

Tropics and subtropics

- humid and sub-humid 2 - 3 3 - 5 5 - 7

-arid and semi-arid 2 - 4 4 - 6 6 - 8

Temperate region- humid and sub-humid 1 - 2 2 - 4 4 - 7

-arid and semi-arid 1 - 3 4 - 7 6 - 9


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\

Characteristics of the hypothetical reference crop

2.6.1.Empirical methods of estimating ETo

The FAO groups of scientists have screened 31 empirical formulae for predicting the ETo the

following five methods will be discussed, which are used under different climatic conditions:

1. Pan evaporation2. Blaney-criddle method3. Hargreaves Method4. Thornthwaite Method5. Radiation method, and6. Modified penman method

The modified Penman method was considered to offer the best results with minimum possible error inrelation to a living grass reference crop. It was expected that the pan method would give acceptable

estimates, depending on the location of the pan. The radiation method was suggested for areas where

available climatic data include measured air temperature and sunshine, cloudiness or radiation, but not

measured wind speed and air humidity.

Three major steps involved in the estimation of ET of the crop:

Estimation of reference evapotranspiration (ETo) Determination of the crop coefficient (kc), and Making appropriate adjustments to management condition and location specific crop

environment.


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1. Pan evaporation

Evaporation pans provide a measurement of the combined effect of temperature, humidity, wind speed

and sunshine on the reference crop evapotranspiration ETo. Many different types of evaporation pans

are being used. The best known pans are the Class A evaporation pan.

Fig. Class A evaporation pan

The principle of the evaporation pan is the following:

Pan is installed in the field The pan is filled with a known quantity of water (the surface area of the pan is known and the

water depth is measured)

The water is allowed to evaporate during a certain period of time (usually 24 hours). Forexample, each morning at 7 o'clock a measurement is taken. The rainfall, if any, is measured

simultaneously

After 24 hours, the remaining quantity of water (i.e. water depth) is measured the amount ofevaporation per time unit (the difference between the two measured water depths) is calculated;this is the pan evaporation: E pan (in mm/24 hours)

The E pan is multiplied by a pan coefficient, K pan, to obtain the ETo.

EpanKpanEto *= With:

ETo: reference crop evapotranspiration mm/day

K pan: pan coefficient

E pan: pan evaporation mm/day

Determination of K pan

When using the evaporation pan to estimate the ETo, in fact, a comparison is made between the

evaporation from the water surface in the pan and the evapotranspiration of the standard grass. Of

course the water in the pan and the grass do not react in exactly the same way to the climate. Note

reflection of solar radiation from water in the shallow pan might be different from the assumed 23%

for the grass reference surface. Storage of heat within the pan can be appreciable and may causesignificant evaporation during the night while most crops transpire only during the daytime. There are

also differences in turbulence, temperature and humidity of the air immediately above the respective

surfaces. Heat transfer through the sides of the pan occurs and affects the energy balance. To relate

pan evaporation to ETo, empirically derived pan coefficients are suggested to account for climate, type

of pan and pan environment. Therefore a special coefficient is used (K pan) to relate one to the other.


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The pan coefficient, K pan, depends on:

The type of pan used The pan environment: if the pan is placed in a fallow or cropped area The climate: the humidity and wind speed

For the Class A evaporation pan, the K pan varies between 0.35 and 0.85. Average K pan = 0.70.

Normally details of the pan coefficient are usually provided by the supplier of the pan. The panmethod it was suggested that calculations should be done for periods of ten days or longer.

2. Blaney Criddle Method:

If no measured data on pan evaporation are available locally, a theoretical method (e.g. the Blaney-

Criddle method) to calculate the reference crop evapotranspiration ETo has to be used. The most

commonly used theoretical method is the modified Penman method which is described in detail in

FAO Irrigation and Drainage Paper 24. This method, however, is rather complicated.

The Blaney-Criddle method is simple, using measured data on temperature only. It should be noted,

however, that this method is not very accurate; it provides a rough estimate or "order of magnitude"

only. Especially under "extreme" climatic conditions the Blaney-Criddle method is inaccurate: in

windy, dry, sunny areas, the ETo is underestimated (up to some 60 percent), while in calm, humid,clouded areas, the ETo is overestimated (up to some 40 percent).

)8*46.0( += TmeanPETo Where:

ETo = Reference crop evapotranspiration (mm/day) as an average for a period of 1 month

T mean = mean monthly temperature (C)

p = mean monthly percentage of annual daytime hours

To determine the value of p. Table below is used. To be able to determine the p value it is essential to

know the approximate latitude of the area: the number of degrees north or south of the equator

Table: Mean daily percentage (p) of annual daytime hours for different latitudes

North Jan Feb Mar Apr May June July Aug Sept Oct Nov DecLatitude

South July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June

60 0.15 0.20 0.26 0.32 0.38 0.41 0.4 0.34 0.28 0.22 0.17 0.13

55 0.17 0.21 0.26 0.32 0.36 0.39 0.38 0.33 0.28 0.23 0.18 0.16

50 0.19 0.23 0.27 0.31 0.34 0.36 0.35 0.32 0.28 0.24 0.2 0.1845 0.20 0.23 0.27 0.30 0.34 0.35 0.34 0.32 0.28 0.24 0.21 0.2

40 0.22 0.24 0.27 0.30 0.32 0.34 0.33 0.31 0.28 0.25 0.22 0.21

35 0.23 0.25 0.27 0.29 0.31 0.32 0.32 0.3 0.28 0.25 0.23 0.22

30 0.24 0.25 0.27 0.29 0.31 0.32 0.31 0.3 0.28 0.26 0.24 0.23

25 0.24 0.26 0.27 0.29 0.30 0.31 0.31 0.29 0.28 0.26 0.25 0.24

20 0.25 0.26 0.27 0.28 0.29 0.3 0.3 0.29 0.28 0.26 0.25 0.25

15 0.26 0.26 0.27 0.28 0.29 0.29 0.29 0.28 0.28 0.27 0.26 0.25

10 0.26 0.27 0.27 0.28 0.28 0.29 0.29 0.28 0.28 0.27 0.26 0.26

5 0.27 0.27 0.27 0.28 0.28 0.28 0.28 0.28 0.28 0.27 0.27 0.27

0 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27


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3. Hargreaves Method:

The Hargreaves (Hargreaves and Samani, 1985) method requires only maximum and minimum daily

air temperature observations and it can be applied on daily, weekly, decadal or monthly time steps.

( ) ( ) amean RTTTET 8.170023.05.0

minmax0 +=

Where all temperatures are in 0C and ET in mm/day. The mean temperature is calculated as

0.5(Tmax+Tmin). Ra is the extraterrestrial short wave radiation in mm/day. (If Ra is given in MJm-2

d-1

,

then division by 2.45 yields the vale in mm/day). Unless unusual weather patterns exist, theHargreaves method should agree within 15% of the Penman and Penman-Monteith calculations.

The most important parameters in estimating ETo, are temperature and solar radiation. Although

relative humidity is not explicitly contained in the equation, it is implicitly present in the difference in

maximum and minimum temperature. The temperature difference (TD) is linearly related to relative

humidity.

4. Thornthwaite Method:

Thornthwaite developed an equation to predict monthly evapotranspiration from mean monthly

temperature data. The small amount of data needed is attractive because often it needs to be predicted

for sites where few weather data are available. M.E. Jensen et al. (1990) warn that Thornthwaites

method is generally only applicable to areas that have climates similar to that of central U.S, and it isnot applicable to arid and semiarid regions.

Thornthwaite found that evapotranspiration could be predicted from an equation of the form

a

oI

TET

=

1016

Where: ETo = Monthly reference crop evapotranspiration (mm/month)

T = Mean monthly temperature (C)

a = is the location dependant coefficient

I = is the annual heat index described below5.1

12

1 5

=

=

i

iTI

and the coefficient a is given by32 000000675.00000771.00179.049.0 IIIa ++=

5. Radiation Methods:

Evapotranspiration is controlled by available energy and the availability of evaporated water to be

transferred from the surface through turbulent transport. The transfer processes are a function of wind

speed and amount of water vapour close to the surface. However, Priestley and Taylor (1972) showed

that evapotranspiration is well described by net radiation, air temperature and pressure for large well-watered surfaces. Radiation methods use solar radiation coupled with air temperature data to estimate

the reference evapotranspiration ETo.


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The Priestley-Taylor (1972) equation has the form

GRET no

+

=

Where is usually taken as 1.26. ET0 is in mm/day, Rn is the net radiation, G the soil heat flux inMJm-2d-1, the latent heat of vaporization in MJkg-1 and and are as defined for the Penmanequations (kPa

0C

-1). Shuttleworth (1993) recommends to take =1.74 for arid climates, =1.26 in

humid climates.

The Makkink(1957) method is commonly used in western Europe

12.045.2

61.0 +

= so

RET

Where ET0is in mmd-1

. Rsis the incoming solar short wave radiation in MJm-2

d-1

. Constant 2.45 is the

latent heat of vaporization at about 20 oC

6. Modified penman method:

In 1948, Penman combined the energy balance with the mass transfer method and derived an equation

to compute the evaporation from an open water surface from standard climatological records of

sunshine, temperature, humidity and wind speed. This so-called combination method was further

developed by many researchers and extended to cropped surfaces by introducing resistance factors.

The FAO Penman-Montheith equation is a close, simple representation of the physical and

physiological factors governing the evapotranspiration processes.

The FAO Penman-Monteith method to estimate ETo:

where

ETo :reference evapotranspiration [mm day-1],

Rn :net radiation at the crop surface [MJ m-2 day-1],

G :soil heat flux density [MJ m-2 day-1],

T :mean daily air temperature at 2 m height [C],

U2 :wind speed at 2 m height [m s-1],es :saturation vapour pressure [kPa],

ea :actual vapour pressure [kPa],

es - ea :saturation vapour pressure deficit [kPa],

:slope of vapour pressure curve [kPa C-1],

:psychrometric constant [kPa C-1].

( )

( )2

2

34.01

273

900)(408.0

U

eeUT

GR

ETasn

o ++

+

+=


15/34

By A.W.

15

Net Radiation

Net radiation (Rn) is the balance of the shortwave and longwave radiation streams, such as

Rsw and Rlw are the shortwave and longwave components and the arrows denote the direction of the

flux, generally expressed in units of Watts per square meter (Wm-2). Thus, the Rn is the difference

between total upward and downward radiation fluxes and is a measure of the energy available at the

ground surface.

Shortwave radiation

Shortwave radiation from the Sun penetrates through space to the outer edge of the atmosphere

unimpeded by the vacuum of outer space. If one places a surface oriented perpendicular to an

incoming beam of light, 1.94 cal cm-2 min-1 of solar radiation will be received. This value is known

as the solar constant but actually varies by a small amount as the Earth-Sun distance changes throughthe year. Once solar radiation begins to penetrate through the atmosphere this amount begins to

decrease due to absorption and reflection.

A portion of the incoming solar radiation is absorbed by the surface and a portion is also reflected

away. The proportion of light reflected from a surface is the albedo (). Albedo values range from 0for no reflection to 1 for complete reflection of light striking the surface. Albedo can be expressed as apercentage (albedo multiplied by 100) that for some is easier to understand. For instance, grass has an

albedo of about 0.23. This means that of the incoming solar radiation that strikes the grass, 23% of it is

reflected away.

Net shortwave radiationis the difference between incoming and outgoing shortwave radiation

expressed as:

( ) sswswns RRRR == 1

Solar radiation Rs

As the radiation penetrates the atmosphere, some of the radiation is scattered, reflected or absorbed by

the atmospheric gases, clouds and dust. The amount of radiation reaching a horizontal plane is known

as the solar radiation, Rs. For a cloudless day, Rs is roughly 75% of extraterrestrial radiation. On acloudy day, the radiation is scattered in the atmosphere, but even with extremely dense cloud cover,

about 25% of the extraterrestrial radiation may still reach the earths surface.

If the solar radiation, Rs, is not measured, it can be calculated as:

Where Rs : solar radiation (MJ/m2day)

n : actual duration of sunshine (hour)

N : maximum possible duration of sunshine or daylight hours (hour)


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16

n/N : relative sunshine duration (-)

Ra : extraterrestrial radiation (MJ/m2day)

as, bs : regression constant, expressing the fraction of extraterrestrial radiation reaching the

earth on overcast days (n=0)

as + bs: fraction of extraterrestrial radiation reaching the earth on clear days (n= N)

Rs is expressed in the above equation in MJ/m2 day. The corresponding equivalent evaporation in

mm/day is obtained by multiplying Rs by 0.408. When no actual solar radiation data are available (as=

0.25 and bs= 0.50).

The actual solar radiation reaching the evaporating surface depends on the turbidity of the atmosphere

and the presence of clouds which reflect and absorb major parts of the radiation.

Relative sunshine duration (n/N)

The relative sunshine duration is another ratio that expresses the cloudiness of the atmosphere. It is theratio of the actual duration of sunshine, n, to the maximum possible duration of sunshine or daylight

hours N. In the absence of any clouds, the actual duration of sunshine is equal to the daylight hours (n

= N) and the ratio is one, while on cloudy days n and consequently the ratio may be zero. In the

absence of a direct measurement of Rs, the relative sunshine duration, n/N, is often used to derivesolar radiation from extraterrestrial radiation.

As with extraterrestrial radiation, the day length N depends on the position of the sun and is hence a

function of latitude and date.

Wheresis the sunset hour angle in radians


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17

Annual variation of the daylight hours (N) at the equator, 20 and 40north and south

Extraterrestrial Radiation (Ra)

Solar radiation incident outside the earth's atmosphere is called extraterrestrial radiation. On averagethe extraterrestrial irradiance is 1367 Watts/meter2 (W/m2). This value varies by 3% as the earth

orbits the sun. The earth's closest approach to the sun occurs around January 4th and it is furthest from

the sun around July 5th.

The local intensity of radiation is, however, determined by the angle between the direction of the sun's

rays and the normal to the surface of the atmosphere. This angle will change during the day and will

be different at different latitudes and in different seasons. The solar radiation received at the top of the

earth's atmosphere on a horizontal surface is called the extraterrestrial (solar) radiation, Ra.

The extraterrestrial radiation, Ra, for each day of the year and for different latitudes can be estimated

from the solar constant, the solar declination and the time of the year by:

Where

Ra : extraterrestrial radiation [MJ m-2day-1],

Gsc : solar constant = 0.0820 MJ m-2min-1,

dr : inverse relative distance Earth-Sun,

s : sunset hour angle [rad],

: latitude positive for the northern hemisphere and negative for the southern hemisphere [rad], : solar decimation [rad].


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18

Annual variation in extraterrestrial radiation (Ra) at the equator, 20 and 40north and south

Inverse relative distance Earth-Sun (dr)

The earth revolves around the sun in an elliptical orbit with the sun at one of the foci. The amount ofsolar energy reaching the earth is inversely proportional to the square of its distance from the sun. The

mean earth sun distance ro is called one astronomical unit

1AU = 1.496*106km

The minimum sun-earth distance is about 0.983 AU approximately in 3 January and the maximum

approximately 1.017AU approximately in 4 July. In long-term cycles, those distances are influenced,

however slightly, by other heavenly bodies and the leap year cycle. However, the relative sun-earth

distance drfor any day of the year is known with considerable accuracy.

Where J is the number of the day in the year between 1 (1 January) and 365 or 366 (31 December).

Solar decimation ()

The declination angle, denoted by , varies seasonally due to the tilt of the Earth on its axis of rotationand the rotation of the Earth around the sun. If the Earth were not tilted on its axis of rotation, the

declination would always be 0. However, the Earth is tilted by 23.45 and the declination angle varies

plus or minus this amount. Only at the spring and fall equinoxes is the declination angle equal to 0.

The declination of the sun is the angle between the equator and a line drawn from the centre of the

Earth to the centre of the sun. is angle between centers of the earth to center of sun to the equatorialplane J is the number of the day in the year

Where J is the number of the day in the year between 1 (1 January) and 365 or 366 (31 December).

Sunset hour angle (s)

Used to describe the earth's rotation about its polar axis. It is the angular distance between the

meridian of the observer and the meridian whose plane contains the sun.

s= arccos [-tan () tan ()]

Net longwave radiation (Rnl)

The solar radiation absorbed by the earth is converted to heat energy. By several processes, the earths

surface loss this energy. The earth, which is at a much lower temperature than the sun, emits radiative

energy with wavelengths longer than those from the sun. Therefore, the terrestrial radiation is referred


19/34

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19

to as longwave radiation. The gases of the atmosphere are relatively good absorbers of longwave

radiation and thus absorb the energy emitted by the Earth's surface. The absorbed radiation is emitteddownward toward the surface as longwave atmospheric counter-radiation(L) keeping near surface

temperatures warmer than they would be without this blanket of gases. This is known as the

"greenhouse effect". The earths surface both emits and receives longwave radiation. The differencebetween outgoing and incoming longwave radiation is called the net longwave radiation, Rnl. As theoutgoing long wave radiation is almost always greater than the incoming long wave radiation, Rnl

represents an energy loss.

The difference between incoming and outgoing longwave radiation is net longwave radiation

expressed as:

= lwlwnl RRR

The rate of longwave energy emission is proportional to the absolute temperature of the surface raised

to the fourth power.

Where Rnl : net outgoing longwave radiation (MJ/m2day)

: Stefan-Boltmann constant (4.903*10-9MJ/m2day)Tmax,k : maximum absolute temperature during the 24-hour period(K=0C+273.16)

Tmin,k : minimum absolute temperature during the 24-hour periodea : actual vapour pressure (kpa)

Rs/Rso : relative shortwave radiation (1.0)Rs : measured or calculated solar radiation (MJ/m2day)Rso : calculated clear-sky radiation (MJ/m2day)

Clear-sky solar radiation (Rso)

The calculation of the clear-sky radiation, Rso, when n = N, is required for computing net longwaveradiation.

For near sea level or when calibrated values for as and bs are available:

Rso= (as+bs)Rawhere

Rso : clear-sky solar radiation[MJ m-2day-1],

as+bs : fractionof extraterrestrial radiation reaching the earth on clear-sky days(n = N).

When calibrated values for asand bsare not available:

Rso=(0.75 + 2 l0-5Z)Ra

Where

Z station elevation above sea level [m].


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20

Soil heat flux (G)

The soil heat flux, G, is the energy that is utilized in heating the soil. Since the soil heat flux is small

compared to Rn it may be ignored. Complex models are available to describe soil heat flux. Because

soil heat flux is small compared to Rn, particularly when the surface is covered by vegetation, As themagnitude of the day or ten-day soil heat flux beneath the grass reference surface is relatively small, it

may be ignored and thus:Gday= 0

For monthly periods:

Gmonth, i= 0.07 (Tmonth, i+1- Tmonth, i-1)

or, if Tmonth, i+1is unknown:

Gmonth, i= 0.14 (Tmonth, i- Tmonth, i-1)

Where

Tmonth, i : mean air temperature of month i [C],Tmonth, i-1 : mean air temperature of previous month [C],

Tmonth, i+1 : mean air temperature of next month [C].

Air temperature (T)

Tmaxand Tmin for longer periods such as weeks, 10-day's or months are obtained by dividing the sum ofthe respective daily values by the number of days in the period. The mean daily air temperature (Tmean)

is only employed in the FAO Penman-Monteith equation to calculate the slope of the saturation vapour

pressure curves () and the impact of mean air density (Pa) as the effect of temperature variations on

the value of the climatic parameter is small in these cases. For standardization, Tmeanfor 24-hour

periods is defined as the mean of the daily maximum (Tmax) and minimum temperatures (Tmin) ratherthan as the average of hourly temperature measurements.

In practice K = C + 273.16

Slope of saturation vapour pressure curve ()

For the calculation of evapotranspiration, the slope of the relationship between saturation vapour

pressure and temperature, , is required. The slope of the curve (Figure 11) at a given temperature isgiven by.

Where


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21

slope of saturation vapour pressure curve at air temperature T [kPa C-1],

T air temperature [C],

Mean saturation vapour pressure (es)

As saturation vapour pressure is related to air temperature, it can be calculated from the airtemperature. The relationship is expressed by:

Where

e(T) saturation vapour pressure at the air temperature T [kPa],T air temperature [C],Due to the non-linearity of the above equation, the mean saturation vapour pressure for a day, week,

decade or month should be computed as the mean between the saturation vapour pressure at the mean

daily maximum and minimum air temperatures for that period:

Using mean air temperature instead of daily minimum and maximum temperatures results in lower

estimates for the mean saturation vapour pressure. The corresponding vapour pressure deficit (a

parameter expressing the evaporating power of the atmosphere) will also be smaller and the result will

be some underestimation of the reference crop evapotranspiration.

Actual vapour pressure (ea)

The actual vapour pressure can also be calculated from the relative humidity. Depending on the

availability of the humidity data, different equations should be used.

where

ea : actual vapour pressure [kPa],

e(Tmin) : saturation vapour pressure at daily minimum temperature [kPa],

e(Tmax) : saturation vapour pressure at daily maximum temperature [kPa],

RHmax : maximum relative humidity [%],

RHmin : minimum relative humidity [%].

For periods of a week, ten days or a month, RHmaxand RHminare obtained by dividing the sum of the

daily values by the number of days in that period.

In the absence of RHmax and RHmin, another equation can be used to estimate ea:


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22

Where RHmeanis the mean relative humidity, defined as the average between RHmaxand RHmin.

Vapour pressure deficit (es- ea)

The vapour pressure deficit is the difference between the saturation (es) and actual vapour pressure (ea)

for a given time period.

Psychrometric constant ()

The psychrometric constant, , is given by:

Where : psychrometric constant [kPa C-1],P : atmospheric pressure [kPa],

: latent heat of vaporization, 2.45 [MJ kg-1],cp : specific heat at constant pressure, 1.013 10-3[MJ kg-1C-1],

: ratio molecular weight of water vapour/dry air = 0.622.

Energy required to change a unit mass of water from liquide water to water vapour in a constant

pressure and temperature is called latent heat of vaporization. In other words, 2.45 MJ are needed to

vaporize 1 kg or 0.001 m3 of water.

Atmospheric pressure (P)

The atmospheric pressure, P, is the pressure exerted by the weight of the earth's atmosphere.

Evaporation at high altitudes is promoted due to low atmospheric pressure as expressed in the

psychrometric constant. The effect is, however, small and in the calculation procedures, the average

value for a location is sufficient.

Where P : atmospheric pressure [kPa],

z : elevation above sea level [m],

Wind speed (U2)

Wind speeds measured at different heights above the soil surface are different. Surface friction tendsto slow down wind passing over it. Wind speed is slowest at the surface and increases with height. For

this reason anemometers are placed at a chosen standard height, i.e., 10 m in meteorology and 2 or 3 m

in agrometeorology. For the calculation of evapotranspiration, wind speed measured at 2 m above the

surface is required. To adjust wind speed data obtained from instruments placed at elevations other


23/34

By A.W.

23

than the standard height of 2m, a logarithmic wind speed profile may be used for measurements above

a short grassed surface:

U2 =

4.87

ln(67.8 5.42zu z

Where U2 : Wind speed at 2 m above ground surface (m/s)

Uz : measured wind speed at z m above ground surface(m/s)

Z : height of measurement above ground surface (m)

Adjusting wind speed data to standard height


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By A.W.

24

2.7. Crop Evapotranspiration (ETC)

The crop evapotranspiration under standard conditions, denoted as ETc, is the evapotranspiration from

disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions, andachieving full production under the given climatic conditions.

Experimentally determined ratios of ETc/ETo, called crop coefficient (Kc) are used to relate ETc to

ETo or ETc = Kc*ETo. Differences in leaf anatomy, stomatal characteristics, aerodynamic properties

and even albedo cause the crop evapotranspiration to differ from the reference crop evapotranspiration

under the same climatic conditions.

ETc = Kc*ETo

Where ETc crop evapotranspiration (mm/day)

Kc crop coefficientETo reference crop evapotranspiration (mm/day)

Most of the effects of the various weather conditions are incorporated into the ETo estimate. The crop

coefficient Kc represents an integration of the effects of four primary characteristics that distinguishthe crop from reference grass: (1) Crop height, (2) Albedo (reflectance) of the crop-soil surface, (3)Canopy (stomatal) resistance, and (4) Evaporation from soil surface.

2.7.1. Factors determining the crop coefficient

The crop coefficient integrates the effect of characteristics that distinguish a typical field crop from the

grass reference, which has a constant appearance and a complete ground cover. Consequently,

different crops will have different Kc coefficients. The changing characteristics of the crop over the

growing season also affect the Kc coefficient. As evaporation is an integrated part of cropevapotranspiration, conditions affecting soil evaporation will also have an effect on Kc.

i) Crop Type

Due to differences in albedo, crop height aerodynamic properties and leaf and stomata properties, the

evapotranspiration from full-grown, well-watered crops differs from ETo.

The close spacings of plants and taller canopy height and roughness of many full grown agricultural

crops cause these crops to have Kc factors that are larger than 1. The Kc factor is often 5-10% higherthan the reference (where Kc = 1.0), and even 15-20% greater for some tall crops such as maize,

sorghum or sugar cane.

ii) Climate

The Kc value in literature are typical values expected for average Kc under a standard climatic

condition, which is defined as a sub-humid climate with average daytime minimum relative humidity

of 45% and having a calm to moderate wind speeds averaging 2 m/s. Variation in wind after the

aerodynamic resistance or the crops and hence their crop coefficients.

Variations in wind alter the aerodynamic resistance of the crops and hence their crop coefficients,especially for those crops that are substantially taller than the hypothetical grass reference. More arid

climates and conditions of greater wind speed will have higher values for Kc. More humid climates

and conditions of lower wind speed will have lower values for Kc.


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25

Typical Kcfor different types of full grown crops

iii) Soil Evaporation

Differences in soil evaporation and crop transpiration between field crops and the reference surface are

integrated within the crop coefficient. The Kc for full-grown crops primarily reflects differences intranspiration, as the contribution of soil evaporation is relatively small.

After rainfall or irrigation, the effect of evaporation is predominant when the crop is small andscarcely shades the ground. For such low-cover conditions, the Kc coefficient is determined largely by

the frequency with which the soil surface is wetted. Where the soil is wet for most of the time from

irrigation or rain, the evaporation from the soil surface will be considerable and Kc may exceed 1. On

the other hand, where the soil surface is dry, evaporation is restricted and Kc will be small and mighteven drop to as low as 0.1

iv) Crop Growth Stages

As the crop develops, the ground cover, crop height and the leaf area change. Due to differences in

evapotranspiration during the various growth stages, the Kc for a given crop will vary over the

growing period. The growing period can be divided into four distinct growth stages: initial, crop

development, mid-season and the late season.

1. Initial stage:The initial stage runs from planting date to approximately10% ground cover. During the initial period,

the leaf area is small, and evaporation is predominantly in the form of soil evaporation. Therefore, the

Kc during the initial period (Kcin) is large when the soil is wet from irrigation or rainfall and low

when the soil surface is dry.

2. Crop development stage:The crop development stage runs from 10% ground cover to effective full cover. Effective full coverfor many crops is at the initiation of flowering. For row crops where rows commonly interlock leaves

such as beans, sugar beats, potatoes and corn. Effective cover can be defined as the time when some


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26

leaves of plants in adjacent rows begin to intermingle so that soil shading becomes nearly complete, or

when plants reach nearly full size if no intermingling occurs. Another way to estimate the occurrenceof effective full cover is when the leaf area index (LAI) reaches three. LAI is defined as the average

total area of leaves (one side) per unit area of ground surface. As the crop develops and shades more

and more of the ground, evaporation becomes more restricted and transpiration gradually becomes themajor process. During the crop development stage, the Kc value corresponds to amounts of groundcover and plant development.

3. Mid-season stage:

The mid-season stage runs from effective full cover to the start of maturity. The start of maturity is

often indicated by the beginning of the ageing, yellowing or senescence of leaves, leaf drop, or the

browning of fruit to the degree that the crop ET is reduced relative to the reference ETo. The mid-

season stage is the longest stage for perennials and for many annuals, but it may be relatively short for

vegetable crops that are harvested fresh for their green vegetation. At the mid-season stage the Kcreaches its maximum value. The value of Kc (Kcmid) is relatively constant for most growing and

cultural conditions.

4. Late season stage:

The late season stage runs from the start of maturity to harvest or full senescence. The calculation forKc and Etc is presumed to end when the crop is harvested, dries out naturally, reaches full senescence,

or experiences leaf drop. The Kc value at the end of the late season stage (Kcend) reflects crop andwater management practices. The Kcend value is high if the crop is frequently irrigated until harvested

fresh. If the crop is allowed to senescence and to dry out in the field before harvest, the Kc end value

will be small.

The generalized crop coefficient curve is shown in Figure below. Shortly after the planting of annualsor shortly after the initiation of new leaves for perennials, the value for Kc is small, often less than 0.4.The Kc begins to increase from the initial Kc value, Kc ini, at the beginning of rapid plant

development and reaches a maximum value, Kc mid, at the time of maximum or near maximum plant

development. During the late season period, as leaves begin to age and senesce due to natural orcultural practices, the Kc begins to decrease until it reaches a lower value at the end of the growing

period equal to Kc end.

FIGURE . Generalized crop coefficient curve for the single crop coefficient approach


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27

2.7.2. Length of growth stages

FAO Irrigation and Drainage Paper No. 24 provides general lengths for the four distinct growth stages

and the total growing period for various types of climates and locations. This information has been

supplemented from other sources and is summarized in Table below.

TABLE . Lengths of crop development stages* for various planting periods and climatic regions (days)

Crop Init. (Lini) Dev. (Ldev) Mid (Lmid) Late (Llate) Total Plant Date Region

a. Small Vegetables

Cabbage 40 60 50 15 165 Sept Calif. Desert, USA

Carrots 20 30 50/30 20 100 Oct/Jan Arid climate

15 25 70 40 150 April MediterraneanOnion (dry)

20 35 110 45 210 Oct; Jan. Arid Region; Calif.

Onion (green) 25 30 10 5 70 April/May Mediterranean

b. Vegetables - Solanum Family (Solanaceae)

Tomato 30 40 40 25 135 January Arid Region

c. Roots and Tubers

Potato 25 30 30/45 30 115/130 Jan/Nov (Semi) Arid Climate

d. Legumes (Leguminosae)

Beans (dry) 20 30 40 20 110 May/June Continental Climates

e. Fibre Crops

Cotton 30 50 60 55 195 Mar-May Egypt; Pakistan; Calif.

j. Forages

k. Sugar Cane

* Lengths of crop development stages provided in this table are indicative of general conditions, but

may vary substantially from region to region, with climate and cropping conditions, and with cropvariety. The user is strongly encouraged to obtain appropriate local information.

Tabulated Kcvalues

Table below lists typical values for Kc ini, Kc mid and Kc end for various agricultural crops. Thecoefficients presented are organized by group type (i.e., small vegetables, legumes, cereals, etc.) to

assist in locating the crop in the table and to aid in comparing crops within the same group. There is

usually close similarity in the coefficients among the members of the same crop group, as the plantheight, leaf area, ground coverage and water management are normally similar.


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28

Table: Typical values for Kc ini, Kc mid and Kc end for various agricultural crops

Crop Kc mid Kc endMaximum Crop Height

(h) (m)

a. Small Vegetables 0.7 1.05 0.95

Cabbage 1.05 0.95 0.4

Carrots 1.05 0.95 0.3

Onions dry 1.05 0.75 0.4

b. Vegetables - Solanum Family (Solanaceae) 0.6 1.15 0.80

Sweet Peppers (bell) 1.05 0.90 0.7

Tomato 1.15 0.70-

0.90

0.6

c. Vegetables - Cucumber Family (Cucurbitaceae) 0.5 1.00 0.80

Pumpkin, Winter Squash 1.00 0.80 0.4

d. Roots and Tubers 0.5 1.10 0.95

Potato 1.15 0.75 0.6

Sweet Potato 1.15 0.65 0.4

e. Legumes (Leguminosae) 0.4 1.15 0.55

Chick pea 1.00 0.35 0.4

f. Perennial Vegetables (with winter dormancy and initially

bare or mulched soil)

0.5 1.00 0.80

Strawberries 0.40 0.85 0.75 0.2

g. Fibre Crops 0.35

Cotton 1.15-

1.20

0.70-

0.50

1.2-1.5

Maize, Field (grain) (field corn) 0.3 1.20 0.60-

0.35

2

Rice 1.05 1.20 0.90-

0.60

1

k. Sugar Cane 0.40 1.25 0.75 3

l. Tropical Fruits and Trees

Banana

- 1styear 0.50 1.10 1.00 3

- 2ndyear 1.00 1.20 1.10 4

Coffee

- bare ground cover 0.90 0.95 0.95 2-3

- with weeds 1.05 1.10 1.10 2-3

Palm Trees 0.95 1.00 1.00 8

Pineapple - bare soil 0.50 0.30 0.30 0.6-1.2

- with grass cover 0.50 0.50 0.50 0.6-1.2

Tea

- non-shaded 0.95 1.00 1.00 1.5

- shaded 1.10 1.15 1.15 2


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2.7.3. Construction of the Kc curve

Only three point values for Kcare required to describe and to construct the Kccurve. The curve such as thatshown in Figure below is constructed using the following two steps:

1. Divide the growing period into four general growth stages that describe crop phenology or development

(initial, crop development, mid-season, and late season stage), determine the lengths of the growth stages, and

identify the three Kcvalues that correspond to Kc ini, Kc midand Kc endfrom Table

2. Construct a curve by connecting straight line segments through each of the four growth stages. Horizontallines are drawn through Kc iniin the initial stage and through Kc midin the mid-season stage. Diagonal lines are

drawn from Kc inito Kc midwithin the course of the crop development stage and from Kc midto Kc endwithin the

course of the late season stage.

FIGURE . Crop coefficient curve

2.8. Irrigation water requirement

The irrigation requirement (NIR) for crop production is the amount of water, in addition to rainfall

ground water contribution and existing soil moisture, that must be applied to meet a crop'sevapotranspiration needs without significant reduction in yield. In other words It is the depth of

irrigation water, exclusive of effective rainfall, ground water contribution and available soil moistureconsidered for timing the irrigation to meet the consumptive use requirement of the crop.

NIR = ETc (Pe + GW + S) (2.23)

Where NIR net irrigation requirement (mm/day)

ETc crop evapotranspiration (mm/day)Pe effective rainfall (mm)

GW ground water contribution (mm)

S available soil moisture (mm)


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Calculation of Effective Rainfall (Pe)It is that portion of precipitation, which has fallen during the base period of a crop and is available to

meet the evapotranspiration need of the crop. In a broad sense, effective rainfall is the portion ofrainfall that has not lost by surface run-off or through deep percolation below the root zone. Generally,

rainfall effectiveness increases with high ET rates, greater allowed soil moisture depletions, and large

soil water storage capacities.

The following different methods are used for calculation of effective rainfall.

Fixed Percentage Effective Rainfall

The effective rainfall is taken as a fixed percentage of the monthly rainfall;

Effective Rainfall = % of Total Rainfall

Normally losses are accounted 10% to 30% , thus a lies between 0.7 and 0.9 An average value of 0.8will be adopted.

Dependable Rain

An empirical formula developed by FAO based on analysis for different arid and sub-humid climates.

This formula is as follows:

Effective Rainfall = 0.6 * Total Rainfall - 10 ... (Total Rainfall < 70 mm)

Effective Rainfall = 0.8 * Total Rainfall - 24 ... (Total Rainfall > 70 mm)

Empirical Formula for Effective Rainfall

This formula is similar to FAO formula (see Dependable Rain method above) with some parametersleft to the user to define. The formula is as follows:

Effective Rainfall = a * Total Rainfall - b ... (Total Rainfall < z mm)

Effective Rainfall = c * Total Rainfall - d ... (Total Rainfall > z mm)

where a, b, c, and z are the variables to be defined by the user.

Method of USDA Soil Conservation Service (default)

The effective rainfall is calculated according to the formula developed by the USDA Soil

Conservation Service which is as follows:

Effective Rainfall = Total Rainfall / 125 * (125 - 0.2 * Total Rainfall) ... (Total Rainfall < 250 mm)Effective Rainfall = 125 + 0.1 * Total Rainfall ... (Total Rainfall > 250mm)

2.9 Field irrigation requirement (NIR)

It is the amount of water required to meet the net irrigation requirements plus the water lost through

the application method includes water loss in percolation in the field watercourse, field channels and

field application of water. If ois water application efficiency:


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o

NIRFIR

=

2.10. Gross irrigation requirement (GIR)

It is the amount of water required to satisfy the field irrigation requirement and the water lost as a

conveyance losses in the distribution up to the field. If c is the water conveyance efficiency

c

FIRGIR

=

GIR is used for channel design

2.11. Soil moisture irrigation relationship

Extending down from the ground surface is the soil zone or root zone, which is defined as being the

depth of over burden that is penetrated by the roots of vegetation. This zone is most important fromirrigation point of view, it is this zone from which plants do take there water supplies.

When a water falls over the ground, a part of it gets absorbed in the root zone, and the rest flows over

the ground under action of gravity.

Field capacity

Immediately after a rain or irrigation water application, when all the gravity water has drained down to

the water table, a certain amount of water is retained on the surface of soil grains by molecular

attraction and by loose chemical bonds (absorption). This water cannot drain under the action of

gravity and is called the field capacity.

The field capacity is thus the water content of the soil after free drainage has taken place for a

sufficient period. The field capacity will depend on the soil texture and soil structure, in average it is

attained 24 hours after watering from saturation. In soil with low permeability, a true field capacitydoes not exist because the velocity of drainage is low from the beginning and can continue at the same

low rate for several days. Nevertheless it is often useful to measure the water held in such soils (48 hr

after watering). The water moving downward through the profile during the period from addition of

water to attainment of field capacity condition is, of course, available for plant use.

Field capacity is expressed in percentage by volume or by weight of the oven-dry soil sample. Field

capacity consists of two parts:1. Water which is attached to the soil molecules by surface tension against a gravitational force,

and can be extracted by plants by capillarity; this water is called capillary water

2. Water which is attached to the soil molecules by loose chemical bonds, and cannot be removedby capillarity, and is called hydroscopic water.

sampledriedovenWt

retainedwaterofweight

sampledriedovenWt

soildriedovenWtsoilSampleWtFC =

=

Let as consider: Area of 1m2

Depth of root zone = d m

Dry unit weight of soil = d kN/m3


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Vw is volume of water stored in a unit area.

Hence, the depth of water stored in the root zone in filling soil up to the field capacity is:

w

dw

dFCd

**=

Total field capacity of water cannot be utilized by the plants; plants can extract water from the soil till

the permanent welting point is reached. Permanent welting point isdefined as the minimal point of soil

moisture the plant requires not to wilt. Beyond this point is the water content at which plants can no

longer extract sufficient water for there growth.

It is therefore, becomes evident that the water which is available to the plant is the difference of field

capacity water and permanent welting point water. This is known as available moisture or maximum

storage capacity of soil.

Available Moisture=Field capacity - Permanent Welting point.

Readily available moistureIs the portion of the available moisture which is most easily extracted by plants is approximately 70 to

80% of the available moisture

2.12. Depth and frequency of irrigation

Soil moisture is consumed by plants through their roots. It, therefore, becomes necessary that

sufficient moisture remains available in the soil from the surface to the root zone depth.

At any time, the moisture content in the soil should be between the field capacity and optimum

moisture content, the moisture between this two is called available moisture and readily available

moisture is 70 to 80% of the available moisture.

w

dw

d

ww

ds

dFCV

d

VFC

VretainedwaterofWtFC

soilsampledryWt

retainedwaterofWtFC

**

**1

*

=

=

=

=


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When watering is done, the amount of water supplied should be such that the water content is equal to

field capacity. Water will gradually be utilized consumptively by plants after the water application,and the soil moisture will start falling. When the water content in the soil reaches the optimum value,

fresh doses of irrigation may be done so that the water content is again raised to the field capacity of

soil.

The frequency of irrigation is controlled by the amount of available water contained in the root zone of

the soil and the consumptive use rate.

If d- root zone depth

FC- field capacityMO- optimum moisture content

dw- depth of water to be given during each watering

w- unit weight of water

d- unit weight of dry soil

( )

w

d MOFCddw

= *

Irrigation IntervalNumber of days between two successive irrigations during the period consumptive use of the crop is

known as irrigation interval. It depends on:

Consumptive use rate (ETc) Amount of available soil moisture in the root zone depth between field capacity and

starting moisture level for irrigation.

Irrigation scheduling is the programming of the timing of irrigation to initiate and to stop, as well as

the amount of application water to the crops. The objective of the scheduling could be maximizing thereturn, minimize irrigation cost, maximum yield, limited water supply, or minimize ground water

pollution, but independently of the objective, the appropriate irrigation scheduling strategy should be

to prevent water stress to the crop through the growing season or during the sensitive growing stages

of the crop.

Frequency of irrigation will be:

CU

dwFI= days

CU is daily consumptive use


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Documents

IRR Chapter 2