CROP IRRIGATION WATER REQUIREMENTS

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CRIWAR 3.0 for Windows
CROP IRRIGATION WATER REQUIREMENTS
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Outline of the topic

• Powerpoint presentation
• Demonstration of the program
• Exercises with Criwar

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Evapotranspiration
the plants extract water from the soil this
water leaves the plant during the day through the
stem and the leaves
effective root zone
After M.G.bos
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Definitions in Criwar
Potential evapotranspiration ETp is the
evapotranspiration from cropped soils that have
an optimum supply of water.
Effective precipitation is that part of the total
precipitation on the cropped area, during a
specific time period, which is available to meet
evapotranspiration in the cropped area.
In CRIWAR, ETp is the volume of irrigation water
required to meet the crops potential
evapotranspiration during a specific time period,
under a given cropping pattern and in a specific
climate
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What does CRIWAR calculate?
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Field water balance
Precipitation
ET
Irrigation
Drainage
Capillary rise
Seepage
Rootzone
Groundwater
Groundwater
After M.G.bos
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Influencing factors in Criwar
The following factors have effect on the water
requirement of a crop

• The climate
• The type of crop
• The growth stage of the crop

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Influence of the climate on crop water needs
Climatic factor Crop water need High
Low Sunny cloudy Hot coo
l Low (dry) high (humid) Windy little
wind
Sunshine Temperature Humidity Wind speed
Highest crop water needs in hot, dry, windy and
sunny areas
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Crop water needs as compared to standard grass
Not all crops have the same water needs. The crop
water requirements vary
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The influence of growth stage on crop water needs
1. Initial stage Germination and early growth of
the crop. Soil surface hardly covered by the
crop canopy (ground cover lt10)
2. Crop development From the end of stage 1 to
effective full ground cover of 70 to 80. Note
that the crop has not reached its mature height
yet  
3. Mid-season From the attainment of effective
full ground cover to the start of maturing of the
crop. Maturing may be indicated by discolouring
of leaves or falling of leaves.
4. Late season From the end of the mid-season
stage until full maturity or harvest of the crop
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CALCULATING EVAPOTRANSPIRATION
In the past Empirical correlation methods to
estimate the potential evapotranspiration.
These were often only valid for the local
conditions and hardly transferable to other
areas.
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CALCULATING EVAPOTRANSPIRATION
Presently most of the calculation methods for
evapotranspitation are based on 3 physical
requirements in soil plant - atmosphere

• continous supply of water
• energy to change liquid water into vapour
• a vapour gradient to maintain a flux from the
  evaporating surface to the atmosphere

Penman was the first to apply this so-called
Combination method
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Penmans formula
  D Rn - G g E0
-------- ------------ ----------
Ea D g g
D g lt radiation term gt
lt aerodynamic term gt
Eo open water evaporation rate (kg/m2
s) D proportionality constant dez/dTz
(kPa/?C) Rn net radiation (W/m2) G heat flux
density into the water body (W/m2) l latent
heat of vaporisation (J/kg) g psychometric
constant (kPa/?C) Ea isothermal evaporation
rate (kg/m2 s)
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The Penman method

• Penman method
• Estimate the evaporation from an open water
  surface
• and use that as a reference evaporation
• Reference evaporation crop factor potential
  evapotranspiration
• Etcrop Eo Kc

• Data required are
• air temperature
• air humidity
• solar radiation
• wind speed

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Two other methods to calculate potential
evapotranspiration

• The FAO Modified Penman Method
• The Penman-Monteith Approach

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The FAO Modified Penman Method
Instead of open water they used the
evapotranspiration from a reference crop Etg
defined as An extended surface of an 8 to 15
cm tall green grass cover of uniform height,
actively growing, completely shading the ground
and not short of water
Again there are a radiation term and an
aerodynamic term
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Main differences are

• Different short wave reflection coefficient (0.05
  water, 0.25 grass)
• More sensitive wind function
• Adjustment factors for local condition compared
  to assumed standards

The formula reads ETp Kc ETref Kc
ETg
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The Penman-Monteith Approach
There was evidence that the Modified Penman
method over-predicted the crop water
requirements   Monteith developed an equation
that describes transpiration from a dry,
extensive horizontal and uniformly vegetated
surface that fully covered the ground, optimally
supplied with water. Canopy and air resistances
to water vapour diffusion were introduced.
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The characteristics of hypothetical reference crop
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The Penman-Monteith Approach

• The main differences with the modified Penman
  method are
• Different reflection coefficients
• Different aerodynamic resistance, resulting in a
  different wind function\
• Modification of the psychromatic constant ?

ETp Kc ETref Kc ETh
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Relationship modified Penman vs Penman-Monteith
Eth 0.85 ETg
Crop coefficients introduced for Modified penman
method could still be used with Penman-Monteith
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The modified Hargreaves method
ET0,mh 0.0013 x 0.408RA x(Tav 17.0) x (TD
0.0123P)0.76
RA extraterrestrial radiation (MJ/m2 per
day) Tav average daily temperature
(Celsius) TD Tmax Tmin P average monthly
precipitation
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Criwar uses 4 crop stages
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The crop coefficient for the initial growth stage
in CRIWAR
During the initial growth stage, the value of the
crop coefficient, Kc1, depends largely on the
level of ETref and on the frequency with which
the soil is wetted by rain or irrigation. The
figure shows the relationship between Kc, ETref,
and the average interval between irrigation turns
or significant rain.
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Other crop stages
Values for the mid-season and late leason crop
stages are derived from tables based om field
research. During the crop development stage, a
straight line interpolation is Assumed to find
the Kc2 value
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EFFECTIVE PRECIPITATION
CWR ETp - Pe
Effective precipitation is that part of total
precipitation on the cropped area, during a
specific time period, which is available to meet
the evapotranspiration in the cropped area.  

• Not all precipitation is effective
• part evaporates
• part become surface runoff
• part will recharge the groundwater
• only that part that will be stored in the
  rootzone and that becomes readily
• available soil moisture will be taken up by the
  roots to meet the crops
• evapotranspiration needs

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Method to calculate for effective rainfall in
Criwar
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USDA method to calculete Pe
The average monthly effective precipitation can
not exceed the total monthly rainfall, nor
the total evapotranspiration
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Effective precipitation formula as used in CRIWAR
Criwar uses the following semi empirical formula
to calculate Pe Pe ?(1.253P0.824 2.935) X
100.001ETp
Pe effective precipitation (mm/month) P
total precipitation per month (mm/month) ETp
total crop evapotranspiration per month
(mm/month) ? correction factor depending on
depth of application term
When the irrigation water application Da 75
mm/turn then ? 1.0 If da lt 75 mm/turn
then ? 0.133 0.201 ln Da or If Da gt 75
mm/turn then ? 0.946 7.3 x 10-4 x Da
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CWR module of criwar
CRIWAR can be used to

• Estimate CWR by variation in planting dates
• Estimate CWR for different cropping patterns
• Estimate CWR with different varieties

A Criwar cropping pattern can exist of 40
different crops in one calculation. The same
crop can be used more than once in one cropping
pattern (staggering crops)
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Data requirement

• General data
• Meteo data
• Crop data
• Cropping pattern

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Main Screen
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General data
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Meteo data
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Cropping pattern data
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Crop factor file
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Report screen
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Report Screen
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Range of input values to be used in
Meteorological file
Description parameter Range Dimension
Latitude 0 Latitude 66 Degrees N or S
Altitude -500 Altitude 4500 Metres
Height wind speed measured 0 Height 15 Metres
Temperature 0 Temperature 45 Degrees oC
Precipitation 0 Precipitation 1000 mm per period
Sunshine hour 0 Sunshine r 24 Hours per day
Relative humidity 0 Rhum 100 Percent
Wind speed 0 Wind 15 Metre per second
Maximum rel. humidity rhum Rhmax 100 Percent
Wind speed ratio day/night 0 Ratio 5 Dimensionless
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Range of Crop Input Parameters