1. CROP IRRIGATION WATER REQUIREMENTS
CRIWAR 3.0 for Windows
CROP IRRIGATION WATER REQUIREMENTS

2. Outline of the topic
Powerpoint presentation
Demonstration of the program
Exercises with Criwar

3. = evapotranspiration (ET )
evaporation
transpiration +
= evapotranspiration (ET )
an open water surface
the soil
the leaves and the stem of the plant
During the day water
Ecapes as vapour to
The atmosphere from:
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

5. 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

6. CWR = ETp - Pe What does CRIWAR calculate? where,
ETp = potential evapotanspiration
Pe = effective precipitation
Criwar calculates the crop irrigation water requirements (CWR)
of a cropping pattern in an irrigated area on a daily base:

7. Field water balance Precipitation ET Irrigation Drainage
Depth to ground-water
Drainage
Capillary rise
Rootzone
Seepage
Groundwater
Groundwater
After M.G.bos

8. The growth stage of the crop
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

9. Influence of the climate on crop water needs
Climatic factor Crop water need
High Low
Sunny cloudy
Hot cool
Low (dry) high (humid)
Windy little wind
Sunshine
Temperature
Humidity
Wind speed
Highest crop water needs in hot, dry, windy and sunny areas

10. Crop water needs as compared to standard grass
Not all crops have the same water needs. The crop water requirements vary
- 30%
- 10%
same as standard grass
+ 10%
+ 20%
Citrus
Olives
Grapes
Cucumber
Radishes
Squash
Carrots
Crucifers
Lettuce
Melons
Onions
Peanuts
Peppers
Spinach
Tea
Grass
Cacao
Coffee
Clean cultivated nuts & fruit trees
e.g. apples
Barley
Beans
Maize
Cotton
Tomato
Potatoes
Oats
Peas
Sorghum
Soybeans
Sugarbeet
Sunflower
Tobacco
Wheat
Paddy rice
Sugarcane
Banana
Nuts & fruit trees with cover crop

11. 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 <10%)
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
Small plants  evaporation more important than transpiration
During initial stage, crop water need about 50% of mid-season

12. 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.
Examples:
Blaney and Criddle 1950; based on air temperature + day length
Turc 1954; based on air temperature + radiation
Jensen and Haise; based on air temperature + radiation

13. 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’

14. < radiation term > < aerodynamic term >
Penman’s formula
D Rn - G g
E0 = Ea D + g g D + g
< radiation term > < aerodynamic term >
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)

15. Etcrop = Eo * Kc 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

16. Two other methods to calculate potential evapotranspiration
The FAO Modified Penman Method
The Penman-Monteith Approach

17. 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

18. Main differences are: ETp = Kc * ETref = Kc * ETg
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

19. 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.

21. The characteristics of hypothetical reference crop

22. 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

23. Eth = 0.85 ETg Relationship modified Penman vs Penman-Monteith
Crop coefficients introduced for Modified penman method could still be used with Penman-Monteith

24. The modified Hargreaves method
ET0,mh = x 0.408RA x(Tav +17.0) x (TD – P)0.76
RA = extraterrestrial radiation (MJ/m2 per day)
Tav = average daily temperature (Celsius)
TD = Tmax – Tmin
P = average monthly precipitation

25. Criwar uses 4 crop stages

26. 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.

27. 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

28. 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 crop’s
evapotranspiration needs

30. Method to calculate for effective rainfall in Criwar

31. USDA method to calculete Pe
The average monthly effective precipitation can not exceed the total monthly rainfall, nor the
total evapotranspiration

32. Effective precipitation formula as used in CRIWAR
Criwar uses the following semi empirical formula to calculate Pe
Pe = (1.253P0.824 –2.935) X ETp
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 < mm/turn then  = ln Da or
If Da >= mm/turn then  = x 10-4 x Da

33. 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)

34. Data requirement
General data
Meteo data
Crop data
Cropping pattern

35. Main Screen

36. General data

37. Meteo data

38. Cropping pattern data

39. Crop factor file

40. Report screen

41. Report Screen

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

43. Range of Crop Input Parameters