1. IRRIGATION PLANNING AND DESIGN: CROP WATER REQUIREMENTS

2. Crop Water Requirements
Plant roots extract water from the soil so as to enable the plant to live and grow.
Some of this water does not remain in the plant, but escapes to the atmosphere as water vapour through the plant’s leaves and stem.
This process is called transpiration.(The loss of water in the form of vapour from the plant canopy to the atmosphere )
Transpiration takes place mainly during the day.
Water from open water surfaces escapes to the atmosphere as water vapour, again mainly during the day.

3. Evaporation and Evapotranspiration
The same happens to water on the soil surface and to water on the plant leaves and stem.
This process is called evaporation.(the diffusive process where water transforms from the liquid phase into the vapour phase and escapes to the atmosphere, this process requires energy and the energy is mostly provided by the sun)
The water need of a crop thus can be termed evapotranspiration, i.e. it consists of evaporation and transpiration.
This water need of a crop is also called consumptive use.
For healthy plant or crop growth, it is essential to ensure that at no stage is the soil water content less than the consumptive need.

4. Crop water requirements (CWR) : Definition
The depth of water needed to meet the water loss through evapotranspiration of a disease free crop growing in large fields under non-restricting soil conditions including soil water and fertility, and achieving full production potential under the given growing environment (Doorenbos and Pruitt, 1977)
The crop water need is usually expressed in mm/day or mm/ month or mm/season.

5. Crop water requirements : cont.
Suppose that, during the hot months in the lowveld of Zimbabwe, the water need of sugarcane is 12mm/day.
This means that each day the sugarcane requires a water layer of 12mm over the whole area on which the crop is grown.
If the area is 1 hectare (ha) , then the volume of water required is 120m3 [i.e., (12/1000)m x 10000m2).

6. Crop water requirements : cont.
CWR depend, to a large extent, on:
The climate : temperature, wind velocity, humidity, incoming solar radiation and cloud cover.
Soil factors: Water table depth, soil moisture availability and soil surface properties, reflectivity, thermal conductivity, texture, chemical characteristics, Hydraulic conductivity and temperature.
Crop factors: plant morphology, crop type, variety, crop geometry, extent of plant cover, stomata density, rooting characteristics, length of crop growing season, growth stage
Water factors: Frequency of irrigation and quality of irrigation water,
Management factors: Tillage, fertilizers, plant protection, weed management and irrigation schedule

7. Effect of climate on Crop Water Need (CWN)
Climatic factors
These factors affect water needs in different but combined ways.
A given crop will require more water per day if grown in a sunny and hot climate than when it is grown in a cloudy cool climate.
When it is dry, the crop water needs are higher than when it is humid.
When conditions are windy, the crop will use more water than when it is calm.

8. Effect of climatic factors on CWR
Crop Water Needs High
Crop Water Needs low
Wind speed
Windy
Calm
Humidity
Low (dry)
High (humid)
Temperature
Hot
Cool
Sunshine
Sunny (no clouds)
Cloudy (no sun)

9. Effect of crop type on CWN
The influence of crop type on CWR is important in two ways;
Daily water needs; Crop type has an influence on the daily water needs of a fully grown crop. i.e. the peak daily water needs.
For example a fully developed sugarcane crop need more water per day than a fully grown crop of carrots.
Seasonal Water needs; Crop type has an influence on the duration of the total growing season of the crop and hence the total amount of water required by the crop per season.

10. Effect of crop type on crop water needs cont.
This can be exemplified by crops that can be termed short duration such as peas which have a 90 to 100 day growing season and those that are long duration such as sugarcane with a growing season extending to 300 days.
There are also perennial crops such as citrus.
Naturally, crops with long season duration will have higher seasonal water needs than those that are short season.

11. Effect of crop type on daily water needs
Reference is made to a fully grown crop in determining CWN for Irrigation Design.
Such crops are at their peak period of the crop water needs.
Some crops require more water than the standard grass while others require less and other , more or less the same amount.

12. Effect of crop type on the seasonal CWN
The time of the year in which the crops are grown is also important.
The variety of a given crop that is grown in cooler months will require much less water than the same variety grown in hotter months.

13. Effect of growth stage of the crop on CWN
When a crop is small, evaporation from the soil surface is much more significant than transpiration.
As the crop develops towards full growth, transpiration becomes more prominent than evaporation.
Thus a fully grown maize crop requires more water than a maize crop which has just germinated.
This is due to a large foliage from which transpiration can take place while evaporation is subdued because of shading from the leaves.

14. Effect of growth stage of the crop on CWN
The growth stage of most crops can be conveniently divided into four stages;
Initial stage, development stage (vegetative stage), mid season stage (or flowering and fruit filling) and late season stage.
During the initial stages crop water needs are estimated at approximately 50% of the crop water needs during the mid season stage when the crop is fully developed.
During the vegetative stage crop water needs increase from 50% to the maximum level.

15. Effect of growth stage of the crop on CWN
Maximum water need is reached at end of the end of the vegetative stage which is beginning of the mid season stage.
During mid season, water needs are at their maximum.
The water needs during the late season differ depending on the type of crop;
With fresh harvested crops water needs remain the same during the late season as it was during the mid season stage.
For the dry harvested crops , generally the water needs decrease substantially , sometimes to as little as 25% of that during the mid season stage.

16. Soil Water
In addition to what was mentioned earlier, uptake of water by crops in the root zone varies but generally for most field crops it is as follows;
40% of water is extracted in the first quarter of the root zone,
30% from the second quarter,
20% from the third quarter and
10% from the last quarter.

17. Irrigation Method
Generally have no effect on CWN if the irrigation system is properly designed, installed and operated.
For drip irrigation however, this method has no effect on Etc at full or near ground cover.
But before attaining 70% ground cover, Etc is reduced because of suppressed evaporation, which takes place only in few wetted spots.

18. Management practices
The management factors adopted for improving productivity and yield are usually associated with increased evapo-transpiration.
Plant population affects Etc through evaporation and transpiration as a function of ground cover.
Mulching reduces evaporation and hence Etc.
Wind breaks depending on their size can reduce Etc by 5-30% in windy , warm and dry conditions , because of their effect on wind velocity.
Weeding reduces competition for moisture hence may reduce evapo-transpiration from the field and hence CWR

19. The reference crop concept
If the same crop is grown in different climatic zones it will have different water requirements.
Because of this, in determining CWR we have to take what is called a reference crop.
We take this reference crop and determine how much water it requires in different climatic regions.
The general crop used as a reference crop is grass i.e., alfafa or lucerne (Doorenbos and Pruitt, 1977).

20. The reference crop concept cont.
With grass as a reference crop, we then would adjust the water needs of all other crops against it for the given climatic conditions.
The daily water needs of a reference crop are usually termed reference crop evapotranspiration (Eto) or potential evapotranspiration.

21. Reference crop evapotranspiration : definition
The rate of evapotranspiration from an extensive surface of 8 to 10cm tall grass cover of uniform height , actively growing, completely shading the ground and not short of water . Doorenbos and Pruitt (1977)

22. Evapotranspiration and Consumptive use
Consumptive use (Cw) = Etc + Water used by crop plants for metabolic activities(Ww).
The term evapotranspiration is often used synonymously with consumptive use since the amount of water metabolically used by plants is hardly one percent of the evapotranspiration value.

23. Methods of Determining ETo
Eto can be calculated by a variety of methods that include both empirical and scientifically derived approaches.
The choice of method depends on the data that is available and the desired accuracy.
The empirical methods for estimating Eto that are recommended by FAO for use under different climatic conditions are:
Blaney- Criddle method,
Radiation method,
Modified penman method and the
Pan evaporation method

24. Steps involved in Estimating of CWR using empirical methods.
1. Estimation of Eto
2. Determination of crop coefficients Kc
3. Making appropriate adjustments to location specific crop environment.

25. Input Data Required for estimating Eto Source: Panda, 2003
Method
Meteorological Data Required T RH Wv Ss Ra E
Environment
Blaney-Criddle * -
Radiation
(*)
Pan Evaporation
Modified Penman
*= Measured Data
0=Estimated Data
Ra= Solar Radiation
(*) = Used if available, but not essential
E = Open pan evaporation
Wv= Wind velocity
Ss = Sun shine hours

26. The Evaporation Pan Method
An evaporation pan gives a measurement of the combined effect of radiation, temperature, humidity and wind speed on evaporation from open water source.
Pan and its environment influence measured evaporation , especially , when it is placed in cropped field instead of open fallow field.
To relate pan evaporation to ETo , empirically derived pan coefficients ( pan factors) are suggested to account for climate , type of pan and pan environment :
Eto = kp x Epan
Where; kp is the pan factor and Epan is the pan evaporation measurement (usually in mm/day).

27. Pan Coefficients (kp) under different conditions (source Panda, 2003)
Wind velocity
Windward
RH Mean
(km /day)
side distance of pan from crop (m)
Low (<40%)
Medium (40-70%)
High (>70%)
Light (<175) 1
0.55
0.65
0.75 10
0.85
100
0.70
0.80
1000
0.50
0.60
Moderate ( )
0.45
Strong ( )
0.40
Very strong (>700)

28. Wind Velocity estimates (Panda, 2003)
Indication
m/s
Km /day
Classification
Leaves start rustling
<2
<175
Light
Twigs move, paper blows away
2-5
Moderate
Small branches move , dust rises
5-8
Strong
Small trees start moving
>8
>700
V strong

29. The Pan Method Cont.
The method requires measured pan evaporation , estimated mean RH , estimated wind velocity (U in km/day at 2 m height) and a pan in cropped or fallow field.
It is simple inexpensive and reliable method for estimating ETo.
As can be seen from table on pan coefficients above;
The kp is high if:
The pan is placed in a fallow area,
The humidity is high and,
The wind speed is low
Kp is low if:
The pan is placed in a cropped area,
The humidity is low and,
The wind speed is high

30. The Pan Method Cont.
The most commonly used evaporation pan is called the Class A pan.
The kp for the Class A pan varies between 0.35 and 0.85 with an average kp of 0.70.
Details of the Class A pan and its use under Zimbabwean conditions are found in Meterlakamp (1978), Prestt (1986) and Makhado (1988).
In summary the use and application of the Class A pan in determining Eto is as follows;

31. The Pan Method Cont.
The pan is installed in the field or the area of interest,
The pan is filled with a known quantity of water,
The water in the pan is allowed to evaporate over a period of time , normally 24 hours, as well as added through rainfall,
After the set period , say 24 hours, the remaining water (i.e water depth) in the pan is measured,
The amount evaporation per unit time is calculated , and this is the pan evaporation (Epan) in 24 hours, and,
The Epan is multiplied by kp to obtain Eto.

32. Worked example Determine Eto (mm/day) for the following conditions:
Class A pan with a kp value of 0.70
Water depth in pan on day1 = 200mm
Water depth in pan on day2 = 192mm (after 24hrs)
Rain fall During 24hrs) = 0mm
What would be the Eto if there had been 3mm of rainfall?

33. Solution: Epan = 200-192 = 8mm/day Eto = 8x 0.70 = 5.6mm/day
Epan = = 5 mm/day
Eto = 5 x 0.70 = 3.5mm/day

34. The Modified Penman Method
It is a popular method, it requires measured data on temperature, humidity, wind speed, sunshine, radiation (if available) and an estimate of the environmental conditions.
it integrates all this data in estimating Eto and because it is so comprehensive in its data requirements,
It is the most accurate of methods used to determine Eto. It is basically applicable under any weather conditions.
The details of this method and its use in determining crop water requirements are obtained in the FAO publication (Doorenbos and Pruitt, 1977) or Panda,2003 .

35. Computer programs for Estimating ETc
To date there is large number of these models available. An example is CROPWAT.
This is a comprehensive program developed by FAO (Smith, 1992) for determining crop water requirements, irrigation water requirements and scheduling of irrigation.
It is also used in irrigation project planning. CROPWAT is based on field water balance.
It offers a wide variety of formulae, e.g. Modified Penman, Blanney-Criddle and Pan Method to determine Eto. CROPWAT comes together with CLIMWAT (Smith, 1993) , which gives climate data for hundreds of locations around the world including Zimbabwe.

36. Other methods of measuring ETo
Lysimeter
Evaporimeters
Autometers
etc

37. The Crop factor (kc) Sometimes called crop coefficient,
Gives the effect of crop characteristics on crop water needs;
It gives the relationship between reference crop evapotranspiration (Eto) and the actual crop evapotranspiration (Etc. )
Takes into account the effect of crop characteristics, time of planting, stage of crop development and general climatic conditions on crop water needs.
It can be expressed as follows;
Kc = Etc/Eto (dimensionless, i.e.no units)
Rearranging this equation;
Etc = kc x Eto (mm/day)

38. Crop factor kc cont.
In general, kc values are greater than unit (one) for leafy crops, such as tomatoes and maize, which transpire more than grass (reference crop).
For the reference crop, the value of kc is one. Kc values are less than 1 for crops/ plants with waxy leaves, like citrus and sugar beets, which transpire less than grass.
Kc values closely match the four crop growth stages discussed earlier.
It is necessary to determine kc values for the different crops, it is also necessary to know the total length of the growing season and the length of the various growth stages.
Figure below illustrates typical kc values variations for crop through a growing season.

39. Source USDA, 1991

40. Kc Values cont.
It is generally recommended that kc values are determined for local conditions they want to be used under, but they can also be obtained from tables.
The table below gives some kc values for common field crops for the various growth stages.

41. Some kc values for common field crops for the various growth stages.
Initial Stage
Vegetative stage
Mid-season Stage
Late Season
Beans (green)
Cabbage/ carrot
Cotton
Maize (Grain)
Onion (green)
Peanuts
Peas (fresh)
Potatoes
Soya beans
Spinach / lettuce
Sunflower
Tobacco
Wheat
0.35
0.45
0.40
0.50
0.70
0.75
0.80
0.60
1.10
1.05
1.15
1.00
0.90
0.85
0.55
NB: Because kc depends on climate(especially humidity(RH)and wind speed), the given values above should be adjusted as follows; reduce by 0.05 if RH > 80% and wind speed is < 2m/sec, increase by 0.05 if RH <50% and wind speed is > 5 m/sec.

42. Determination of Crop Water Needs (Etc)
With information on Eto and Kc ( as discussed in sections above), Etc is calculated from the following formula also mentioned above;
Etc = kc x Eto (mm/day)
Note that Etc can be calculated on a daily (mm/day), weekly (mm/week), Decade (mm/10days) or monthly basis (mm/month) depending on the use to which the information is going to be put.

43. Worked Example on the calculation of Etc.
Determine the CWR (mm/dy) and mm/month) and seasonal water needs (mm and m3) of a crop of tomatoes, transplanted on the 1st of May , for the following given conditions.
Note: kc has been adjusted for the growth stages and the months. Eto is mm/day
Mth
Jan
Feb
Mar
April
May
Jun
July
Aug
Sept
Oct
Nov
Dec
Eto
5.0
7.5
8.4
10.2
9.8 Kc
0.45
0.70
0.95
1.15
0.90

44. Solution:
Mth
Jan
Feb
Mar
April
May
Jun
July
Aug
Sept
Oct
Nov
Dec
Eto
5.0
7.5
8.4
10.2
9.8 Kc
0.45
0.70
0.95
1.15
0.90
Etc/dy
2.25
5.25
7.98
11.7
8.82
Etc/Mon
69.8
157.5
247.4
362.7
264.6
Seasonal water need = = 1102mm
Seasonal water needs per ha = (1102/1000)m x10000m = 11020m3
It is important to note that the accuracy and reliability of one’s estimates of Etc depends on the accuracy and reliability of the input data, the “garbage in garbage out “principle also applies here.

45. Table below gives some approximate values of seasonal crop water needs
Table below gives some approximate values of seasonal crop water needs. These values vary widely with place and crop variety. Source: Doorenbos and Pruitt (1977)
Crop
Crop Water Needs (mm/total growing season)
Alfalfa (standard grass)
Beans
Cotton
Maize
Onion
Peanut
Peas
Potatoes
Rice
Soya beans
Sugarcane
Tomatoes
Wheat ?

46. Effective Precipitation
Effective precipitation (rain fall) is that part of rainfall that is effectively used by the crop to meet its crop water needs.
It is governed by a number of factors that include;
The amount, intensity, timing, duration and frequency of rainfall,
The initial soil moisture conditions,
The type of crop and stage of growth,
Drainage conditions, and
Conditions of storage in the field.

47. Methods of Determining Effective Rainfall
Dependable Rain Formulae
Applicable in areas with a slope of 4-5%.
Empirically derived from an analysis carried out from different arid and sub- humid climates:
Pe = 0.8 x P -25 for P > 75mm/ month
Pe = 0.6 x P -10 for P < 75mm/ month
Where Pe = effective precipitation (mm/month) and P = precipitation (mm/month).
Note that Pe is always equal to or larger than zero.
Task: Determine the effective precipitation for the following monthly rainfall, P, in mm; 5,20,40,75,95,160,230,380.

48. Methods of Determining Effective Rainfall cont.
USDA. Soil Conservation Service Method
Pe = P x ( P)/ for P< 250mm
Pe = P for P> 250mm
Where Pe and P are as previously defined.
Task: for the data under task on dependable formulae, calculate the effective rainfall using the USDA-SCS method.

49. Determining Effective Rainfall Cont.
Fixed Percentage of Rainfall Method
Considers losses due to run off and deep percolation:
Pe =P
Where Pe and P are as previously defined and  is a fixed percentage accounting for deep percolation and runoff losses.
Generally these losses are taken to be about % and so  is in the range
Task: for the data under task on dependable formulae, calculate the effective rainfall using the fixed percentage method and for an  = 0.8, and compare the estimates of Pe from the three methods discussed.

50. Determination of Net Irrigation Water Needs (IWNnet)
Also called net irrigation requirements (IWR)net;
IWNnet =Etc- Pe – Gwc
Gwc = ground water contribution (normally assumed =0) → IWNnet =Etc- Pe
Three likely scenarios;
Sufficient rainfall : all the water needed for crop growth is supplied by rainfall, irrigation is not required →IWNnet = zero.
No Rainfall. : All water needs of the crop have to be met by irrigation. IWNnet = Etc
Some Rainfall : Irrigation needed to supplement rainfall; Thus, IWNnet = Etc -Pe

51. Worked example
Determine IWNnet on a monthly and daily basis, for the following given data and conditions. Use the dependable Rain Formulae to estimate effective precipitation.
Month
Jan
Feb
March
April
May
Etc (mm/mon) 60
120
154
208
187
P (mm/mon) 80 58 44 12

52. Solution: Month Jan Feb March April May Etc (mm/mon) 60 120 154 208
187
P (mm/mon) 80 58 44 12
Pe(mm/mon) 39
24.8
16.4
IWRnet (mm/mon) 21
95.2
137.6
IWRnet (mm/day)
0.68
3.40
4.44
6.93
6.03

53. Determination of Gross Irrigation Water Needs (IWNgross)
Gross irrigation water requirements take into account losses that take place in the field, in conveyance, run off and deep percolation losses , as well as other needs such as crop germination , dust suppression , frost protection, leaching requirement and crop cleaning.
Worked example.
IWNnet are estimated at 80mm and system efficiency is 75% (i.e. losses are 25%), what is the gross irrigation water requirement?

54. Solution: IWRgross = IWRnet/Efficiency = 80mm/0.75 = 106.7mm
In the case when there is need to apply extra water for other needs, then this is added to the net irrigation water needs.
Typical examples in Zimbabwe include water needed to help with seedling germination and that required for frost protection.
Technically speaking this water is not used by the crop for evapotranspiration.

55. Determination of crop water needs and IWN: Summary

56. Crop Critical Stress Periods
For many crops there are critical periods during the growing season when a water deficit or stress is detrimental to crop yield.
The critical period for a number of commonly grown crops under irrigation is given in table below.

57. Critical periods for water stress, symptoms, and some other considerations for several important crops Source USDA,1991

59. Yield-Evapotranspiration Relationships
The amount of water evapotranspired to produce the highest crop yield at a given location will depend on the climate, soil, and characteristics of the specific crop.
A supply of irrigation water is essential for sustained high levels of crop productivity.
If a water deficit develops in the soil beyond a threshold level for the specific stage of growth, the resulting water stress will reduce ET, and crop yield will be reduced proportionately.

60. Yield-Evapotranspiration Relationships
Studies to develop yield-ET functions have been conducted under non limiting salinity conditions.
This is not usually the case in reality.
Under saline conditions, a reduction in ET usually result in a greater reduction in yield as compared to non saline conditions.

61. Concepts of Production Functions
The production function provides a useful means of analyzing water-productivity relations.
Water response functions for a variety of crops have been developed.

62. Yield-Evapotranspiration Production Functions
For many crops and growing conditions, the relationship between ET and yield is linear up to ET values that result in maximum productivity;
This is especially true for crops where the aboveground biomass represents yield.
This type of response is illustrated in figure below.
Approximately 850mm were required to achieve a maximum yield of 24 tons per hectare).

63. Source: USDA, 1991
1 ton / acre = 2.24 tons/ ha

64. Yield-Evapotranspiration Production Functions
Figure below shows a relationship between cotton lint yield and ET that is nonlinear.
The relatively complex nature of vegetative-reproductive growth partitioning of cotton accounts for the slight curvature for this function;
Other crops, such as maize and sorghum, have been shown to have linear functions between seed or reproductive growth and ET.

65. Source: USDA, 1991
1 pound/acre = kg/ha

66. Yield-Applied Water Relationships
Figures above show that an applied water (AW) function progressively departs from the ET function as ET and applied water increase.
This results primarily from increased drainage below the root zone and larger amounts of AW remaining in the soil profile at the end of the growing season which is directly related to the level of management.
Adding additional water beyond that associated with achieving maximum yield may
frequently be associated with yield reduction.

67. Yield-Applied Water Relationships
Mechanisms that may be responsible for the yield loss include;
Leaching of nutrients,
Reduced aeration,
Excessive vegetative growth at the expense of reproducing seed yield.

68. Evapotranspiration –Yield relationship
Where
ETc = Actual crop Evapotranspiration
ETo = Maximum evapotranspiration
Ya = Actual yield
Ym = potential maximum yield ,
Ky = is a constant, the yield response factor
Relative yield is the ratio/proportion of actual yield to maximum yield,

69. Design Duty
Design duty is the crop water needs at peak conditions expressed as an equivalent continuous flow (for example measured in l/s/ha instead of mm/day).
This information is used to size irrigation water supply pipelines or canals.
Generally, peak water demand in the range of 2 to 10mm/day is equivalent to a continuous flow of 0.23 to 1.16 l/s/ha.
Taking an average design duty of 1l/s/ha , it means for each hectare we irrigate the supply canal /pipeline must be able to carry a flow of 1l/s i.e. the system capacity would be 1l/s assuming that we are irrigate 24hr/day and our field is 1 ha in size.

70. Example on the determination of design duty from IWR
Assume the IWRg is 10mm/day. Then;
Design duty = {(10/1000)m/day x 10000m2/ha} 1000l/m3 / (24 x60x60)s/day
= 1.16 l/s/ha

71. Task
Determine IWNnet on a monthly and daily basis, for the following given data and conditions. Use the dependable Rain Formulae to estimate effective precipitation. Then determine the IWRg and the design duty for the data, assuming a system efficiency of 80%. Assuming you had a flow of 15l/s, how much area would you be able to irrigate per day at this peak design duty?
Month
Jan
Feb
March
April
May
Etc (mm/mon) 60
120
154
208
187
P (mm/mon) 80 58 44 12

72. Use of CWR and IWR Evaluating the need of storage reservoirs
Determining the capacity of irrigation systems.
Formulating the policy for optimal allocation of water resources and decision-making in day-to-day operation and management of irrigation systems.
Important for planning, design and operation of irrigation and water resources systems.
for assessing the adequacy of water resources,

73. Problems of Incorrect determination of IWR
Serious failures in the system performance and to the waste of valuable water resources.
Inadequate control of the soil moisture in the root zone, and may cause water logging, salinity or leaching of nutrients from the soil.
Inappropriate capacities of the irrigation network or of storage reservoirs,
Low water use efficiency and to a reduction in the irrigated area.
Over-estimating IR at peak demand may also result in increased development cost.