irrigation engineering presentation

1. CROP WATER REQUIREMENT
(CWR)

2. IWR
Crop water needs depends on:
• Climate
• Crop type
• Growth stages

3. Evapotranspiration
ET – Evapotranspiration from surface
T- Transpiration from leaves
E - Evaporation

4. Evapotranspiration
 Factors affecting evaporation
 Soil Surface Moisture
 Climate (Temp, Wind, Rel. Humidity)
 Definition
 also known as consumptive water use
 expressed as mm/day, mm/month, mm/season

5. Evapotranspiration
Factors affecting transpiration
 Plant variety and type
 Stage of growth
 Soil moisture
Climate
Crop Health and fertility

6. Climatic Factors Affecting ETo
 Cool
 Humid
 Cloudy
 Little or no wind
 Hot
 Dry
 Sunny
 Windy
ETo

7. Evapotranspiration
Methods for predicting ETo
Mass transfer- moisture moves away vs mixture of
air-vapour gradient
Energy balance-heating water-no temp rise-
evaporation
• Penman
• Jensen-Haise
• Radiation
Empirical
• Blaney-Criddle
• Thornwhaite

8. Methods of Estimating ETo

9. Pan Evaporation Method

10. Pan Siting

11. Pan Evaporation Method
 Advantages
• real-time evaporation rates
• on-site data
• relatively easy
 Disadvantages
• data influenced by pan placement and type
• water in pan stores and releases water differently
than crop
• Edge effects on heating of pan

12. Pan Evaporation Method
ETo = Kpan x Epan
Epan : Evaporation from pan (standard pan is Class A Pan)
Kpan: Pan factor determined by avg daily wind speed and mean relative humidity
Varies between 0.35 to 0.85. Average value is 0.7
Given Pan Evaporation Data for seven days of a month: 8.2, 7.5, 7.6,
6.8, 7.6, 8.9 and 8.5 mm/day. Mean wind speed= 1.9 m/s and RH of 73%.
Description Calculation/Condition Result (mm/day)
Wind speed is light U<2 m/s
RH is high RHmean>70%
Average Epan (8.2 + 7.5 + 7.6 + 6.8 + 7.6 +
8.9 + 8.5)/7
7.9
Average ETo 0.7 x 7.9 5.53
Example 1

13. Blaney-Criddle Method
 One of the oldest methods of estimating ET
a bit simplistic
mean monthly temperature
monthly percent of annual daylight hours
crop coefficient
 Advantages
Simple
Does not require too many data
Most of the data is readily available

14. Blaney-Criddle Method
f = p (0.46 Tmean + 8) ETo = cf
Tmean : Mean daily temperature (0C)
p : mean daily percentage of annual day time hours
C : Adjustment factor which depends on relative humidity, wind
speed and sunshine hours
Given: Monthly average max and min temperatures of Werer (9°16'N) are
35.5 and 19.6 C respectively; medium RH, sunshine hr and wind speed.
Example 2
Description Calculation/Condition Result
Tmean (35.5+19.6)/2 27.6
p for 9°16'N 0.28
Factor (f) 0.28 x (0.46 x 27.6+8) 5.8 mm/day
Adjusted ETo Cf from fig 1, II, 2 7 mm/day

15. Modified Penman
• ETo = Reference evapotranspiration (mm/day)
• Rn = Net radiation at the crop surface (MJ/m2 per day)
• G = Soil heat flux density (MJ/m2 per day)
• T = Mean daily air temperature at 2 m height (°C)
• u2 = Wind speed at 2 m height (m/sec)
• es = Saturation vapour pressure (kPa)
• ea = Actual vapour pressure (kPa)
• es - ea = Saturation vapour pressure deficit (kPa)
• Δ= slope vapour pressure curve [kPa °C-1]
• Δ slope vapour pressure curve [kPa °C-1],
• γ= psychrometric constant [kPa °C-1].

16. Indicative Values of ETo
Climactic
Zone
Low (<15ºc) Medium (15-
25 ºc)
High (>25ºc)
Desert/Arid 4-6 7-8 9-10
Semi-Arid 4-5 6-7 8-9
Sub-humid 3-4 5-6 7-8
Humid 1-2 3-4 5-6

17. Crop Water Requirement
Crop water needs depends on
•Climate
•Crop growth stage
•Crop type
–LGP: Longer period
–peak daily water need

18. ETc-Standard & non-standard conditions

19. Computation of IWR

20. Factors-IWR

21. General Procedure for Calculating ETc
under Standard conditions
Calculate ETo
Select lengths of growth stage
Select single coefficient - Kc
Select values- Kcini, Kcmid, Kc end
Adjust Kcini for wetting frequency
Adjust Kc mid & Kc end to local conditions
Construct Kc curve
Read Kc values from the curve ETc = Kc x ETo

22. What is Crop Factor, Kc?
• The effects of weather conditions are captured in the
ETo estimate.
• The effects of crop transpiration and soil evaporation
are combined into a single Kc coefficient.
• Kc combines differences in soil evaporation and crop
transpiration rate b/n the crop and the grass
reference surface
• Kc varies with the specific crop characteristics
• Kc = f(crop type, climate, soil evaporation,
growth stage,)

23. Kc Curve

24. ETc Calculation Procedure
1. Select the type of crops to be grown
2. Establish planting dates
3. Determine lengths of crop growth stages
4. Determine Kc values for each growth stage
5. Calculate ETc from
ETc = Kc x ETo
Where,
ETc = Crop water requirement
Kc =Crop factor
Kc =f(crop type and growing stage)

25. Steps for Determining Kc Values
• Determine growth stages of a crop (initial, mid season and late season)
• Select Kc values for growth stages
• Draw a curve through the points
 Plot kc values at midpoints of growing periods and connect them
 Construct a curve by connecting straight line segments through each
of the four growth stages
 Read kc values for any period during the growing stage
• Read Kc values for each selected period at mid point of the period
• Kc can be determined mathematically,
t=day number within the growing season Kcpr = Kc of the previous stage
Kct= Crop coefficient on day t Kcnext= Kc of the beginning of the next stage
Lstage= Length of the stage under consideration ΣLpr= sum of the lengths of all previous stages
 
pr
next
stage
pr
pr K
Kc
L
L
t
K
Kct 







 




26. ETc Calculation Procedure

27. Kc Values
Crops
Initial Crop
development
Mid-
season
Late &
harvest
Depth of Root
system (cm)
Depletion
level (%)
Seasonal
Cabbage 0.45 (20) 0.75 (25) 1.05 (60) 0.90 (15) 40-50 0.45
Carrot 0.45 (20) 0.75 (30) 1.05 (30) 0.90 (20) 50-100 0.35
Cotton 0.45 (30) 0.75 (50) 1.15 (55) 0.75(45) 100-170 0.65
Maize 0.40 (20) 0.75 (35) 1.15 (40) 0.75 (30) 100-200 0.60
Onion 0.50 (20) 0.75 (45) 1.05 920) 0.85 (10) 30-50 0.25
Pepper 0.35 (30) 0.75 (35) 1.05 (40) 0.90 (20) 50-100 0.25
Potato 0.45 (25) 0.75 (30) 1.15 (30) 0.75 (20) 40-60 0.25
Rice 1.05 (30) 1.2 (30) 1.2 (80) 0.9 (40) 1
Sorghum 0.35 (20) 0.75 (30) 1.11 (40) 0.65 (30) 100-200 0.55
Sesame 1.1(20) 1,1 (30) 1.1 (40) 0.35 (20) 2 0.6
Tomato 0.45 (25) 0.75 (40) 1.15 (40) 0.80 (25) 70-150 0.40
Wheat 0.35 (15) 0.75 (30) 1.15 (65) 0.70 (40) 100-150 0.55
Alfalfa 0.35 0.85 100-200
Banana 0.50 1.1 50-90

28. Calculation Example
Sewir Irrigation Scheme- North Shewa, Amhara
Crop Onion Pepper
Planting date 16 September 16 July
Harvesting date 30 December 25 December
Soil type clay Clay
Note
•Months and growing stages do not correspond. Also do ETo and Kc
•Yet ETc has to be determined on monthly basis

29. Cropping Pattern
Crop J F M A M J J A S O N D
Pepper 16
Onion 16

30. Climate Data
ETo for Shewa Robit
Month ETo (mm/day) Rainfall (mm)
J 3.3 15.3
F 4.1 75
M 5.2 49.4
A 5.5 126.4
M 6.2 29.3
J 6.3 16.2
J 5.6 249.2
A 5 0
S 4.9 110
O 4.7 21.3
N 4.2 15.6
D 4 0

31. Step 1 - Growth Stages-
Growth Stages Onion Pepper
Duration
(days(
Months Duration
(days(
Months
Initial 20 15 Sept-5 Oct 40 16-30 Dec
Development 50 6 Oct-25 Nov 45 26 Aug-10 Oct
Mid season 20 26 Nov-15
Dec
50 11 Oct-30 Nov
Late season 15 16-30 Dec 25 1-25 Dec
Total 105 160
Manual- 3 IWR Annex III

32. Step 2-Kc values
Growth Stage Onion Pepper
Initial 0.5 0.35
Mid Season 1.05 1.05
Late Season 0.85 0.90
Determine Kc Values

33. Step 3- Kc Curve for Onion
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Initial Development M-season Late



Sept Oct Nov Dec

34. Step 4-Monthly Kcs
Sept Kc = 0.5, 15 days, Oct Kc =0.5, 5 days
Oct Kc=(0.5+0.83)/2= 0.665, 25 days
• Kc (Oct) = 0.5 x 5/30 + 25/30 x 0.6665 =0.64
Nov Kc = (0.83+1.05)/2 = 0.94, 25 days
Kc (Nov) = 0.94 x 25/30+ 1.05*5/30 =0.96
Dec Kc=(0.85+1.05)/1=0.95, 15 days
Kc (Dec) = 15/30 x 1.05 + 15/30 x 0.96 =1 =(1.05+.96)/2
Determine Kc for onion for Each Month
Assume 30
days for
each month

35. Kc by Calculation
 
pr
next
stage
pr
pr K
Kc
L
L
t
K
Kct 







 



Kpr=kci=0.5
Kcnext=Kcm=1.05
t=33 days
Lpr=20 days
Lstage= 50
Kct=0.5+ (33-20)/50 x (1.05-.5)
= 0.5+0.26 x 0.55
=0.64

36. Summary of ETc Values
Month Sept Oct Nov Dec
ETo 4.9 4.7 4.2 4.0
Growth Stage Initial Development Mid Season L.
Season
LGP (days) 20 50 20 15
Kc (GS) 0.5 0.75 1.05 0.85
Kc (month) 0.5 0.64 0.94 0.95
ETc (mm/day) 2.45 3.2 3.9 3.8
ETc (mm/mon) 37 96 117 114
Seasonal ETC = 364 mm

37. Multiple Crops
Crops Information
Crop Planting
Date
Harvesting
Date
LGP (days) Kc Values for 3
stages
Sesame June 15 Dec15 20 30 40 25 0.35 1.1 0.35
Groundnut May 15 Oct 25 25 35 50 20 0.45 1.0 0.75
Onion Oct 15 Jan 25 20 45 20 15 0.50 1.05 0.85
Estimate Seasonal Crop Water Requirements

38. Kc Curve for Multiple Crops
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
LGP(days)
Kc
Values
Sesame
Groundnut
Onion

39. CWR for Multiple Cropping
No Item A(%) M1 M2 M3 M4 M5 Total
A Crop 1 P1 kc11 kc12 kc13 kc14
B Crop 2 P2 kc21 kc22 kc23 kc24
C Crop 3 p3 kc31 kc32 kc33
D (Wt
Kc)
(pi/100 x kci) kc1 kc2 kc3 k4 k5
1. ETo
2. ETc = (d ) x (1)
3. Pre-Irrigation
4. CWR = (2) + (3)
5. Rainfall, P
6. Pe by formula
7 NIR = (4)-(6)
8 GIR = (7)/Eff

40. CWR_for_ Multiple_Crops.xls
Table 1 Bategarmama irrigation site, Elevation 1625m asl , Crop water Requirement calculation sheet

41. Example 2
crop SEMPT. October November DECEMBER Total
growing
Onion 30 30 30 10 105
Kc
Eto(mm/mon)
Etc(mm/day)
Etc(mm/month)
nth
Eto
(m
m/d
ay)
ET
o
(m
m/
mo
nth)
ETc
(m
m/
mo
nth)
ETc
(m
m/
mo
nth
2 To
mat
o
15 30 30 30 25 130
Kc/
mo
nth
Eto
(m
m/d
ay)
ET
o
(m
m/
mo
nth)
ETc
(m
m/
mo
nth)
ETc
(m
m/
Table 1 Bategarmama irrigation site, Elevation 1625m asl , Crop water Requirement calculation sheet

42. End of CWR

43. Table 1 Bategarmama irrigation site, Elevation 1625m asl , Crop water Requirement calculation sheet