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
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,)
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
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
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
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