Lecture (13)

Direct Measurement of ET : (Lysimeters)

Change in soil moisture storage

= (Inflow – Outflow)Δ S = (ID + P) – (DP + ET) in depth ; ID = IR × TRDrz (θf – θi) = (ID + P) – (DP + ET)

Where, θi and θf = are measured initial and final soil moisture ID, P, and DP can be measured, then determine ET

Selection of the Crop Coefficients: (Kc)Stage # 1 initial (germination)Stage # 2 crop developmentStage # 3 Flowering (mid-season)Stage # 4 Fruits (later season)

Factors affecting Kc are:

- Crop characteristics (height, shape of leaves, ground coverage,..,etc)

- Crop planting date- Length of the growing season- Rate of crop development

Selection Approach

• Initial Stage: for a given (predicted) irrigation frequency and predetermined Eto during this stage; obtain Kc from figure (4-8).

• Mid-season stage : for a given climate (humidity and wind speed); select Kc from table (4-10).

• For the end of the late season stage: select value of Kc from same table.

• For given periods and dates of crop growing stages, plot the Kc-time function.

Calculations of ET:

0April

2

4

6

8

10

12

May June July Aug. Sept. Oct.

ETo

(mm

/day

)

Month

7.50

9.00 8.80

10.00

7.506.75

0April

0.2

0.4

0.6

0.8

1.0

1.2

May June July Aug. Sept. Oct.

K1= 0.34

K3 = 1.13

K4 = 0.60

Cro

p co

effic

ient

, K

c

Month

Initial20

days

Crop development

days35 40

days

Mid season Late

days30

0April

2

4

6

8

10

12

May June July Aug. Sept. Oct.

ET (m

m/d

ay)

2.553.06

5.76

9.06

9.94

11.30

10.2

16 4 26 9 22 18 13

5.63

Month

17

Irrigation Frequency:

- Irrigation frequency (irrigation intervals)varies along the growing season

- For a fixed irrigation interval we need to vary ID (irrigation depth)A

ccum

ulat

ed E

T (m

m)

0

200

400

600

800

1000

May June July Aug. Sept. Oct.

R.A.W.

34 mm

Month

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

R.A.W.

Tota

l ET

Dep

th =

934

mm

II min

II max