2.3 Evapotranspiration
One way to improve water use efficiency and optimize plant
production is to provide crops only with the water they need based on the
climate-plant-soil relationship. Therefore the concept of evapotranspiration
(ET) is the base for the right amount of irrigation water that should be
applied.
Water supplied to crops is lost from the soil through direct
evaporation and transpiration into the atmosphere (Fonteh and Assoumou, 1996).
It is difficult for us to isolate the two mechanisms on a field with growing
crops. As such the two losses are usually combined to give evapotranspiration
(ET). Knowledge of ET enables us to predict the soil moisture deficit (SMD) for
irrigation. We express the ET as a rate of loss of water e.g. 5 mm/day. ET
could also be referred to as the consumptive use i.e. the total amount of water
a crop takes from the soil as it grows. Designs of irrigation systems are
usually based on the period of the growing season with the maximum consumptive
use. Systems are designed with capacities to supply water when demand is the
highest. We can differentiate between two types of ET: reference crop ET and
actual ET. Reference crop evapotranspiration (ETo) is the water use
of a vigorously growing reference crop under full cover, when water is not
limiting (Fonteh and Assoumou, 1996). The crop or actual evapotranspiration
(ETa) is the actual amount of water lost from the soil during field
growing conditions. The reference crop ET is related to the actual by the
equation:
(2.7)
KC is a coefficient accounting for crop maturity and water
stress under which the plant is growing. KC values vary with the crop, its
stage of growth, growing season and prevailing weather conditions. From the
equation (2.7) the actual ET is obtained from the reference. This is because
depends only on climatic factors and hence, it is easier to obtain
and then apply the crop coefficient (KC). ETa can
be obtained directly for example, by determining the moisture content (MC) of
the soil between a given time interval. However, this approach is slow and
tedious and is used only as a check on indirect methods.
Table 2.1: Length of crop growth developmental stages
for various planting periods and Climatic regions
|
Stages of Development
|
Plant date
|
Region
|
|
|
|
|
Crop characteristic
|
Initial
|
Crop Development
|
Mid- season
|
Late
|
Total
|
|
|
Banana 1st year
|
|
Stage length, days
|
120
|
90
|
120
|
60
|
390
|
March
|
Mediterranean
|
Depletion Coefficient, p
|
0.35
|
>>
|
0.35
|
0.35
|
-
|
|
|
Root Depth, m
|
0.30
|
>>
|
>>
|
0.80
|
-
|
|
|
Crop Coefficient, C
|
0.5
|
>>
|
1.1
|
1.0
|
-
|
|
|
Yield Response Factor, Ky
|
|
|
|
|
1.2-
1.35
|
|
|
Banana 2nd year
|
|
Stage length, days
|
120
|
60
|
180
|
5
|
365
|
February
|
Mediterranian
|
Depletion Coefficient, p
|
0.35
|
>>
|
0.35
|
0.35
|
-
|
|
|
Root Depth, m
|
0.30
|
>>
|
>>
|
0.8
|
-
|
|
|
Crop Coefficient, C
|
1.0
|
>>
|
1.2
|
1.1
|
-
|
|
|
Yield Response Factor, Ky
|
|
|
|
|
1.2-
1.35
|
|
|
|
Source: Allen et al., 1998
Table 2.2: Monthly Kc values of Banana for
tropical climate
Months after planting
KC
|
Crop Developmental Stage
|
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
13
|
14
|
15
|
|
0.4
|
0.45
|
0.5
|
0.6
|
.7
|
0.85
|
1.0
|
1.1
|
1.1
|
0.9
|
0.8
|
0.8
|
0.95
|
1.05
|
|
shooting
|
Harvesting
|
|
Source: Allen et al., 1998
| 
