4.3 Water requirements in each plot 4.3.1 System
requirements
The system water requirements vary with respect to the system
in place. From the database, the quantity of water that was needed by each plot
was obtained. The number of sprinkler heads which function simultaneously and
the efficiency of the system concerned were considered in these calculations.
Figure 4.3 shows a report of a query to calculate the system requirement using
MS Access 2003.
The various systems found in the PHP group and their
characteristics are: Big gun (canon) system
Rain bird big guns: 60 m3/h at 5 bars
Rainfall depth: 10 mm/h
Microjet system
Rondo type: 300 l/h at 1.5 bars
Rainfall depth: 3 mm/h
Undertree system
Rain bird type: 620 l/h at 3.0 bars
Rainfall depth: 4.78 mm/h
Figure 4.3: System water requirement as calculated in MS
access
4.3.2 Crop water requirements
Climatic data for a period of 20 years (1989-2008) from
meteorological stations in the plantation were used in this study. These
stations provide rainfall and pan evaporation data.
The average annual rainfall varies from 2400 mm to 3200 mm and
distributed as thus in the plantation, with Mantem being the highest in
elevation and Bonanadam the lowest in altitude:
Mantem : 3200 mm/yr
Loum : 3200 mm/yr
PHP-haut : 2800 mm/yr
Dia-Dia, Bonandam : 2600 mm/yr
Sir, Mpoula : 2500 mm/yr
Four : 2400 mm/yr
There seems to exist a relationship between altitude and
rainfall and enables us to distinguish the high altitude plantations (Mantem
and Loum), low altitude plantations (DiaDia, Bonandam, Mpoula, Sir and Four)
and intermediate altitude plantations (PHP-haut). The evapotranspiration was
calculated in the database by applying a pan coefficient, Kp of 1.1 as
indicated by Allen et al. (1998) and the crop coefficient as given by
the database.
The RAW was found to vary by a factor of 10 with respect to
the type of soil and by a factor of 4 with respect to root depth. The RAW was
considered to be 10 mm for every 10 cm of root depth.
A probability study for the risk of non satisfaction of the
crop water requirements was done on the 20 years climatic data available. All
the rainfall data was assumed to be effective because of the following
reasons:
· Rainfall in the dry season are the most susceptible to
modify the terms of the water balance equation. We thus considered that during
this period due to the physical properties of the soil (high water retention,
high hydraulic conductivity), the quantity of water loss as run-off and
drainage is negligible.
· Rainfall data obtain were already cumulated for each
month and it was therefore difficult to distinguish rainfall that are less than
5 mm/day as proposed by Smith et al. (1998).
· Most of the soils of the group are andosols and
hence have a high capillarity. This upward movement of water is thus considered
to compensate for the non effective rainfall.
Table 4.4 gives the probability of satisfaction of irrigation
requirements for a total of 20 years so as to better schedule irrigation with
the use of the database knowing the risks that could arise I these water
requirements are not fully satisfied. The table shows a summary of the average
monthly water requirements for banana in the area.
Table 4.4: Probability of satisfaction of crop water
requirements (requirements in mm)
|
Jan
|
Feb
|
Mar
|
Apr
|
May
|
Jun
|
Jul
|
Aug
|
Sep
|
Oct
|
Nov
|
Dec
|
1 yr/2
|
>102
|
>69
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
>31
|
>98
|
1 yr/5
|
>137
|
>127
|
>50
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
>82
|
>127
|
1 yr/10
|
>144
|
>139
|
>65
|
=0
|
=0
|
=0
|
0
|
0
|
0
|
0
|
>123
|
>139
|
1 yr/20
|
172
|
138
|
124
|
64
|
34
|
36
|
0
|
0
|
0
|
9
|
133
|
149
|
|
If we consider a probability of satisfaction of crop water
requirements 1 yr out of 5, then we need to apply a total of
137 mm of water per month, an average of 32 mm/week.
If this risk of satisfaction is considered to be 1
yr/10, the quantity of water to be brought in through irrigation is
144 mm per month, giving an average of 33.5 mm/week.
Taking a risk factor of 1:20, that is, satisfying the crop
water needs 1 out of 20 years, the water required by the crops will be 172 mm
per month giving an average requirement of 40.2 mm/week for an
average root depth of 50 cm.
With respect to the irrigation systems this water requirement
could further be adjusted by applying the efficiency of the system,
Keff. Table 4.5 thus shows the weekly dose of water to be applied
taking into consideration the various satisfaction probabilities and the
irrigation system concerned.
Table 4.5: Irrigation dose (mm) for two irrigation
systems
Probabilty of non satisfaction Undertree
Microjet
of irrigation water Keff =0.8 Keff =0.9
requirements
1 year/2
|
30
|
26
|
1 year/5
|
40
|
35
|
1 year/10
|
42
|
37
|
0 risk
|
50
|
45
|
|
This water requirement is been fractioned and applied three
times in a week for soils with light textures as shown in the irrigation
calendar in Appendix II. Thus for an application of 40 mm a week, the
application could be 13.3 mm in 3 days.
When this water requirement is calculated, depending on the
events of the previous day, the value is adjusted in the database. If for
example, we simulate a situation where the effective rainfall is say 10 mm and
that ETc is 4 mm then for a 21.3 mm crop demand, we would apply only
7.7 mm after a reduction of Peff and ETc. This shows that
7.7 mm of water could be saved. This could lead to reduction in the pumping
time and cost of operation and increase in marginal profits.
| 
