In arid and semi-arid climate, where the study area has been carried out, according to the results of the climate classification, the most relevant method to improve agriculture production is irrigation.
To decide where and how to irrigate, natural conditions, available types of crops and technology, previous experience, costs and benefit analysis, should be considered. In order to avoid negative phenomena from irrigation practices, such as soil erosion and salinisation, an evaluation system for irrigation purposes must be developed.
In order to evaluate land suitability for irrigation in the study area a parametric system (Sys et al., 1991) was applied; this method is based on the standard granulometrical and chemical-physical soil characteristics. The evaluation was carried out in order to estimate land suitability for small scale surface irrigation, so excluding more advanced techniques such as drop irrigation, that could yield totally different results. Only potential land characteristics were taken into account but nothing is here reported about effective irrigation possibilities, i.e. about irrigation water availability.
The factors influencing the soil suitability for irrigation can therefore be subdivided in the following four groups:
The different land characteristics that influence the soil suitability for irrigation are rated and a capability index for irrigation (Ci) is calculated according to the formula:
Ci = A/100 * B/100 * C/100 * D/100 * E/100 * F/100
Where: Ci: capability index for irrigation; A: rating of soil texture; B: rating of soil depth; C: rating of CaCO3 status;
D: salinity/alcalinity rating; E: drainage rating and F: slope rating.
The capability classes are defined according to the value of the capability (or suitability) index (Ci) (Table 28)
|
Capability index |
Class |
Definition |
Symbol |
|
>80 |
I |
Excellent |
S1 |
|
60-80 |
II |
Suitable |
S2 |
|
45-60 |
III |
Slightly suitable |
S3 |
|
30-45 |
IV |
Almost unsuitable |
N1 |
|
<30 |
V |
Unsuitable |
N2 |
Table 28 - Capability indexes for the different capability classes
For slope class, texture, soil depth, calcium carbonate status, salinity and alkalinity, drainage, a weighted average was calculated for the upper 100cm of the soil profile then the considered factors were rated according to Table 29-Table 34.
|
Slope class (%) |
Rating |
|
|
Non terraced |
Terraced |
|
|
0-1 |
100 |
100 |
|
1-3 |
95 |
95 |
|
3-5 |
90 |
95 |
|
5-8 |
80 |
95 |
|
8-16 |
60 |
85 |
|
16-30 |
50 |
70 |
|
>30 |
30 |
50 |
Table 29 - Rating of slopes (after Sys et al., 1991)
|
Textural class |
Rating |
||
|
<15% Gravel |
15-40% Gravel |
40-75% Gravel |
|
|
Clay |
65 |
65 |
55 |
|
Loam |
90 |
80 |
70 |
|
Sand |
30 |
25 |
25 |
|
Silt |
90 |
80 |
70 |
|
Sandy clay loam |
95 |
85 |
75 |
|
Silty clay loam |
100 |
90 |
80 |
|
Loamy sand |
55 |
50 |
45 |
|
Sandy clay |
75 |
65 |
60 |
|
Silty clay |
85 |
95 |
80 |
|
Clay loam |
100 |
90 |
80 |
|
Sandy clay |
80 |
90 |
75 |
|
Silt loam |
90 |
80 |
70 |
Table 30 - Rating of textural classes for irrigation (after Sys et al., 1991)
|
Soil depth (cm) |
Rating |
CaCO3 (%) |
Rating |
||||||||
|
<20 |
30 |
>50 |
30 |
||||||||
|
20-50 |
60 |
25-50 |
60 |
||||||||
|
50-80 |
80 |
10-25 |
85 |
||||||||
|
80-100 |
90 |
0.3-10 |
100 |
||||||||
|
>100 |
100 |
<0.3 |
90 |
||||||||
|
Table 31 - Rating of soil depth (after Sys et al., 1991) |
Table 32 - Rating of CaCO3 (Sys et al., 1991) |
||||||||||
|
Na+ (%) |
Electric conductivity (mS) |
||||||||||
|
0-4 |
4-8 |
8-16 |
16-30 |
>30 |
|||||||
|
0-8 |
100 100* |
95 90* |
90 80* |
85 70* |
80 60* |
||||||
|
8-15 |
95 90* |
90 80* |
85 70* |
80 60* |
75 50* |
||||||
|
15-30 |
90 80* |
85 70* |
80 60* |
75 50* |
70 40* |
||||||
|
>30 |
85 70* |
80 60* |
75 50* |
70 40* |
65 30* |
||||||
|
(*) Clay, Silty clay, and Sandy clay |
|||||||||||
Table 33 - Rating of salinity and alkalinity (after Sys et al., 1991)
|
Drainage class |
Rating |
|
|
Clay, silty clay, sandy clay, silty clay loam |
other textures |
|
|
Excessively drained |
100 |
100 |
|
Somewhat excessively drained |
80 |
100 |
|
Well drained |
85 |
95 |
|
Moderately drained |
65 |
80 |
|
Somewhat poorly drained |
55 |
70 |
|
Poorly drained |
45 |
55 |
|
Very poorly drained |
30 |
50 |
Table 34 - Rating of drainage classes (after Sys et al., 1991, modified)Results and discussion
The results obtained for the study area are summarised in the Table 35 and Figure 24.
Figure 24 - Land suitability for irrigation
|
Class |
Land unit |
Area |
|
|
km2 |
% |
||
|
S2 |
10, 12, 22, 23, 6, 31 |
42,65 |
6,59 |
|
S3 |
1, 8, 9, 11, 13, 14, 18, 34 |
229,73 |
35,51 |
|
N1 |
7, 4, 17, 19, 20, 21, 25, 26, 27, 28, 29, 32, 35, 37, 33 |
157,55 |
24,35 |
|
N2 |
2, 3, 5, 15, 16, 24, 30, 36 |
168,40 |
26,03 |
|
Not Relevant |
U, R, Q |
12,04 |
1,86 |
Table 35 - Absolute and relative extent of different classes of suitability for irrigation
No highly suitable areas are found in the study area. Moderately suitable land units are located in the alluvial valley (western part of the study area), and in the forest of Ben Slimane, where, of course, irrigation transformation is purely theoretical. The largest part of the agricultural areas were classified as marginally suitable, the most limiting factors being physical parameters such as slope, soil texture, and soil depth. This does not exclude the presence of small favoured areas where conditions can be much better. Chemical factors like salinity and CaCO3 content are almost never a limiting factor throughout the whole study area.
On the plateau, large areas were classified as unsuitable mainly due to physical soil limitations: that cannot be overcome under the current management, but the area could be conveniently irrigated with different techniques. In fact, some scattered spots of trees and vegetable orchards with drop irrigation are occasionally present, demonstrating the feasibility of these techniques.
In the eastern escarpments, steep slopes and shallow soils lead to permanent unsuitable conditions for surface irrigation.