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4.2.1 Land suitability for irrigation

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.

Methodology

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.


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