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Results

Figures 10 and 11 show the net and gross irrigation water requirements for potential cropping pattern and potential irrigation efficiency with effective rainfall. Table 8 summarizes the figures for each of the 84 zones. NIWR and GIWR for the potential scenario with effective rainfall were further combined with the 136 basic units of this study to obtain individual NIWR and GIWR for each of these units through GIS.

The results have been compared with figures available from country studies (national water master plans, projects, etc.). The comparison shows that the methodology yields relatively accurate regional estimates of IWR that are suitable for the present study. Discrepancies with country studies find their origin mostly in the assumptions made on cropping pattern, cropping intensity and irrigation efficiency, and are discussed in details in Chapter 6.

The influence of cropping pattern zones on the quality of the output is of prime importance. Important differences in irrigation water requirements in adjacent zones is one of the consequences of this approach. For instance, in Burkina Faso, areas located north of the 1000 mm annual rainfall line have a gross potential water requirement of 500 mm per year, while areas located just south of this line need more than 2800 mm per year. This artificial break is due to the choice of the irrigated cropping pattern zones, where it was decided that no rice was cultivated under 1000 mm of rainfall per year. Within the cropping pattern zones, the boundaries of irrigation water requirement zones follow rainfall trends.

The estimates used for cropping intensity and irrigation efficiencies, to obtain the gross irrigation water requirements from the net figures, also have a direct influence on the results presented on the final maps.

The differences in irrigation water requirements between adjacent zones is directly related to the density of the climate stations' network. In low-density areas, such as the Sahara and southern Africa, differences in IWR between adjacent zones are high (up to 600 mm/year gross requirements) as the low station density does not allow the delineation of HIWR zones with smaller differences. A high density of the station network in the rest of the continent, in combination with rainfall raster maps, has resulted in differences of a maximum of 200 mm/year gross requirements between adjacent zones.

Figure 8: Irrigation cropping pattern zones. List of cropping pattern zones.

1 Mediterranean coastal zone
2 Saharan oases
3a Semi-arid to arid savannas in West-East Africa
3b Semi-arid/arid savanna (Somalia - Kenya - Southern Sudan)
4a Rice: Niger/Senegal rivers
4b Rice: Gulf of Guinea
4c Rice: Southern Sudan
4d Rice: Madagascar tropical lowland
4e Rice: Madagascar highland
5 Egyptian Nile and delta
6 Ethiopian highlands
7 Sudanese Nile area
8 Shebelli-Juba river area in Somalia
9 Rwanda - Burundi - Southern Uganda highlands
10 Southern Kenya - Northern Tanzania
11 Malawi - Mozambique - Southern Tanzania
12a West and Central African humid areas above the equator
12b Central African areas below the equator
13 River affluents on Angola - Namibia - Botswana border
14 South Africa - Namibia - Botswana desert and steppe
15 Zimbabwe highland
16 South Africa - Lesotho - Swaziland
17 Awash river area in Ethiopia
18 All islands (Comoros - Mauritius - Seychelles - Cape Verde)

TABLE 7: Irrigation cropping patterns for the 24 zones

1. Mediterranean coastal zone

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

summer

vegetables


p

-

-

-

-

-

-

-

-

h


40

40

winter

wheat

-

-

-

-

h






p

-

15

15

winter

fodder

-

-

-

h






p

-

-

25

25

all year

arboriculture

-

-

-

-

-

-

-

-

-

-

-

-

20

20















100

100

2. Saharan oases

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

summer

vegetables



p

-

-

-

-

-

-

-

h


30

30

winter

wheat

-

-

-

-

h






p

-

30

30

winter

fodder

-

-

-

-

h






p

-

20

20

all year

arboriculture

-

-

-

-

-

-

-

-

-

-

-

-

20

20















100

100

3a. Semi-arid to arid savannas in West-East Africa

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

wet

maize/sorghum







p

-

-

h



90

100

dry

vegetables

-

-

-

h






p

-

-

20

20















110

120

3b. Semi-arid/arid savanna (Somalia. Kenya, Southern Sudan)

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

wet

maize/sorghum




p

-

-

-

h





40

50

wet

cotton




p

-

-

-

-

-

h



30

50

dry

vegetables

-

-

h







p

-

-

30

20















100

120

4a. Rice - Niger/Senegal rivers

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

main

rice

-

-

h








p

-

100

100

secondary

rice






p

-

-

-

h



80

100















180

200

4b. Rice - Gulf of Guinea

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

main

rice






p

-

-

-

h



100

100

secondary

rice

-

h









p

-

50

100















150

200

4c. Rice - Southern Sudan

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

I

rice

-

-

h








p

-

100

100

II

rice













80

100















180

200

4d. Rice - Madagascar tropical lowland

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

main

rice

-

-

h








p

-

100

100

secondary

rice




p

-

-

-

h





30

100















130

200

4e. Rice - Madagascar highland

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

wet

rice

-

-

h








p

-

100

100

dry

vegetables





p

-

-

-

-

-

h


10

10















110

110

5. Egyptian Nile and Delta

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

winter

wheat

-

-

-

-

h






p

-

40

40

winter

fodder

-

-

-

h






p

-

-

60

60

summer

maize





p

-

-

h





50

50

summer

rice






p

-

-

-

h



30

30















180

180

6. Ethiopian highlands

cropping season

main crops

cropping calendar

cropping intensity



J

F

M

A

M

J

J

A

S

O

N

D

actual

potential

wet

maize






p

-

-

-

-

h


40

70

wet

vegetables






p

-

-

-

-

h


60

30

dry

vegetables

-

-

-

-

-

h





p

-

10

60















110

160

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